Quote:
Originally Posted by Jeffhs
Why would an open loop antenna in an AC-DC radio cause garbled reception (if the radio receives anything at all) on very strong stations? I would think an open loop would kill all reception, even if the radio were literally within spitting distance of a local station's transmitter towers. I was only receiving the local station in my area (1kW, two miles or so distant from my apartment) on the AM band of my C845 with one end of the loop open; however, the sound was weak and extremely garbled, as if 60-cycle AC hum were entering the audio stages due to a defective filter capacitor or an H-K short or leakage in an audio tube (although H-K shorts in AC-DC radios usually kill the set entirely, since the short will remove filament voltage from all tubes ahead of the shorted one). The tipoff that the problem was unlikely to be the filter capacitors or tubes was there was no hum whatsoever on FM.
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In most designs using a loop antenna, the loop provides the DC return path to ground for the converter tube's grid. When that path is broken, the grid is free to 'float' and becomes exquisitely sensitive to ambient AC fields in the air.
When a station is tuned in, its signal is heavily modulated by the resultant hum making it sound garbled. And it's weak because the tube's bias polnt is wacked 'way out, cutting the gain*. But when you tune off-station, you don't hear the hum at all. You hear it only when a station is tuned in. Thus the effect is called 'tunable hum'.
And it affects only the AM in an AM/FM set.
*Loss of gain also occurs due to the resonant circuit (of the loop and tuning cap) being broken.