Shot down in the middle of nowhere....

[SkippyUK]

Please bear with me - all this needs telling or you won't believe it....
There I was taking advantage of a short spell of sunshine and low wind to fly my small 3D Yak about when I kept losing control in one particular area of sky (low down, not high up).
I stopped flying and decided to check the footie scores on my phone - imagine my surprise when the phone was indicating wireless access point available.
Why was I surprised?? Well I was down in the dunes at least 3/4 mile from the nearest houses or habitation so where was the signal coming from? nobody else was in sight for at least 250 metres so it could not be another phone.
As I walked back towards my car the signal disappeared but walk back 10 paces and it was full strength - confusing.
Curiosity got the better of me and I spent the next hour walking about like Mr Bean with phone held high backtracking the signal to find one of these attached to the outside of a cottage and it was pointing towards a sailing boat moored maybe a mile and a half or more away in the estuary.
Further investigations on the net have revealed that these routers and antennas are becoming popular to beam Internet from somewhere with good broadband to somewhere with poor or no broadband and they have a range of 50Km.
All I can say is that my Yak with 2.4GHz radio is being 'shot down' whenever I fly through the beam ( I think It's called swamping).

Just a heads up that technology is being used to expand WiFi without consideration to it's effects of other users and may just cause models to be shot down with no apparent reason.

[WolstonFlyer]

Hi Skippy, yes it sounds like the house is providing a WiFi link to the boat and the radio beam is strong enough to swamp the connection between your TX and RX.

The 2.4 radio band is very congested and will probably get worse over time.

[Former Member]

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[Martin Whybrow]

That's rather worrying; being an RF engineer, I did some calculations:

The bandwidth of 802.11g is 22Mhz, which is effectively 22 of the 80 available channels for RC use (that's a drastic oversimplification, but it will do!). The power output of an 802.11g router is limited to 100mW or 20dBm and that antenna has a gain of 24dBi, so the effective power is 44dBm, compared to your 24dBm (the stubby antenna has a gain of 4dBi) AT THE SAME DISTANCE. At 1 1/2 miles away, that equates to your radio control transmitter at 240m away (using the inverse square law), so there would be sufficient power when you're at a fair distance, but only if your equipment doesn't switch to another pair of channels. The type of system in use would probably make a significant diifference in this case; assuming the Tx wasn't in the direct Line of Sight of the router's signal, the Tx would think the channels were clear at switch on (for systems like DSM) and could possibly allocate them for the session.

[Former Member]

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[Simon P]

If I understand Martin, he reckons the RC equipment might at least be partially to blame in so far as it being suceptible in the first place. Frequency hopping spread spectrum RC equipment that utilises the entire band should not be affected in this way should it? In other words if your RX and transmitter combo amount to something like DSM, or even DSM2, then you're susceptible but probably not if using DSMX. The former pair deciding to occupy a narrow portion of the band at switch on and the latter continually hopping all over the entire band regardless of the status of the band at switch on. Martin, please let me know if I understood. Thanks!

[Martin Whybrow]

Spot on Simon

[John Privett]

Martin, is that 24dBi directional antenna actually legal to use connected to a "standard" 100mW router?

I can't seem to find any conclusive info but thought it was the effective power output (EIRP?) that was limited and so bolting one of these directional antennae to the side of your house would require a much reduced signal into it from the router, or risk a visit from the man from Ofcom... Or am I wrong?

[Vecchio Austriaco]

May be it itime to offer my old 35MHz Futaba equipment

I thought I will never find somebody who is interested! this could change with this thread!

Have it and I stay happily at 2.4GHz

VA

[Olly P]

John - I would have to look up the regs, but the easy way to check is to phone OfCom. The enforcement and regulation teams are very helpful generally, as well as knowing there stuff. if you send them a link and a sketch of where they will go out with the detector gear and see if anyone is being naughty.

I had a good chat with them when obtaining permisison to switch some gear on for testing which is not normally used in the UK....

[SkippyUK]

Before we all go off on a brand bashing contest, I purposely did not say which brand of radio I was using but I will add that the receiver is a cheap as chips GC one which is only for park flyers so may not have the best RFlink in the world - the post was more to inform you all that there is some gear coming into use which should be considered in the investigations of unexplained crashes - particularly when near to the ground such as on landing approach????.

I will certainly be adding a quick check using my phone before I fly somewhere I'm not used to.

I've checked availability of these in the UK and they are being sold in limited quantities by ebuyer including (according to comments) using a signal booster to increase range - possibly more spurious signals.........

Looking at images of the antenna and access point, they both have CE marks on so presumably are OK for sale and use - Joe Public would think so anyway.

Skippy

[Dave Miller]

I have a recollection of a similar problem occurring 30years ago when microwave links were being set up one of which crossed our flying field and resulted in a patch of sky becoming a no-fly zone. We were all using 35mhz then; so much for progress.

[kc]

Whilst there are a few experts discussing things perhaps they can answer my question and perhaps solve all our problems.
Why can't we have 35 Mhz Rx that only reacts to our own personal Tx? i.e.some sort of PCM that is matched to just one Rx?

And why do we only get Model Match on Spektrum /JR? Is there some sort of patent part which nobody else can reverse engineer/ imitate?

[Olly P]

Posted by kc on 15/11/2013 11:13:39:
Whilst there are a few experts discussing things perhaps they can answer my question and perhaps solve all our problems.
Why can't we have 35 Mhz Rx that only reacts to our own personal Tx? i.e.some sort of PCM that is matched to just one Rx?

And why do we only get Model Match on Spektrum /JR? Is there some sort of patent part which nobody else can reverse engineer/ imitate?

OK kc - taking your Q's one at a time:

We could use packetised data via 35Mhz - I have done it on other similar systems, but it is relatively slow - possibly too slow for RC type use. Also everyone on the same frequency would have to be using it, otherwise there could be some very interesting results (frequency as in crystal channel, not 35Mhz)

Model Match - I'm not sure how the JR/Specktrum system works, it may be something built into the code, which will be protected - I just don't know enough about it. My assumption would be that without a feedback signal from the Rx, the Tx will assign a specific Rx code to each model and without this part of the signal the model will ignore it. A fairly simple bit of code, but would need to be in both Tx and Rx and the encoding would need to be set up to support it.

[GONZO]

As the band becomes more clutered and the noise floor rises no doubt people will start to use **LINK** these as well, possibly in addition to the directional aerial. I think that works out to something like 1000W erp in the beam. If an Rx is suffering from a swamped/overloaded front end then I can't see as it matters if the rc signal is FH, its just not going to work. AFAIK All our RC Rxs use very little if any front end RF filtering/selectivity and as such would be equally susceptable to out of band strong signals. As a point of interest the latest multi copter offering from DJI uses 5.8gHz for the RC link and 2.4gHz for the video FPV link. Both these bands are licence free where all users must accept any interference from all other users.

So, whos going to be the first to develope FH 35mHz gear???

[kc]

Olly, thanks for your answers. That's what Futaba etc should be working on, not on making thousand pound Tx that do a thousand things we dont need! We have our own 35 Mhz slots that we should use and protect not repeat the Citizens Band problems with 2.4Ghz
We dont need frequency hopping, we just need a radio that only our own Rx reacts to. PCM worked so why couldn't it be unique to one particular Rx Tx combination?
Lots of people have garage door openers which seem to use some unique system - as an experiment I drove up the road pressing my opener and nobody elses door opened!

[GONZO]

KC, you could try one of these **LINK**

[Tim Mackey]

Where does the GUID system fit in with all of this "interference" situation?

....I had always believed that Globally Unique Identification was designed to provide exclusivity of channel(s) within the whole 2.4G system.

[Former Member]

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[GONZO]

Posted by Tim Mackey on 15/11/2013 12:59:47:

Where does the GUID system fit in with all of this "interference" situation?

....I had always believed that Globally Unique Identification was designed to provide exclusivity of channel(s) within the whole 2.4G system.

As I understand it the GUID determines the hopping sequence plus the GUID becomes relevant part way through the Rx reciever proccess when the Rx decides wether to use a portion of the received data based on wether it has a valid GUID or not. If not then it discards that portion of data (there are other reasons to discard) and waits for the next bit. Thats why controll response gets slow in the face of strong interference (slower update) before the Rx drops into fail safe. None of this gyrating about the sky like used to happen with some of the 35mHz gear. If an Rx is swamped/blocked by a v/strong transmission other than the desired one then the RC info does not get that far through the Rx and after the designed time without valid data it drops into failsafe.

Anyone feel up to giving a block by block break down of a RC Rx with descriptions of the process caried out by each? I can do it easily for the old 35mHz systems, from Tx through to servo, but retired 17 years ago and am not that conversant with FHSS systems

Something to read **LINK**

[Olly P]

Tim - the analogy I use is this (OK, not for this application but close enough)

You are driving a car and the satnav volume is stuck on low, but you can here it. You pick up a friend and he turns the radio up, you now can't hear the sat-nav.

Now imagine the satnav is your Tx, and the radio is the "random interference."

You can't make out anything worth hearing because you are 'swamped' at the front end...

And you don't give that friend a lift again!

[Bandit]

Great analogy Olly P

[graeme jones]

One of my clubs went "2.4 only" some time ago, banning 35 on "safety grounds". I've continued to use 35 for some models at other sites with no problems.

I still have and use a 35MHz frequency checker. Is there a similar device available for 2.4 GHz?

Graeme

[Peter Christy]

Posted by graeme jones on 15/11/2013 15:52:45:

I still have and use a 35MHz frequency checker. Is there a similar device available for 2.4 GHz?

Graeme

Sort of. There's a thing called WiSpy: http://www.metageek.net/

which plugs into a laptop and can be used to scan 2.4 GHz. The problem is that just because you detect a signal, doesn't necessarily mean it will interfere with you! Spread Spectrum is somewhat akin to Black Magic, and the normal rules of logic don't apply!

The problem the OP had is that someone was using a very high gain antenna, which produces an extremely high field strength within a narrow beam. However, looking at the picture of said antenna, although the beam is very narrow horizontally, it doesn't look as if it would be that narrow vertically! Which of course is a problem for our aircraft, but not other WiFi users!

Technically, the user of the antenna is almost certainly in breach of the rules, as the 100mW erp figure *includes* any antenna gain. A model control transmitter is typically fitted with an antenna of 2 dB gain, and has an output of 60mW. 60mW x 2 dB = 100mW!

However, since 2.4 GHz is an unlicensed band, Ofcom have little incentive to monitor misuse. They will usually investigate if provided with credible evidence of misuse that is consistently inconveniencing other users of the band, but it isn't high on their list of priorities! If they were jamming local TV broadcasts, or emergency services, it would be a different matter!

Years ago, we used to have to get a license from the Post Office for RC equipment. Many rejoiced when that requirement was dropped, but all it meant was that we lost any right to expect action except in the most serious cases of interference.

I have no idea why clubs are branding 35 MHz as "unsafe". With proper discipline, it is perfectly safe - indeed it is arguably safer as it is exclusively ours! It is only unsafe if the users are too lazy to use a pegboard properly......!

 

Edited By Peter Christy on 15/11/2013 16:39:21