Satgen598  Interference on RS13    by GM4IHJ (BID SGEN598) 2000-09-09

Murphy's Law is operating this morning . Albeit for the benefit of,
the good guys for once. Having been asked to write about a curious
intruding signal which has been sitting on the lower end of the RS13
downlink for a month . It has not appeared this morning.

The signal comes from somewhere south of Scotland. It has appeared
regularly every day around 0700z. It is weak at first but builds up
over about an hour to become a positive nuisance. Some days it fades
by 1330z , but on others it does not fade until 1530z or so. So it
appears to be received via the ionosphere, with the absence of any
minute by minute fading suggesting that it is single hop ionospheric
rather than long distance multiple hop propagation. 

With its central stronger elements clearly audible over a band from
29.457 to 29.461 MHz it appears to be composed of a string of
separate unmodulated CW carriers, spaced one ever 10 Hz
approximately. So what is it? the FFT DSP Spectral analyser reveals
it as very constant, unchanging hour after hour. Suggesting that it
is perhaps the product of some industrial process or maybe even a
games machine , rather than some new form of communication.

What ever it is, it is perhaps indicating that radio amateurs need to
discuss how much the bands have changed over recent years given the
very limited attention that licensing authorities seem to give to
intruders. It is very easy to pretend that low power devices can be
introduced in all amateur bands , with little effect on their
licensed users. But this completely ignores the fact that low power
signals propagate worldwide very easily and while allowing one or two
to sneak in may not do much damage , the overall impact of what is
more like a free for all, can be very damaging.

Picking a quiet site in a remote Scottish village is no protection at
all. These low power devices and the radio toys that get into every
childs cupboard, fill the air waves with their sparks and squeeks. As
is only to be expected from what is mostly, the product of a system
where price is all important and adherence to radio interference
suppression regulations is nil.

FFT DSP equipment producing print outs of the radio spectrum , have
been with us for several years now, and these records can be studied
to examine just how much the situation has deteriorated, as
interference has grow almost exponentially. Present day traces are
full of fixed frequency signals , slow drifting signals and sweeping
irregular wandering signals of quite high power. Indeed few traces
taken in year 2000 show less than 4 of these intruders per kilo hertz
of bandwidth. Many traces show far more. Can anything be done about
this ? Checking ones own equipment may produce some slight reduction.
Indeed one microcomputer at IHJ proved to be completely clear whereas
the other, of a different make, required considerable attention. But
what ever you do, you are stuck with the problem of signals coming in
over the air. Particularly in UK, where the radio authorities have
always been far more lax than those in say the USA.