> This continued for several overs, and included letters to
> station managers, with suggestions as to how this spur
> could possibly be generated. It eventually died on the
> vine. The "ham" engineer really seemed to be in a fog on
> the whole topic.
While bad connections are often thought of as causes in
transmitters, that generally only applies to bad connections
near or in the receiving end.
Near or in the transmitting antenna any bad connection is
almost always quickly "healed" by arcing and becomes at best
a very intermittent problem.
The common cause of this mixing is sufficient RF making it
back down the feedlines into a transmitter. Good modulation
linearity or an efficient PA stage demands having a very
sharp switching waveform at radio frequencies...and this
means the transmitter is a very efficient mixer for anything
injected into the transmitter's output device. The
conversion or mixing loss can be just several dB!
When two stations are very close together or using the same
antenna any amount of RF backfeeding the PA stage of either
transmitter will be converted to new frequencies. The
transmitter's main frequency (and its harmonics) becomes a
"local oscillator".
Take the 1550 and 1680 case. We have every sum and
difference of the "local oscillator" and its harmonics, so
we have a new signal possible every 130kHz up and down from
the transmitters. Since this spur is 1810 the cause is
1680*2-1550 = 1810. This means the mixing causing the 160
problem is almost certainly in the 1680 transmitter where a
tiny bit of 1550 is modulating the 1680 PA stage. This is
because the problem is the *second harmonic of 1680* minus
the fundamental of 1550.
The first thing they should check is the notch in the
multicoupler that removes 1550 from the 1680 transmitter.
This would be the parallel tuned circuit that is in series
with a series tuned circuit going to the 1680 input. If they
sent a copy of the multicoupler schematic I could tell them
how to adjust it. If the notch isn't deep enough then the
best thing to add would be a 1810 trap on the 1680
transmitter or an additional stage of 1550 notch on the 1680
transmitter.
It isn't simple to notch at the antenna but it could be
done. It is much easier to notch at the offending
transmitter.
73 Tom
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