The FM Capture Effect and other Amateur Radio magic...

Foundations of Amateur Radio Today I'm going to talk about magic. In the past I've made mention of the magic that is Amateur Radio. There are those who think that our hobby isn't magic and that everything that we do in this field is understood and documented. I think that this is both wrong and unhelpful, since there is much to learn, much to discover and much to invent. Amateur Radio isn't dead, it's full of life, full of things that are continuing to develop, evolve and grow. Let me give you an example. In radio there is a phenomenon called the "FM Capture Effect". Explaining how you experience it is simple. If you have two FM transmitters on the same frequency, and you're using an FM receiver, one of the FM transmitters will win, that is, you'll hear one and not the other. Unlike in AM and SSB transmissions, where you hear both at the same time, the FM Capture Effect causes the receiver to pick one over the other. As an aside, it's because of this effect that aviation and HF communication mostly prefer AM based communication. Imagine two pilots trying to talk to the tower at the same time, one is heard and the other not. Back to the magic. We can describe that this thing happens. We can show it happening, we can even measure the signal strength difference that causes it to happen, 0.17dB according to one document I read. We can use formulas to describe our FM signal, we can use simulations to emulate it, but in the end, the closest we can get to the how and why is: This happens, we know it happens, it happens under these circumstances, but precisely how, we're not sure. A thesis I read on the subject by Park Soon Sang at the Naval Postgraduate School in Monterey, California, published in 1989, spends many pages saying all these things and finishes off with: "The simulation results establish that the low pass filtering portion of frequency demodulation accounts for the capture effect of FM receivers." and goes on to say: "It is recommended that the capture effect be verified using an operating experimental system in which system parameters can be controlled and accurately measured." Or in other words, we built a software simulator to learn about this phenomenon. This simulator suggests that the low pass filter causes this, but you really should make an actual set-up to test this. If I'm less vague, we simulated it, it looks right, but we're really only guessing, so test this in the real world. Now, before you get all huffy. I'm saying that a phenomenon that has existed since the first FM transmission in 1936 is still being explored and investigated and the jury is still out as to what precisely causes it and what the parameters are. As Arthur C. Clarke wrote in 1973: "Any sufficiently advanced technology is indistinguishable from magic." -- by that definition, Amateur Radio is clearly magic. So, when you next scratch your head about what the future of our hobby has in store, the answer is almost certainly covered by the very same author: "The only way of discovering the limits of the possible is to venture a little way past them into the impossible." Amateur Radio, it's magic and there is more to discover. I'm Onno VK6FLAB