GMRS/FRS frequencies

Table 1: GMRS/FRS frequencies
Ch. # Description Frequency (MHz)
1 GMRS/FRS 1 462.5625
2 GMRS/FRS 2 462.5875
3 GMRS/FRS 3 462.6125
4 GMRS/FRS 4 462.6375  (mid)
5 GMRS/FRS 5 462.6625
6 GMRS/FRS 6 462.6875
7 GMRS/FRS 7 462.7125
8 GMRS 2 462.5750
9 GMRS 4 462.6250
10 Emergency 462.6750
11 GMRS 1 462.5500  (low)
12 GMRS 3 462.6000
13 GMRS 5 462.6500
14 GMRS 6 462.7000
15 GMRS 7 462.7250  (high)

The first thing we need to do is to know the frequencies we operate on so that we can calculate the correct lengths for out antenna. A list of the GMRS/FRS frequencies can be found in table 1. The important frequencies for us are the highest and the lowest which we need to get the average or center frequency of the range we are building the antenna for:

(462.5500MHz + 462.7250MHz)/2 = 462.6375MHz

We need to build the antenna for the middle frequency of 462.6375MHz which happens to be the frequency of GMRS/FRS channel 4. That means that this antenna will be “tuned” for this frequency however, the bandwidth is wide enough, you will not notice a degradation of signal on any of the other channels. As a side note, many dual-band (GMRS/FRS) radios will not broadcast at a full 2 watts or higher on the shared GMRS/FRS frequencies, but instead will broadcast at the .5 watt that is standard for FRS transmission. That, at least, is how my Midland G-28 radios behave.

Measurements

Once we have the frequency, determining the wavelength (λ) is relatively easy. With the basic wavelength measurement determined by the frequency, we’re ready to get the real measurements for the antenna dimensions.

Of course, all dimensions are metric, the accepted measurement system of all civilized, modern countries 8^).

λ = speed of light / frequency
λ = 300000km/s / 462.6375MHz
λ = 0.6485m (64.86cm)

One thing to note is that electromagnetic waves travel slower in metals and other solid materials than they do in a vacuum. In the case of our material, uninsulated stainless steel, it travels about %95 the speed of light (which in a vacuum travels about 300,000km/s). This is known as the velocity factor of a conductor.

GMRS/FRS J-Pole Measurements (λ’ is adjusted for the velocity factor)
j-pole design
The measurements are metal to metal surface, not center to center.
f = 462.6375MHz
λ = 0.6485m
λ' = λ * .95
= 0.6160m (61.60cm)
A 3/4 λ' = 46.2cm
B 1/4 λ' = 15.4cm
C 7.0104 / f = 1.5cm
D 6.7056 / f = 1.5cm

On to the construction method, description and pictures…

Related posts:

  1. PL576Ws goes back, WRT54G comes homeUnfortunately the ProView PL576Ws LCD monitor, as nice as it...