2.4 interference from FPV?

[Simon Chaddock]

I normally fly alone but periodically some FPV flyers arrive that use some serious kit with remote mounted ground station aerials.

A few days ago I tried to fly a simple EDF (DSM2 rx) whilst two of them were in the air but got no further than 30 ft before control was lost and it crashed. The weather conditions were ideal.

The plane had flown several times the previous day (whilst alone) with no problem and after some repairs flew just as well the following day!

I know DSM2 can be subject to interference in a 'busy' RC environment but I was not aware FPV could create the same sort of issue.

I can only hope DSMX is more secure.

[Essjay]

Simon, I would imagine if the FPV fliers were, like you, using 2.4 for flight controls, then it's more than likely they were using 5.8gHZ for the video transmission. So it's difficult to see how their kit could have caused the problem, unless they were using some high powered non-approved kit. Maybe DSM2 can be affected more as you say.

I'm no expert by a long chalk on FPV, so maybe someone else will be along with a better explanation.

Steve

Edited By Essjay on 28/05/2017 11:51:44

[Peter Christy]

Its possible that they were using 459 or 433MHz for control and 2.4 GHz for video, as this set up can offer better range (though 433MHz is not legal in the UK). Your comments about them using "serious kit" and ground station aerials indicates that they might have been running illegally high power levels.

If so, then a 2.4 GHz video signal is likely to wipe out a large chunk of the entire band, and it is unlikely that any system would have survived. That's why we have rules and regulations, and when they are ignored, chaos ensues!

Having said that, DSM-2 is generally pretty good in that it won't activate unless it can find free channels to use. However, if they switched on *after* you, and were using boosted 2.4 GHz for video, you stood no chance.

Some sort of LBT system, such as FrSky's, might have stood a better chance, but its performance would still be seriously degraded in the circumstances described above.

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Pete

[Phil 9]

people used to use channel checkers for 35Mhz is there something similar you could use to check for likely 2,4 interference as 2.4 seems and increasingly busy frequency range

[Peter Christy]

I don't think there's a cheap option available. I've got one of these:

**LINK**

Its a pocket spectrum analyzer that covers all the bands we use including 27, 35, 40, 459 MHz, 2.4 GHz and 5 GHz (for video). There are cheaper models that cover only specific bands, but none of them are particularly cheap. I think the 2.4 GHz only version is around £100

Another option - but it requires a laptop - is WiSpy:

**LINK**

Still not cheap, only covers 2.4 GHz (plus 5 GHz on some models) and needs a laptop. I think the BMFA actually have one of these, which can be lent out to clubs who suspect interference problems. Check with BMFA Head Office.

There used to be on called the Ubiquity Airwave - a bit like WiSpy, only cheaper, but it appears to be no longer available.

If anyone knows of a cheap but effective 2.4 GHz monitor, I'd love to hear about it! Nice little hole in the market for someone like HobbyKing?

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Pete

[onetenor]

I have seen monitors on a U.S. site from $ 100 to $600 but can't recall which one. It could be a club purchase given the prices .

[Frank Skilbeck]

Posted by Peter Christy on 28/05/2017 12:17:28:

Some sort of LBT system, such as FrSky's, might have stood a better chance, but its performance would still be seriously degraded in the circumstances described above.

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Pete

Why would LBT help you, it checks if the channel before you transmit and if it's busy it doesn't transmit, and if folklore is to be believed the Frsky international firmware (which isn't LBT) is superior,.

Note of the FPV fliers were using illegal 2,4 gear it's likely that it was high power and it transmits on a single channel (i.e it doesn't hop like the Tx) but higher powers lead to some splatter on adjacent frequencies, if for some reason your DSM2 has picked the wrong channels then it could have been affected, where any hopping system (like DSMx) would have a better chance.

I'm not sure why FPV fliers need serious kit for legal line of sight flight, my 5.8 ghz legal systems work just fine and it's usually my spotter telling me to turn back as the model is getting to the limit of his sight..

[Simon Chaddock]

I don/t know if their transmitters were of higher power but I certainly couldn't see the planes some of the time!

[MaL]

There is a thread on singlechannellersreunited about a 2.4Ghz handheld band scanner. It uses an arduino nano, an nRF24L01 board and an oled display.

All the construction details are in this thread.... **LINK**

Total cost about $15 and it works very well indeed, but it can only show you signal strength where it is....not 200 ft up!!

 

Edited By MaL on 29/05/2017 12:26:45

[Peter Christy]

Frank,

Video on these systems is normally transmitted as an analogue signal rather than digital, and would typically have a bandwidth of some 4-5 MHz for standard definition (if digital it might be a fair bit higher!). That whole chunk would be completely and continuously occupied during operation of the video transmission. If they had two up at once, you can double that. This would fill up the available spectrum pretty quickly. LBT should be able to contain itself within the available frequencies left, ignoring those already occupied.

Of course, the reduced spectrum availability would certainly have an impact - this usually takes the form of increasing latency in the control response, but it *might* have allowed sufficient control to be retained to land.

As for FrSky's EU firmware being inferior to the rest of the world, that's a little off topic here, so I'll confine myself to saying there is little or no evidence for that. Note that I'm not promoting FrSky's system here, I did say "such as"! As far as I'm aware, all the manufacturers have had to introduce LBT in order to meet the media utilisation requirements. Its just that FrSKy's is perhaps the best known. If you don't use LBT, you are restricted to 10% media utilisation, IIRC.

MaL: Yes! I should have remembered that project! Seems to me that it would make an excellent construction article for RCM&E! (Is the editor reading this?)

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Pete

[kc]

Does that mean 2 or more sets legal of FPV prevents safe use of 2.4ghz for normal flying?

Perhaps Simon wishes he still had his 35mhz gear!

[Former Member]

[This posting has been removed]

[Peter Christy]

"Legal" FPV gear on 2.4 GHz is restricted in its power output to 10mW - way below the 100mW allowed for model control. On 5.8 GHz, 25mW is allowed. Any decent R/C system shouldn't have a problem with this. The trouble is that some are using illegally high powers - I've seen kit advertised as having 1 watt output! Against those kinds of output, a standard R/C system has no chance, no matter what kind of magic smoke and mirrors is used in the modulation system!

5.8 GHz tends to have less range than 2.4 GHz for video transmission, which is why some are adopting very high power outputs on 2.4, and then using some other frequency for control, typically 459 or 433 MHz, but 35 MHz is also very viable.

Fortunately, all the UHF frequencies won't travel beyond the horizon, and will be effectively screened by any large hills in the way. But if someone is using a high power video transmitter on 2.4 GHz just down the road, you could well be in trouble.

Remember, 2.4 GHz is a shared band, and there are other legal users with just as much right to use it as us. Fortunately, most of those other users tend to be in urban areas, not out in the sticks like us! Its illegal users who are the problem, just like in the bad old days of CB and 27MHz! Luckily, there aren't too many people around running illegally high powers.

If you want an exclusive frequency, stick to 35 MHz!

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Pete

[Don Fry]

Now I am told.

[Frank Skilbeck]

Posted by Peter Christy on 29/05/2017 14:21:37:

Frank,

Video on these systems is normally transmitted as an analogue signal rather than digital, and would typically have a bandwidth of some 4-5 MHz for standard definition (if digital it might be a fair bit higher!). That whole chunk would be completely and continuously occupied during operation of the video transmission. If they had two up at once, you can double that. This would fill up the available spectrum pretty quickly. LBT should be able to contain itself within the available frequencies left, ignoring those already occupied.

Of course, the reduced spectrum availability would certainly have an impact - this usually takes the form of increasing latency in the control response, but it *might* have allowed sufficient control to be retained to land.

 

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Pete

Pete, I understood that most LBT systems were not "active" in that they just don't transmit if there's something on that channel, where a non-LBT system would transmit anyway, so the benefit is to other users who have already hopped on that channel not the actual transmitter about to transmit, but the effect is the same for LBT and non-LBT one doesn't transmit the other is interfered with.

Comment about Frsky LBT was a bit tongue in cheek as if you read the forums, many users strongly advocate that non-LBT international (not the first EU iteration that didn't use LBT but lower utilisation), is superior,

I use a European certified system which is fully updated so i assume it's LBT.

BTW I do concur that there is a sizable minority probably using illegal video transmission systems, indeed there are some manufacturers now offering 900 mhz transmitter modules with selectable power outputs leaving it up to the user to decide if they want to be legal or not, the only reason for the higher power output is for long range operation.

Edited By Frank Skilbeck on 30/05/2017 08:13:52

[kc]

What are we going to do about any illegal users? Should Simon approach these FPV pilots and chat to them and try to find what power they are using ( they will probably brag about it if caught unawares!) and then report them? To whom?

[Peter Christy]

Frank: Most manufacturers are particularly cagey about what their various protocols actually do, so its difficult to be sure how any of them will behave in the presence of a strong interfering signal. The other problem, of course, is the need to retain synchronisation between the transmitter and receiver. One would hope that not transmitting on an occupied frequency would lead to a faster "hop" to a free one, but how many do this is a matter of conjecture. Unless someone out there has information they would like to share?

kc: This harks back to the bad old days of CB and 27 MHz. The band was completely overwhelmed by illegal operators, and the authorities had neither the manpower nor the inclination to pursue the matter, maintaining that "it wasn't a problem"! Ha!

To be honest, it is a very small minority operating illegally high powers, and the chances of coming across a group doing this near your patch is very small. But of course, when it does happen, it is extremely annoying - not to mention potentially dangerous.

However, 2.4 GHz is an "un-licensed" band, meaning that as we pay no license fee, we cannot expect the powers-that-be to have much incentive to act. I would strongly advise against any personal confrontation. However, it *might* be worth making a note of any car registration numbers associated with suspected illegal activity and passing them on to Ofcom. You never know, if the same numbers keep cropping up, they may eventually act!

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Pete

[Geoff S]

It's not only 27Mhz that suffered from illegal users. At RR(Hucknall) MAC one 35Mhz channel was banned (72 IIRC) because several models had been lost because of severe interference. It eventually was tracked down to anglers in a nearby pond using bait boats controlled by 35Mhz RC systems which happened to use that band.

My first forays into RC were on 27Mhz but that was for model yacht racing when interference didn't (usually) result in model loss. But I think that was after the craze in what the numerous radio amateurs at work called perjoratively 'council house radio'.

Geoff