66 Silicon chip Australia’s electronics magazine siliconchip.com.au
terfering signal could be heard many
kilometres from the repeater.
I started monitoring the repeater
input frequency in my car as I went
about my normal activities, to try to
get an idea of where the signal was
strongest. The interference could be
heard all over the place with varying
strength with no discernible pattern,
although I had the impression that el-
evation could have been a factor.
I obtained a list of pager station loca-
tions and tried approaching several of
these, however, the signal strength of
the transmitters up close was so high
that it swamped my receiver input and
made it impossible to make any mean-
ingful observations.
I solved that problem by connect-
ing several spare cavities between my
antenna and the receiver input, care-
fully tuned to the repeater frequency.
I could now get right next to a pager
station and maintain normal sensitiv-
ity of my receiver.
This approach eventually bore fruit
as I was confident that I was hearing
only the interfering signal, even when
close to a pager station. One day, my
travels took me further east than usual,
and I began to receive much stronger
signals than before as I moved towards
the coast. Eventually, as I topped a rise
to Bilgola Plateau, a flat area right near
the coast, the interference came roar-
ing in at S9+.
At first, I thought something had
gone wrong with my gear, but it all
checked out. Maybe I was seeing the
actual interference! By inserting an
attenuator into the receiver input and
driving around a bit, I was able to es-
tablish that the signal was coming from
a tower on the edge of a public park
on the Plateau.
There was a hut next to the tower,
and the door was open. People were
working in there, so I approached
them, introduced myself, explained
what I was doing, and asked if they
could tell me anything about the pag-
ers. They said no, they did mobile
phones, but they could give me a num-
ber for “the pager blokes”.
I asked them if I could have a look at
the pager gear. They showed me some
very impressive rack-mounted trans-
mitters with large heatsinks. Appar-
ently, they were quite high-powered
units. Connected to the transmitter
outputs were, guess what, cavity reso-
nators! These were not the home-brew,
copper-pipe devices I was used to see-
ing, but were nicely finished commer-
cial units.
I called the number the mobile
phone guys had given me and spoke
to a very helpful technical officer who
listened patiently to my tale of woe. He
said he would send a technician out to
investigate the problem. When I sug-
gested that the technician visit the re-
peater site to see the problem for him-
self, he agreed, and we arranged a time.
In due course, several members of
the repeater committee met the Telstra
technician, complete with his spec-
trum analyser, on site. After a coffee
break to help our new friend recover
from his long drive, we connected his
spectrum analyser in line with the re-
peater and antenna via a T-piece, after
assuring him that we had disabled the
transmitter!
Monitoring the repeater’s receiver
audio while watching the spectrum
analyser screen we could plainly see
and hear the interference. Our new
friend agreed that it was a pager signal,
and that it probably was an intermod-
ulation product of two networks. He
said he would investigate the problem.
By the next morning, we were de-
lighted to find that the interference had
gone; but it returned at a lower level
that afternoon.
Our friend phoned to explain what
he had discovered. He had found a
faulty cavity at the Bilgola site and
having no spares at that time, had
swapped it for a good one from the
Parramatta site.
His thinking was that Parramatta
would cause us less interference since
it was shadowed from our repeater by
the mountains to some degree. He had
ordered a new cavity.
He was right, the interference level
was lower, and by setting the repeater’s
squelch level higher, we were able to
stop it from triggering the repeater. This
was not ideal because it effectively re-
duced the sensitivity of the repeater,
but at least we didn’t have to listen to
the repeater triggering constantly. It
would do until the new cavity arrived.
I was curious as to why the pager
units, which were transmit-only de-
vices, should have cavities connect-
ed to them. Our friend explained that
when several transmitters on similar
frequencies are feeding antennas near-
by, the transmitted signals from each
antenna are induced into the adjacent
antennas.
With no cavity resonators, therewould be nothing to stop these in-
duced signals being fed back into the
power amplification (PA) stages of the
other transmitters. These PA stages are
highly non-linear (Class C), and when
the transmitter is triggered, a whole
spectrum of frequencies would be pro-
duced from the mixing of the transmit-
ter output with the extra signals picked
up by its antenna.
This whole mess would then be
amplified and anything that could get
through the PA stage’s output circuit
would be transmitted. These ‘dirty’
signals would be induced into the ad-
jacent antennas as before, in turn gen-
erating a mind-boggling array of new
modulation products.
The cavity on each transmitter pre-
vents this by blocking the induced sig-
nals from reaching its transmitter and
rejecting any spurious outputs from
its transmitter.
A few weeks later, I received a call
from our Telstra friend to say that he
had installed the new cavity. I was able
to report to him that there was now ab-
solutely no interference, and to thank
him profusely for his diligent attention
to our problem. Full marks to Telstra
and their staff!
Since then, a tone squelch system
has been installed in the repeater. A
sub-audible tone imposed on the user’s
audio is required to open the squelch,
Preventing the repeater from being
triggered by rogue signals. However,
this does not prevent legitimate traf-
fic from being subject to interference
while the repeater is being used.1970s Marantz 1120 amp repair
J. W., of Hillarys, WA did a mate
a favour a few years ago and fixed a
fault in his trusty Marantz amplifier.
Now something else has gone wrong
and so it’s back on the workbench for
some more surgery...
A few years ago, I repaired a friend’s
Marantz 1120 stereo amplifier (circa
1968). He rang last week to report an-
other fault in the amplifier. I made a
house call to check it out and found
everything was working except for
the phono input. There was no sound
from the right channel. I disconnected
the myriad cables from the unit and
took the amp back to my workshop to
check it out.
I connected the phono input to an
iPod and wired up some speakers in
my workshop and found the fault was
still present – no output from the right