Yes it is possible to safely use 1.3 GHz video for FPV with 2.4 GHz radio control, providing the following rules are followed:
- Carry out a range test to ensure your video transmitter is not interfering with your 2.4 GHz radio control before you fly. All set-ups are different and will produce different results.
- 2.4 GHz has a safe range of around 1km. Unless you have a reliable RSSI monitor, don’t fly beyond this range.
- Range is reduced when the receiver is not line of sight (a couple of trees is OK but a building will block the signal).
- Ensure the video transmitter is of reasonable quality and not over driven. 400mw is more than enough for medium range FPV flights.
- Ensure the video transmitter and receiver antenna has as much separation as possible (In my test they were kept at least 300mm apart).
- Ensure the receiver antenna/s are well sighted and correctly oriented.
The test equipment, my Tricopter based on the RC Explorer design:
How do I know this is safe? Well I carried out the following tests:
I have read many times that you shouldn’t use a 1.3 GHz video transmitter with a model using 2.4 GHz radio control. The risk is that harmonic transmissions generated by the video transmitter fall within the 2.4 GHz radio control band. Harmonics are multiples of the original frequency, I use 1280 MHz for my FPV video and the first harmonic is therefore 2560 MHz which is just above the band used by 2.4 GHz radio control systems (2400 MHz to 2483.5 MHz). So in theory there shouldn’t be much of an effect providing the harmonic interference is not too wide band.
Since I don't take anything for granted I decided to test the theory and see for myself the effect 1.3 GHz transmissions have on 2.4 GHz. I firstly tested my DX6i (DSM2) and AR7000 receiver. I got 800m range in a ground test with the VTX on, and 850m with the VTX off. So there is an effect, but in this test it was limited. I have been flying FPV with this set-up on a tricopter and quadcopter for around 10 months and have not had any loss of signal.
Short Range FrSky Test
I recently built a new tricopter and installed a FrSky system. I tested the system before flight to ensure there was no conflict with the 1.3 GHz video transmitter.
Equipment used - Turnigy 9x with V8HT FrSky module, placed on a plastic garden chair (low power range test on), antenna set vertically. Receiver was a V8FR (HV). The Supply voltage to the transmitter was 7.8volts. Video transmitter equipment used was a 400mw unit from BEVRC, set on channel 9 (1280 MHz) with homemade inverted vee tuned for 1280 MHz. A low pass filter was not used.
I walked away with the tricopter held at waist height until the orange receiver LED started blinking.
With video TX ON I got 85m before the orange light started blinking.
With video TX OFF I got 101m before the orange light started blinking.
I then decided to make the test even harder by pointing the Turnigy 9X antenna at the receiver. This has the effect of reducing the power of the transmitted signal to a minimum (maximum is when the TX and RX antennas are parallel to one another, i.e. both vertical).
With video TX ON I got 68m before the orange light started blinking.
With video TX OFF I got 72m before the orange light started blinking.
So again there is an effect but according to FrSky you need a minimum of 30m to pass the low power range test, so even with the video TX on the system was well within safe limits.
I already had a cheap low pass filter from BEV RC but had not used it since it reduces the video range significantly. I fitted the filter and only got 63m, a reduction of 5m! The only reason for this I can think for this is that the filter is poorly matched to 1280 MHz and caused more problem than it solved.
For higher power systems a good quality low pass filter is highly recommended and should virtually eliminate harmonic interference in the 2.4 GHz band.
Full Range Test
Full range test method - Turnigy 9x on full / normal power, placed on a plastic garden chair, antenna set vertically. The Supply voltage to the transmitter was 7.8 volts. I walked away with the receiver held at waist height until the orange LED started blinking.
My route was down a lane lined with trees, then up a gentle hill where near line of sight is achieved (one or two trees get in the way at times).
I got the first blinking LED at 412m (lowest point of the route with a couple of trees between TX and RX).
I got the next LED blink at 790m, and after this the link was lost. However, beyond 790m there was a hedge and trees in the way, so line of sight it would have done more.
With the video TX off I was able to go to 920m, but again it would have done more if there were no tree and hedges in the way.
Receiver Antenna Orientation
On my tricopter I have both antennas vertical. I did this on the basis that in flight the antennas remain near vertical most of the time, and it would be better to have both antennas near to their optimum position.
At the limit of range, just before the receiver LED started flashing I tilted the receiver antennas forward / backward and left / right. This had a limited effect on the received signal strength, certainly within 45 degrees roll / pitch there was very little effect.
Mobile Phone Effects?
As a final test I decided to see if my mobile phone had any effect on the results. At the limit of range, just before the receiver LED started flashing, with the video TX on, I turned on my mobile phone. This had no effect at all. For reference my phone is a 3g phone with all radio services active (Wi-Fi and Bluetooth on).
To be certain your FPV flights are safe, test your own system and make sure it is working well before flying.
I hope this information helps you and goes some way to busting a few myths!
You may also be interested in my video antenna tests which show that 400mw gives ample range for medium distance FPV flights, providing you have the right antenna!