Codan Radio Radio Repeater Systems Training Guide - Manual (Page 42)

TRAINING GUIDE | RADIO REPEATER SYSTEM
Chapter 4: Installation and Maintenance Page 34
INTERFERENCE
Interference occurs when undesired signals prevent the reception of wanted signals. It can be caused by
either internal or external sources. The use of poorly adjusted transmitters or receivers are one example
of on-site noise or interference. Sparking electrical sources such as power drills and miscellaneous
co-located microprocessor telemetry equipment are other examples. In addition, random noise from
electrical and heavy snow or sleet storms may cause interference on your system. Transmitter noise
from other transmitters, not necessarily nearby, can cause interference that is particularly diffi cult to
solve. Nulling out interfering signals with antenna patterns is one effective method of reducing or
removing this problem.
By far the most common interference problem is inter-modulation interference (commonly called
“intermod”). It is the product of two or more signals beating together to produce interfering signals to
your receive frequency. One of your own on-site transmitters may be part of the problem, beating with
some other unknown signal.
A complete listing of all transmitter and receiver frequencies and their users, including your own, within
a reasonable distance of the site (say two kilometers) is necessary to calculate the many combinations
of intermod that can occur. There are computer programs available to calculate possible interfernce
through intermod. The following example shows the necessary calculations for a three repeater (A, B
and C) site:
One of A’s transmit frequencies is 138.615 MHz; one of B’s transmit frequencies is 138.885 MHz. C’s
receive frequency is 138.375 MHz. Two times A’s frequency minus B’s frequency (or 2A-B) equals 30
kHz off C’s receive frequency (277.23 - 138.885 = 138.345). Depending on other variables such as
distance between transmitters, distance between transmitters and receiver, antenna installations, etc,.
this difference of 30 kHz may or may not cause an intermod interference to C’s receiver. This particular
equation is fairly easy to solve as it means that the second harmonic of A’s frequency is the basic culprit.
It may be that A’s transmitter is not correctly tuned, allowing the second harmonic to dominate, or it may
require a fi lter be installed (on A’s transmitter) to trap out that second harmonic. The co-operation of
all on-site users is required to eliminate intermod interference. If a site has been interference free and
a new user causes problems, it is usually the new user who pays for the purchase of any necessary
fi ltering.
In many cases, the licensing authorities will have done the initial work on this problem before allowing
you to do your installations. Therefore, early in the project process, you should discuss with them your
radio communications problems and proposed solutions. If you intend to use existing frequencies, it is
in your own interests to have any intermod information available before starting your repeater system
design.