[KYHAM] KEN Training for April 14: Considerations for Amateur Repeater Operations Part 1

[Ron Dodson]

Thanks to Tony Whobrey, KY4SP for the following article on
repeaters. Part one of a two part series.


There are a number of issues that are can be overlooked by
one who proposes to install a �ham� repeater. This article
will
address a few of the notable ones from the top down. 

Arguably the most important factor that determines a
repeater�s usefulness is the location and quality of its
antenna system.
While it is obvious that in most cases the antenna should be
located as high as possible, be sure that the tower or mast
will be
able to withstand the effects of adverse weather. If your
installation is done on a clear summer day, it is easy to
forget that your
antenna and its supporting structure must survive winter
winds that may be accompanied by an inch or more of ice on
the entire
assembly. Most of the well-known makers of land mobile
antennas offer a model suitable for amateur use, at an
(unfortunately)
similar price. Since the spring thunderstorm season will
bring winds of sometimes-phenomenal velocity as well, masts
and
mounting hardware should be of a quality that will not give
you second thoughts when severe weather occurs.
Weatherproofing
of cable connections should also be done with this in mind.

The feed line used in a repeater system plays a more
critical role than that used in a simplex installation, in
that it must
simultaneously receive and transmit without signal
degradation. Some popular brands of foil shielded cable have
been known to
fail in repeater service after only a few months of
operation. In most instances it is a good idea to limit feed
line losses to 2 db
or less, this can require relatively expensive cable and
fittings on frequencies of 440 MHz and higher. It takes less
than 50 feet
of standard RG-8 cable to account for 2 db of signal loss at
440 MHz, while Andrew LDF-5 (7/8inch) Heliax is good for
over
200 feet. Use the best feed line you can arrange for, and
test it before it is installed; as few things are more
irritating than finding
out that your cable is defective after it has been
painstakingly installed on the tower. Once again, keep the
wrath of Mother
Nature in mind when securing and weatherproofing feed line
and any needed jumpers, you will need more here than just a
roll
of electrical tape. Don�t forget to bring the cable down and
back up before entering your repeater shelter, this drip
loop will
help keep water out of the structure. The feed line should
be properly grounded just outside of the shelter entry
point; note that
lightning protection devices (Polyphasors, etc.) that may be
installed just inside the shelter do not eliminate the need
for this
ground. 

Duplexers (cavity filters) may be the most misunderstood
part of a typical repeater, and are accordingly the
component most
often subject to needless adjustment. Once properly
adjusted, duplexers should require only an occasional
(perhaps yearly)
inspection and only very minor adjustment, if any. When you
believe that the duplexer requires a significant change in
settings,
closely inspect the remainder of your system before taking
action. Problems such as water in the feed line or a
defective
antenna harness are often misdiagnosed as duplexer trouble;
these already laborious repairs become even more burdensome
if
someone has �helped� the problem by turning all of the
duplexer cans. While it is possible to adjust duplexers
reasonably well
with basic test equipment (signal generator, receiver,
wattmeter, and dummy load) if you have plenty of time, the
job is quicker
and easier with a tracking generator. Use double braid or
semi-flexible (Andrew Superflex) jumpers between your
repeater
and duplexer, this is no place for rg58 cable. 

Though a typical amateur repeater sometimes seems virtually
un-used for days, periodically there may be several stations
conversing for an hour or longer. It is not at all difficult
for three or four stations in casual conversation to key up
a repeater
nearly continuously, since each operator is only responsible
for a portion of the total usage. Your repeater will almost
certainly
require a transmitter capable of continuous duty operation.
Repeaters used for Skywarn or other quasi-emergency
operations
should unquestionably be capable of continuous operation, to
prevent an untimely failure. While numerous amateur
repeaters
have been successfully constructed using surplus mobile
equipment, you should be aware that a typical duty cycle
rating for
mobile equipment is 20 percent (2 minutes on, 8 minutes off)
transmit operation. Use of these units will usually dictate
a
substantial cut from rated power output and the addition of
forced air cooling for repeater service. 

Even a �commercial� repeater may have to be down-rated when
used for amateur service, since commercial applications
typically involve brief, two-party communications that
demand somewhat less than 100 percent duty cycle. Uniden�s
ARU251
repeater is rated at 25 watts at 20 percent (EIA) duty
cycle, with a continuous rating of 15 watts; comparable
units
manufactured by others have similar ratings. The two more
common makes of vintage commercial equipment, Motorola and
General Electric, are found as both continuous and
intermittent duty stations; power ratings for some models
are as low as 2
watts, while others deliver 375 watts. Be sure that you know
what you have, as these units look remarkably alike to the
casual
observer, unless viewed side by side. Intermittent duty
stations are analogous to the previously mentioned mobile
units, and
should be de-rated accordingly. Any transmitter intended for
repeater service should be able to remain continuously keyed
for
a minimum of one hour without noticeable overheating of the
power amplifier or station power supply.