Why Use Linear Polarized Antenna on Radio Transmitter (And Not Circular Polarized)

by Oscar
Published: Last Updated on

Circular polarized antennas (CP) are far more popular for FPV setups, but why most radio transmitters still use linear polarized antennas (LP)? In this post we will explore the differences and explain the advantages of CP and LP antennas for radio transmitters.

Further Reading: What are the differences between linear and circular polarized antennas?

LP and CP antennas performance are similar at the ideal conditions. However things get a bit different when you change antenna orientation.

If you put the receiver antenna at an angle to the transmitter antenna, the signal will be weakened. This is especially worse for linear antennas, you could get as much as 20.7dB in signal loss at 90 degree alignment, that’s over 90% reduction in range!

Further Reading: How to calculate range with dB?

It’s true that circular polarized antenna has much less signal loss due to antenna  misalignment – only 0.33dB at 90 degree compared to the 20.7dB of LP.

But why do we still prefer to use LP antennas on radios?

First of all, linearly polarized antennas are smaller and lighter than circularly polarized antennas.

Also antenna size is related to the signal frequency, lower frequencies normally require larger antennas. Radio transmitters normally use lower signal frequency such as 2.4GHz and 900MHz, and so the antennas will be larger than those in 5.8GHz.

This is how big a 2.4GHz CP antenna is.

Transmitter and receiver should have matching antennas, i.e. both sides should have either CP or LP antenna, because mixing them will result in at least 3dB signal loss.

As you can imagine, having a large CP antenna on the drone will be highly inconvenient, and easy to damage it in a crash, therefore the more compact LP antennas are a far more popular choice. To match that, LP antenna should be used in the radio.

To mitigate signal loss from antenna misalignment, diversity receiver can be used. Diversity receiver has two LP antennas, and when you place them at 90 degree to each other, they are always in good alignment with the transmitter antenna, and therefore signal loss can be reduced.

With all that said, you can use CP antennas on the radio transmitter, but you should also have matching CP antenna on the receiver, the size and weight might not be a good idea.

Here are some good aftermarket antennas for your radio:

2.4GHz:

900MHz (Crossfire / R9M):

Before placing your order, make sure they have the correct SMA or RP-SMA connector.

 

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3 comments

Argle Bargle 8th May 2021 - 2:30 am

Is this the same for 5.8 gig video?

Reply
Oscar 8th May 2021 - 4:30 pm

No it only applies to radios. CP antennas are still preferred for video.

Reply
Claudio 18th April 2021 - 11:07 pm

Good theory.
I have been piloting various DJI drones in Europe for years, but in FCC mode and only on 2.4 Ghz, using an efficient double antenna for the RC, the CP helix antennas like those made by FPVLR, or the also very good cheaper clones made by a Chinese company, almost always placed on a tripod and connected to the RC via a cable a few meters long.
Without RF amplifiers with the Phantom 4 series and with the help of 3 W or 4 W amplifiers when flying with Inspire 2 and everything is fine, excellent signal received at the ground station, almost always at full strength, even at a distance of 5.5-6 km with some clouds between, and the drone very high in the sky.
I fly around and above the mountains almost always far from strong radio environmental noise.

Reply