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I recently bought a (used) radio controlled airframe and buried deep inside the tail section is an antenna, broken out to some coax, with an SMA connector within the core of the fuselage. There is no mention of this antenna in the manuals, so I assume it was added by the previous owner. I've tried multiple ways to physically see the antenna (removing it, using a flashlight to see through the tail section, taking apart the tail section), but have not succeeded in physically accessing the antenna.

Based on the frequencies used in radio controlled aircraft, my guesses are that it's either

  • 433MHZ for a UHF control link
  • 1280MHZ for an analog video link
  • 2.4GHZ for either control/telemetry or video
  • 5.8GHZ for video

Given that I plan to have a 2.4GHZ control link and a 1280MHZ video link, it would be very beneficial to use the antenna built into the aircraft, however I have no information on the antenna or the polarization.

Are there easy ways to figure this out? My only guess is a very controlled situation with equal power transmitters of each frequency and an RSSI meter, rotating to check polarization as well, or a frequency sweep and SWR meter, which at these frequencies is not cheap.

Edit: Would it be reasonable to measure the current consumption of a transmitter of each frequency with a properly matched antenna, and then with this mystery antenna? My concerns are:

  • The current draw of the transmitter may not change measurably just due to a mismatched antenna, especially since the transmitters have voltage regulators built in
  • Damage to the transmitter with a mismatched antenna (5.8GHz tx transmitting through a 433MHz antenna - works out to about 3.5 wavelength)
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it is most likely a quarter-wave "whip". Measure it in centimeters, multiply by four and then back-calculate the frequency.

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  • $\begingroup$ I don't have access to the antenna, it is buried within the tail of the aircraft (80cm long 5cm diameter tube, blocked at both ends except room to run cables through bulkheads). I agree that it's likely a quarter wave whip, but I can't see or measure it $\endgroup$
    – Brydon Gibson
    Jan 29, 2019 at 19:06
  • $\begingroup$ do you have a friend who is an amateur radio guy? $\endgroup$
    – niels nielsen
    Jan 29, 2019 at 19:08
  • $\begingroup$ I do not, I figured that was my best bet. I don't need very much accuracy as I know it will be one of the four listed frequencies, so a hack that could get me reasonably close would be enough to go off of (think 400MHz accuracy - very vague) $\endgroup$
    – Brydon Gibson
    Jan 29, 2019 at 19:09
  • $\begingroup$ Hmmm............ $\endgroup$
    – niels nielsen
    Jan 29, 2019 at 19:10
  • $\begingroup$ That's why I'm here, for the tough questions :). Edited with a thought $\endgroup$
    – Brydon Gibson
    Jan 29, 2019 at 19:23

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