At night, one of the local AM stations receives significant interference from only one co-channel station. I'm quite sure that I live outside of the NIF contour of the local station, but when the interference from the co-channel station fades down, the local station comes in fine. For kicks, using nothing but a radio and a clock with a second hand, I "measured" the frequency difference between the two stations. I counted 10 loudness peaks in 15 seconds or 40 peaks/minute. 40 peaks/minute = 0.67 peaks/sec = 0/67 Hz. Or does it? Are there one or two peaks of the sub-audio heterodyne per beat-frequency cycle? I would say that the beat frequency is mixing with the higher-frequency audio, and if so the beat frequency signal (that is, the SAH) forms a modulation envelope. I think that there might then be two audio peaks per beat-frequency cycle and the beat frequency would thus be 0.33 Hz. Which is correct in this case? Is the frequency difference between the two stations 0.33 Hz or 0.67 Hz? This is driving a little crazy!
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Sub-audio heterodyne (SAH)
- Thread starter DanStrassberg
- Start date
I think the number of peaks per second would be the actual frequency difference between the two carriers. It is not uncommon for stations to offset their carrier frequencies by agreement to reduce this fading effect. Station A could run +10 Hz and station B -10 Hz to produce a subaudible 20 Hz beat that most receivers would ignore. Analog TV stations did the same thing (except that the FCC specified them do it) to eliminate beats in the video. They commonly used a 0, + or - 10 kHz offset.
ncradioeng said:
Thanks! I wonder if a carrier offset of anything like 10 Hz would be practical in this case. The interfering station should have no problem; it has a two-tower array. The local station, however, has a five tower array with a VERY tight teardrop pattern that suppresses radiation to an unbelievable extent around a 240-degree arc. I wonder if changing the frequency by ~10 ppm would mess up the adjustment of the array. Oh, and BTW, the local station is diplexed with another station that also has a five-tower array (same five towers) and whose night pattern is nearly as tight as that of the first station. A high-voltage AC power line is located a short distance from the array and is oriented roughly perpendicular to the line of towers. The towers of the AC line had to be detuned at the two stations' carrier frequencies in order to bring the patterns into compliance.
The lower the better on carrier difference. Anything near 20 Hz and you begin to sound awful in your fringe area.
And it's not just offsets and your own stability, it's the other station's stability and accuracy too.
In other words, your transmittter might hold within +/- 10 hz of a perfect 0.00 khz setting, but if your closest, significant co-channel runs 30 hz too high, you'd sound better in most areas if you also ran 30 ( or maybe only 20 ) hz high.
If they drift, you always have a moving het frequency.
I zero beat my pt 15 osc daily against a local 50 kw AM. But the nearest co-channel is a traffic report signal 25 miles away.
I really OUGHT to go off the air, zero beat to the distant 1620, then come back and zero my carrier on that reference.
The traffic transmitter is not as nearly as close to being on "zero.zero" as commercial broadcast seems to be.
This would eliminate low moaning sounds in the audio for fringe listeners.
Deep fades are not really a possibility or a concern for part 15 AM, so sounding as good locally as possible is important.
And it's not just offsets and your own stability, it's the other station's stability and accuracy too.
In other words, your transmittter might hold within +/- 10 hz of a perfect 0.00 khz setting, but if your closest, significant co-channel runs 30 hz too high, you'd sound better in most areas if you also ran 30 ( or maybe only 20 ) hz high.
If they drift, you always have a moving het frequency.
I zero beat my pt 15 osc daily against a local 50 kw AM. But the nearest co-channel is a traffic report signal 25 miles away.
I really OUGHT to go off the air, zero beat to the distant 1620, then come back and zero my carrier on that reference.
The traffic transmitter is not as nearly as close to being on "zero.zero" as commercial broadcast seems to be.
This would eliminate low moaning sounds in the audio for fringe listeners.
Deep fades are not really a possibility or a concern for part 15 AM, so sounding as good locally as possible is important.
The FCC AM frequency tolerance is +/- 20 Hz. I doubt anything in the antenna system would be affected by drift inside this 40 Hz window. If it was, it would not play well with modulation applied.
Crawford used to purposely offset the carrier of their AM Stereo stations. This would make the fluttering worse for skywave and fringe-area listeners, but would reduce platform motion for their local C-Quam listeners. However, now that they're gung-ho for IBOC, Crawford's stations should have their carriers exactly on-frequency, because iBiquity requires AM IBOC stations to lock their carrier frequency in the IBOC exciter using GPS as the reference.
satech said:
AHA! The station that is the only co-channel contributor to the NIF of the station in question runs IBOC. The station in question does not. Helps to explain (even if it doesn't _completely_ explain) the very small (0.33-Hz) difference between the two stations' carrier frequencies. If the station in question could lock its carrier to the same reference, which would--I presume--produce a zero difference between the average carrier frequencies (even if not the "instantaneous" carrier frequencies), I don't know whether the co-channel skywave interference would become more or less objectionable. I think it's pretty clear that if an AM could pick whether its nighttime skywave interference came from one station or a large group of stations that produced the same RSS of 10% skywaves, the large group would be much less intrusive even if all of the stations that are contributors to the NIF were precisely locked to the same reference frequency.
Have used the +-4 Hz offset when running C-QUAM. Years later a new co-channel shows up and is plainly heard in the 20 to 10 mV/m service area just after sunrise & before sunset. Had to offset -4 Hz from them and it did help in the Metro. However nothing will help those trying to listen outside of the 10 mV/M contour.
Platform motion is cool. If you're in vehicle with front and rear speakers, the effect is "audio vertigo",
where the brain says "we're not spinning", but the ears say " Oh, yes we ARE!."
I once listened to WLS at 200 miles in CQUAM while the Cuban seemed to run in circles around my audio perspective,
first clockwise, then counterclockwise.
I don't know why this effect should bother anyone. It's not a problem, it's a feature!
where the brain says "we're not spinning", but the ears say " Oh, yes we ARE!."
I once listened to WLS at 200 miles in CQUAM while the Cuban seemed to run in circles around my audio perspective,
first clockwise, then counterclockwise.
I don't know why this effect should bother anyone. It's not a problem, it's a feature!
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