Serious 2.4Ghz Problem arising.

[GONZO]

5.8gHz is also a 'catch all' ISM band for future use.

[Wolfie]

The interference models quoted are not actually correct.

2,4 GHz systems will be using either Direct Sequence Spread Spectrum or Frequency Hopping Spread Spectrum.

 

It is not a case of whether the channel is 'busy' but whether the channel is either busy or contains energy at that exact frequency at that exact instant in time.

Take the example of DSSS, the energy is effectively spread over a considerable bandwidth and the result is a few mW per MHz bandwidth. It requires a correlation filter to re-assemble this spread energy back into a coherent signal that can be demodulated. If the correlation filter is is not an exact match the output is also spread spectrum. This is why many units can operate in the same channel.

The problem with DSSS is when the interfering signal is extremely larger that the wanted one. Then the spread spectrum output of the correlation filter is a similar level to the matched correlated signal. Called the 'near - far effect', when this happens then interference will occur.

[Peter Beeney]

Wolfie,

Is it possible that you might have any information on the relative power levels at which this might happen; and perhaps of even more interest the physical distances involved? For instance, if the interfering tx output levels were at 1W, and say the model control tx was 400 metres away from the model, how close would the 1W tx have to be to be able to cause a problem? And again with the the control tx at 600 metres? I’m assuming just the bog standard rx set up here.

And still guessing, would this ‘interference’ take the form of the simply the throttle shutting down and the servos remaining at the last known good position, or maybe stepping to a pre-set position, if that had been programmed in, rather than the servos acting in a random manner?

I suspect the model is going to more or less crash, rather than land in any sort of fashion, anyway, whatever happens, but what I was sort of thinking was that if say a FPV vehicle caused this to happen, at a reasonable height, the model would then go out of control. And go into a dive. Assuming the FPV doesn’t dive as well, to stay close, is it likely the model may get far enough away to be able to regain control? Or is this likely to be a real blanket cover? A recovery would be indicated by the throttle opening, provided the stick had been left in the Full Throttle (FT) position, (trying to keep in line with another recent tread here!!).

Lots of ifs, buts and maybes here, and to say nothing of the speculation on my part, too.

PB

[Wolfie]

With DSSS modulation we need to consider the IF recovered to the demodulator. Say for example that the demodulator has an 'unremarkable' 30 dB margin for signal/noise.

To swamp the wanted signal at the demodulator would require a similar difference in signals in the near/far effect.

That amounts to a distance ratio of about 32 times.

ie if the receiver was 10m from one transmitter and 320m from the other.

A good demodulator would be significantly better and a coherent one would be better still.

 

As to what form the interference would take, that is unknown. It depends totally on the receiver decoder software (failsafe or other) and is not related to the interference mechanism.

[Peter Beeney]

Wolfie,

 

Thanks for your reply. I must admit, I was thinking a bit more hopefully, too. Going way way back, when I was struggling with this stuff, it was steam radio in those days, comparatively speaking, as I remember a cheap transistor radio might have a s/n ratio of 40dB, whilst a first class tuner might be as high as 80dB. If we get really optimistic here, and say that a coherent demodulator might have a s/n ratio of 75 dB, then perhaps we can make some sort of rough estimate on the distances/powers involved.

I sincerely hope I’ve got this right, for obvious reasons, but if 30dB is equal to a distance ratio of approx. 32 times, then 40dB will be 100 times. 50dB equals 320 times and 75dB would equate to 5,600 times. 80dB has climbed to 10,000 times. I think I’m also right in saying that to get to the 30dB ratio difference on our model, if the control tx and the FPV are the same distance away then the FPV tx would not have to have an output of of 1 watt. The output would have to be 100 watts. 70dB requires a megawatt. With regard to the actual interference, I’m pretty sure the decoder wouldn’t pass any rubbish to the servos. That would have to be a step back. It must be some sort of pre-set servo position.

Trying to take all this wild speculation on a apiece, GONZO mentioned that the FPV pilots are increasing the 35MHz tx aerial length to get increased range. Would this be up to quarter wave, around 6ft 8ins long? I’d have thought that to get a nice solid link at two miles you might need to up the power a bit as well, perhaps not quite so easy with a standard transmitter; but maybe it’s not needed, I’m just guessing.

But, which ever way it is, and assuming the wayward FPV bird is being controlled by 35 MHz and not 430 - 440MHz UHF, this might have some sort of implication for the remaining ordinary club man using 35, if the two disciplines are being flown close together. I might have to fire my scanner up again. By the same token, the FPV model could also be affected, if it were 2 miles away from the controlling pilot and just happened to fly over another 35 tx on the same frequency, at say about a thousand feet, I wouldn’t give very much for it’s chances. I’m not sure there is enough room for two radios on the same channel on narrow-band 35. And, I suspect, the channels can be changed quite quickly on a synthesised outfit. An unlikely story, and highly improbable; but perhaps not entirely impossible.

All in all, if I were a FPV pilot using high powered illegal equipment, I think I would want to stay as isolated as possible. It seems to me, in some respects, that he may well be more vulnerable than the 2.4 model pilot. Especially if he’s on 35...

Perhaps what we want now is some manufacturers data, if available, or if someone has tried any experiments to see what actually happens when you mix this stuff. I found occasionally that things didn’t always behave as I thought they were going to behave!

Thanks again. It gets interesting!

PB

[GONZO]

Peter,

Correction - FPV flyers extend the length of the RX aerial not the Tx aerial. Rx aerials are normally extended to just over 2mtr(1/4 wave) for UK flying in remote areas that have been checked for club locations.

An FPV 'trick' is to fly with the bottom section of the Tx aerial not extended so that when the RCRx RSSI level displayed on the OSD display in the googles indicates a failing (out of range) signal the aerial can be fully extended and a turn back to home initiated.

[Peter Beeney]

GONZO,

Very humbly sorry, I think I shall have to go to Specsavers, my mistake entirely. That should teach me how to read the posts properly. I guess the pilots checking for the presence of clubs works both ways, in fact, the possibility of getting accidentally shot down yourself may be the greater.

Not sure about your abbreviations, taking a guess, but is that the Radio Control Receiver - Received Signal Strength Indicator? So presumably there is an on-board sensor which can monitor the incoming 35 signal. But I reckon that must be obvious. I shall take more notice of my friends FPV when the sun shines again and he comes out of hibernation.

When we were flying 27 in amongst the CB radio we would close the tx aerial up a couple of sections, sometimes more, and carry on flying, to try and increase the chance of seeing interference. Admittedly this was with thermal soarers, and generally overhead but sometimes they would be very high. It never ever seemed to make any difference.

I like the ‘and a turn back to home initiated!’ Do we always know which way is home? I notice that the Graupner top of the range receiver now has a giro and GPS facility. I wondered about this, but could there be an opening here, perhaps.

Do you have a FPV? I must admit, so far I’m certainly not very convinced that it’s any great threat to normal club flyers, I think it’s going to require some proper proven evidence to really establish the facts. Looks like speculation to me, but then I was also wrong at the top of this post, too, so maybe I’m still on track to maintain a perfect record! The necessary corroboration will now probably pop up tomorrow!

PB

[GONZO]

The RSSI feed is taken directly off the RCRx (any frequency band). There are various threads detailing where to solder the pick up for a number of recievers.

To see the RSSI you would need an OSD. Bruce Simpson of RCModelreviews is doing a series on his entry into FPV. THIS is a vid of a typical display(no RSSI). The display can be switch on - off - partial. You can see there is a compass and centre bottom an indicator showing home direction.

Check out, from the start, his series of vids on his FPV adventure and also HERE for more written details of the equipment used.

I don't have a FPV plane but intend to. I have done months of reading and still have lots to learn. It seems to me that a lot of people are quick to condem without even fully reading and understanding in depth the other side. Without my extensive reading I would have been unaware of the units that are used that can give auto stabilisation/altitude hold and auto return to home. I used to fly (parachuting - hang gliding - PPL with a share in a plane) and FPV is the closest I can get to this now.

THIS VIDEO shows an FPV plane with a 2.4gHz VTx filming a mates plane being controled with 2.4gHz RC (Spektrum DX7). The pilots (both flying within LOS) stood 30mtr apart on the ground to avoid the RC Tx interfering with the FPV VRx at the ground station(there is slight video banding). I PM the FPV pilot for these details and he confirmed that the other pilot experienced NO problems.

Edited By GONZO on 05/03/2011 09:45:09

[Chris Johnson 1]

Hi Guys,

I've been watching this thread with interest the last few days. I must admit a lot of it goes straight over my head with all this RSSI/DBi blah blah. All I am interested in is if I switch my gear on it works. Like most drivers I guess not knowing anything about their engines.

Anyway, I agree with a lot that is said on here. I also did the CB/model experiment years ago and also could not get a glitch on the model only inches away from the CB antenna.

 

I have also "chased" a non FPV 2.4 Ghz plane with my 2.4 FPV equiped plane and again not a glitch.

As said on here, the only interference is from the 2.4 Ghz radio Tx on the 2.4 Ghz Vrx display.

 

If any of you guys are interested in getting into FPV there are many forums including this one, but the best I've seen so far is www.fpvuk.org which is 99% FPV'ers.

There is a lot of very good information available on there and it's certainly not the usual run of the mill dare I say it, "old farts" club. (although i'm in it so it could be) most of the guys there are quick to help each other out but also equally quick to tell someone they are doing something wrong.

The equipment and legalities are clearly displayed on the home page links so anyone is in no doubt what is and is not legal.

Having said that, it wouldn't matter if it was in font 72 font size if someone chooses to ignore it.

Apparently there's a thread somewhere that is actively "witch hunting" an apparent maverick FPV'er as the LOS flyers keep falling from the sky. I know the guy they are referring to and he hasn't flown FPV for over 2 weeks as his gear was u/s. but they were still falling from the sky and still blaming him. I know this to be fact as I have just sent him a new 10mW Vtx yesterday to get him flying again.

As someone on this thread said it could be someone somewhere with a WiFi booster in the nearby housing estate.

We are organising a spring meet in May and will be demonstrating a lot of gear to the LOS flyers as well as giving them the opportunity of a passenger trip "on the goggles"

It's not a secret location but numbers will be limited as it's a small club site.

Anyone interested in coming over can find the details on the aforementioned website.

I think there's enough disagreement in this world without modellers getting at it as well.

Let's all live in harmony not harmonics

Cheers Guys

 

 

[GONZO]

'Old Farts' indeed. I'm there as well - come to think of it you may be right, I get my state pension next year.

No, an 'old fart' (flatulitus ancientis) is a state of mind not an age in years!

[Tony Smith 7]

"LOS" ??

[flytilbroke]

Line Of Sight

 

 

[Peter Beeney]

Chris,

Yes, perhaps it’s possible the only thing you can say say in favour of dB’s is that they do give you some sort of relative power levels on which to decide if you have a problem. For example, I think the housing estate WiFi booster would really have to be a considerable Booster, enough to cause other problems amongst the local population! So anyone using it might not be that popular generally!

However, having said that, I’ve been having a little look around to see if I can find some more info., something I’ve not done previously. I found one little posting on a forum in the US of A, it’s quite revealing. The gentleman that wrote the post said that his last employer modified a Futaba tx to bypass the aerial and send the signal to a 50W amplifier as a backup controller for a UAV. Standard components used and costs less than 500 dollars. The range is, and I quote, - ‘It has safely operated UAV’s out to 70 miles.’ They’ve also noticed another possible snag, sending the goodies home, - ‘The trick is that you need an equally powerful transmitter on the aircraft video.‘ 50W presumably? There were no details of power or frequencies used. And there’s just one small detail to add, - ‘Also, how do you do frequency deconfliction with everyone else flying RC’s within 70 miles of your location.’ I think what he was trying to ask was how do you avoid shooting down everyone else on the same frequency within 70 miles? That’s more than 15,000 square miles. As the song says, Every thing’s big in America! This was on 72 MHz. He did qualify this in his next post, saying he’d left that employ because he didn’t agree with it. It might have crossed his mind that there could be some irate modellers knocking on his door at some point. Or could that be knocking his door in?

He did have one piece of advice, again I quote, - ‘I recommend trying high gain directional antennas, first. I believe Eagle Tree now has an antenna auto-tracking capability with their new hardware.‘ Antenna auto-tracking capabilities? Is this getting complicated, or wot? This was back in 09, I wonder how far this has got?

I once read a book, way back, about making your own directional aerials for 2.4 WiFi devices. Provided you measure quite accurately it’s fairly easy, some types are tin cans etc.

There is another acronym, OAP! I’m familiar with this one at least, I’ve been practising this for a number of years now! Remembering the lessons is the big stumbling block, or at least I’m told it is, I justly forget…….

 

PB

 

[TWilson]

FPV flyers are getting a bad name and it is easy to blame a model crashing on a rogue FPV system, when it could be any number of things, but it is in our nature to want to point a finger anywhere other than ourselves, when in fact the likelihood of any video transmitter bringing down an RC model is almost impossible.

 

Firstly, if a 2.4 FPV video transmitter of even the highest output was close to a DMS or FASST system then the FPV flyer would not be able to fly FPV as he'd have far too much interference from the RC transmitter, you would also be able to ask him to stop flying as he'd have to be extremely close.

 

Secondly, I own a 2.4 1000mW video transmitter, commonly used in the UK for CCTV systems, I bought it a few years ago not knowing it was no legal for use in the UK and I have never used it on a model, however after hearing that FPV flyers were being blamed for glitches on 2.4 models I decided to do some test as I fly both Futaba and Spetrum systems. I did extensive tests ranging from having the transmitter and receiver antenna touching, removing the FASST receiver antenna, using low power modes on the RC gear and range tests, and not once did I get a single problem with either the DSM or FASST RC systems, both worked perfectly - I did however have glitch on a 35mhz system.

 

That said, the most common output power for FPV is 500mW, which is less than many 2 way radio systems freely available, there are also radio enthusiasts that have the ability to transmit 20w on 2.4ghz, so I'd suggest that if you do have any radio issues then look further than FPV.

 

[Tom Wright 2]

Interesting and good to see some actual tests but perhaps not conclusive.

 

TW2.

[Chris Johnson 1]

There you go TW, it doesn't matter what proof we find, nothing will ever convince some people.

I have flown FPV on 2.4 Ghz Vtx in formaton with another model on 2.4 Ghz flying controls and not a glitch from either model or loss of video on my goggles. What better evidence can you get.

Chris

[TWilson]

I would be more than happy to lend my 1W VTX for trial purposes, I did the research because 2.4ghz is full of such a diverse variety of devices and wondered why FPV is singled out - just look around the house and see just how many things operate on 2.4 and are they a problem?

 

If you look at how FASST and DSM skips across the band you can then understand that it is almost impossible for a device on a fixed frequency to interfere as FASST jumps all over 2.4 and DSM on part of it - I guess if you had a very poor video transmitter that bleeds over then there could be a small chance but it would have to be very powerful and very close.

 

[Tom Wright 2]

Chris and Tom.

To be fair the real life testing is the best evidence yet and i commend you both for taking the trouble,but in the real word i doubt if OFFCOM would accept your findings as formal proof, i am not anti FPV at all i think the technology is great, but it does not alter the fact that 100MW is the legal limit so you cant blame people for showing concern when its clear much higher level RF transmissions on 2.4 GHz can be with in range

 

TW2.

[Chris Johnson 1]

Sorry to correct you TW2 but 10mW is the legal limit for FPV 2.4 Ghz Vtx's and 25mW for 5.8Ghz Vtx's.

I agree with TW, look at how many other domestic devices are on 2.4Ghz...WiFi, mobile phones etc.

I think that someone, somewhere has seen FPV and "Trappy" mentioned and just jumped on it with no solid evidence. (I'm sure everyones heard of that "Ambassador" of FPV flyers.)

I have no doubt there are some FPV'ers out there who flaunt the rules as there is in every sport/hobby to get one up on everyone else but that's human nature I guess.

www.fpvuk.org always discourages this but it will happen I know.

We are also governed by OFFcom and the CAA as regards the guidelines but if some flyers are not members of either BMFA or FPVUK and fly without insurance and not at a club then they will one day come unstuck big time I think.

I don't get to fly often as I am working in Afghanistan but I do enjoy flying at a great club, when I am home, whose members embrace FPV fully and as yet we have never had a problem. As I said way back in this thread, if sensible frequency precautions are adopted and adhered to I doubt there ever will be a problem.

 

[Tom Wright 2]

Chris.

Are you correcting the right Tom ?

Tom W stated on post 15.31.00 and i quote "The most common output power for FPV is 500 mw."

 

TW2.

[TWilson]

I said the most popular, not what was legal, 10mW is the maximum you can transmit from a model, with a license you can go to 500mW but it's not legal to transmit from the air regardless of license.

[Tom Wright 2]

Sorry Tom W. your statement is quite clear it was not qualified with any reference to legality, so if 500 mw is the most popular could it be said most FPV operations are not legal?

 

TW2.

[Peter Beeney]

I think we’ve been here before. Back in the days of 27 MHz there were all the stories of the illegal CB radios, including those of some people that filled their cars up with powerful amplifiers that could broadcast wall to wall. Wall Street to the Great Wall of China, perhaps? No, maybe not. Anyway, we never experienced any interference, even when the CB convoys were going past. The early model shows carried on as usual, accompanied by the mandatory ‘interference monitoring team’. There were plenty of crashes blamed on interference, it certainly got a big press, but I wonder just how many incidents were the actual genuine unknown article.

Then 35. Again I’ve never seen even one case of genuine interference, so I would have to say that as far as I’m concerned at least 35 has never been a problem. Shoot downs have happened, but that’s a local problem; as is 35 and electric model inter-action; and all the other little glitches etc. that we’ve investigated have proved to be modeller induced. I’ve said many times that I monitored the airwaves with a scanner a great deal a few years back and never found anything. But I will admit that doesn’t necessarily mean it’s not there. At the sites where there is evidence claimed it always seems to be speculative, but I’d have thought that any signal that the receiver ‘sees’ can be seen, recorded and identified by a scanner. I’ve never seen any reports that any signals have been found on these frequencies. At a guess, and this is speculation on my part, but if there there were many incidents on 35 then it’s possible that the attitude at airshows for instance would not be quite so nonchalant. Sure, there is the ever-present spectrum analyser, doing the monitoring, but by by the time any interfering signal has shown up on the screen it may be a bit late. The models will have gone in.

Now we have 2.4. To what extent is this monitored at shows? How is it done? Is it possible to define which frequencies will have an effect on a receiver and at what signal level? It seems to me that interference capable of affecting 2.4 model radio receivers would in fact affect all the receivers that are switched on. In other words, a multiple shoot-down. Are there any cases of this happening? Looking at this more closely, I’d have though that an interfering signal would not be able to affect the servos, as in PPM modulation, 35 MHz. Will it not be the case that the wanted signal is prevented from reaching the rx so then it will just go to it’s pre-set failsafe situation? Even the PCM modulation had that facility, MPX and others are now doing the same with PPM receivers.

I’m in the gang that says that FPV is not going to be a problem. At least until I see any evidence to the contrary I shall consider that it’s very unlikely that the FPV vehicle could transmit at the necessary signal strength. What, for instance, would be the power requirements and thus the size (and weight!) of the battery? I’ve seen a few problems with 2.4, far more than 35, but none of it is anything to do with interference. Other than local, actually on the model. I've just been looking at one this week, a tx that can be made to stop transmitting at will and then start again. Sometimes it will do it all on it’s own. Sounds like an ideal candidate for making models crash to me!

PB

[Tom Wright 2]

Peter.

I think your , post puts the situation into a understandable and logical position ,and no doubt backed by a deeper know how that's been kept at bay, to suit the not so technically minded ,if i have read and interpreted the recent forgoing posts correctly ,it leaves us with a rapidly growing annex to our hobby with the majority of the participants transmitting illegally if this is the true case then should some conventional modelers register concern?

 

TW2.

[TWilson]

Tom, I think you are a classic example of why FPV has a bad name, have you not read anything in recent posts? I don't want to be rude but there are so many 2.4ghz devises transmitting on far greater power than any FPV flyer is able to, just have a look on google and you will find loads of equipment far more powerful. Most, if not all the FPV flyers I have met have a got their Amature Licence, including myself, which make them, I no longer fly FPV, a pretty responsible group of people, as it is very technical and very expensive you will only get very dedicated and skillful people participating, those not so switched on will never get far. The classic RC plane flyer will always have a problem with people that fly anything different, this is why so many helicopter pilots get such a hard time.

 

Unfortunately FPV is here to stay, the BMFA and CAA will need to workout some proper guidelines but the levels of output from a video transmitter is tiny. How could you be concerned by a modeler flying a half watt tx a couple of miles away when there could be a radio ham with a 20w transmitter on the same frequency much closer.