(Note: I didn't write this, but it does a good job explaining GMRS & FRS, so I am passing this on to you. Eric)
Everything You Need to Know
After licensing and installing a General Mobile Radio Service (or GMRS) repeater, I have learned a lot about GMRS and Family Radio Service (or FRS) equipment, the laws concerning them, and some of their performance characteristics. But allow me to back up a bit…
In the early Spring of 2001—about the time the hams of east Texas were coping with the Columbia Space Shuttle disaster—KC5UOZ was working on an ATV mobile van, including a 60-foot crank up tower. It occurred to me that this van, equipped as it was, could someday have a valuable service role for a future disaster response team—just as we were witnessing in east Texas. In a role like this, I thought, “Couldn’t the van be equipped with a portable repeater to facilitate emergency communications?” I considered an amateur 440 MHz repeater, but quickly rejected that idea because 1) most hams don’t have 440 MHz handie-talkies and 2) at such an incident, there would likely be many volunteers who were not licensed hams anyway. On the other hand, a GMRS repeater could serve non-hams and hams alike who could be equipped with more affordable handheld GMRS radios. And, since FRS radios operate on similar frequencies, maybe these inexpensive radios could also be used.
To that end, I applied to the FCC as control operator for a GMRS repeater, and was granted the license KAF7259. The repeater output frequency is 462.700 MHz; the input frequency is 467.700 MHz; with a PL tone of 97.4 Hz. Tests with my repeater mounted in the KC5UOZ ATV van have demonstrated that, with handheld 5-watt GMRS units, this repeater setup has a useful range of 10 to 15 miles. I can now imagine a possible scenario:
1. The van (and portable repeater station) might be parked in a disaster search area.
2. Workers use GMRS radio communications as much as 10 to 15 miles away from the van.
3. Workers operate under the privileges of my GMRS license.
4. Worker reports could be relayed to an Emergency Communication (EmComm) station by a control operator in the ATV van.
5. With the aid of a yagi antenna mounted at the van (as high as 60 feet), the EmComm station could be 30 miles or more from the van.
This sounded like a wonderful opportunity to make use of handheld GMRS radios. And lately, you’ve probably noticed that FRS radios use almost the same frequencies, are even less costly than GMRS equipment, and are starting to appear packaged as combination GMRS/FRS radios.
To further explore these possibilities, I found it necessary to get a bit more technical. Let’s first look at how the FCC has mapped out the frequencies and capabilities for these two services (as detailed in Part 95).
General Mobile Radio Service (or GMRS)
The GMRS has 8 frequency pairs designated for GMRS repeater use, with the input and output frequencies separated by exactly 5 MHz, as listed in Table 1. Notice that we can use the kilohertz values of each pair as “channel designators” (e.g., “channel 550,” “channel 575,” and so forth).
Thus, all GMRS stations will monitor the 462 MHz frequencies above for transmissions coming from repeater stations. Stations wishing to use those repeaters must use an offset of +5 MHz to achieve the 467 MHz input frequencies (i.e., just as 2-meter repeater users use a +600 kHz or –600 kHz offset for 2-meter repeater inputs).
However, the FCC rules also permit GMRS simplex operation on the above 462 MHz frequencies. Consequently, GMRS stations may transmit on the 462 MHz repeater output frequencies to achieve simplex communications, as well as the 467 MHz input frequencies for repeater communications.
Of course, the FCC rules specify power restrictions for these frequencies. Transmissions on the frequencies listed in Table 1 are permitted a maximum power output of 50 watts and a maximum FM deviation of ±5 kHz. So, at these power levels, we can imagine high-power operations from base stations, repeaters, and mobile (i.e. automobile-mounted) rigs, or low-power operations from hand-held transceivers. Hold on to this information for a moment.
The GMRS also authorizes the use of 7 intermediate or interstitial frequencies, as shown in Table 2. These frequencies are located midway between each of the Table-1 frequencies.
The use of these interstitial frequencies by GMRS users are more restricted than the Table-1 frequencies. According to the FCC rules, the interstitial frequencies are
1. solely for simplex use by mobile units and “small base stations,” and
2. limited to 5 watts effective radiated power (ERP) at a maximum deviation of 5 kHz.
A “small base station” is a base station that has an antenna that extends no more than 20 feet above ground or an existing structure on which it is mounted. Thus, for example, an antenna mounted on the ATV mobile van at 60 feet could not be used to transmit on these frequencies. Furthermore, in light of the 5 watts ERP restriction, not only would the radio power of a typical base station have to be reduced, but the gain of the antenna must also be taken into consideration. So, it’s clear that these interstitial frequencies are intended to be used primarily by handheld GMRS radios, or at most by other GMRS radios operating at a low power level with minimal antennas. However, note that even these transmissions are permitted a maximum deviation of 5 kHz. Again, keep this bit of information in your mind, as we will return to it later.
Family Radio Service (or FRS)
Let’s now turn our attention to the Family Radio Service (or FRS). The interstitial frequencies in the 467 MHz band are given solely to FRS users, as shown in Table 3. They were designated channels 8 through 14 (for reasons that will be more clear in a moment).
The equipment manufacturers pressured the FCC to allow the FRS and GMRS users to share some frequencies. To that end, they included the 7 interstitial GMRS frequencies in the FRS, too. These became FRS channels 1 through 7, as shown in Table 4. This meant that FRS users could both listen and talk to GMRS users on the interstitial frequencies.
Finally, FRS users were also allowed to communicate on the GMRS simplex (and repeater output) frequencies. For FRS radios, these became channels 15 through 22, as shown in Table 5.
Thus seven channels are exclusive to FRS, and fifteen channels are shared with GMRS—for a total of twenty-two FRS channels. (For a summary of all the GMRS and FTS frequencies and their overlap see the table here.)
However, compared to GMRS radios, FRS radios are very restricted in power. The FCC rules restrict FRS radios to:
1. a maximum power output of ½ watt,
2. a maximum FM deviation of ±2.5 kHz, and
3. an antenna that remains attached to the handheld unit (i.e., a non-detachable, “rubber duck” antenna).
Comparing GMRS and FRS
This leads to some possibly surprising contrasts between GMRS and FRS operations.
1. The most obvious difference is power. Most GMRS radios can operate at 50 watts. Even on the shared interstitial frequencies, GMRS radios are allowed 5 watts ERP. FRS radios, on the other hand, are never permitted more than ½ watt power output.
2. GMRS radios can use gain antennas to achieve rather impressive territorial coverage. Even on the shared interstitial frequencies, they are permitted raised antennas that will increase the range of their 5-watt transmissions. FRS radios, on the other hand, may not improve their antenna performance beyond the attached “rubber-duck” antennas attached to the transmitter by the manufacturer. (One manufacturer (Radio Shack - ed.) has creatively packaged the transmitter and antenna as a mag-mount unit with a remote microphone attached by a cable.)
3. While the FRS channels share many frequencies with GMRS, they are not allowed to transmit on the GMRS repeater input frequencies. Thus, inexpensive FRS radios cannot serve as inputs to GMRS repeaters.
4. Lastly, the FCC has specified differences in maximum allowed deviation for the two services. For FM transmissions, loudness of audio is not dependent on the strength of the signal (as is the case with AM), but rather on the amount of deviation. Recall that GMRS radios are allowed a ±5 kHz deviation, while FRS radios are allowed only ±2.5 kHz deviation. The Modulation Index is the amount of deviation divided by the modulating frequency (i.e., the audio we’re trying to send). Using 2.5 kHz as the upper end of the voice frequencies, for GMRS radios we calculate a Modulation Index of 2 (that is, 5 kHz/2.5 kHz). For FRS radios, we get a Modulation Index of 1 (that is, 2.5 kHz/2.5 kHz). Comparing these indices indicates that on the shared frequencies, GMRS radios are going to sound about twice as loud and clear as FRS radios and, consequently, have a better signal to noise ratio.
What Does It All Mean?
In summary, be careful that you’re not confused by the growing popularity of “combination FRS/GMRS radios.” First, if you read the fine print accompanying these radios, you’ll discover that the use of GMRS frequencies requires a license, requiring a rather substantial fee (compared to an amateur radio license fee). If you were to operate under the auspices of a licensed GMRS entity (such as an existing GMRS-licensed repeater owner or small business), you might be able to avoid the cost of your own GMRS license.
Second, realize that while these combination radios may be advertised as 2-watt or even 5-watt radios, those power levels apply only to the GMRS frequencies. When set to the FRS channels 8 through 14, the power is automatically limited to ½ watt. When operating at the higher levels on the shared frequencies, you are implicitly operating in the GMRS. (Are you licensed to do so?)
Third, due to the deviation limitations imposed by the FCC, the use of a mixture of FRS radios and GMRS radios on shared frequencies during a special or emergency event is going to be marked by noticeably decreased signal-to-noise ratios by transmissions from the FRS radios.
And last, while it’s fun to imagine using the inexpensive FRS radios with the GMRS repeaters, most all of the combination FRS/GMRS radios do not include the GMRS repeater input frequencies. At this time, I’ve only found one unit that does: the Motorola Talkabout (Model T-7200). (It also happens to receive 8 NOAA weather frequencies in the 162 MHz band.) However, it has a price tag around $110—a lot higher than the more common $25 combination radios (that, as we’ve said, will not work the repeaters).
FRS and GMRS radios can have a place in the amateur radio operator’s arsenal of tools. But know the limitations before you buy.