Topband: Fwd: BCB Interference

[Jim Brown]

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