Demystifying the Dark Art of Antenna and Frequency Selection

DISCLAIMER: Transmitting/receiving on some of the frequencies listed below and/or transmitting above certain powers on certain frequencies may be illegal in your country. I claim no responsibility for this so check with your local government (FCC for the U.S.) for any restrictions.

I decided to make this guide because I was tired of seeing people speculate as to what the range was of their FPV setup (or any RF setup) and wanted to find some cold, hard technical facts. This will be totally theory-based as I do not have the resources to try each and every combination I will list below. These calculations are based on an unobstructed line-of-sight signal with no electronic interference. However, the real world presents many variables that result in less-than-perfect wireless performance, such as mismatched impedance, electronic noise, building obstructions, reflected signals, etc. This means that you can use these values as an absolute maximum for transmission. Any further and the signal-to-noise ratio will drop below 1.

All of the work I did was done in MATLAB and I will be including the (fairly simple) source files so you all can recreate it as well. It was nearly completely based off of Friis' transmission equation.

I'm going to preface this by saying that essentially any range for an FPV setup is possible, but range grows logarithmically with transmission power and antennas with high enough gains may not even be realizable. Alas, we can still get some pretty serious range out of some simple antenna setups. When I was thinking about designing my FPV setup, I tried to think what the limiting factors would be. I came up with:

1. Range of my Spektrum DX6i transmitter (about 9/10 of a mile)
   
2. Range of line-of-sight view in case of FPV failure (about 1/4 of a mile)
3. As a corollary, I thought I might also want to integrate an autopilot into my FPV setup so I figured I wouldn't stop with those distances listed above and try and push the envelope.
   

For the figures below, I used the following gains as generalizations for different types of antennas. Also, I couldn't find specific values for receiver sensitivity so I chalked it up to manufacturer quality. I went with -81 dBm for it. I also included a 15 dB fade margin to make the values a bit more realistic. The following values were retrieved from spec sheets of commonly purchased antennas:

1. Skew-Planar Wheel: 1.0 dBi
2. Cloverleaf: 1.2 dBi
3. Dipole (Rubber Ducky): 3.0 dBi
   
4. Patch: 8.0 dBi

The great part about including the source means that if you want to try out other specific values you have for antennas then you can feel free to do so! If you don't have a MATLAB license, the source code should still compile in GNU Octave/PyLab/SciLab/etc. Here's a link to another calculator I used to estimate/verify transmission range which includes Fade Margin calculation and signal loss due to cable impedance: RF Link Budget Calculator

Finally, enough talk! Let's see some pretty plots!

Here we have an example of a Cloverleaf transmitting on the aircraft and a Skew-Planar Wheel receiving on the ground station. The range is not that great, but it is omni-directional and circularly polarized:

Next up we have an example of a Cloverleaf transmitting on the aircraft and a circularly polarized patch antenna receiving on the ground station. The range is significantly better than the previous example, but you always have to have the patch antenna pointing towards the aircraft or else you'll get 26 dB loss:

Now here we have two dipole antennas. Keep in mind these are omni directional, but the radiation pattern means they both either have to have the same polarization (vertical and vertical, horizontal and horizontal, etc) otherwise there is a 26 dB loss:

A linearly polarized dipole antenna with a linearly polarized patch antenna:

I also found some high end directional antennas that claimed 16 dBi so I wanted to include one of those in use with a Cloverleaf:

And finally, just out of morbid curiosity, a high-end circularly polarized patch antenna transmitting to another high-end circularly polarized patch antenna!

After seeing all of these findings, the ideal setup (in my opinion) would be a Cloverleaf transmitting to a Skew-Planar Wheel receiver (and a Circularly Polarized Patch Antenna if you have a diversity controller.)

I hope that everyone enjoys this article and finds it informative and also that I've fulfilled one of the three Es from the FliteTest mission statement! If you have any questions or think I made a mistake (I more than likely did) feel free to let me know in the comments and I'll do my best to remedy them!