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Real Impact of AGC on AM Coverage

G

GeneSavage

I've heard for years that the denser your signal on AM, the further you would cover... up to a 50% improvement in coverage!

The more I've thought about it, the less sense it made. I get that it's Amplitude Modulation, and the more you can modulate the signal, the louder it will be through the noise on the other end... but 50%??

So, let's take an extreme case scenario, and tell me what you would expect:

If you have a part-15 AM transmitter set up, all within legal limits but at its max... and you play a (reasonably compressed but not clipped like many modern CDs) recording of a pop song:

A) Without external compression but peaking at 100% (or 125%, I suppose).

B) With a compressor like Breakway or the Omnia 9 or the like, with all controls set to 100% / blowtorch.

Would the song be louder but go out of range at the same distance, or would the coverage be increased... by inches, by feet, by yards, by miles?

Obviously I've taken all other variables out: no skip, identical terrain, etc.

I'm curious if the benefits of expensive processing on AM really matches the coverage gain.
 
AM works differently than FM. On FM, dense modulation just masks noise. On AM, there is actually more power applied to the transmitted signal as the positive modulation is increased, however, with the modulation being close to symetrical, the extra power doesn't really translate into a lot of extra coverage. Even if you modulated enough to double the power, you only gain 3dB which buys you a relatively small increase in range. Back in the AM radio heyday more than 50 years ago, some bright engineers got the idea of modulating at several hundred percent, so they would buy 50kW transmitters for 5kW stations. In that case, there were real gains from the extreme positive modulation. That is why there is now an FCC rule limiting positive modulation to 125% and another rule limiting the maximum size of transmitter that an AM station can own. For instance, a 1kW station cannot legally own a functional 5kW transmitter.
 
On my part 15 AM, there is no increase in range, but the difference is worth the trouble.
Instead of sounding weak at the fringe, the audio is much more full and rich sounding right out to the end of coverage.
I still fade out to nothing between 1/2 mile to a mile depending on direction and proximity to the nearby river.
But it SOUNDS like a 50kw blowtorch AM top-40 rock monster from the 60s dx on skywave.
 
I would advise my 2-way FM radio customers to whisper into their mics when the intended receiver was on the fringe. The opposite of shouting into CB (AM) mics.... just sayin.
 
That's because you were dealing with narrowband FM. The less you deviate the carrier, the narrower the receiver can go to minimize noise and adjacent channel pickup. Modulation has no effect on power on FM.

On AM, modulation=power. That's why you have these crazy CB operators with a deadkey of 5 watts and a "swing" to 35 watts. (or 500 and 5000 depending upon the nutcase behind the mic)
 
WNTIRadio said:
That's because you were dealing with narrowband FM. The less you deviate the carrier, the narrower the receiver can go to minimize noise and adjacent channel pickup. Modulation has no effect on power on FM.

On AM, modulation=power. That's why you have these crazy CB operators with a deadkey of 5 watts and a "swing" to 35 watts. (or 500 and 5000 depending upon the nutcase behind the mic)
Click to expand...
Where's the 'like' button. :)
 
I put an Audimax to help the front of a CRL am amigo years ago.

It made quite a notable improvement in how the fringe SOUNDED.

Cant say it made a whopping difference with this particular 250watt graveyard's actual
coverage area.
 
Back in the 1970s, the old KOIN Radio, CBS afilliate from the beginning, would receive co-channel interference in the fringes. In 1977, they were purchased by Taft and switched to CHR, increasing positive modulation at the same time. I never again heard co-channel interference in the fringes!
 
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