After my post Estimating the Maximum Range of a 100mW Futaba S-FHSS RC System I received a query about the maximum range of 5.8 GHz video links.
I started by looking at RF noise levels in the 5.8 GHz band. I struggled to find much information here – unfortunately this band was not included in the spectrum survey that I used for the previous article. Recommendation ITU-R P.372-8 shows (in Figure 3) that all sources of natural noise are below the thermal noise floor. AD Spaulding measured noise levels in several locations for Technical Evaluation of the 2.45 and 5.8 GHZ ISM Bands for Intelligent Vehicle Highway Systems and found that “The background is quite low in the 5725-5875 MHZ band, especially since there is currently no ISM equipment operating in this band”. However these measurements were made in 1995, so there is probably more activity in the 5.8GHz band these days due to WiFi. However for the sake of this initial investigation, I am going to assume that the only noise present is thermal. I’ll update it if I can find a more recent RF noise survey covering this band.
The next question is what is the required carrier to noise ratio (CNR)? FPV video uses frequency modulation (FM) because above a certain threshold CNR, an FM signal enjoys a significant baseband signal to noise ratio (SNR) advantage over other modulations types such as Single Sideband (SSB). However as the CNR of the FM signal falls below the threshold value, the SNR of the recovered baseband signal falls rapidly, and the advantage is lost. I could not find any published image quality tests at low CNR for FM video, so I decided to use the FM threshold as the required SNR as it is the level below which the video quality will rapidly deteriorate. With modern FM receivers (using a PLL or similar demodulator) the threshold CNR is about 7 dB. I assumed a receiver noise figure of 3 dB, giving an overall CNR requirement of 10 dB. Since the bandwidth of FM video is typically 36 MHz (which is why Raceband channels are spaced 37 MHz apart) this means the required signal power at the input to the receiver is -88.4 dBm (-174 dB for thermal noise density plus 75.6 dB for bandwidth factor plus 10 dB for CNR).
Note that I am using CNR to mean the ratio of the received RF signal to noise, and SNR to mean the ratio of the baseband signal after demodulation to noise.
I assumed that the gain of an omni-directional antenna is 0 dBi. The quoted gain for the TBS Triumph is 1.3 dBic, so this allows 1.3 dB margin to allow for the fact that the video receiver may not be in precisely the direction of maximum radiation, and for coax losses etc. For the receiver antenna I again assumed 0 dBi for an omni-directional antenna, 5 dB for a medium gain directional antenna such as a Spironet patch, and 10 dBi for a high gain directional antenna (for example, the VAS Pepperbox). For the two directional antennas I have subtracted 3 dB from the nominal gain figures to account for antenna alignment anywhere within its half-power beamwidth.
With these assumptions, the calculation is straightforward. For a 200 mW VTX the effective isotropic radiated power (EIRP) is 23 dBm (23 dBm VTX power plus 0 dBm for the antenna gain). For an omnidirectional receive antenna, the maximum acceptable path loss is therefore 23 dBm (EIRP) – -88.4 (Rx signal power) = 111.4 dB. Applying the Friis Transmission Equation gives a maximum range of 1.5 km.
The following table shows the range in kilometers predicted for each combination of transmitter power and receiver antenna type. The transmitter antenna is assumed to be a 0 dBi Omni in all cases.
Range (km) | Omni (TBS Triumph) |
Med. Gain (Spironet Patch) |
High Gain (VAS Pepperbox) |
---|---|---|---|
25 mW | 0.5 | 1.0 | 1.7 |
200 mW | 1.5 | 2.7 | 4.9 |
600 mW | 2.7 | 4.7 | 8.4 |
1W | 3.4 | 6.1 | 10.9 |
2.5 W | 5.4 | 9.7 | 17.2 |
Please remember my opening caveat. This assumes zero noise (other than the 3 dB receiver noise figure). If you are operating near sources of 5.8 GHz RF noise, such as WiFi or other FPV video transmitters, then your range may be considerably less.
Also, I should mention that this is a “back of the envelope” type calculation that one would use as an initial rough estimate, for example to eliminate clearly infeasible proposals. It’s not intended as a substitute for the detailed measurement, modelling and calculation that one would perform when designing an RF communication system.
Finally, I know that Costi_n has claimed a 24km flight with only a 200 mW 5.8 GHz video transmitter. But he was using a 37cm parabolic dish antenna, which should have a gain of about 25 dBi. Performing this calculation for a 200 mW transmitter with a 24 dBi receive antenna (allowing 1 dB for beamwidth) predicts a maximum range of 24.3km.