On Tue,11/3/2015 10:41 AM, Dennis Egan wrote:
After reading the article by N0AX in QST early this year, there may be
rectification going on in one or more pieces of equipment. Since switching
diodes in different gear could cause a mixing problem, I'm also planning to
remove the feedlines from the back of all amplifiers and looking for the
problem to go away. I have a handful of disc caps to bypass rotor, stackmatch,
and other control cables. I'm also bringing an RF probe, so I can listen for
IMD products at various pieces of equipment.
Hi Dennis,
Intermod like this can be generated ANYWHERE, both with your own
equipment and wiring, and with equipment and wiring totally unrelated to
you or your station. It then re-radiates as the intermod products. No
amount of filtering within your antenna system or in your radio will
help that. Mixing can occur on virtually any poor connection that forms
a diode -- anything from rain gutters to wiring for power systems, CATV
antennas, telephone, etc. It can be building metal.
Some suggestions. Try to identify the AM stations involved in the mix.
The object is to get a handle on how strong they are at the problematic
QTH. Use this FCC link
https://www.fcc.gov/encyclopedia/am-query-broadcast-station-search
to find stations near you. Plug in your Lat/Long and enter a distance of
20 miles or so, using the "Stations within a Radius" search (Scroll down
to find it.)
In Chicago, I had intermittent intermod from a couple of 50kW Clear
Channel stations that were nearly 20 miles away, and from a lower power
station that was much closer. I never did track it down.
To find the wiring that is detecting and re-radiating, get a portable RX
that you can tune to the affected bands, put a suitable RX antenna on
it, and poke around your QTH, extending the radius until it gets
stronger. I have two good choices in my arsenal -- a Kenwood TH-F6A
talkie and a Tecsun PL660 (discontinued, still available, I think,
replaced by the PL880). Both have internal loopstick antennas. The
Kenwood talkie has pretty poor sensitivity outside the VHF/UHF ham
bands, so you've got to be pretty close to the source to hear anything.
The Tecsun radios have much greater sensitivity on MF and HF.
A transceiver in a car with an antenna tuned to the affected bands can
also help to get close to the IMD source.
73, Jim K9YC
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