Should VTX ever be lower freq./higher power than the control transmitter?

Question

I can find video transmitters from 5.8GHz down to 433MHz with the latter being ideal for longer range, but I don't see nearly as much diversity as far as control transmitters. Am i correct in saying that a lower frequency transmitter will almost always go much further than that of a higher frequency, with transmit power being secondary in affecting range? The extension of this would be that the control link would be lost far before the video if the video is TX'd on 433MHz and control on 2.4GHz. I assume that this would be dangerous since, for example, if video is gradually lost, you have a chance to return the craft to a location if better signal, but you don't have obvious warning signs for control being lost.

Edit - I know that there are 72MHz control transmitters, but i don't know of any way to use them on a quadcopter.

Answer 1

There's a few factors that you're missing. The first is that the RC world is dominated by traditional line-of-sight control, and 2.4GHz provides more than enough range to fly a model plane to the limits of vision. There aren't many long range system because few people need them. Even for FPV, long distance flying means a long walk or losing your model when you crash.

The other is that control is very low band-width compared to video. A typical 2.4GHz protocol includes error detection and can reject a lot of packets and you won't notice so long as one or two get through every second - maybe even less when you're at extreme range and not manoeuvring hard. A video signal needs most of the data to get through to be flyable. Analog video has no error handling or rejection, and while digital systems do, there isn't enough spare bandwidth to ignore any data. Even a corrupt picture is better than black screen!

Good RC systems these days do include RSSI (Received Signal Strength Indication) in their telemetry, so you know when you're reaching the limits of your range - and models with GPS can be set to return-to-home automatically if they loose the RC connection.

Finally, it's entirely possible to use 72MHz on a quad, with a FC that takes individual inputs for each channel - however the range is no greater than on 2.4GHz, and there was rarely any interference rejection on the old analog radios, we used to use the glitches as an indication that we were flying too far away!