Hunting down sources of intermod on the AM MW BC band

[wa2ise]

I think I'm seeing intermodulation on the
AM broadcast band here in northern NJ. A station WMTR 1250kHz apperas to be
getting interference from WABC 770 and WINS 1010, two very strong stations
here.

3rd order inter-modulation between two strong signals follows this equation:

Fb > Fa

2Fa - Fb is a 3rd order IM product below the frequency of Fa. And 2Fb - Fa
is another 3rd order distortion product which falls above Fb.

2*770 MHz - 1010 MHz = 530 MHz.

2* 1010 MHz - 770 MHz = 1250 MHz, the WMTR frequency.

However, third order IM can be generated by any junction of dissimilar
metals. This is now called Passive Inter-modulation.

Receivers with high RF selectivity reject most strong undesired signals but
I don't think any are still around. That required a tuner RF amplifier and a
3 gang tuning condenser.


the intermod was getting really bad, as it's been raining in NJ the past few days. So I figured it would be a good time to hunt it down. Using a portable AM radio to sniff around, I hunted around various cables around here. Turns out it was my ham radio setup. The antenna is a vertical designed to work from 3.5 to 30MHz, and is mounted on a metal pole in the ground in the back yard. And the ham radio transciever has an external 12VDC power supply, with a green wire safety ground tied to the negative 12V terminal. So now I have a large loop of wire, the antenna coax cable from the antenna and its ground about 40 feet (I used RG8 for less attenuation), to the transciever, and then there is the safety ground conductor on the house wiring running something like 50 feet to the electric servive entrance ground. Break this loop and the intermod goes away. I now think that there was a loose coax connector on the back of the rig, and some corrosion acting like a diode, and not a protection diode in the rig minus 12V line. As I didn't want to run the power supply with a disconnected safety ground, I found that a short piece of wire to commect the power supply safety ground to the antenna coax outer shield, to jumper over that "diode", also killed the intermod. Later on, tightening the loose connector also kills it.