Circularly Polorized Antenna


#1

Found a circularly polarized 1090 MHz antenna on ebay. Compared to the FlightAware antenna, the circularly polarized antenna results are disappointing.
I will revert to the FlightAware antenna.


#2

Transponders transmit vertically polarised signals.

Circular polarisation is normally used for satellite communication. Even then you need to ensure that you use the correct direction. Left or right handed.


https://oscarliang.com/linear-circular-polarized-antenna-fpv/


#3

The ADSB antenna on planes are vertically polarized. Your antenna needs to match the same polarization to get the best signal.

I haven’t heard of anyone using a circular polarized ADSB antenna. The only benefit is that when a plane banks or turns the circular polarized antenna will not lose signal while the vertical antenna will lose 1-4dB of the signal (depending on the angle of the bank). But you will have a 3-4 dB less signal from using circular polarized antenna all the time due to having the wrong polarization (vertical vs circular).

I see circular polarized antenna on quad copters. I think they are very common considering they are mass produced and cost almost the same as a vertical antenna.


#4

There is a constant 3db loss from vertically polarised to circularly polarised antenna, no matter what the bank angle of the aircraft is (it’s consistent).

There is up to 26db loss from vertically polarised to horizontally polarised (quite a steep turn for an aircraft!).

The use of circularly polarised antenna on quadcopters is twofold:

  1. To significantly reduce ‘interference’ from reflected signals - when a vertically polarised wave is reflected, it’s mostly still vertically polarised, it just arrives at the receiving antenna slightly behind the main intended wave. When a CP wave is reflected, it becomes the opposite hand, i.e. it starts out as Right Hand, gets reflected and becomes Left Hand polarised - again, there is a 26db loss between polarity and minimal interference at the receiving antenna.
  2. As the video from the quadcopter is channelised, it’s possible to get more quads in the air by using opposite hand circularly polarised antennas on adjacent channels (minimising interference).

Back in the early days of quads, it was quite a challenge to use 5.8GHz because of reflected signals, omni directional circularly polarised transmitting antennas changed the whole game.

I played with a homebrew omni directional Skew Planar CP receiving antenna at 1090MHz and by bending the elements, it’s possible to get near(er) vertically polarised reception. I simply haven’t experimented any further, but IIRC the result wasn’t too shoddy, I just stuck with my homebrew stripline collinear.

Nigel


#5

Nice information from someone with a lot more experience on using circular polarized antennas. I haven’t made a CP antenna so this might be a good side project when I have the time.

Passenger planes don’t do crazy bank angles due to not wanting passengers to throw up. There isn’t much to gain from using a CP antenna for normal plane tracking. CP antenna are probably better for acrobatic and military plane tracking.


#6

Thank you for your reply. I always try to spread the good news about FlightAware. -paul


#7

Thank you for the excellent technical information. I guess that I was thinking about FM broadcasting and how they switched to circular polarization. Does not apply to aeronautical radio.
If circular polarization is used to track satellites, then I should be ready when FlightAware starts tracking Elon Musk’s sports car. :slight_smile: thanks again.