Once the initial euphoria of the first FPV flight has passed, one of the first questions that enters a FPV newcomer’s mind is ‘how far can I fly‘.
In the FPV world, the answer to that question boils down to the performance of two RF links, the uplink (control), and the downlink (video).
This document will attempt to demystify RF link range calculations, boiling them down into some easily understandable blocks.
Where to start… lets start here, with the major blocks to consider in an RF link;
- Transmitter power, generally specified in mW (milliWatts), but more useful in dBm(1)
- Transmitter antenna gain
- Free space loss
- Receiver antenna gain
- Receiver sensitivity
NOTE: The knowledgeable reader will have realized that I left out connector, and cable losses, but these are generally not a big factor in the FPV world, so we will quietly ignore them.
In Simple Terms…
We transmit some RF energy, using an antenna with a certain amount of gain, loose a bunch of energy in free space, pick it up with an antenna with some more gain, and then feed it to a receiver. The receiver must be sensitive enough to identify the transmitted signal above the ‘noise’.
Lets start with some simple numbers to explain how it works. We’ll dive into more detail a litte later.
So I have a transmitter emitting 500mW (at least that is what the datasheet says). The math gets a lot easier if we work in terms of ‘dBm’ so lets convert 500mW into dBm (any of the online tools will make this easy). We arrive at 27dBm (or 27dB relative to a milliWatt (thats the ‘m’)).
Now, for transmitter and antenna gains, lets start with a simple omni-directional dipole, which comes in at about 2dBi of gain.
Next thing is to figure out this ‘free space loss’ thing. This is how much your transmitted RF signal will get attenuated for a given distance (for a given frequency). Lets start with 1 km as a nice round number. Doing a little math (or cheating with the calculator below) we arrive at a free space loss of 108 dB.
So now the math gets easy. The signal received at the receiver’s input connector is the following:
27 + 2 – 108 + 2 = -77 (dBm)
(27dBm transmitted, plus 2dB for the Tx antenna gain, minus 108dB for free space loss, plus 2dB for the Rx antenna gain).
Now, lets take a typical receiver sensitivity figure of -94dB, The received signal in this case will be 17dB above the receiver’s sensitivity, a fairly healthy margin (and known as the Link Margin).
Confused? Amazed at how simple it is?
Lets work through another simple example, something common in the FPV world.
At the receiver we have a SpiroNET Patch for 5.8GHz, with a gain of 13dBi, hooked to a Uno5800 Receiver.
We want a link that will take us 5km out.
So, start with the hard part, lookup the free space loss at 5.8GHz for 5km, which is 122dB.
Now, the primary-school math:
28 + 2 – 122 + 13 = -79 (dBm)
So for a 5km link using this equipment, we have a signal at the receiver’s SMA connector of -79dBm, or 15dB above the sensitivity level of the receiver.
This is a fairly reasonable link margin, and shouldn’t cause too many surprises.
The Other Factors…
So the description above explains one of the major factors to consider in the estimation of RF range. There are some others, which can seriously ‘skew’ the numbers:
1. Antenna Radiation Pattern
Antennas are never perfect, even an ‘omni’ directional antenna is not truly omni-directional (more like a doughnut). Flying directly above the pilot, with an omni-directional antenna on the plane, and on the ground, is never a fun experience.
Patch antennas are directional, some of them highly directional, with radiation patterns like a flashlight beam. Keep the model in the beam, and all is well, drop out of the sides of the beam, and the fun factor decreases rapidly.
When two (or more) RF signals arrive at a receiver ‘out of phase’, the resulting received signal is always attenuated. RF signals propagate like waves in the ocean. Take two waves which arrive half a wave length apart, and the result is a calm ocean.
In RF terms, take a direct ‘line of sight’ wave, and one reflected from the ground, and the effect is the same.h
Multipathing is reduced significantly when using Circular Polarized antennas. After a single reflection (or equally an odd-number of reflections), circularly polarized waves reverse their polarization, and arrive at the receiver antenna with a polarization opposite to that of the antenna.
3. Other Stuff
In addition to these factors there are a whole slew of other effects, which can skew the range calcuation, including things like the amount of water floating around in the air . For these, at least in the FPV world it is best to keep a ‘link margin’ of around 10-12dB.
(1) Power in dB (a logarithmic unit) is as simple as addition and subtraction, for example, a 27dBm trasmitter, followed by a 6dB attenuator, results in 21dBm (27 – 6).