Purpose of this document:
This page was created in an effort to answer some of the most common questions that are asked of the frequency coordinator, provide an understanding of why a frequency coordination body is necessary, to clarify some frequency coordination policies, and to de-mystify the process.FAQ Index:
It should be understood that the needs and actions of the frequency coordinator are not arbitrary, but are based on technical knowledge of the systems involved, familiarity with the coverage of many of the involved sites, and past experience with the amateur community in terms of its needs, expectations, and anticipated future requirements.
While every attempt has been made to make this page as informative and clear as possible, it is likely that something was overlooked. In matters of policy, the ultimate authority is a document called The Policies of the Frequency Coordinator and its interpretation by the frequency coordinator, not this document. Please note that this page addresses frequency coordination based on the Utah bandplan and that certain frequency recommendations may not apply in other parts of the country.
Here is a quick index of topics covered in this FAQ - click to jump to that topic.
|"Why do we even have frequency coordinators, anyway?"||Back in the
early part of the century, radio pioneers Marconi and
DeForest tried to use radio to relay results of a boat
race to two competing newspapers. The Interference between
the two stations was so bad that neither one had much
success. It became clear that for two transmitters to
operate at the same time, they needed to operate on
different frequencies with adequate spacing between them.
Thus began the science of frequency allocation and
The FCC doesn't assign particular frequencies to amateurs. In day-to-day operation, amateurs simply listen to a frequency before transmitting to assure it is available. But for some kinds of stations, notably repeaters and some fixed auxiliary stations, it is almost mandatory that they operate on fixed frequencies. The remote locations of these stations and the need for filters using cavities make these stations hard to operate on changing frequencies. Additionally, for users to find them, their frequencies must be constant and well-known.
The problem of finding frequencies for repeaters and auxiliary stations so they do not interfere with each other is given to a frequency coordinator. In the early days of repeaters, frequency coordinators simply made sure that stations close enough geographically to have an "overlap zone" used different frequencies. As available frequencies filled up, frequency coordinators effectively gained authority to say that there is no space for a proposed system on a particular band. When several amateurs vie for a single available frequency, a frequency coordinator may have to make choices based on which proposed system will provide the greatest benefit to the largest number of amateurs.
|"How does someone get to be frequency coordinator in the first place?"||The Utah VHF Society was
formed in 1968 to promote the installation and use of
VHF/UHF repeaters in the state of Utah. Part of this
task involves being a coordination body which operates as
a central clearinghouse for information on the usage of
the VHF/UHF spectrum within the state, as well as
functioning as an arbitrator to help prevent/resolve
disputes that occasionally arise.
The office of Frequency Coordinator is an elected one, chosen yearly at the election meeting. For the past several years this meeting has occurred immediately after the annual swapmeet which is usually held in February or March. Although it is not required by the bylaws, the frequency coordinator should be someone who possesses a reasonable degree of technical skill and have the resources to make informed coordination decisions.
It should be pointed out that the
legitimacy of established frequency coordinating
entities has long been established in the eyes of the
FCC: As spelled out in FCC section §97.201 (see below) the actions of the frequency
coordinator carry weight in disputes. For several
decades now, the Utah VHF Society has been recognized as
the official Amateur Radio Frequency Coordinating entity
by the ARRL and other national coordinating bodies, and
it is through such affiliation that recommendations by
the VHF Society Frequency Coordinator pertaining to
disputes that may arise tend to be strongly considered.
For a recent example of this the FCC's working with frequency coordinators to help prevent/resolve disputes, read the article FCC Commends Band Plans in Enforcement Letter.
|"My repeater broke/quit working/was struck by lightning/buried in snow and will be off the air for a while. What do I do?"||One of your first orders of business
should be to contact the Frequency Coordinator, IN
WRITING, and explain:
Please note that you are expected to either return your system to operation in a timely fashion or relinquish the frequencies if you have no further plans to do so!
If there are extenuating circumstances that may explain delay in notification or return of the system to operation, the frequency coordinator will consider these on a case by case basis.
|"What are these 'Bandplans' that I keep hearing about?"||Although the FCC has already defined
specific portions of each band for use with specific
modes, there exists a need, in a somewhat less formal
manner, to further divide the bands according to the types
of operation encountered in everyday operation.
While these "band plans", unlike the FCC's defined
segments, do not carry the weight of law behind them they
are an integral part within the framework of a number of
widely recognized "gentleman's agreements" that, by
general consensus of the occupants, play a large part in
determining which operations may occur and where.
For more information, go to the UVHFS Utah VHF/UHF Bandplan page.
|"Why in the heck are Utah's 70cm repeaters 'backwards' from everyone else?"||In most parts of the
country, UHF (450 MHz) repeaters operated by government
and commercial entities use a "Low-Output, High-Input"
split: That is, they use a positive
Many areas (including Utah) are different: The standard is to use a negative split (i.e. "Low-Input, High-Output") - and for good reason. In the early days of 70cm repeater operation, it was immediately noted that the best sites for locating repeaters already had existing UHF transmitters on-site. Because commercial users use a positive offset, their transmitters were (in some cases) just a few hundred KHz above the top of the ham band.
This provided a very good case for avoiding a positive split on the amateur frequencies: Placing a repeater input just a few hundred KHz away (or even a few MHz) from a UHF transmitter (like a multi-hundred watt 454 MHz paging transmitter) was just asking for trouble. Using a negative split for the amateur repeater places the receiver an additional 5 MHz farther away, making co-location with commercial users more practical.
|"I want to put up a repeater with a phone patch at my house. Gimme a frequency."||Unused
repeater pairs, especially 2 meter repeater pairs,
are virtually nonexistent in all metro areas in the
country. The Wasatch front is no exception.
Occasionally, a repeater pair will become available by attrition (the owner/trustee moves away, loses interest, dies, and no one takes over) and/or by mutual agreement to implement protection measures such as tone access, directional antennas, site location chosen to intentionally limit coverage. If frequency sharing is to be considered, all parties agree, in writing, to accept the potential of some degree of interference from each other as well as spell out the responsibilities of the parties involved and the means by which these problems are to be resolved.
Unfortunately, pairs don't become available very often on 2 meters (and, increasingly, on other bands as well.) For this reason, in fairness to the largest number people, when a repeater pair becomes available (on any band) first consideration is given to those who can put the repeater in a location that will benefit the greatest number of people and/or provide a service and/or demonstrate a technology that has the potential of furthering the art of communications. Quite frankly, a limited-coverage repeater at someone's house with a closed phone patch doesn't fulfill this requirement.
|"I have already bought/was given/stolen a repeater. It is worth a lot of money! You must give me a frequency now!"||WRONG!!!
You just can't buy a frequency - and spending a pile of money on a bunch of equipment isn't going to create a frequency where none was available. True, you may have better equipment and/or location than other repeaters that are on the air, but that doesn't enable one to exercise any sort of "eminent domain" on someone else. If you find yourself in this unfortunate situation, you have several choices:
|"I have listened on xxx MHz and I haven't ever heard anything. Can't I put my repeater there?"||Just because
you can't hear anything on a particular frequency doesn't
mean that it is not being used. Especially if you
are using a handie-talkie, you may just not be able to
hear it. It's possible that you may be able to hear
something on that frequency just across the valley.
Even if you can't hear anything anywhere in the valley with state-of-the-art equipment it doesn't mean that the frequency can be used without interference. Repeater site A may not be able to hear a trace of repeater B and vice-versa, but if there's an area somewhere between the two where users can access both sites, it may not be practical to share the frequency. In other words, the places where you have to check aren't just where you are going to put the repeater, it could be somewhere totally unexpected and not just in the midpoint between the proposed site and the nearest existing one on the same frequency. The experience of the frequency coordinator (and others) can be invaluable in determining the suitability of various sites in this respect.
If you do find a frequency that you think can be shared, and if you can come to some agreement (in writing!) with the other potentially affected user(s) of that frequency as to how you can implement a frequency-sharing plan, and the plan is a sound one, then some serious consideration will be given to the proposal. Frequency re-use plans that are carefully considered and implemented are encouraged by the frequency coordinator.
|"Who is hoarding all of these frequencies, anyway?"||They aren't
being hoarded. Owing to geography, usage patterns,
specific monitoring location, the use of subaudible tone,
or the fact that the repeater may be temporarily
off the air, a frequency may appear to be vacant
when, in reality, it is very much in use. Keep in
mind that there are locations in the valley where
it is possible to key up a repeater on almost every pair
on 2 meters! For a list of all 2 meter and 70 cm
repeater pairs and how they are being used around the
Wasatch Front, go to the "2
Meter Repeater Pair Utilization along the Wasatch
Front" and the corresponding 70cm pair utilization page.
Another thing: Before you consider a frequency too strongly, make sure that it is actually in a repeater subband! (Yes, coordination requests like that are frequently received...)
|"On 2 meters, we could put more repeaters on the air if we went to a 15 KHz spacing, right?"||This is an
oversimplification of the problem. As it turns out,
you cannot put two repeaters only 15 KHz apart and
have usable results unless you take certain steps:
In Utah, that is definitely not the case: The vast majority of the population lives along the Wasatch Front. Furthermore, there are only a limited number of sites that have reasonable coverage (i.e. atop a mountain) and so the existing repeaters tend to be located in clusters. Finally, since they are atop mountains, they cover large geographical areas and are thus poor candidates for the 15 KHz spacing. In the 80's when there was a big push to adopt either 15 or 20 KHz spacing, the VHF Society carefully considered both options. Upon changing to a 15 KHz spacing "on paper" it was soon discovered that if 15 KHz spacing were to be adopted, it would allow fewer repeaters along the Wasatch front than the 20 KHz spacing!
By the way, are you wondering why, for example, the 146.61 repeater on Abajo peak does not seem to fit in the Utah bandplan? That's because it doesn't! Because of its remote location and proximity to Colorado, it has been worked into Colorado's bandplan, which is 15 KHz.
|"I notice that repeater frequencies like '146.85' and '145.40' aren't being used. Can I put my repeater there?"||The simple answer is no, as
Utah is on a 20 KHz bandplan. The nature of FM does
not allow two channels 15 KHz (or closer) to each other to
be used simultaneously without mutual interference.
In areas (such as California) where they use 15 KHz
channel spacing they have to go through great pains to
permit 15 KHz channel spacing (see the above
Q&A about 15 KHz spacing.)
If you did put two repeaters just 10 KHz apart (even if one was relatively weak at a particular location) there would be enough splatter to open the squelch of a receiver on either channel (both the repeater's receiver and the user's receiver! And no, running subaudible tone is not the answer!)
(If you still think that adding a subaudible tone will help, you should read this first.)
|"I'm looking at the VHF Society repeater list and I see some repeater 'test pairs' marked 'SNP.' What are those for?"||There are several so-called "Shared,
Non-Protected test pairs" (SNP) that are set aside for
special purposes. Some uses of these pairs might
Along the Wasatch Front, all new 70cm and 2 meter repeaters are initially placed on one of these test pairs. Why? As it turns out, most proposed repeaters are never built! In the past, when repeater pairs were more readily available than now, a pair was simply assigned to these "paper" repeaters. At one point, years ago, there were nearly as many "paper" repeaters as real ones - and it's an unfortunate truth that trying to reclaim long-unused pairs often resulted in resentment as the holder perpetually "still planned to put it on the air..." Assigning proposed new repeaters to the SNP allows testing of the repeater - if it is ever built - but does not tie up valuable spectrum if it isn't.
The available test pairs are as follows:
|"If you can't coordinate
simplex frequencies, then why can you tell me what I can't
use for simplex operations?"
||You may be wondering why,
if frequency coordinators deal only with "coordinated"
frequencies, why they have any business dealing with
"simplex" frequencies at all? First of all, one must
remember that one of the primary duties of the frequency
coordinator is to keep track of the uses of all
amateur VHF, UHF, and Microwave frequencies to make sure
that chaos doesn't result from ill-considered
operations. Closely related to this is to make
certain that those operations that do occur on the
bands in question are, in fact, compatible with the other
operations that occur on that same band.
Clearly, not all possible needs can be accommodated by our limited amount of spectrum, so a combination of common sense, technical savvy, and cooperation is required in order to allow as many users to peacefully co-exist on the band as possible.
When it comes to simplex operations, one must realize that frequencies may be coordinated if they are not listed in repeater lists. Why is this? It is because that it is not only repeaters that are coordinated, but so are control, auxiliary, and special-purpose links - and it is not necessarily in the best interest of the operators of those systems to publish certain details of their operation. One reason for such apparent "secrecy" would be for an inter-system link - that is, one that ties one repeater to another: It is often the case that in a system of this sort, direct access of the system by an uninformed user on this frequency would, in fact, disrupt the system.
For this and other reasons, a close relationship is maintained between the frequency coordinator that the person who is keeping track of local simplex usage see the "Simplex Frequency Manager" on the UVHFS Simplex Frequency Usage page.: Because both people are familiar with the technical aspects of radio communications and the types of systems that are using the various frequencies, it is best that those who need to request additional frequencies keep close contact with them.
|"What is the deal with these 'Coordinated' simplex frequencies?"||Believe it or, there is really only one
"coordinated" simplex frequency on 2 meters and it is
146.52. This is a de facto coordination that
provides for a common frequency on which contacts may be
While simplex frequencies are not coordinated, they are assigned to various groups. Most of these groups are localized and thus it would make little sense for them to use a wide-area coverage repeater (assuming that there were enough repeaters to accommodate them in the first place.) By publicizing the use of various simplex frequencies by these groups, potential conflicts (such as scheduling of nets as well as interference issues) may be minimized.
This list of simplex frequency usage is the basis of "gentleman's agreements." It is a matter of courtesy that one would refrain from using a frequency during the scheduled activities of a particular group. Likewise, it would be improper for anyone to discourage the use of a frequency by anyone other than a member of the group to which the use of a particular frequency is attributed. Remember, the amateur radio frequencies are a shared resource!
If it is determined that use of a frequency results in interference with another group or to a coordinated system, it is imperative that the frequency coordinator be notified immediately to facilitate resolution!
For a list of Wasatch Front simplex frequency usage, click here. It should be noted that this list of simplex frequencies is managed by John Mabey, W7CWK and is subject to review (and revision) by the frequency coordinator.
|"I have a mobile/portable repeater that I want to set up. Give me a frequency."||For the reasons outlined above,
assigning a full-time repeater pair to a user that will
only be using that pair on rare occasion can hardly be
justified. For this reason, one of the "test pairs"
may be assigned for the occasional mobile/portable
It is in the best interest of the amateur community that these mobile/portable repeaters are just what they claim to be: That a "temporary" mobile/portable repeater is not to turn into a permanent one! If possible, information on who is using a test pair (and where) will be made available online.
|"I want to leave my radio on the 'Crossband Repeat' mode for (some event.) What frequency can I put this on?"||Many radios have the ability to
function as a crossband repeater where signals are
automatically received on one band and retransmitted on
another, usually between 2 meters and 70 cm.
Generally when these radios are operated in this manner
they are configured to retransmit what they hear on 2
meters on 70cm and can switch automatically to
retransmitting what they hear on 70cm on 2 meters.
Unfortunately, they are rarely operated legally! Most radios capable of crossband operation cannot automatically perform the function of a legal ID: In most cases, it may not be possible (or practical) to ID the link in both directions as required. (Note: YOU may be identifying your station as you transmit through the crossband to, say, a repeater, but your crossband may not be identifying legally as it retransmits the repeater on another band.) Additionally, the FCC rules require that some means of control be implemented in the event the repeater needs to be shut down (such as in the case of a malfunction that causes interference to another repeater or radio service.) Remember that most radios in crossband repeat mode will be stuck in transmit as long as there is a signal on the other band's input and in that state remote control, if available, may not even be possible.
It is possible to operate some radios
as a remote controlled station instead of a
repeater. In that case, you can remotely operate a
radio that is transmitting/receiving signals from one
band to another and it can be argued that this is not
a repeater. It should be remembered that this is only
the case if there is, in fact, a definite means of
remote control on a frequency on which it is legal to do
Assuming that the operator has taken the trouble to provide a legal ID and control mechanism in the case of a crossband repeater, there is often the tendency of many crossband repeater operators to forget that the device they are operating is a repeater and may be operated only in the repeater subbands!
|"Where can I operate my simplex repeater?"||A simplex
repeater is a "store and forward" device. That is,
it operates on just one frequency and, when it
hears a transmission, it records that transmission and at
the end of the transmission it plays back what it
"heard." Typically, these repeaters have 20-30
seconds of recording time (much more makes them very
tedious and awkward to use...) As in the case of
crossband repeaters (mentioned above) the frequency
coordinator considers that they really are
repeaters and must be operated in a repeater
Some suggested operating frequencies on 2 meters include those in the 146.42 to 146.58, and 147.400 to 147.58 area, avoiding the most heavily-used 146.52, 146.54, 147.42, 147.54, and 147.60 MHz simplex frequencies. Always listen before using a frequency and make yourself aware of regular users - and let them know what you plan to do. To prevent the simplex repeater from being an unintended nuisance and potentially making the frequency unusable to any others, configure the system such that a subaudible tone is required for access.
A WORD OF WARNING: The
FCC rules could be interpreted to regard a simplex
repeater in the same manner as a full-duplex
repeater. If this is the case, the repeater must
be operated in a portion of the band in question where
repeater operation is allowed. For example,
a simplex repeater may not be operated below 144.500 MHz
or in the range from 145.5 to 146.000 MHz.
The rules also state that, for repeaters, some means of control is required. This could be a person at the repeater site, or some electronic/remote means of control. Because of some recent "incidents" involving simplex repeaters, it has become apparent that not all simplex repeaters are equipped with a means of remote control - or even an automatic IDer! In these recent incidents, it was the intention by the repeater's operator to be able to manually control the equipment and/or manually ID, but in these cases he/she simply forgot to shut the equipment off or left it on when they were unable to quickly return to the control point. In these cases, the identity and location of the now-interfering simplex repeater was unknown, owing to complete lack of an automatic ID.
If you are able to set up a simplex repeater so that it can be operated lawfully (e.g. with ID, appropriate control, etc.) any of the suggested simplex frequencies where repeaters may be legally operated can be used (aside from the most popular ones such as 146.52, 146.54, etc.) and that you are certain that its operation will have minimal impact on those who already monitor/operate there. You should also configure any simplex repeater to use subaudible tones so that casual or random operations on the frequencies involved don't activate the repeater.
|"Where can I operate my simplex Echolink/IRLP/whatever node?"||These days, it is common
to read/hear about people who have connected their radios
to the internet providing virtual linking all over the
known universe. Typically, this is done using
Echolink or IRLP - although a number of other similar
There are a number of "high profile" IRLP systems - such as the full-time links operated by WA7GIE and the IRLP node on the 146.76 repeater that is open to UARC members. These offer excellent, wide-area coverage for both "conventional" repeater users and those connected via the internet and are connected to high-reliability links.
It is also common for people to want to put up their own "private" Echolink or IRLP nodes - but there's a problem: These usually are NOT associated with a repeater and, therefore, the frequency of operation cannot be readily "coordinated" owing to the widespread use of all simplex frequencies on 2 meters and many simplex frequencies on the 222 and 440 MHz bands. For this (and other) reason(s) it is recommended that simplex node operation NOT be carried out on 2 meters as this band is so heavily-used by a lot of different groups.
Why not "coordinate" such frequencies? This has not been traditionally done as amateur radio operators are free to choose their own operating frequency - with the possible exception of currently-coordinated repeater frequencies - the latter authority having been granted by the FCC to bona-fide coordinating bodies: In case you didn't already know, the Utah VHF Society is, in fact the "bona-fide" coordinating body in Utah!
Simply plopping a node on any old simplex frequency - especially on 2 meters - is NOT a good idea - and for several reasons:
If you wish operate a node on a simplex frequency, you should:
got an idea: Can't we re-use the same
frequencies if we all run really low power - like the
||In the world of
cellularized communications systems (such as cell/PCS
phones, and many emerging data networks) the re-use of
frequencies is made possible via the use of, among other
things, transmitter power control
This is not a new idea: FCC §97.313(a) clearly states that "An amateur station must use the minimum transmitter power necessary to carry out the desired communications." This decades-old rule makes sense, as not only is excessive power wasteful, but has the increased probability of causing interference to other stations on the same or nearby frequencies.
With the increased pressure on spectrum, commercial entities (such as wireless telephone companies) have also capitalized on this idea - and taken it to further levels:
While, at first glance, it might seem that such schemes could be implemented via amateur radio, this is only partially true:
As can be seen from the examples, while frequency-reuse on amateur frequencies is possible, it is a simple fact that the extent to which this may be done is limited by our system topology - which includes several important things:
Again, remember that it is NOT a good idea to attempt to use subaudible tones on simplex frequencies in an attempt to achieve frequency re-use!
|"Nobody owns a frequency. Why can't I operate wherever I choose?"||While it is
true that no amateur owns a particular frequency,
the FCC rules state the following:
- Section §97.201 of the Amateur Service rules:
(c) Where the transmissions of a repeater cause harmful interference to another repeater, the two station licensees are equally and fully responsible for resolving the interference unless the operation of one station is recommended by a frequency coordinator and the operation of the other station is not. In that case, the licensee of the non coordinated repeater has primary responsibility to resolve the interference.This clearly gives the party that is operating a coordinated system priority over those who are not - a fact re-emphasized by recent FCC enforcement action.
This does not address the use of those non-repeater operations that aren't coordinated, such as various simplex frequencies. Since the amateur service is largely self-policing, there are a number of "gentleman's agreements" that offer recommendations and provide guidelines as to what may be operated where. It is in all of our best interest to follow these guidelines as well as the recommendations of the frequency coordinator. If a dispute should arise that, for whatever reason, cannot be resolved by the parties involved, it is recommended that the frequency coordinator be consulted for adjudication.
|"If no-one owns a frequency, then why are there these 'closed' repeaters and/or autopatches?"||This can be a point of
contention for some. While it is true that no-one
owns a frequency, there are some repeater owners/operators
that wish to limit the access of their repeaters to a
certain subset of the amateur population (such as club
The cases where this may be so includes:
|"Why have subaudible tone (a.k.a. 'PL') access on a repeater?"||In the amateur radio tradition of
cooperation and public service, most repeater operators
strive to make their repeaters available to
everyone. (There are a few exceptions, such as
linked repeater systems, etc. that strongly encourage
membership - for more information, read the section
above.) In light of this, repeaters have
traditionally operated COR (Carrier Operated Relay.)
That is, the repeater is activated if there is a signal
Unfortunately, COR can be activated by any signal - even ones that are not supposed to be there. Most of the repeaters in the mountain west are located on sites with good vantage points such as mountaintops and ridges. By necessity, these sites are often crowded with many other radio users. It is an unfortunate fact that, when multiple transmitters are co-located, some "mixing products" occur. These products (also known as intermod) can be created in the receiver itself, in other transmitters, in metal-to-metal contacts of nearby structures, and conductors, to name a few. There are also those occasions when a nearby transmitting system may be malfunctioning (or just of poor design) and radiating signals on frequencies where it should not, or any combination of the above.
In an ideal world, these other transmitters (and receivers!) would all operate independently of each other and not cause any sort of QRM. Unfortunately, this is the real world. Noise, spurious emissions, and intermodulation products do get created and they do get into other systems. As population densities increase and radio sites become more and more crowded, these problems are becoming increasingly more common.
While good system design will minimize these sorts of problems (and that's assuming that those sharing your site have designed their systems well) but not always will they be completely avoided. If everything has been tried (as far as reduction of interference is concerned) then the last choice may be to make the repeater tone-access only.
It should be remembered that adding
tone access will not solve the problem, but it
may mitigate it. A common scenario is that a
repeater will work perfectly most of the time but
occasionally, it will start squawking, making noise, and
key up (kerchunk) at random. A repeater that does
this, even occasionally, is very tedious to
monitor. This is a case where it might make sense
to put tone access on a repeater.
All of the above applies to
digital tone-signaling schemes (such as DPL) as
As with other aspects of repeater operation, tone squelch operation (including frequency) should be coordinated with the Frequency Coordinator. This helps the Frequency Coordinator maintain accurate records and, in cases of frequency re-use issues, prevents the same tone from being assigned to two repeaters with potential overlap issues. Furthermore, this allows accurate tone information to be made available for publication (if this is desired) in repeater directories and databases.
Finally, when a tone is needed, it is recommended that a 100 Hz tone be used where possible. This is the ARRL recommended frequency for open repeaters with tone access. In this area, 88.5 Hz is commonly used by the ARES groups while 123 Hz is used by the Ogden group. Keep in mind that there are a few people that have radios that are capable of only one tone frequency (for all channels) and that the use of many different tones may complicate the programming of already difficult-to-use radios and the plethora of tones makes access to these repeaters more difficult.
One other instance where tone access may be desirable is to minimize the effects of repeater overlap. As an example, there are two 146.34/146.94 repeaters in Utah: One is located atop Farnsworth Peak, just west of Salt Lake City, and there is another one on Frisco Peak, located north of Cedar City in the southwestern quadrant of the state. These repeaters are quite distant from each other, but there are a few points (in the middle of the desert around Delta, Utah and in various parts of Utah county) where it is possible to access both repeaters simultaneously. As rare as this is, it was decided that each of these two repeaters should be equipped with subaudible tone access (different frequencies, of course) to reduce the effects of a user's operation into one affecting the other. Remember that if you are accessing two repeaters at once, part of the burden is for you to do what you can to reduce this interference potential: This might include cessation of operation, operating at a lower power, changing location, or using a directional antenna.
It should be important that the use of tones on repeaters is a decidedly different technical issue from using them on simplex frequencies: In the case of repeaters, the interference potential amongst the repeaters and the groups of users is well-known. In the case of simplex use, the fact that users' geographical location may be random and the fact that simplex frequencies are expected to be shared makes the use of tones (or digital codes) on these frequencies a bad idea! (Read the next topic for more info on the use of tones on simplex.)
|"What about using subaudible tones (a.k.a. 'PL') to allow more people to 'share' a simplex frequency?"||Several years
ago, someone decided that, if the different local groups
using the same simplex frequency all used their own
subaudible tone frequencies, that this frequency could be
"shared" and all of these different groups could use this
same frequency at once without the different groups
hearing each other.
They were right - these groups were no longer hearing each other.
The problem was that a lot of the people trying to talk amongst themselves in these groups could not reliably hear each other, either! Why? It's the nature of radio (and, in particular, FM) communications: The strongest signal will always win - tone or not! If the strongest signal that "captures" your receiver doesn't have "your" tone, you will hear absolutely nothing! The same is also true of signals that are of "approximately" equal strength as well: Neither signal will be clearly audible due to the interference and any tones (or codes) cannot be properly decoded!
A common occurrence in this situation was as follows: One station was receiving the tone-encoded transmission from another station. Suddenly, the signal seemed to disappear for a few seconds - and then reappear. What happened? Another signal of the same or stronger strength appeared on frequency - but without the tone frequency for which your receiver was set - and covered up the other station. With the tone missing, corrupted, or invalid the receiver faithfully muted the speaker.
What about "Digital Squelch" systems? Again, the issue is not trying to find a "unique" code to tag a transmission, but the fact that signals of similar strength will simply obliterate each other: This obliteration (unintentional "jamming") will make any coding that you may have applied irrelevant!
Tone and digital squelches are not conducive to the sharing of simplex frequencies!
Perhaps a more important reason for not using subaudible encoding/decoding on simplex frequencies is that it goes against the very notion of sharing a simplex frequency: If your subaudible tone decoder is active, you cannot hear any other activity on that frequency and if you transmit, there is a good chance that you will disrupt their communications as well!
Read the previous topic for info as to why a tone is OK on a repeater.
|"It sounds to me like getting a repeater on the air is tough. How do I go about actually getting one on the air?"||No-one said that it would
be easy. Here are some things that you should keep
in mind and some questions you should ask yourself if you
are serious about putting up a repeater:
|"I done some 'figgering here, and it looks like I can shoehorn in a repeater/link if it uses a split that is opposite everyone else's on that part of the band. Can I do this?"||Generally speaking, putting a reverse
repeater pair in amongst the rest of the repeaters is not
a good idea. For one thing, most users are at
"ground level" - that is, their simplex coverage is rather
limited as compared to that of a mountaintop repeater, so
their signals to a repeater's input are going to be
relatively limited in their coverage area. A
repeater, on the other hand, is often located where it
will offer the best coverage for the area intended - and
that may overlap other repeaters that may be on the same
frequency and quite far away.
If the two repeaters share the same output frequencies, then they don't really "know" of each other's existence (i.e. they will not cause interference to each other, although those people that happen to be in overlap area(s) will likely experience some interference) because their output frequencies are the same. Take these same two repeaters and invert one of them so that the output of one is the input of another, and vice-versa, and you can have some real problems. If there is the slightest chance that they will "hear" each other, then one will need to put a subaudible tone encoder on the inputs to prevent feedback, for starters. Additionally, if one of the repeaters is transmitting and (even weakly) gets into the other repeater, it will effectively "desense" that repeater, making it useless for repeating weak signals that are intended for it - and tone access will do nothing to prevent this problem. (Read this to learn of an instance where tone access was a bad idea...)
If both of these repeaters are on mountaintops, say, it is likely that they will have to be 200+ miles apart from each other (in many cases, even farther!) to keep such repeaters from bothering each other. Additionally, it must be remembered that there are occasional (but not extremely rare) tropospheric propagational enhancements that may cause paths between such repeaters suddenly improve dramatically (just ask a VHF/UHF weak-signal person!) or appear when no previously known path was known to exist.
In any event, placing an upside-down repeater pair amongst other repeaters necessitates much wider geographical spacing of the repeaters on that frequency - making that frequency unavailable for any use (or re-use) over a much larger area than with a conventional split.
Occasionally a "reverse" pair seems ideal for linking two sites together. If this link is not acting as a repeater, per se, the above considerations need to be recognized in appropriate context: If you are operating a hub-and-spoke system (such as the Intermountain Intertie or the SDARC system) then you may have sufficient justification to "tie up" a pair over an already large area. Recognize, however, that there are some instances where it may be more advantageous to link a system on another portion of the band (the 430 or 420 portion, in the case of 70cm) or on another band entirely rather than "hog" a pair. It should be noted that in these sorts of systems, directional antennas are often used.
In short, while there may, on specific occasions, be definite technical merits of using an inverted pair, it generally does not lend itself to the most efficient spectrum use.
|"D-Star will save us
all!!! We should be putting up D-Star repeaters
||A lot has been said about
D-Star recently. If you don't already know, D-Star
is a digital radio system, capable of carrying both voice
and data, that includes networking and messaging
capabilities and at the time of this writing, D-Star
radios are available only from Icom.
D-Star uses a proprietary (e.g. secret) voice
encoding/decoding scheme called AMBE: Because of its
proprietary nature, "open-source" versions of this codec
are not available. This voice data
stream is then carried on a data stream that uses an open,
published protocol that allows for control,
identification, message-handling and routing - amongst
Others have successfully been able to retrofit other radios by adapting an Icom D-Star module intended for other Icom radios and interfacing it with an analog radio using a simple computer interface. It is also worth mentioning that a few have bought the proprietary AMBE codec chip from DVSI (the owner of the intellectual property rights associated with AMBE) and made their own interface device or have used a "D-Star Dongle" - a device that includes the DVSI codec that can be attached to a computer. Most of these "alternate" means of generating/receiving D-Star are beyond the means of many amateurs, however.
It should be noted that the voice compression used in D-Star is somewhat similar in its operation to the voice compression used in digital cell phones, satellite telephones, and VOIP telephone circuits. Because of the heavy amount of compression, the subtle qualities of the voice are noticeably altered - particularly in the presence of strong background noise.
The digital nature of D-Star - despite its intrinsic error-correction capability and tolerance of signal degradation - should not excuse the system designer from considering the limitations of this - or any - system in the analog/RF domain. Because of the nature of this and other digital systems, signal degradation due to interference, multipath, path loss, or dropoff in transmit or receive system performance can be masked: Unlike with analog systems, casual observations of the D-Star signals being "heard" may not provide many clues as to the problems leading to system degradation - that is, it is difficult to discern whether signal loss is due to a weak signal, multipath, or interference from another source - say powerline noise, spurs from a computer, or intentional interference. Another consideration is that, at the time of this writing, there are relatively few tools available to the maintainer of a D-Star system to aid in the diagnosing of system problems: Existing test equipment is of limited use in the determination of overall system performance and the diagnosing of problems. The semi-proprietary nature of the D-Star system (e.g. the "closed" nature of the AMBE voice coding plus the fact that, currently, only Icom is supplying gear) makes it unlikely that D-Star specific test gear is likely to be available soon, although that doesn't rule out possibility of the modification of existing D-Star radios or adapting the use of existing analog test gear to provide some of this functionality. (The Utah VHF Society has produced a web page with information about using analog test gear for analysis of D-Star systems - it may be found here.)
One of D-Star's potential strengths is its digital nature. D-Star's original intent was to allow for a network of linked systems - connected locally or via the internet - that could allow users to talk to each other from anywhere in the world. This network is growing and various practical challenges imposed by peculiarities of the original design of the open-sourced linking protocol are gradually being addressed.
The Utah VHF Society is looking closely at D-Star and other emerging digital voice and data technologies. One of the challenges is to provide spectrum for the testing, evaluation, and operation of these types of systems without significantly impacting more "traditional" operations. Despite the very crowded conditions on 2 meters and 70cm, effort has already been made to provide spectrum for some of the first systems to be put online.
If your group wishes to put a D-Star system on the air, here are a few things to remember:
|"Yaesu has its own digital audio system
that is sort of like D-Star. How is it
||More recently than
D-Star's introduction, Yaesu has introduced their version
of digital voice for the VHF and UHF amateur bands.
While it uses the same type of audio codec as D-Star, it
uses a different form of modulation allowing twice the
data to be sent in less than twice the bandwidth.
What this means - in theory - is that it should be
possible to obtain better weak-signal performance than
with D-Star since more bits may be sent to provide error
correction. This also means that more data may
accompany digital voice transmissions if so-desired.
While D-Star uses a modulation form called MSK (Minimum-Shift Keying) at 4800 baud, the C4FM signal used by Yaesu - and other commercial digital audio communications systems - uses four separate levels which means that for every "baud", two bits may be sent rather than just one as is the case for MSK which means that the C4FM system used by Yaesu conveys 9600 bits per second of data as compared to D-Star's 4800 bits per second. For a variety of technical reasons, the bandwidth occupied by this C4FM signal is comparable to the average bandwidth of an analog FM voice signal - which is to say that it is a bit wider than a D-Star signal. Being that it is digital, however, it doesn't have the occasional "bursts" of wide bandwidth associated with voice peaks found on a typical FM signal so the level of adjacent-channel interference is much more consistent and thus it is more likely that it could be made to co-habitate at 15 kHz channel spacing than an analog FM voice signal.
Yaesu's implementation has two voice mode: A "wide" digital voice mode and a "narrow" digital voice mode. On the air, they take up exactly the same bandwidth as the difference is related to how much of the 9600 bits per second being sent is dedicated to voice. In the "wide" mode, more bits are used for voice so the audio quality is quite good - very noticeably better than that of D-Star. In the "narrow" mode, however, the bit rate dedicated to voice is much lower and the audio quality noticeably reduced - perhaps slightly worse than that of D-Star - and in that mode one can send much more auxiliary data on the transmission along with the voice payload. Interestingly, testing has shown that the "narrow" mode appears to tolerate data errors better than the "wide" mode and thus has an approximate 1dB weak signal advantage while still retaining intelligibility.
At this point it is not known if/how the digital audio signals from the C4FM signals may be routed on the internet in the same way as voice is. It is worth noting, however, that unlike D-Star, there is much easier access to the "analog" side of the world so that it is theoretically possible that a Yaesu digital repeater could interface directly with the older analog-interface linking systems such as IRLP, D-Star or Yaesu's own WIRES system.
|"D-Star is digital so its audio quality and range has to be better, right?"||D-Star uses a lossy
voice codec: The term "lossy" means that at least some
of the information in the original audio has been
discarded in order to reduce the amount of bandwidth (e.g.
number of bits) required to represent some semblance of
the original sound. The higher the compression, the
lower the bit rate, and the more information has been
It is important to be aware of the effects of this "lossy" compression on speech, however. D-Star uses a class of codecs referred to as "MBE" or MultiBand Excitation - specifically AMBE (Advanced MultiBand Excitation,) a proprietary codec developed by DVSI. The MBE codecs work somewhat differently from other, more-familiar audio compression schemes such as MPEG, in that they are more-specifically designed to reproduce speech.
In a nutshell, the codec takes advantage of the fact that human speech consists primarily of two types of sounds: Voiced sounds (a fundamental frequency and its harmonics - as in vowels) and unvoiced sounds (such as consonants) that are more-or-less unvoiced bursts of noise. Simply put, the codec decides of the sound is voiced, and if so, it decides which of the harmonics are present and their relative amplitudes; When an unvoiced sound is produced by the user, it analyzes the properties of the noises.
This sort of codec is quite good at mimicking human speech - even at very low (lower than 5 kbits/second) bit rates. As you might guess, as the bit rate is lowered, the representation of the original voice sounds become less precise as more information about the original voice is discarded. A very serious problem of intelligibility arises with these low-rate codecs in the presence of extraneous noise: As the amount of non-voice sounds added to the speech increases, the codec has increasing difficulty in proper reproduction of the speech, as there's often no way for the codec to "know" what is speech, and what isn't, plus the codec's limited data bandwidth becomes more occupied with trying to reproduce increasingly complex combined sounds: This has the inevitable result that the precision of the reproduction of the speech degrades noticeably. When this happens, the codec itself can produce truly odd artifacts when it mistakes the extraneous noise (or the combination of extraneous noise and a voice) for speech!
Note: Most current public-safety implementations of digital voice systems operate at a coding rate higher than that used by D-Star and often include additional noise-canceling techniques that are applied prior to final encoding. This has the implication that the lower coding rate of D-Star makes it even more susceptible than the systems used by the public safety users to the deleterious effects of extraneous noise on speech intelligibility.
This problem has become tragically apparent in the public safety field where there are numerous examples of fire fighters, policemen, and other safety workers having had their lives put in jeopardy (or lost!) because of a breakdown in communications due to various quirks in the modern, narrowband digital voice systems. These incidents have related both to the topology of the system (the fact that they require end-to-end communications through a trunked system rather than having the option of something like simplex that allows in-situ, direct unit-to-unit communications) to the "digital cliff" problem (that is, the inability to determine the actual quality of the RF signal in order to determine how reliable the radio link is) to the breakdown of the speech codec itself due to extraneous noises, such as those from sirens, nearby engines, or even speech muffled by a breathing apparatus: These problems have been increasingly apparent as the use of the digital systems has become more widespread, causing a number of public safety agencies to put the brakes on the rollout of new systems, or revert back to the old, analog systems. Recently, there has been talk of putting a halt on the FCC-mandated rollout of these narrowband systems until these problems have been successfully addressed.
While it is unlikely that many of these instances would affect the amateur radio operator to the extent that they impact public safety workers, it is well worth keeping apprised of these issues and the lessons learned. These lessons include the effects of extraneous noise on the intelligibility of transmitted speech, plus providing education to the user of a digital system about the "digital cliff" that is encountered at the fringes of the digital system's communications range.
is digital so it doesn't bother analog users, right?"
"D-Star is digital, so it can ignore analog users, right?"
"To save space, we can use both both analog and D-Star on the same channel because they won't bother each other, right?"
One who tunes in a D-Star signal on an analog receiver will hear in "buzzy" noise and by just listening to this "noise" the user of an analog receiver will not be able to determine who it is that is transmitting. Remember that a D-Star signal will interrupt and jam an ongoing conversation of those using an analog mode.
Likewise, if an analog signal appears on the same frequency as a D-Star signal, it is likely that the D-Star receiver will simply mute - or, possibly, give a few odd-sounding syllables, grunts and noises as it tries to make sense out of the mess - and D-Star signals are more easily "damaged" and rendered unintelligble by an analog signal than an analog signal by a D-Star signal!
In either case, you cannot operate D-Star and Analog signals on the same frequency and expect them to co-exist in a practical manner. D-Star users have an advantage, however, in that they can program their receivers to detect the presence of an analog signal on the same channel.
This situation is not unlike the use of subaudible or digital-coded squelch on simplex frequencies: Operators doing so may have the illusion of privacy or even the mistaken notion that such operation allows improved sharing, but this is not the case!
|"Because D-Star/C4FM/APCO P-25/Mototrbo
is digital, you have to use special gear to 'DF' it,
There are two general types of DF (Direction Finding) techniques:
For the phase-detection systems, it turns out that many of the systems designed for analog reception will work with a digital audio signal as well, although the "bandwidth-limited noise" nature of the D-Star/C4FM and other digital audio signals may reduce accuracy and sensitivity of some units somewhat - depending on their design and filtering.
Even the signal modulation of these digital audio modes is "digital" in its nature, it is still just an analog signal so one would continue to use the same analog receiver as before in conjunction with the signal meter or direction-finding unit. Of course, if one wanted to "hear" the audio being transmitted on the digital signal being tracked you would need to use a receiver capable of receiving that digital mode on a separate antenna to do so.
|"D-Star is digital and so
much narrower than analog that we can put a whole
bunch of them in the space of one analog signal,
||There has been some
mention of how spectrum-efficient D-Star is as compared
with analog signals and how several D-Star signals can be
crammed into the same space as one analog signal:
One oft-cited instance is the simultaneous operation of
several D-Star signals spaced only 6.25 kHz apart from
each other. While this sounds like an impressive
feat, cursory examination of the bandwidths of the
transmitters, receivers, and link margins will immediately
reveal that this is NOT a good thing to
do! (It is worth
noting that some Icom radios cannot tune in 6.25 kHz
steps anyway, requiring a minimum channel spacing of 10
kHz anyway - see below.)
As it turns out, as of the time of the original writing of this article relatively little had been done to carefully analyze how D-Star signals will co-exist with each other - and with existing analog signals - in the real world, using real radios that people own. In order to answer some of these questions, we decided to make some careful measurements using typical radios. Details of such measurements may be found on this page:
Based on the test data as well as frequency and spectral analysis, the following are recommendations of the Utah VHF Society:
Remember: The above are minimum spacing recommendations. Depending on the specific situation, there may need to be other considerations based on the necessity to protect existing and proposed systems.
A word of warning: Some Icom D-Star capable radios (such as the IC-2200H) cannot tune in 6.25 kHz steps: When considering channel frequencies, choose carefully to make certain that you do not inadvertently exclude users by selecting frequencies to which their radios cannot be tuned!
For 23cm DD (128kbps) mode operations, the currently-recommended channel spacing is 500 kHz - read here for more information about this recommendation.
|"Yaesu's C4FM digital voice is as narrow as D-Star and we can pack more of those channels in too, right?"||Newer than D-Star, Yaesu's
own digital voice format using a modulation type called
C4FM - which stands for 4-level FSK - has a digital voice
codec that operates like that used by D-Star - but it is not
compatible with D-Star.
Rather than using plain old FM (MSK, actually - which is a special case of FM) Yaesu has chosen a modulation scheme in which two bits are sent per baud rather than just one with D-Star. What this means is that the baud rate used by D-Star and Yaesu's C4FM are the same, the latter, sending two bits per baud means that the raw data rate for the C4FM is 9600 bits per second as opposed to D-Star's 4800 bits per second. What this means is that in the "Digital Wide" mode, much better audio fidelity is possible with the Yaesu implementation than with D-Star as most of the bits being sent are dedicated to voice. In "Digital Narrow" mode far fewer bits are used for sending voice and the audio has a more "compressed" sound than in "Digital Wide."
In order to preserve the signal integrity as much as possible, C4FM's bandwidth is comparable to a narrowband voice channel which means that the receiver bandwidth is about the same as that used for standard +/- 5 kHz deviation FM signals. In measurements of C4FM signals it has been determined that the -6dB bandwidth is on the order of 8 kHz, the -20dB bandwidth is approximately 13 kHz and the -40dB bandwidth is about 18 kHz: Please note that these measurements are relative to a modulated C4FM carrier using a 300 Hz resolution bandwidth which means a similar comparison to an unmodulated carrier would yield "better" numbers in terms of occupied bandwidth.
In measuring the bandwidth of a Yeasu C4FM receiver it was found to be approximately 13 kHz wide at the -6dB points, 16 kHz wide at the -20dB points and 19 kHz wide at the -40dB points which makes it comparable to that of a standard (perhaps somewhat narrow) FM receiver intended for +/- 5 kHz analog FM.
What this means is that one would coordinate and space C4FM signals in much the same way as +/- 5 kHz deviation analog FM signals. Considering that like D-Star, C4FM's modulation is quite consistent as compared to FM which can have transient peaks of wider bandwidth from the voice energy, a digital signal like this is likely to be a better neighbor than an analog signal! This also means that it is more likely that one could successfully put two C4FM signals 15 kHz apart than you could two analog FM signals!
|"Once my frequency is coordinated, it is mine, right?"||Remember: No-one owns
a frequency. To prevent valuable spectrum from being
tied up needlessly, there are several limitations on
frequency coordination. For example:
|"What's this about 'Frequency Sharing?' How can one 'share' a repeater frequency?"||Actually, frequency
sharing is done all of the time in other places. On
HF, for instance, frequencies are re-used all of the
time. As an example, numerous groups could use the
same frequency on 75 meters during the daytime and, if
they were far enough apart (daytime signals on 75 meters
only cover a area of several hundred miles radius) the
groups would never be aware of each others'
existence. Likewise, in the commercial radio world,
frequency sharing of VHF/UHF frequencies and their re-use
is the rule rather than the
exception! (Cellular telephone operation, for
example, is an extreme example of such re-use!)
Effective sharing on VHF/UHF requires a bit if forethought, however. Clearly, one would wish to avoid the placement of two systems on the same frequency in the same town (unless, of course, the two owners of the systems can come to some agreement as to how this is to be done...) but re-use by multiple systems separated geographically can work, provided that a few thinks are taken into account:
need frequencies for my emergency services group -
give me some!"
||One of the most important
aspects of amateur radio is that of community
service: This is, in fact, one of the key
justifications for the very existence of amateur radio and
its use of valuable frequency spectrum and such use is
spelled out in the FCC rules in the portion known as the
"Basis and Purpose" and you may have even heard people
refer to amateur radio-related community service as
"paying the rent." To aid in this aspect of amateur
radio, there are a number of groups that organize and
provide training for amateur radio operators to serve in
these roles. This nature of these organizations and
training is widely varied, but it almost always includes
structure and technique for effective communications under
different and adverse conditions.
It should come as no surprise that for training and organizational purposes, many of these groups have certain frequencies on which they conduct their activities. Often, these groups use a combination of repeaters (their own or, with permission, existing repeaters belonging to other groups) as well as simplex frequencies to facilitate these communications, organizational and training activities.
It should also be recognized by members - and especially, organizers of these groups - that the radio spectrum is a very limited resource and its use requires the careful coordination of the activities of many diverse groups to avoid conflicts that might impede the effective communications. Practically speaking, there are simply not enough frequencies for each group to have their "own" frequency!
While it might seem to a good idea if each group had its "own" frequency, this would not necessarily be a good thing. If the intent of a group is to communicate, breaking the organization into too-small groups would unnecessarily complicate the passage of traffic. Conversely, too-large a group can make the proper prioritization and the management of the sheer bulk of traffic difficult: Given the two choices, however, the latter case is arguably easier for skilled operators to manage, as it is always easier to deal with what you do know when dealing with the large group than with what you do not know if you are trying to deal with many smaller, fragmented groups!
One welcome challenge often faced by organizers of such groups is there are often individual, energized members, each being willing to contribute to the betterment of the group as a whole. One aspect of this challenge includes the absolute need to make sure that these eager, members do not unnecessarily duplicate efforts - particularly when those efforts may, when the "big picture" is considered, actually decrease the overall effectiveness of the group as a whole.
One request often made of the frequency coordinator is that by a zealous individual that wishes to have additional frequencies for their local emergency services group. While this may sound like a reasonable request, the coordinator must make sure that such a request is, in fact, one that receives the blessings of the organizers of the group as a whole and not an isolated effort by a well-meaning enthusiastic member: You can surely imagine the chaos that would ensue if each individual took it upon his/herself to try to obtain the limited spectrum resource without appropriate regard to the needs of the organization at large! Without careful management of the spectrum, large groups can become unintentionally fractured with much duplicated effort resulting in an overall decrease of communications effectiveness and efficiency. This is not to mention very real and practical concerns about unintentional interference between the many groups that might result if their efforts are not appropriately coordinated!
To avoid such unnecessary duplication of effort and inefficient fragmentation, organized groups do not permit individuals to undertake their own, isolated efforts in such important matters as frequency coordination and the like. While not always well-publicized, many of these groups have stated policies about such efforts, directing their members to work within the system (and not individually) to maximize both communications efficacy and organizational efficiency.
For an example of one such directive, see the document: "Emergency Response Communications (ERC) Simplex Frequency Policy for the Wasatch Front." While this document specifically applies to the ERC group in particular, its stated goals and intentions are generally applicable to any group that has the need for amateur radio spectrum use to facilitate their communications. Taking the ERC group as an example, individual members should not undertake frequency assignments on their own, but rather work with the group to assure the maximum effect of their individual contributions!
|"I noticed that
my newer radio has FM-Narrow on it. What about
using that to get more frequencies?
have a radio of fairly recent vintage, you may have
noticed that it has an "FM Narrow" (FM-N) mode.
Also, you may be aware that digital voice modes such as
D-Star take up less bandwidth than conventional FM
The "conventional" FM that has been used for decades uses +/- 5 kHz deviation and has receivers that are a bit over 15 kHz wide. As it turns out, for FM the occupied bandwidth is approximately the sum of twice the deviation plus twice the highest modulated frequency - a formula called "Carson's Rule", so for standard FM voice, that comes out to:
( (5 kHz) * 2 ) + ( (3 kHz) * 2 ) = 16 kHz
Where 5 kHz is the deviation and 3 kHz is the highest audio frequency.
And to make the receivers work properly, its own filters must be pretty close to that 16 kHz figure calculated above. In Utah, our standard channel spacing is 20 kHz which means that we can comfortably put two channels that far apart and not have them bother each other. In some parts of the country, 15 kHz channel spacing is used and it turns out that this is too close to avoid interference which is why special considerations are made when coordinating two repeaters that might be spaced that far apart - usually, they put two such repeaters as far apart from each other as possible. (Note: Since most people in Utah live along the Wasatch Front, we would have fewer available channels if we had both spaced them at 15 kHz and had to keep them geographically separated!)
Narrowband FM uses +/- 2.5 kHz deviation instead of +/- 5 kHz deviation, so if we run it through that same equation:
( (2.5 kHz) * 2 ) + ( (3 kHz) * 2 ) = 11 kHz
Where 2.5 kHz is the deviation and 3 kHz is the highest audio frequency.
At this point, you may have noticed something: Even if we turned our deviation down to zero (something that is impossible to do and still get voice through it) the equation shows that we'd still occupy at least 6 kHz. If you turn the transmitter's deviation down - which is the equivalent of quieter audio - you have to turn up the audio on the receiver to compensate, but if you have ever used FM you know that there's always a little bit of background "hiss" - particularly on weak signals. What this means is that you will hit the point of diminishing returns when the hiss so loud on all but the best signals as to be distracting!
It's worth noting that if you transmit/receive narrowband on a wideband channel, others will likely complain about your low audio and your receiver may experience "squelch clamping" (e.g. the squelch closing on audio peaks - very annoying!) - but you will likely be able to communicate.
One problem with narrowband operation is the receiver. As noted above, you need to have the receiver's filtering to be slightly wider than the signal itself because of several; factors:
So, knowing the above, how narrow can you go when using narrowband FM and get away with it? The answer is a bit tricky, but you cannot put two narrow FM signals closer than about 12.5 kHz and expect to avoid undesirable interaction (e.g. interference) between the two. Even at 12.5 kHz spacing, there have been reported some instance of undesirable interference between two channels, but apparently not as bad as one would experience with "normal" (+/- 5 kHz deviation) FM at 15 kHz.
What this means - in theory - is that you could put narrowband channels 12.5 kHz apart and get away with it in many situations!
There are some complications that make this harder to do than you might think:
Unlike with digital modes, there's a bit of compatibility between wide and narrow FM rigs so it would be very easy to accidentally to transmit on a narrowband channel using a transmitter set to wideband mode with a user causing havoc to THREE channels! This usually doesn't happen between "normal" FM and digital modes since the radio will automatically set the bandwidth on the mode!
As mentioned above, digital modes can
take up less bandwidth - and there isn't the "operating
wideband on narrowband" channels problem - but the same
issues about channel spacing and frequency stability
still apply: You about have to clear a block of
adjacent narrowband channels (analog or digital!) in
order to save much spectrum at all!
|"If someone gets on my repeater and does illegal things, whose responsibility is that?"||This isn't a particularly
easy question to answer as each situation is unique,
sometimes resulting in a slightly different interpretation
of the rules. There are a few things to keep in
Do you have any questions/comments for the frequency coordinator? If so, send an email.
For more information, you might want to read the "So you want to put up a repeater?" page.
Return to the Utah VHF Society home page.
Updated 1 March, 2016
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