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Old 04-06-2017 | 01:37 PM
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rhklenke
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Originally Posted by darryltarr
As mentioned earlier one of the major contributing factors at our club is Drones, without generalizing many of the drone guys are a decent bunch, very sensible and have some really nice equipment (DJI, Yuneec etc.) which are CE and FCC approved I might ad. However, we also have some other gadgets (home built FPV’s to mention a few), which are NOT regulated and/or approved, and are different to the common DRL FPV’s. Hover one of these close to your TX and your telemetry warning goes crazy (Spectrum DX18).

[snip]


The "higher end" multirotors (i.e., DJI, Yuneec) use a Wi-Fi link to send their video. Wi-Fi uses Direct Sequence Spread Spectrum to spread the signal over a wide bandwidth so that to other users of the band, it really just looks like a rise in the noise floor. Another system using DSSS or a combination of DSSS and FHSS, like most RC radios, can work around a DSSS-based Wi-Fi link without much problem. We've actually flown a UAV with a 2.4 GHz Wi-Fi link on the aircraft to transmit video to the ground and the Futaba FASST system has no problems with it - at least not that we noticed.

The "cheap" drones, especially the FPV ones, use a simple FM video transmitter, which can be in the 2.4 GHz band or the 5.8 GHz band. If its in the 2.4 GHz band, it can cause problems with 2.4 GHz RC systems as it is a noise source that is much more concentrated in a given area of the spectrum. Since DSM2 chooses two frequencies in the 2.4 GHz band and then doesn't change them, its understandable that a 2.4 GHz FM video link could clobber it. If the video link happens to be close to the 2 frequencies that DSM2 originally selected when it was turned on - bang, your link is dead. Other RC systems that frequency hop, can work around a 2.4 GHz FM video link, but they usually become unreliable in that situation, as you have seen.

A contributing factor is that in order to get a good, reliable video link, most FPV guys run as much transmit power as they can get (legally or otherwise).

The solution is to force all of your FPV guys to go up to the 5.8GHz band. Most FPV guys in the US are already there it seems. We have FPV users at our club (a fair percentage of the "regular" flyers at the club are messing around with FPV racers...), but their video links are all on 5.8 GHz, so it hasn't been a problem.

An interesting and related story... We were working on a UAV research project where we needed a high-power video link, so we went out and bought a 1.2 GHz FM video transmitter (1 watt). We installed it on our aircraft and did a ground range check - which was OK. Unfortunately, shortly after launch, the RC link (Futaba FASST) went into failsafe and the plane landed in the trees. Subsequent investigation with a spectrum analyzer revealed that the 2nd harmonic of the 1.2 GHz video link, which was right in the middle of the 2.4 GHz band, was almost as powerful as the primary frequency at 1.2 GHz and was also very wide. The result was that the Futaba FASST link checked out on the ground OK, but suffered significant range reduction in the air.

Needless to say that we "adjusted" the cheap video transmitter with a 5 lb sledge hammer to eliminate the problem from reoccurring...

That same "adjustment" technique could be applied to the errant drone that wanders too close to a pilot on the flight line!

Bob
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