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Pardon the possible simplicity of this question but I am incredibly new to antenna design. So, I settled on a simple loop antenna, made of 14awg copper and about 18inches circumference (tuned to 644Mhz) hooked to a Balun. I picked that size because that is the weakest channel I wish to receive. The antenna picks up a pile of other channels just fine but still gets no viable signal on that channel. I know that I cant just go about making things larger arbitrarily so how do I increase gain? Can I somehow add other loops (I think I saw a picture of this) and if so how do I chain these together?

N.B. I am height limited and this is stuck inside so the old advice of put it higher and remove obstructions is difficult.

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Can you report the calculations that have you made for the circumference? – clabacchio Jan 20 '12 at 16:16
I used an online calculator @ csgnetwork.com/freqwavelengthcalc.html. I used 644Mhz as the input and clicked full wave – Bob Roberts Jan 20 '12 at 21:02
You'd probably do better with a directional antenna, e.g. a small Yagi. – Paul R Jan 20 '12 at 21:36
@PaulR, I always thought loops/bow-ties were the choice for uhf due to the higher frequencies acting on magnetic resonance (Warning: I may not have any idea what I am talking about) – Bob Roberts Jan 20 '12 at 21:56
@bobnix, the standard is called a whip antenna actually. It is the electric field couterpart of a loop. A loop is a magnetic field dipole, a dipole is an electric field dipole. – Kortuk Jan 20 '12 at 23:51
up vote 1 down vote accepted

I'll just write down some theory, based on what I've studied.

Your problem may be cause by the fact that you realized a full wave antenna: this size gives you a very high input resistance, that makes difficult to transfer power to the receiver.

Try an half-wave antenna, that is the recommended size. That's also the reason because the other channels work better, because the peak of the resistance is centered where the length of the antenna is equal to the wavelength.

Resistance over L/lambda for simple dipole antenna

This image shows the radiation resistance shown by the simple dipole antenna related to the antenna length/wavelength ratio. I'm sorry that i couldn't find the same figure for the loop antenna, but I hope it gives the idea.

About the bow-tie antenna, it's like the folded dipole (and so also similar to the loop antenna) but it has a wider resonant band, so it's less sensible to frequency change.

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Oh, I thought the idea of half or quarter waves were just to reduce size and otherwise bigger is better. Like I said in the OP, I am a radio neophyte. – Bob Roberts Jan 20 '12 at 22:21
That chart is very helpful. Not only for its contents but I remember seeing a chart like that for loops and being generally lost as to what I was seeing. – Bob Roberts Jan 20 '12 at 23:10

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