Mobile experiments with high power.

Foundations of Amateur Radio

A little while ago I had the chance to use a mobile radio with high power. I used it to learn about the coverage of our local repeater, but also to hear what the effects were when two local radios were both using the same repeater and high power at the same time.

I made all manner of observations and wondered how much of what I observed was real and how much was a case of me adding two and two together and coming up with five. Immediately after I made those observations I received emails from around the globe explaining in great detail what was going on.

Suffice to say that there was some disagreement among the emails, but overall they lead to a few new things that I'd not considered.

One comment was that the two radios, not quite side-by-side, but in two cars nearby was similar to the operating environment of a repeater, that is a receiver and a transmitter sitting in close proximity. Initially I didn't cotton on to the analogy and it took several readings to understand, but the outcome is that, as I suspected, the receiver is being overloaded by a local transmitter which is putting out a big signal that is overwhelming the electronics in the receiver, something that a repeater deals with every time you key it up. The short version of this is called de-sensing. I'm still reading about how it exactly works on the inside, something for another day.

In a repeater the issue is dealt with by filtering the outgoing signal and filtering the incoming signal, making sure that only the desired information makes it to where it needs to go. Two random radios bolted to two cars don't have any such filtering and no way to reject the unwanted signals. Adding filters to both cars might fix the issue, but then we weren't trying to fix anything, just to learn what was going on.

Another thing I learnt was that FM receivers don't need an AGC, since the volume of a signal is related to how much it deviates from the central signal, not how much signal there is, which is why the microphone gain setting on your radio determines the volume, not the level of power. To be clear, enough of your signal needs to get to the other end for it to work, but after that, you're just wasting electrons. If you need a visual for that in FM the height of the signal doesn't matter once it's high enough, the wobble determines how much volume there is. In AM, there is no wobble, the height determines the volume.

Incidentally, if not enough of your signal gets to the other end, then your weak signal might be overtaken by another signal and the so-called "FM capture effect" happens, where the low signal gets effectively rejected in favour of the higher one.

Interestingly Amateur Radios can have an FM AGC which can be used to determine the signal strength, which makes your S-meter behave more like it does on HF, but if you recall, an S-meter is really a guess-o-meter since every manufacturer has their own "standard" and two radios are unlikely to experience the same S-level for the same signal. Don't misunderstand, I'm not maligning the S-meter, just pointing out that your S-5 and my S-5 are unlikely to be the same.

So, the more I peel away from my little mobile experiment, the more I unearth in the wake of the experiment. Such is the joy of Amateur Radio. Be curious, investigate and learn.

I'm Onno VK6FLAB