Foundations of Amateur Radio

The other day I had the fortune of using a club station with an Advanced License whilst mobile using more than my usual 5 Watts. I was driving in convoy away from the main local 2m repeater and was interested in finding out what the coverage might look like and how high power might make a difference. When I say high power, that's 50 Watts FM on 2m.

I learnt several things during my journey.

First of all, line of sight is still true, even with more power, but there is more margin for error. Little obstacles that would cause a 5 Watt signal to be absorbed before it got to the other end are all but invisible with full power. Don't get me wrong, a hill is a hill and a radio signal won't go through it, but trees, houses, and all the other little things we humans build on the planet each obstruct a little bit of signal. Higher power gives you more margin and less effect on the resulting audio.

The next thing I learned is that two cars, travelling in convoy can overwhelm each other with power. The way you experience this, is that the other car starts talking, and all you hear is mush. As they drive away from you, at say a traffic light, their signal becomes clearer, to the point where it's back to normal. The way it sounds is almost as if the signal to noise ratio is being adjusted, more signal, less noise as the distance increases - a very strange sound to hear.

As it was explained to me, the phenomenon relates to the receiver being overwhelmed by the very strong signal nearby. I'm guessing that the AGC, Automatic Gain Control is reducing the sensitivity of the receiver, to compensate for the strong transmission nearby, which in turn means that it is unable to hear the repeater which is relaying the same signal on the receive frequency. As the distance from the strong transmission increases, the AGC compensates, increasing the gain, thus making the receiver able to hear the repeater.

There wasn't the opportunity to experiment too much, being in a moving vehicle, but I wonder if changing the AGC setting would have made a difference. I suspect that there is more going on than just the AGC, since the signal from the nearby transmitter, the one in the other car, is still exciting the receiver, even if the gain is being adjusted. I suspect, but don't know for sure, that theoretically, I would have been able to hear both signals, the outgoing one from the other car, even though it's 5 MHz off frequency and the incoming signal from the repeater. Likely there was a minuscule delay between the two, perhaps even to the point of suppressing each-other.

Another question that comes to mind is a phenomenon called the "FM Capture Effect" as well as the so-called "near-far problem" and I'm wondering if this is another aspect of what I was hearing and if we had a local AM repeater, would it exhibit the same behaviour?

Now, as you might have guessed, there is very little in the way of research in my comments here, but that wasn't the point of what I'm talking about. The point was that a slightly new operating environment introduced me to concepts I'd never considered, never even really knew about, other than in a theoretical sense and I was able to actually see, well, hear, this in real life.

Given my track record with over 300 different episodes, you can take for granted that I'll be digging deeper into the experience to see what I can learn and to see if my initial observations bear any relationship to reality, or if I'm adding two and two together and coming up with five.

I'm Onno VK6FLAB