Station design for EMC
Less likely to cause interference problems FM CW PSK31
Certain classes of emission are more emc friendly than others. Frequency
Modulation (FM) is the least likely to cause interference because there are
no amplitude variations on the constant amplitude carrier wave. Therefore,
there will be no envelope detection in the receiving equipment. Morse
transmissions (CW) and the new PSK31 HF data mode are fairly EMC friendly
because relatively low power is possible to maintain contact over long distances.
More likely to cause interference problems AM and SSB
Amplitude Modulation (AM) and Single Sideband (SSB) are the most likely types
of transmission to cause interference because of the syllabic nature of the
radiated signal and the fact that receiving and audio equipment, under certain
conditions, is able to detect (extract the audio modulating signal) from
the modulated carrier wave. As it is likely that SSB will be your
most used mode of communication and it is essential that you understand that
you can easily cause problems to others.
What can you do regarding aerials
All antennas should be correctly adjusted for the frequency on which they
are operating for the best VSWR (Voltage Standing Wave Ratio). Possibly the
best antennas - from an emc point of view - are centre-fed dipoles using
a BALUN at their centre feed point when fed with coaxial cable. Long wire
antennas have the potential to cause interference and should be correctly
tuned with the aid of an antenna tuning unit (ATU). Most HF transceivers
have a built-in ATU these days - although it may come as an optional extra.
EMC problems for VHF/UHF can be minimized by good location of aerials as
far away from houses as possible and also to locate them as high as possible.
Operating power
Operate with as low a power as possible and keep transmitting antennas away
from neighbours receiving antennas and Hi-Fi equipment to avoid trouble.
Think of the neighbours
It is more than possible that your neighbour's receiving equipment lacks
immunity from strong nearby signals. In such cases it is necessary to gain
their confidence and carry out a few tests to determine the real source of
the problem. Never admit causing interference - although it might seem the
case to the neighbour - the problem is more likely to be due to lack of immunity
to strong RF fields of the TV and Hi-Fi equipment.
Always refer to it as an EMC problem - Never interference. Ask your neighbour
to keep a log and see if it ties up with your transmissions which will be
properly recorded in your log book!!
Such log book on both side of the EMC problem will be useful when checking
for correlation is helpful during investigation of EMC problems.
Other systems can be affected
Also security alarms, lights and similar devices can lack immunity and be
triggered by an amateur transmitter. |