Oscillators On After Signoff

[N4GBK]

A strong AM transmitter signal can get into the FM exciter and cause all sorts of spurs.

I was at an AM on 1400, there was another AM nearby on 900. There would be a signal on 500 kHz that caused problems with maritime distress near the port. FCC required both stations to put a filter in the tuning networks. Even with one or the other station off the air, there was mixing going on in the tuned circuits from the two stations. You could ground the tower with the station off the air and the interference would go away. It was heard for at least 10 miles when 500 kHz was quiet.

 

[crainbebo]

Sometimes KKPZ 1330 Portland's carrier never shut off after 12AM, meaning CJYM was still under the carrier.
I have not heard any oscillators after sign off however...

-crainbebo, who just did post 3500!!

 

[audioguy]

The college station I engineered had a Harris FM-5H3 with a TE3 exciter. The exciter was on anytime the filaments were powered up. I never measured the power to the antenna; it would only have been whatever leaked through the PA. It must have been in the milliwatt range but you could hear it anywhere on campus. As we were normally a 24 hour station, this didn't happen very often. We had to warn the station staff not to send any audio to the transmitter when we were "off the air". But yes, you could hear the "oscillator" within a few blocks of the transmitter.

 

[BobOnTheJob]

I've seen AM signals get into FM exciters on more than 1 occasion. If you have an 1100khz AM and a 93.1 FM on the same stick, you'll sometimes hear the AM audio (distorted but identifiable) on 92.0 & 94.2. Double shielded cable at the exciter composite input will often fix it. That got me thinking so I hooked my 400' longwire to the composite input of an exciter. The modulation indicator moved to about 1/3 scale and when tuning above and below the exciter's frequency, I could hear numerous AM stations. My best composite DX "catch" was 0.7mhz from the exciter frequency--WLW--which is 90 air miles (!) away.

 

[KR4BD]

I've seen AM signals get into FM exciters on more than 1 occasion. If you have an 1100khz AM and a 93.1 FM on the same stick, you'll sometimes hear the AM audio (distorted but identifiable) on 92.0 & 94.2. Double shielded cable at the exciter composite input will often fix it. That got me thinking so I hooked my 400' longwire to the composite input of an exciter. The modulation indicator moved to about 1/3 scale and when tuning above and below the exciter's frequency, I could hear numerous AM stations. My best composite DX "catch" was 0.7mhz from the exciter frequency--WLW--which is 90 air miles (!) away.

Bob...Here's a good one I'd like explained. We have a station near here (Lexington, KY) on 1380. They really don't pay attention to what gets on the air. Their automation often fails and they often don't power down at night. Three nights ago, they had an open carrier on ALL NIGHT on their 5kw day power. Faintly, I could CLEARLY HEAR WCBS-880 under (on?) their carrier. This was on two different radios/antennas. I AM SURE what I was hearing was WCBS.... ????

 

[HGR1290]

Sounds like WHGR may have induced a bit of RF on the composite cable of WUPS-FM, thus causing the spurs.

I recall, a station I worked at had a loaner Gates transmitter that we borrowed after our BC5P went up in flames. The gates was a 1kW, after midnight, when we would sign off, you could still hear the carrier from the osc, 5-6 miles out.