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I have accumulated a large amount of R/C gear over the years. I have several antennas which are not labelled as to their original use. This antenna is either for 5.8ghz, 2.4ghz, or 910mhz.

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The connector is SMA with a male pin, threaded on the inside, and the markings on the mat are in mm.

Is there a way to tell what frequency it is designed for by looking at it? I tried calculating wavelengths, and the antenna is shorter than the wavelength of these three frequencies.

I would also like to know if there is a general solution to this problem...

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Would this be better at electronics.se? –  dmckee Jul 14 '13 at 21:09
    
    
@dmckee: Will keep that in mind for next time. –  foobarbecue Jul 16 '13 at 1:18
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No, you can't really tell by looking at it, at least when all the details are covered with a skin so you have no idea what might be inside. If those markings are in mm (it would be helpful to say), then it's probably 910 MHz, but again, there is no way to know without measuring it.

You don't even know if this is a self-contained antenna or one intended for use with a ground plane. That makes a factor of two difference in size right there. Then these kinds of fully assembled "rubber duck" antennas have reactive components inside, so you really can't guess much what frequency it will resonate at without measuring.

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They were indeed mm (edited). What's the cheapest / easiest way to measure the resonant frequency? –  foobarbecue Jul 15 '13 at 5:40
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@foobar: There is no easy way, and therefore no cheap way, at these frequencies. –  Olin Lathrop Jul 15 '13 at 11:33
    
What if I built a cheap spectrum analyzer like this one? rtl-sdr.com/rtl-sdr-used-as-a-spectrum-analyzer –  foobarbecue Jul 16 '13 at 1:17
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