[CambridgeWorks]

I remember a lowband fm talkthrough system and office controller of about 35 years ago. Complaint was tx from office ok, rx was weak and talkthrough had weak rx as well. It was a farmer. Testing tx rx performance on base and a couple of mobiles, all appeared ok. So, with no mobiles more than a short distance away, it was nff. A couple of days later, same complaint. Again, all tested ok and with no distant mobiles available I swapped over the tx and rx feeders and said "let me know if improved". BTW, the 2 dipoles were at about 90ft and spaced correctly. A bit longer before the next complaint of same poor rx on the base. This was my only lowband fm scheme so I had no base station gear I could swap out. Knowing talkthrough rx and base rx were both at fault and thinking it could only be an intermittent fault with the R17fm receiver, I started a complete system check to prove everything else was ok. The rx mute was at about 0.5uV with a direct signal into the socket, but when I checked the tx/rx antenna isolation by reconnecting the rx aerial and feeding sig gen into the tx aerial, it needed a far larger signal to open the 0.5uV mute than what I expected. This set me thinking. Interference maybe?
To summarise, it turned out that a local 11kV powerline insulator was arcing and hitting the rx with wideband noise. Being fm, it used a noise mute circuit. This also made the rx deaf when connected to the aerial. Like I said, it was my only lowband fm system as around 10 other local lowband systems were AM. If it had been the usual AM the fault would have been seen immediately. When the insulator was replaced, one happy customer.
Unusual fault that I never forgot.
Rob