I am sure there is a better way. That better way may just happen to be a magnetic loop antenna.
I was reviewing some old physics text books that I have and I began to contemplate the usefulness of the magnetic component of the oh-so-familiar electromagnetic waves which we use to radiate our radio signals. As it turns out, there are many examples of well designed magnetic loops which behave much as traditional antennas, but with a substantially smaller foot-print. Oh and it should be mostly immune to sources of electrical noise! That is a considerable bonus considering I live in an extremely noisy (electrical) environment.
So now I need a plan.
The theory of operation is quite simple: feed the loop at point “A” and tune the resulting resonant circuit at the opposite side of the structure with a variable capacitor. Building the loop will be trivial at most. It is the sourcing of a capacitor and an antenna analyzer that will be my downfall.
If you are reading this and know where I can get my paws on a variable capacitor (air, vacuum, ceramic) for less than $40 by all means, let me know. The capacitor should be in the range of 5-250pf (or so) with a voltage handling of -at least- 5kV. Yeah, 5kV. I could live with less, however, best engineering practices always require one to design with 50% margin – and in this case, a 5kV capacitor would allow 100W of power. I don’t think I would use 100W as my interests lie in weak-signal modes, however, I personally over-engineer by a margin best measured in logarithmic units.
The next piece of equipment is an antenna analyzer. Yes I realize that I could tune and everything using an SWR meter and an RF source, however, an antenna analyzer is soooo much faster and lets face it – without one, I doubt I would bother with this project.
If I am unable to source a cheap variable capacitor which will handle 5kV, I may break-down and try to fabricate one by hand. The principle is quite simple really – a trip to the local scrap metal shops should yield most of the components. After that, its some seat-time with a dremel tool and many replacement cutting wheels. I’d prefer -not- to have to make a variable capacitor however.
There it is. My plan.
If you can help in any way, please do not hesitate to leave a comment here, or contact me via email or through the Ottawa Valley Mobile Radio Club.
Thanks as always for reading!
73,
James
I may have a variable capacitor for you in my spare parts here. You’re welcome to root through a couple of boxes of them an pick one that suits your project. Let me know via email when you’d like to hook up.