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But Were Afraid to Ask By Larry Bush,

W5NCD and Jo...

Description

Updated October 9,

Everything You Need to Know About GMRS/FRS,

But Were Afraid to Ask By Larry Bush,

W5NCD and John Chamberlain,

AC5CV

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,

and some of their performance characteristics.

But let’s 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—David Bush,

KC5UOZ was working on an amateur television (ATV) mobile van,

that included a 60-foot crank up tower.

It occurred to me that this van,

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 radio 440 MHz repeater,

but quickly rejected that idea because 1) most hams don’t have 440 MHz handheld radios,

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.

since inexpensive FRS radios operate on similar frequencies,

maybe these consumer-grade radios could become part of the solution.

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

Tests with my repeater mounted in the KC5UOZ van have demonstrated that,

with handheld 5-watt GMRS units,

this repeater setup has a useful range of 10 to 15 miles.

I could now imagine a possible scenario: 1) The communications van (and portable repeater station) is parked in a disaster control area.

the EmComm station and/or Net Control could be located 30 miles or more from the van.

This sounded like a wonderful opportunity to make use of handheld GMRS radios—and maybe the new FRS radios.

You’ve probably noticed that 1) FRS radios use almost the same frequencies,

and 3) are often packaged as combination FRS/GMRS 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 services (as detailed in Part 95 of the Title 47 legislation).

I’ll first examine the GMRS,

consider how they might work together.

General Mobile Radio Service (or GMRS) The GMRS has eight frequency pairs designated for GMRS repeater use,

with the input and output frequencies separated by exactly 5 MHz,

We can conveniently use the kilohertz values of each pair as “channel designators” (e.g.,

“channel 550,” “channel 575,” and so forth).

Table 1.

GMRS Frequencies for Repeater Use Designator Repeater Output Freq* Repeater Input Freq 550 462.550 MHz 467.550 MHz 575 462.575 MHz 467.575 MHz 600 462.600 MHz 467.600 MHz 625 462.625 MHz 467.625 MHz 650 462.650 MHz 467.650 MHz 675 462.675 MHz 467.675 MHz 700 462.700 MHz 467.700 MHz 725 462.725 MHz 467.725 MHz * May also be used for simplex communications.

Thus,

GMRS stations will monitor the above 462 MHz frequencies for repeater transmissions.

Stations wishing to use those repeaters use an offset of +5 MHz to transmit on the 467 MHz input frequencies (i.e.,

just as ham radio operators use 5 MHz offsets for 440-band repeaters).

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 for simplex communications,

or on the 467 MHz input frequencies for repeater communications.

GMRS users may transmit on any of the above sixteen frequencies.

However,

GPRS users normally listen on only the eight 462 MHz frequencies.

The FCC rules of course specify power restrictions.

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.

At these power levels,

we can imagine high-power operations from base stations,

or low-power operations from hand-held transceivers.

GMRS antennas using these frequencies must remain under 200 ft altitude.

Table 2.

GMRS Interstitial Frequencies Interstitial Channel Frequency Additionally,

the GMRS authorizes the use of seven 1 462.5625 MHz intermediate or interstitial frequencies,

as shown in 2 462.5875 MHz Table 2.

These frequencies are located midway 3 462.6125 MHz between each of the Table-1 simplex frequencies.

the interstitial 7 462.7125 MHz 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 limited somewhat in that its antenna is raised no more than 20 feet above ground or the existing structure on which it is mounted.

Thus,

an antenna mounted on a 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 transmit power of a typical base station have to be reduced,

but the gain of the antenna must also be taken into consideration.

clearly these interstitial frequencies are intended primarily for 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.

Keep this last fact in mind.

Family Radio Service (or FRS) Let’s now turn our attention to the FRS.

We saw the use of interstitial frequencies in the 462 MHz band in the GMRS.

Interstitial frequencies in the 467 MHz band are given solely to FRS users,

They are designated channels 8 through 14 (for reasons that will become clear later).

Table 3.

FRS Radios Channels 8-14,

FRS Frequencies FRS Channel Frequency 8 467.5625 MHz 9 467.5875 MHz 10 467.6125 MHz 11 467.6375 MHz 12 467.6625 MHz 13 467.6875 MHz 14 467.7125 MHz

Compared to GMRS radios,

FRS transmissions are very restricted in power.

The FCC rules restrict FRS radios transmitting on FRS frequencies to: 1) a maximum power output of ½ watt,

and 3) emissions from an antenna that remains attached to the transmitting unit (i.e.,

Radio manufacturers convinced the FCC to include some overlap in the GMRS and FRS,

so FRS users got the 7 interstitial GMRS frequencies,

These became FRS channels 1 through 7,

This meant that FRS users could both listen and talk to GMRS users on the interstitial frequencies,

albeit under the power and deviation limitations imposed by the FRS.

Table 4.

FRS Radios Channels 1-7,

FRS/GMRS Frequencies But radio manufacturers pushed the FCC a bit more,

FRS Channel Frequency and were allowed to produce so-called “hybrid” FRS 1 462.5625 MHz radios,

that include the eight GMRS simplex 2 462.5875 MHz frequencies,

These hybrids have channels 3 462.6125 MHz designated 15 through 22,

Since 4 462.6375 MHz these GMRS frequencies permit up to 50 watts of 5 462.6625 MHz transmitted power,

users of the hybrid radios can 6 462.6875 MHz also transmit with more power,

more than 7 462.7125 MHz ½ watt.

it is on these that the so-called “22-

channel FRS radios” emit their maximum advertised power (typically 2 watts),

and achieve their maximum range.

according to the Part 95 rules,

and mentioned in the owner’s manual of every hybrid radio,

clearly one needs a GMRS license to transmit on channels 15 though 22.

In allowing the —3—

manufacture and sale of these hybrid radios,

the FCC has effectively created an unenforceable situation (ala CB radio).

When the average consumer buys a hybrid FRS/GMRS radio,

the perception is that he gets a “22-channel walkie-talkie” that works best— because it transmits with higher power—on channels 15 through 22.

But few,

will take the steps to apply for the requisite GMRS license to use those channels.

Table 5.

Hybrid Radios Channels 15-22,

GMRS Frequencies FRS/GMRS Channel Frequency 15 462.550 MHz 16 462.575 MHz 17 462.600 MHz 18 462.625 MHz 19 462.650 MHz 20 462.675 MHz 21 462.700 MHz 22 462.725 MHz

of the twenty-two channels on a hybrid radio,

and eight are actually GMRS channels which,

require a GMRS license to use.

How all these channels fit together can get rather confusing.

For a helpful summary of all the GMRS and FRS frequencies and their overlap,

But again,

let’s reiterate: transmissions on the fourteen FRS channels are restricted to: 1) a maximum power output of ½ watt,

and 3) use of a “rubber duck” antenna attached to the transmitting package.

On the GMRS channels 15 through 22,

although higher power is permitted,

the wattage will be limited by the small handheld package and batteries,

and the transmissions are constrained by the maximum deviation and antenna restrictions imposed by the shared FRS radio package.

Comparing GMRS and FRS This leads to several interesting contrasts between GMRS and FRS operations.

Most GMRS radios can operate at 50 watts.

Even on the shared interstitial frequencies,

GMRS radios are allowed 5 watts ERP.

FRS radios operating on the FRS channels,

are never permitted more than ½ watt power output.

FRS radios operating on the GMRS channels can use more power,

but are realistically limited by the small handheld package and its antenna to about 2 watts max.

Even on the shared interstitial frequencies,

GMRS radios can use raised antennas that will increase the range of their 5-watt transmissions.

FRS radios,

may not improve their antenna performance beyond the “rubber-duck” antennas attached to the transmitter by the manufacturer.

(One manufacturer has creatively packaged the transmitter and antenna as a mag-mount unit with a remote microphone.) 3) While the FRS channels share several 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 different maximum allowed deviation values for the two services.

For FM transmissions,

loudness of audio is not dependent on the strength —4—

(modulation) of the signal (as with AM transmissions),

but rather on the deviation of the signal.

The FCC allows GMRS radios to use a ±5 kHz deviation,

while FRS radios are permitted only half that amount: ±2.5 kHz deviation.

The consequence is that,

GMRS radios are going to sound about twice as loud and clear as FRS radios and,

have a much better signal to noise ratio.

What Does It All Mean

considering the “combination FRS/GMRS radios,” or “hybrids”,

be careful that you’re not distracted by their growing popularity and marketing.

First,

the fine print accompanying these radios reveals that the use of the GMRS frequencies (channels 15 to 22),

where the radios can transmit at their maximum power,

requires a GMRS license from the FCC.

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 individual GMRS licenses.

Realistically,

few hybrid radio users will apply for and obtain the GMRS license.

Second,

realize that the capabilities of these hybrid handheld radios are seriously over-hyped.

They may be advertised as able to communicate “up to 25 miles” or even more

a) ranges in excess of a few miles will seldom happen in real life with trees,

and vehicles blocking most of the signal,

and b) the advertised power levels apply only to the GMRS channels 15 to 22.

The FRS channels 1 to 14,

where the power is automatically limited to ½ watt from a small antenna,

can be useful around a local campsite,

or even a short caravan of vehicles,

but unless you’re communicating between adjacent mountain peaks or two cruise ships at sea,

ranges of several miles will be hard to achieve.

to reach their maximum potential,

the hybrid radio user must use channels 15 through 22.

(They have obtained the requisite GMRS license,

although the GMRS and FRS have some frequencies in common that could be conceivably shared during an emergency or special event,

due to the deviation limitations imposed on the FRS radios,

a mixture of FRS radios and GMRS radios on those frequencies is going to be marked by noticeably decreased signal-to-noise ratios (effectively,

lower volume) from the FRS users.

The Net Control operator will struggle to hear the FRS transmissions.

And lastly—addressing my initial motivation for pursuing this study—while we can imagine how nice it could be to be able to use the inexpensive FRS radios with the GMRS repeaters,

the hybrid FRS/GMRS radios do not include any of the GMRS repeater input frequencies.

(The Motorola Talkabout,

Model T-7200 did

it was rather expensive and is hard to find now.) So,

FRS and GMRS radios can have a place in the amateur radio operator’s arsenal of tools.

For example,

they’re great for communicating at a family picnic or in a short mobile caravan down the highway.

And they might even possibly serve a similar useful purpose for a civilian emergency response team covering a small area.

But as a knowledgeable amateur radio operator,

you’d be well advised to be sure everyone involved knows the limitations and restrictions before buying and including them in the emergency response plan.

Table 6.

Summary of GMRS/FRS Frequencies and Limitations GMRS Frequencies GMRS radios up to 50W output,

±5 kHz dev “550” Repeater Output / Simplex (1)

462.550

Interstitial Frequencies GMRS Frequencies GMRS radios FRS radios FRS/GMRS “hybrids” up to 5W (ERP,

±5 kHz dev up to ½W,

±2½ kHz dev (rubber-duck ant.)      Mobile,

Small base stations* Handheld (ch 1)**

Handheld (ch 15)***

     462.5875 Mobile,

Small base stations* Handheld (ch 2)**

Handheld (ch 16)***

     Mobile,

Small base stations* Handheld (ch 3)**

Handheld (ch 17)***

     462.6375 Mobile,

Small base stations* Handheld (ch 4)**

Handheld (ch 18)***

     Mobile,

Small base stations* Handheld (ch 5)**

Handheld (ch 19)***

     462.6875 Mobile,

Small base stations* Handheld (ch 6)**

Handheld (ch 20)***

     Mobile,

Small base stations* Handheld (ch 7)**

Handheld (ch 21)***

    

Handheld (ch 22)***

462.575

“600” Repeater Output / Simplex (3)

462.600

462.625

“650” Repeater Output / Simplex (5)

462.650

462.675

“700” Repeater Output / Simplex (7)

462.700

462.725

467.550

467.575

467.600

467.625

467.650

467.675

467.700

467.5625

Handheld (ch 8)

467.5875

Handheld (ch 9)

467.6125

Handheld (ch 10)

467.6375

Handheld (ch 11)

467.6625

Handheld (ch 12)

467.6875

Handheld (ch 13)

467.7125

Handheld (ch 14)

467.725

typically ~2W Table created by John Chamberlain,

AC5CV

Author Information Larry Bush,

W5NCD 1331 Western Ridge Rd Waco,

TX 76712 Email: [email protected] Phone: 254-848-5155 Larry Bush while a senior in high school in 1946 was issued the amateur radio call,

W5NCD.

He was the owner of Waco Communications,

a 2-Way radio sales and service company from 1951 to 1995.

He was one of the original founders of Wacom Products,

and radio station KWOW in central Texas.

Now retired,

Larry enjoys exploring the latest electronic benefits of amateur radio,

SSTV,

sending digital images over amateur radio,

Broadband Hamnet.

Larry is a Member and Director of the Heart O’ Texas Amateur Radio Club in Waco,

Texas.

John Chamberlain,

AC5CV 3506 Greenleaf Drive Waco,

TX 76710 Email: [email protected] Phone: 254-855-7731 John Chamberlain AC5CV is a Senior Associate with CORD in Waco,

TX where he develops and writes mathematics and physics curriculum materials.

John has been a ham since 1994 and enjoys applying his computer and science skills to amateur radio,

SSTV,

digital image exchange over amateur radio,

John is a Member (and past Secretary),

HOTLINE Publisher,

and Webmaster for the Heart O’ Texas Amateur Radio Club in Waco,

Texas.