# Is there a way to tell what frequency an unmarked antenna is designed for?

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.

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...

• Would this be better at electronics.se? Jul 14, 2013 at 21:09
• – user4552
Jul 15, 2013 at 3:26
• @dmckee: Will keep that in mind for next time. Jul 16, 2013 at 1:18

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.

• They were indeed mm (edited). What's the cheapest / easiest way to measure the resonant frequency? Jul 15, 2013 at 5:40
• @foobar: There is no easy way, and therefore no cheap way, at these frequencies. Jul 15, 2013 at 11:33
• What if I built a cheap spectrum analyzer like this one? rtl-sdr.com/rtl-sdr-used-as-a-spectrum-analyzer Jul 16, 2013 at 1:17

the two grooves at the base of the antenna just above the elbow identify it as 2.5ghz

a single groove would be 900mhz

• Do you have a reference for this groove as the identifier? Mar 15, 2017 at 12:01
• Oh my gosh! This absolutely does provide an answer to the question if it's correct! Once I've confirmed this I will switch the correct answer to this one. Mar 15, 2017 at 17:16

But you can always open the PVC as I did. And calculate with simple formulas the aproximate central value of the antenna.

My calculations:

L = (c / Hz) * 100 (With c = 299792458 and L full length of wave, ex 4*quarter length)

So Hz = (100/L)*c

For my example:

So it can be an 8,9 (quarter length wave) so undoing my calculations 842Mhz antenna. Or a 5,92cm (Qlw) So 1,266ghz. I don’t know what it does the piece put in the antenna but I think that’s the reason, to shorten even further the antenna.

NOTE: This example is not mine taken from TTN forum