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radioint.txt
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1988-08-20
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4KB
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87 lines
ELECTRO-MAGNETIC INTERFERENCE
Interference to radio receivers has been around a long time.
Remember the pictures of Grandpa in the parlor with his ear
glued to the morning-glory speaker? Lots of fun on a hot summer
night with thunderstorms around!
These days, there is much more than thunderstorms to contend
with. For EMS, Fire, Police and other Public Service personnel,
the radio is an indispensable tool in the performance of their
work. Unfortunately, there are alot of other 'transmitters'
around to affect their ability to communicate in an emergency
situation. Some of these problems are more pronounced at the
UHF and VHF channels.
A recent issue of PC WEEK carried an article 'THE GOVERNMENT'S
RADIO REGULATORS', in which the author cited many cases of
poorly shielded microcomputers being sold around the country.
Of particular interest to Public Service personnel is this
quotation:
"In one city, for example, police radio for several
blocks was wiped out by radio frequency interference
from an insuffiently shielded microprocessor in a
store cash register, FCC officials said."
EMS personnel could be victimized by this sort of occurrence
if, for instance, they were called into a store to attend a
heart-attack victim and could not use their radios. A little
reflection on this and you could imagine other situations
involving police and fire personnel carrying radios rendered
useless by this type of interference.
Microprocessor interference is characterized by buzzsaw-like
tones, with different pitches. Or a soundless carrier, like a
radio station broadcasting no sounds. These may completely
block your receivers. You may transmit and be heard by
receivers outside of the range of the interference, but you
might not be heard by another radio within range of this EMI.
There are many other types of interference which can affect
your receiver. One, called Intermodulation, or 'intermod',
occurs when two or more transmitters on different frequencies
are activated at the same time; their frequencies combine in
ways to produce 'sum' and 'difference' frequencies, one or more
of which may be on your receiver freqency. In these case, you
may hear the voices of those other stations on your channel.
The causes of 'intermod' can be very complex and usually
require the services of an expert to locate and correct the
offending transmitter.
The real point of this is - the time to locate these bad areas
is BEFORE you are called into them to provide services.
Interference of many types can be a violation of FCC
regulations. It's worth your while to locate these trouble
spots before your have to perform near them! Enforcement is
possible if voluntary compliance is not forthcoming.
How to find them? Well, just drive through, or walk through
these areas with your receiver's squelch just barely on, while
listening for the interference. Try to identify the source.
Make a note of the locations where EMI occurs to alert
personnel if they are called into the area. If, in a particular
building, it is possible to locate 'quiet' spaces, these might
be useful to mark on a layout of the building if the problem is
severe and the likelihood of working there is great.
By the way, those 'rubber-duckie' antennas are just great when
it comes to preventing the skewering of your partner or your
own kidneys but, for effectiveness against these kinds of
interference, they are the worst. The reason is, they are
extremely inefficient. If you have an option, plug in a quarter-
wave antenna onto your 2-way portable. There are some
telescoping antennas which fasten neatly onto your radio. When
folded, they are about the size of a pocket tire guage. A
couple of these among your equipment could come in handy.
There are many other types of EMI and solutions, too. This
little text wasn't intended to address them all, even if the
writer knew them all, which he doesn't. The solutions proposed
here do work. I have used them, and maybe they could help you
out of a tight spot.
Larry Bay
15 Sep 1986