64 Silicon chip Australia’s electronics magazine siliconchip.com.au
the bigger magnet. I firmly tacked it
in place using some of the food-grade
silicone sealant that I had left over
from the kitchen reno and gave it a
good 24 hours before re-assembling
the holder to the vane and the vane
to the dishwasher.
After completely resetting the dish-
washer by powering it off at the break-
er and powering it up again, I ran a
test cycle using the magic codes. It
worked perfectly, without any nasty
noises and the bottom rack of dishes
are now cleaning correctly. The dis-
play still glitches now and then, but
$25 is an excellent repair bill, given it
could have been much, much higher.
Of course, the display is still a bit
flakey as that is an unrelated prob-
lem. But I’m so relieved to have clean
dishes again that I’m leaving that fix
for another time...
RF interference at the end of the
rainbow
D. P., of Faulconbridge, NSW went
on a bit of a wild goose chase to try to
track down the source of some strong
radio frequency interference. It took
some time but not only did he figure
out where it was coming from, he also
managed to shut the source down.
Here is how...
The Amateur Radio fraternity
maintains repeaters on various bands
(mainly VHF and UHF). These repeat-
ers are usually set up and maintained
by local amateur radio clubs and are for
the use of all licensed amateurs. The
idea is that one can get good commu-
nications from low-lying or other dif-
ficult locations by virtue of the prime
(radio) location of the repeater.
The repeaters have different input
and output frequencies and are locat-
ed on the best high point that the club
can organise. For the higher bands, a
single antenna usually serves for both
the receive and transmit signals.
You may wonder how that is pos-
sible. The received signal is typically
tenths of a microwatt, while the trans-
mitter output is usually 50W or more,
and the frequency separation between
the receive and transmit frequencies is
relatively small (600kHz, in the case
of VHF repeaters).
The secret is cavity resonators. They
have a very narrow passband, with
extremely high attenuation outside
of it. They can be connected in series
for even better filtering. Most VHF re-
peaters have three or more cavities,
providing a high degree of isolation
between the transmitter and receiver.
Other devices such as hybrid rings are
sometimes also used to enhance the
effect of the cavities.
A few years ago, I joined the repeat-
er committee of my local club in theBlue Mountains, west of Sydney. For
some time, the club’s VHF repeater had
been plagued by interference. The ori-
gin of the interference was unknown,
although it had been positively estab-
lished that it was coming in on the an-
tenna, and that nothing in the building
was causing it.
This had been established partly by
monitoring the repeater input frequen-
cy with various receivers at different
locations. The interference could be
heard well away from the repeater.
The interference consisted of bursts
of a nasty rasping noise and made the
repeater pretty much useless. The in-
terfering signal was strong enough to
open the receiver squelch at regular
intervals, triggering the repeater, re-
transmitting the horrible noise. The
constant bursts of noise were so an-
noying that few people monitored the
repeater any more.
One of our club members had a job
as an engineer in one of the telecom-
munication companies. He became
interested in the interference prob-
lem and connected a VHF antenna to a
spectrum analyser at work. The inter-
fering signal was plainly visible, and
one of our member’s colleagues said
he thought it was a pager signal, albeit
grossly distorted and “chopped up”.
Strangely, though, it was not on any
established pager frequency; it was
definitely in the VHF Amateur band.
Actually, it was centred adjacent to
our repeater input frequency with its
sidebands regularly intruding into our
repeater input passband. It was at these
times that the interference occurred.
I tried listening to the pager frequen-
cies on a separate receiver while mon-
itoring the repeater output but I was
initially confused because sometimes
the pager data seemed to be triggering
the repeater, sometimes not.
Telstra used three pager frequen-
cies at the time. These same frequen-
cies were transmitted simultaneously
from stations dotted around the coun-
try. Signals on one of these frequencies
were definitely unrelated to the inter-
ference, but the other two seemed to
both be contributing to it.
Eventually, by using two receivers,
we discovered that it was when both of
these frequencies were active simulta-
neously that the interference occurred.
So it seemed to be some kind of in-
termodulation effect, but where was it
occurring? It was not in our repeater,
since we had established that the in-