When flying FPV you have to make a choice. Do you use 2.4GHz or 5.8GHz for your video system(*)? If you decide to use 2.4GHz its generally not a good idea to get a 2.4GHz RC system, because that would interfere with your video transmission. Depending on the country you live in, the only option you have left, is the good old 35MHz Band which has one pretty nasty disadvantage. In case someone in range of your plane utilizes the same frequency as you are, your plane will become uncontrollable. Thats why most of the ‘normal’ RC guys moved to 2.4GHz RC systems. But on the other hand, 35MHz RC has one big advantage too. By extending the receiver antenna to 1/2 wavelenth, it offers more range than a 2.4GHz RC system with legal(in germany) transmitting powers. So how can we minimize the possibility of a frequency collision?
First of all, there is the cheap and easy way. If your receiver has a failsafe function, simply power up your plane first. When the receiver locks on to a signal and your ESC armes you can tell that someone already uses your frequency. Now it is a very bad idea to just turn on your transmitter and start flying. Not only would you have VERY limited range but you would risk to bring the other plane down too. Your only option is to change your crystal or location. If you don’t own another crystal(get one) and change your location make sure to drive at least 10km away from your previous one. Even after doing that, check your frequency again and don’t even think of doing any crazy long range stuff as you could fly directly into disaster(towards the other transmitter).
Now lets get to the fancy solution. I recommend getting one of these.
It is a frequency scanner for the 35MHz A- and B-Band. With this, you will be able to monitor the whole band and tell exactly which frequency is already in use. So before powering up your gear you simply turn on this little gadget. The principle of this device is pretty easy. It will simply display the received signal strength at a given frequency. The frequency will be shown on the x-axis and the signal strength on the y-axis. Here is how it looks like.
This is what you will see when the band is free. There are no peaks and the stuff you can see is only background noise. You can adjust the sensitivity of the scanner so that it won’t display the noise but i thought it would be nice to see it for this demonstration. ‘CH76’ in the upper left corner indicates, that the center of the graph shows channel 76.
These two images show, how a signal is displayed. In the first one only one transmitter was active, using channel 78. You can see that it is slightly of the center. In the second picture, a second transmitter was turned on using channel 76.
As you might have guessed, this can be very useful. If you get more than one active signal it is most likely that you are near to a flying field with some ‘old dinosaurs’ still using 35MHz(or if you are lucky some crazy FPV convention) 😉 I would not recommend flying near a normal RC flying field. Either join their club and fly from their field so that no one else will get your frequency, or you stay away as far as you can.
But both methods have a few downsides. First of all, they can’t protect you from someone who just powers up his gear, without checking the frequency first. And second, if you do some long range action, it is still possible that your plane gets into the range of a signal that you couldn’t detect from your starting position. But if you check your frequency first, not only will you fly safer, you will also make things safer for other users of the band. So the things said here apply to your video frequency as well. I hope that you find this information useful and feel free to leave a comment if you think that i forgot some important stuff. Fly safe!
(*) I know that there are 1.2GHz video systems out there, but these are illegal without a HAM-License here. Still i would not recommend to use 2.4GHz RC with 1.2GHz video.