GMRS Radio 101

The GMRS service is readily available for members of the public to gain access to; I thought it would be useful to put together a brief primer to fill in newcomers interested in using the services with a few of the basics:

For information on obtaining a license; see “Getting a GMRS License

Frequently Used Terms

Term Meaning
Channel A common frequency standardized either within a radio service or within a manufacture of radios. The GMRS has 22 channels defined by the FCC, and an additional 8 channels to be used as repeater input frequency (for a total of 30)
CTCSS Continuous Tone Coded Squelch System, also marketed as Private Line (PL), Privacy Code or Sub-Channels, this is a constant tone which the radio sends out when transmitting, that is usually outside of the audible portion of a radio’s receiver and thus not apparent to the user. The tone is used to control the squelch on the receiving end, and lets radios and repeaters better handle channel interference, either from other stations on the same frequency, or other sources. Almost all repeaters require the use of a CTCSS tone, although they can use an open squelch for operation.
DCS Digital Coded Squelch; a digital variant of the CTCSS tone. The radio sends out a continuous data pulse that represents a squelch code, again, typically outside of the audible portion of a radio’s receiver. DCS is less common on repeaters, but still commonly used. It is more resilient to interference than CTCSS, but also more sensitive to weak-signal problems.
Duplex A method of radio operation where users transmit on one frequency/channel, and receive on another. This method of operation is used for operations with repeater stations.
FCC Federal Communication Commission; the branch of government that administers and regulates communication services, including wireless radio.
Frequency The rate at which something oscillates, usually measured in terms of oscillations per second. Megahertz is a unit of measurement for frequency, meaning millions of oscillations, or cycles, per second.
GMRS General Mobile Radio Service; the name of the radio service created by the FCC located around 462Mhz and 467Mhz that is available for licensed use, and what we’re all about!
Repeater A remote radio station that receives and rebroadcasts signals in real time. They are typically located at advantageous sites in terms of height above average terrain, including mountain and hilltops, on top of tall buildings or radio towers, or both! Repeaters can greatly boost the coverage range for radio communication devices, such as walkie talkies, vehicle mounted (mobile) stations, and fixed (base) stations.
Simplex A method of radio operation where users transmit and receive on the same frequency/channel. This is most common for consumer-type handheld and mobile radios, and common in direct radio-to-radio communications
UHF Ultra High Frequency; the name for the broad band in which the GMRS and many other radio services are located

Simplex Operation

Simplex operation is a direct radio-to-radio method of using a radio, in which all users receive and transmit on the same channel; it is the most common type of operation that people are familiar with. You turn on the radio, dial in a channel (or frequency), and start talking. So long as everyone is on the same channel with the same squelch settings, and everyone is in range, all parties can hear one another.

A basic set of handheld transceivers “Walkie Talkies” set to the same channel for simplex operation

Simplex operation generally works well over shorter distances, or when communicating using a base, mobile or handheld station that’s in a high location. Since there is no repeater, coverage is generally limited to a few miles, although depending on topography, much greater ranges may be possible. It is most easily deployed in the field among groups of people.

The GMRS service has the following 22 simplex channels defined by the FCC:

Channel Number Frequency Maximum Power Bandwidth
1 462.5625 MHz 5 W 20 Khz
2 462.5875 MHz 5 W 20 Khz
3 462.6125 MHz 5 W 20 Khz
4 462.6375 MHz 5 W 20 Khz
5 462.6625 Mhz 5 W 20 Khz
6 462.6875 MHz 5 W 20 Khz
7 462.7125 MHz 5 W 20 Khz
8* 467.5625 MHz 0.5 W 12.5 Khz
9* 467.5875 MHz 0.5 W 12.5 Khz
10* 467.6125 MHz 0.5 W 12.5 Khz
11* 467.6375 MHz 0.5 W 12.5 Khz
12* 467.6625 MHz 0.5 W 12.5 Khz
13* 467.6875 MHz 0.5 W 12.5 Khz
14* 467.7125 MHz 0.5 W 12.5 Khz
15 467.550 MHz 50 W 20 Khz
16 467.575 MHz 50 W 20 Khz
17 467.600 MHz 50 W 20 Khz
18 467.625 MHz 50 W 20 Khz
19 467.650 MHz 50 W 20 Khz
20 467.675 MHz 50 W 20 Khz
21 467.700 MHz 50 W 20 Khz
22 467.725 MHz 50 W 20 Khz

*Radios must have integral, non-removable antennas to operate on these channels

You’ll notice that channels 1-7 are located between channels 15-22 in terms of frequency. Channels 8-14 are actually located between GMRS channels as well, in this case the repeater input frequencies which we’ll discuss in a later section. For this reason, channels 1-14 are known as interstitial channels, and have reduced power output and bandwidth requirements as a result, they must fit and play nicely between the main GMRS channels, 15-22, which were the original GMRS channels before the restructuring.

The reason for this arrangement is due to channels 1-14 originally being license free channels the FCC created for the Family Radio Service, or FRS. Since then, the FCC has consolidated the channels for FRS and GMRS into a common set with different power limitations, due to the rampant misuse of the GMRS service allocations associated with the sale of hybrid FRS/GMRS radios to the general public (which may no longer be sold at this time). This means the GMRS has a few more channels it can now use with slightly elevated power, but has to share its channels with the FRS.

FRS traffic is not a huge problem for GMRS in terms of interference, since the FRS is limited to lower power output than GMRS, and radios must come with non-removable antennas, which are usually very inefficient. Regardless, FRS users can definitely be heard and may require jumping around on different channels in crowded areas or use of a coded squelch to filter out. In either case, FRS users are not allowed to use repeater input channels.

One thing to keep in mind is the different power levels available on the different channels; 1-7 are limited to 5 watts, 8-14 are limited to half a watt, and 15-22 are limited to 50 watts. If everyone is using handheld radios, channels 1-7 will work just fine since they don’t typically transmit with more than 5 watts. If mobile or base station radios are involved, it may make sense to switch to 15-22 to take advantage of the extra power. 8-14 are suitable for close range communications within a mile or so, since they are limited to half a watt. As always, it’s good practice to use the least amount of power necessary to maintain reliable communications, so if everyone has access to high power radios, but is within a couple miles of each other, channels 1-7 might still be fine and it helps keep the air waves clean and free of excess interference.

Repeater “Duplex” Operation

We’ve discussed briefly in the previous section that topography and tall buildings can greatly affect the usable range of a radio. UHF radio is generally considered line of sight, although this is a bit of a misnomer, since radio waves will frequently travel beyond what is directly visible due to scattering, refraction, and other forms of interaction with the natural environments.

Taking a look at the landscape around you, a few landmarks probably stand out as visible in a large portion of the location you may be in; these are commonly tall buildings, water and/or radio towers, mountain peaks (even distant ones). These are all features that greatly protrude above the horizon, and for this reason, are often where you will find radio installations, be it for broadcast TV & radio or for two-way communications, including cellular services. As far as two-way radio goes, repeaters often find a home in these sites.

Located in a high-visibility location, repeaters provide enhanced line-of-sight coverage for stations that wish to boost their range. Repeaters work by listening on one frequency, and when receiving a signal, immediately re-transmit the received signal from the high ground; repeaters are unique in that they are able to receive and transmit at the same time.

Repeater providing coverage to two stations that would otherwise be out of range

The reason for transmitting and receiving on different frequencies is due to a number of reasons; primarily since two stations transmitting on the same frequency would cause interference, and if the repeater were to listen to its own transmit frequency, it would not be able to hear anything other than itself (and get stuck in a feedback loop along the way!); for this reason, the two frequencies are separated.

The GMRS service includes the following repeater channels. The input frequency is the frequency on which the repeater listens and a user’s radio will transmit to use the repeater, and the output frequency is the frequency on which the repeater transmits and the user’s radio listens. The change in frequency happens automatically when the user presses the “Push-to-Talk” button on their radio.

Channel Number Input Frequency Output Frequency
15R 467.550 Mhz 462.550 Mhz
16R 467.575 Mhz 462.575 Mhz
17R 467.600 Mhz 462.600 Mhz
18R 467.625 Mhz 462.625 Mhz
19R 467.650 Mhz 462.650 Mhz
20R 467.675 Mhz 462.675 Mhz
21R 467.700 Mhz 462.700 Mhz
22R 467.725 Mhz 462.725 Mhz

Note that the output frequencies on channels 15R to 22R match that of the GMRS high power simplex frequencies (15-22); these channels are shared by simplex and repeater users.

Configuring your radio to use repeaters depends on the make and model, its a little different for each one, and not every type of GMRS radio will be capable of doing this.  You will need to consult your radio’s manual or alter it’s programing to utilize the Texas GMRS Network repeaters. Note that most repeaters require setting a CTCSS (PL/DPL/DCS) tone to activate them; this is discussed in the next section.

Coded Squelch Systems

Just about all radios used for 2-way communications come with a device called a squelch. A squelch serves as a sort of check-valve to turn on the receiver in the presence of a signal; without it, you would be listening to non-stop static between transmissions, it works as a way to keep the radio quiet unless it receives a signal.

A basic squelch looks at only signal strength to determine when to turn on the speaker; however this makes it susceptible to interference, as not every signal a radio picks up is necessarily a signal you want to listen to. In places with lots of interference generating sources, the user can often still be bothered by the squelch opening up and causing a bunch of unwanted noise.

To address this issue; manufacturers came up of a few ways to signal to a receiving radio that the signal source is in fact another radio, by injecting an audio signal that is not likely to occur from external interference. This tone or data pulse is known as a coded squelch, and is most commonly based on the CTCSS and DCS standards. These at times are also marketed under the names Private Line (PL), Privacy Code, Sub-Channel, and a few other variants, but they’re essentially all the same thing using the same tone or pulse standards.

Because CTCSS and DCS can set unique squelch codes, using a code squelch essentially gives the following benefits:

  1. Eliminates random interference from coming through the squelch and being heard on a receiver. This is the primary reason repeaters utilize coded squelches; it prevents them from keying up and re-transmitting a bunch of noise
  2. Lets groups of users use the same channel / frequency, while only being able to hear others in their party using the same squelch code. Users who transmit on the same time in different groups may still cause interference to one another, but it prevents others from being bothered by transmissions unrelated to their party.

Common Operating Practices

It’s important not to take certain formalities too seriously; one should focus on using the radio in a matter that works well for them. With that said; there are a few general rules to keep in mind for starters:

  1. GMRS users must identify their transmissions with their FCC issued call-sign every 15 minutes, or at the end of a series of transmissions.
  2. Foul language is not allowed by the FCC
  3. Keep a brief pause between transmissions so other stations can have a chance to break in if they need to.
  4. When using a repeater, wait until after the courtesy tone to transmit – this is a brief beep or tone you may hear shortly after someone keys up the system. The intent is to give a pause for other stations to break in. Some repeaters will time-out after a few minutes if this is not followed and cease to transmit for a brief duration. Not all repeaters will have this.

 

Special thanks to Tucson GMRS Association for this information.