AM question

Beat me daddy, 8 to the bar

Howdy, beat, oh yeah, I get that every night on my carrier current station, going away from the power lines a bit, and there are a few other stations mixing in with mine. With my Procast above 1600, there's beat too, but not usually as bad.

It's caused by the difference in frequency between your transmitter and the carriers from other stations. The carrier by itself is the most powerful part of an AM signal, so you can hear beating even without audio from the other station being heard.

The standard for broadcast transmitter stability is +/- 20 hertz, so maximum beat frequency could 40 hz, a tone audible as a low hum on larger speakers. I used to hear that on my parents' lowboy Magnavox hi-fi console that had a great AM tuner in it, the speakers would rumble on some stations.

Over the years there have been proposals for greater stability on transmitter carriers, when it was found that a lower beat frequency was less annoying to listeners. They'd do it by comparing the frequency to WWV or these days, GPS clock signals for better than 1 hz stability, or even 0.1 hz.

The fact that we can even get carriers within a few hertz of each other and stay that way for a long period of time with only a few parts in a simple crystal oscillator is a pretty neat feat to begin with!

I love these qualities of AM, even when it seems destructive to listening, it's part of the AM scene for me, and people who don't listen to AM are missing out, in my opinion. It's like a vinyl fan who likes needle noise when playing their records, it makes the experience real to them.

Boomer

This may work well for something like an ASMAX2 because you could connect it right in line.  I wonder however if it could do an accurate wireless reading of something like a Procaster a few inches away from the antenna?

The Beat Generation

Hi Oldies WMRK and Legacy, in my listening, I don't find commercial stations to be 'dead on' frequency, but I think they're tighter than they used to be, it's more like an average of 3-5 hertz, where in old times I regularly heard flutter and even deep audible tones, carriers that might be 30-40 hz off. I was well aware of that kind of thing, since often I'd tune the AM band from one side to the other at night to hear something interesting, so I was hearing a lot of weaker signals and remember the beat situation as it was.

On the other side, I've found it's easy to get commercial stability at home, with good oscillator and enclosure design, and a modern crystal. If you're really into it, you can use an oven on the oscillator, or GPS disciplined oscillator, which Part-15 Engineer introduced to the group a few years ago, and had prototype units for his station as I recall.

Boomer

Clippy, the paper clip

It's good you're going for a lower beat, it should help at night.

Where do you connect the clip on the other end of the wire to, the antenna, or more wire or coil to make it pick up better? That might be some good info for people who can get access to a counter.

I think I see what you did, you set the DIP switches for another frequency with a weak station on it to cause a beat, then zero beat with them. It's a good idea, since the synthesizer will be an exact 10 khz step and zero beat on your own frequency again.

I did something similar before I had a frequency counter, and even now I think it's acceptable to do. I'd just zero beat my oscillator with another station at night, and the performance was totally acceptable.

Boom

It comes with a CB radio!

Well, maybe no CB with the counter you're getting, you wouldn't need it. Looks good, but is probably the same circuit as my BK Precision model from 1983, which is still a great performer though mine only counts to 100 mhz.

You can get your frequency close to zero beat with what you're getting. Over time though there's drift in a transmitter, usually due to temperature change, and either you or the offending transmitter could drift a little, so it's good to check it at times.

Some tech thoughts about beat related to modulation, I notice in newer receivers with DSP, digital signal processing of the RF and IF signals, that any beat can seem to be accented in the speaker, especially when the beat is stronger. I have a couple of DSP portable AM-FM, dial tuned Radio Shack and a Tivdio, both pocket radios, and side by side comparison with an analog portable, the beat seems stronger on the DSP radios.

I think it's happening because the DSP radios mute audio at low RF signal levels, like tuning between stations, so when there's a beat, the RF carrier is dipping below the mute threshold, so the audio is muted at that moment repeatedly, making the beat sound stronger.

That may be a reason that 'MDCL' modulation trick isn't being pushed so much for AM stations these days. A few years ago, the FCC approved the use of MDCL, Modulation Dependent Carrier Level as a power saving technique, so that transmitter carrier power level drops to a lower level at those times when the audio gets quieter, like song fades and between words and sentences on talk.

The theory is that the AVC in a radio can follow the rising and falling carrier, and keep the audio level about the same, and it was found to work without much disturbance to the program.

I can see how a DSP radio with muting might not work so well with MDCL, the swinging carrier could cause unnatural muting in the sound that would affect some listeners.

Another thing I've found about beat is in a strong beat situation, using very high, and brick wall modulation is worse for the sound in the affected receiver, it's more distorted than modulation with less peak density in the usual envelope detector in the presence of a beat.



Boomer