I have been near several AM transmitting sites, usually Class Ds within the protected Class A skywave contour, where the oscillator stays on after the amplifying stages are off. Usually, you can hear the unmodulated oscillator for a few blocks before the Class A overpowers it. Recently, I have noticed a Class B with a silent STA where the oscillator can be detected 15-20 miles away in the Daytime without skywave interference. It is at least 30 dB down from the full power level. Does anyone know the typical power output of an AM BC oscillator, and how much would pass through to the antenna?
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DXing AM BC Oscillators
- Thread starter Schroedingers Cat
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kenglish
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I wonder how old their transmitter is. Maybe it's an old all-tube unit, with a big old crystal oven. They'd leave the tubes on all the time for stability.
If they are silent, maybe they are afraid to leave it off for long, since it might not come back on?
If they are silent, maybe they are afraid to leave it off for long, since it might not come back on?
A lot of us would run our AUX in hollow state days with the filaments on always. But that meant that the low power part of the rig with the oscillator and the audio input were on fully powered. The idea was to allow instant switching if the main transmitter failed. Otherwise, it took 30 to 60 seconds at best to get back on the air... way to long in competitive markets.
My experience was with, at most 5 kw to 10 kw rigs. And what you had was a few watts off the oscillator stage going into the intermediate stage. A 1 kw might have had a 6146 that could output about 30 watts or so efficiently. That would be fed to the input of the final, and anything that got on the air was inductive coupling from that stage.
I'm guessing that even a 50 watt intermediate stage would likely get no more than 5 to 10 watts onto the air. Yes, it was a tuned circuit, but the tuning was only intended to match the input to the tube's specs, not to create RF. On the other hand, my iPhone does a pretty good job of charging that way!
I suppose I could do the math and look at tube specs and get a real number.
Would it be easier to measure the current, square it, and multiply by the impedance? Or direct power measurement? Say, for example when you ran 5 kW, you have 10 amps and 50 ohms. If it was 40 dB down, you'd have 0.1 amps, if the meter measured that low, and 50 ohms, 0.5 watts to the antenna. For the old classic minimum efficiency Class B, 6.31 mV/m inverse field at 1 km.
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Mr Ohm would agree. That is mathematically correct, but we know the impedance of the common point or the coax at the transmitter for a one-tower system. I'm not sure we have the measurement for the effective impedance at the plate of the final tube for a non-"ignited" operation. And I wonder if it would be very different, such as the case between a pair of 833's vs a pair of 4-440A's where you have one with a screen grid and another with a plain old tetrode. At this point, what I remember of engineering from my tech days hits a cliff and totally fails... or falls, to continue the analogy.
Again, I am not sure whether you measure the antenna impedance or the internal impedances of a tube. The transmission is essentially via induction, isn't it?
Mr. Ohm says "Call me Georg".
I've never noted that phenomenon with any AM (MW) station, however I remember it with the first FM station I worked for. We left the filaments on for both the AM daytimer and 3kW FM. There would still be a signal on the FM for a half mile or so. More than once I'm thinking "did I actually turn the transmitter off"?
There was this other small market 1 kW AM Daytimer (before PSSAs) with a 3 kW FM. They separated daily programming from simulcast for a short call in show from the other studio. If the phone line was still potted up after official sign off and before the AM transmitter was shut down, sometimes for several minutes, calls to the studio line would go out over the air, because the sign off cart came from that studio. Girls would call the DJ and ask him whether he wanted to meet to "make love" after his shift was over. And it went out over the air a few times. I happened to hear it driving in my car. No, it wasn't me who was the DJ. The FM exciter would stay on after the transmitter was turned off, and sometimes was left on while the DJ did production (that didn't go out over the air). I heard it because I would sometimes leave the radio on to wake me up at the 6 AM sign on. Sometimes I would fall asleep listening to WLS on the same radio and wake up to WCBS!
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