[eddie_ce]

An interesting one that led me a merry song and dance was when, in my former life I was responsible for blue light comms in a part of northern Germany.

All blue light organisations used specific kit and frequencies exclusive to the emergency services. For our 4m Band scheme we had a standard duplex split of 9.8 MHz. In this incident, the local fire service control room complained of sporadic music on one of their channels, this only happening at certain times of the day. The TX/RX talkthrough system (we always had talk through active) was co-located at a main and remote site. Most control rooms were not usually tied to the hilltop RX via leased lines or radio links (these came later) but received the repeater off air as did the mobiles.

Ah, I thought probably interchannel modulation with one of the TXs on the local radio broadcast mast, however when listening on the output frequency of the fire service repeater I could hear that speech associated with the music was French. Listening to the music I noticed fading associated more with short-wave propagation. Then the penny dropped. When the TX came up the product of its frequency with a 9.8 MHz "interfering" signal fell exactly into the passband of the fire service RX.

Looking on Frequency tables I found that Radio France International transmitted on a frequency of 9.805 MHz. But where did this mixing take place. After many tests it was found to be a corroded antenna connector. While listening to the French station on the fire channel, I found that giving the base of the antenna (98m above ground) a hefty whack with a large hammer "solved" the problem.

Antenna and connector replaced. Problem gone. This was my first and only experience of secondary modulation and the "rusty bolt" effect. Happy days.

Although not connected to PYE etc. I hope this is on topic enough.

I find this thread most entertaining, it brings back many memories.