35MHz Aerial "Amplifiers"

[Peter Beeney]

Steve, - Just purely for interest, and to have a bit of a guess that what goes on here, I’m not sure that choke is quite the right description to use in this case. I think an inductor is just a choke when it’s used on it’s own for a specific purpose, such as in an ESC lead with a few turns round a ferrite bead, this just ‘chokes’ off any stray rf. on the wires.

I remarked in a previous post that when the aerial becomes short relative to the wavelength, or parts thereof, it becomes capacitive. This is not desirable because it increases the aerial resistance or ‘impedance’ which is a combination of resistance and reactance, the reactance part being the resistance to the alternating wave form caused by the capacitance. An inductor is also a reactance to AC, too. Because an inductor has the opposite effect to a capacitor they can be connected in series and then ‘tuned’, that’s adjusted in value so the inductance balances the capacitance, and then they become a straightforward low value resistance, just to one particular frequency, of course. So a series circuit can become an ‘acceptor’ circuit to one individual frequency.

A capacitor and an inductor connected in a parallel circuit becomes a high resistance to one frequency and thus is now a rejector circuit, but it can also be extremely useful to select frequencies and tune radios.

So the value of the inductance, 10 microhenrys, has been established as being correct to balance the capacitance of this short aerial at 35 MHz. Not as good a performance as the proper aerial, I guess, nothing’s perfect, but probably it will be well good enough. Using our transmitter, we really only need perhaps a thousand metres ground range, the air range will be greater. Just about every other type of transmitter will be looking for the maximum range possible. The receiver doesn’t want to be too sensitive either, just good enough to pick up a chain of about 0.45 millisecond pulses from a short distance away, anything else can be ignored.

Also I know the sort of reluctant feeling that abounds when you are trying out new procedures, but in all honesty I suspect the quality of the radio reception has little to do with the size of the test vehicle. I’m sure it will perform equally well in a large, heavy and expensive model as it will in a small, light and cheapo. Just a matter of committing it, I reckon.

So, all in all, I’d say this is about as far away as you could get from using an inductor as a choke, but these are only casual ramblings and it’s all of little or no consequence anyway.

But, a moderator is a moderator after all, and we need to keep him up to speed…….

PB