Anyone up on their antenna theory?

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Macinjosh's picture
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Anyone up on their antenna theory?

Long, long ago (OK so it was 1993, but it makes for a better opener,) I earned my no-code Tech ham license and used to know quite a lot about antenna theory, and be very interested in it. I would crack open the ARRL Handbook and just start reading wherever I landed (it's a shame I can't find that thing now.) I built quite a few 2m and 70cm antennas, and had a blast doing it. J Poles, quads, et cetera.

Fast forward to today, where i discover that it's fun to tinker with homebrew Wifi antennas. Unfortunately, my knowledge has gotten cloudy on the theory, which brings me to my question.

In the case of 2.4 GHz frequencies, where wavelengths are so short, how come designs are typically using 1/2 or 3/4 wavelength elements, and not full wave, or greater than full wave? It seems to me that a longer antenna would provide better reception. It may be intrinsic to the designs that I have been looking at, mainly stuff like this:

http://martybugs.net/wireless/collinear.cgi

So, I'm basically thinking of building a simplistic omni antenna to improve on my Linksys' standard rubber ducks. But more importantly, I can't seem to find an answer to that simple question, which is why not use greater lengths when microwave frequencies make them so short? Just in the name of uber-compactness?

-- Macinjosh

Macinjosh's picture
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To Clarify that...

OK, so I realise that the length and proportion of element(s) in antennas are to be a certain way, due to design. To throw the design out of the equation, why not a 2 wavelength "whip?"

Edit: started thinking it might be that youre affecting the resonant frequency, but is that it? I mean a 1/2 wave 2.4 GHz element is a 1 wave element for a different frequency, is it not?

-- MJ

moosemanmoo's picture
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I'm not an electrical enginee

I'm not an electrical engineer, but a half wavelength is the best you can make. One and a half wavelengths would work slightly worse because of resistance. Or, here, I found a good explaination from http://www.borg.com/~warrend/guru.html

Quote:
When the antenna is approximately one half wavelength long the capacitive and inductive reactances are equal, they cancel, and the antenna is in resonance. As the antenna becomes longer still, the equivalent circuit transforms into a parallel resonate circuit like that shown in figure 1b, often called a tank circuit. This resonance point is reached when the antenna is about one wavelength long and the radiation resistance becomes very high. When the antenna approaches one and one half wavelengths it again looks like a series RLC circuit and at two wavelengths it is back to the parallel circuit; this impedance pattern repeats in increments of one wavelength in length.

Macinjosh's picture
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Last seen: 7 years 4 months ago
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That's exactly what I'm looki

That's exactly what I'm looking for. I knew there was some gotcha, just couldnt pull from the fog what it was. Thanks!

I hear ya on not being an engineer. There was loads of useful learning required for the test; but it was loads of basics. Where you went with it after that was up to you.

-- Macinjosh

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