| April 10, 1999
The Ultimate Repeater Column
GMRS Repeater
Hardware
GUMMERS-101
by Paul Shinn, KAF8333
You can ask Paul technical
questions about
building repeaters by emailing him at gummers@gmrsweb.com
This month, we examine the hardware
required to put a repeater on the air. Of course, even before you apply for
your GMRS repeater and mobile license, you need to know where the repeater is
going to be located. For a whole bunch of information about getting your GMRS
license and selecting a location for your repeater, check out the FAQ and the
bulletin board here at DougWeb Online, the most complete GMRS source.
For the sake of this column, let’s
say that you are putting the repeater at your home. There are a lot of GMRS
repeaters in service matching this description that offer fantastic coverage
and reliability, so this is a good example to use. In the months ahead,
we’ll look at hilltops and large buildings as possible sites for your
repeater, and what to expect from a professional site management landlord.
There is just no substitute for a real
repeater unit. There are some people using two mobiles (one solely for
transmit, the other for receive) with a controller as a repeater, but I have
never seen one of these systems perform as well as a true repeater. Some of the
older Motorola MSY and Micor repeaters can be bought for less that a thousand
dollars at swaps and on the commercial used market, which puts them well within
the financial ball park of a similar wimpy two-mobile setup.
The biggest problem with the mobiles
posing as repeaters is that the heat sink on the transmitter is too small. Keep
in mind that the repeater is continuously transmitting during the entire
two-way conversation. The mobiles and portables using the repeater are only
transmitting about half that time, so they can handle it. Transmitters get hot
real quick when used continuously. True repeater transmitting radios are
designed for continuous operation with little effort.
Since you want to transmit and receive
at the same time (that is what a repeater does), you need to somehow keep the
transmitter and the receiver isolated from each other. Two antennas do a poor
job of this for GMRS. Since your transmitter is going to be on 462.575, that
means your receiver will be only five MHz away at 467.575. That’s close!
The duplexer is the most common method of providing isolation. The isolation
that a duplexer provides is so great that the transmitter and the receiver can
use the same antenna at the same time!
One type of duplexer is called a pass
duplexer. This is usually two filters for each frequency that are in series
which pass the tuned frequency with only minor loss, but the higher and lower
you go from the tuned frequency, the more loss there is. For example: A 5 watt
signal on the tuned frequency will still be almost five watts at the output of
the filters, but a 5 watt signal on a frequency five MHz away would be almost
undetectable with your watt meter!
The other type of
duplexer is called a notch duplexer. This type passes all frequencies with
little loss except for the frequency it is tuned to reject. The notch gets
wider and deeper as you add filters in series. This type of duplexer is not
appropriate for popular transmitter sites. It offers no protection for the
receiver from other transmitters at the site other than your own. Also, it
allows spurious signal from your transmitter to make it out the antenna, which
could interfere with other receivers at the site. A notch duplexer is OK. for
your home located repeater as long as you don't need to use a receiver
pre-amplifier. We will talk about pre-amps below.
The most desirable type of duplexer
uses both methods of isolation and is call a pass/reject duplexer. Some hilltop
radio sites only allow pass/reject duplexers on the two-way equipment.
Since we are taking such careful
measures to filter the transmitter output and the receiver input, we continue
the practice with the cables that connect everything together. Radio Shack coax
should never be used on a repeater installation. Cables that are double
shielded must be used. Some of the smaller cables like RG-142 are OK. for
shorter runs between the radios and the duplexer. My favorite is Andrew 1/4
inch Superflex (TM) for this use. Another good cable, that is perfect for
longer runs is LMR-400. It has less loss than the Superflex (TM) cables, and is
about the size of the RG-8 you're familiar with from the ‘Shack’. I
just wish the connectors were compatible, they're not.
The run to the antenna is the most
important run to use low loss cable on. Since the typical run will be several
wavelengths long, over half of the transmitter power could be lost on its way
to the antenna. The same goes for the signal the antenna captured destined for
the receiver. At the very least, for runs up to 100 feet, you should use 1/2
inch Andrew or Cablewave coax. Anything longer than 100 feet should be at least
7/8 inch cable.
Make sure to use the proper connectors
on the ends of the cables, since adapters add unnecessary loss and the
increased chance of interference.
You may be surprised, but the power
output from the repeater transmitter has very little to do with the range you
can expect. At 462 MHz, the radio waves do not follow the curvature of the
earth hardly at all, and signal refraction is less productive than at lower
frequencies. The antenna elevation makes the most difference in coverage. The
communicating antennas need to ‘see’ each other.
By doing some quick mental
calculations, if you mount almost any gain antenna at an elevation of 40 feet
above ground and using a radiated transmitter power of 10 watts, the 20dbu
contour will be about 15 miles over flat terrain. 20dbu translates into a field
strength of 0.01 millivolts, or about where you start to hear a little scratch
in the audio. If you could use the same transmitted power and the same antenna
on top of a 200 foot tower, the 20dbu coverage extends to 26 miles. Changing
just the transmitter power from 10 to 50 watts (the maximum allowable power on
GMRS) only extends the coverage 1.8%!
When I removed the tube power amplifier
from my repeater once for maintenance, I ran the 12- watt exciter straight into
the duplexer though an adapter cable. When I was using the repeater without the
power amplifier, I could not tell the difference at all. The coverage did not
change. The only noticeable change was that when using a portable inside of a
car, hearing the repeater was a little scratchy at times.
Adding a power amplifier to boost a
GMRS repeater output to the maximum allowed power is, in my opinion,
unnecessary. As long as you can get at least 10 watts from your repeater
transmitter, your will receive predicted coverage, for a given antenna height.
In mountainous terrain, or in large city areas, a higher output power may help
you hear the repeater better in low spots due to the refraction of the signal
radiated. However, this still does not help you get into the repeater from your
mobile!
One item that is a must for any
repeater you will use portables with, is a receive preamplifier. A pre-amp, as
it is called, amplifies the signal that makes it through your duplexer receiver
port before it gets to the receiver itself. Since a pre-amp will amplify
anything sent to it, including noise and other frequencies other than your own,
the use of a pass or pass/reject duplexer is mandatory. Amplified noise and off
frequency signals can overload the front end of your receiver and cause it to
become less sensitive to your desired frequency.
In future columns, we’ll talk
about the setup and optimization of your repeater, including the tuning of your
duplexer. Next month, get ready for the first installment on repeater antennas.
We’ll look at some different types and according to my extensive hilltop
and low-level experiments, what works best for GMRS repeaters
Paul
Shinn.....Clear.
Click Paul's name to send him
mail at Gummers@dougweb.com
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