2.4GHz radio and cellular phone masts

[Daithi O Buitigh]

No problem Gonzo - as a licensed ham I well know what QRM is like (in all shapes and forms) but you are correct in pointing out that 2.4 GHz is a shared band and liable to interference - that's what the 'ISM' clause is in our regulations (Industrial, Scientific, Medical)and we're just told "it's there so just put up with it and don't complain"

 

Although we are limited to 400 Watts (26 dBW) that is at the aerial feedpoint - if it's a stacked Yagi beam, the actual Effective Radiated Power (ERP) will be in the order of Kilowatts (egg-frying time at close range) and any model crossing the lobe of the beam will get the receiver knocked out.

 

What we need is a 2.4 GHz broadband receiver/monitor to check first before one of the big 4 engine or jet jobs goes out of control with disastrous results

 

I remember years back (in 27 MHz days) that CBs were used to monitor the frequency in case one of the chicken-boxers (using illegal rigs) decided to "get rid of the bloop-beeps on his channel". It wasn't illegal to have a CB back then - just illegal to operate it on transmit

 

Daithi, GI7OMY

[Malcolm Fisher]

I can remember being "shot down" several times by CB years ago when operating on 27 MHz and only once since with 35 MHz when a clubmate switched on his PCM transmitter whle I was flying. He was inadvertently on the same channel as me and had a brand new set which he had been told was on a different frequency. Everyone in the club then checked which crystals they were actually using before every flying session.

I've no real personal experience of 2.4 GHz but have never seen any problems when on the slopes with other people all using the system while I am still on 35 MHz.

Malcolm

[Ben B]

We fly next to a hospital (lots of lovely bleeps and other RIF noise), a mobile phone mast (chucking out lots of lovely 2.4Ghz (ish) noise), and a prison (with a mobile phone jammer). And just to add to the fun there's a mahusive mains cable running under our site which supplies most of north London.

 

So have lots of fun Still, you get used to the glitches after a while.

 

The main source of problems is the person who turns up with the FPV 2.4Ghz rig....

[GONZO]

Ben B,

Now you have baited your hook well. I can, given a bit of time, point you to a video of two planes flying in the same part of the sky and the pilots about 60ft apart in the field. One is just LOS 2.4gHz Rc the other is 35mHz Rc with a 2.4gHz video down link. The pilot of the 2.4 Rc plane experienced no problems where as the FPV planes video down link suffered some interference from the others 2.4 Rc. The reason for the separation of the pilots was to minimise the the interference (2.4 RC signals) picked up by the FPV pilots ground station(2.4 Video Rx). The 2.4 RC interference causes banding on the video and can make FPV impossible. In the FPV community it is accepted that 2.4 RC causes more problems for 2.4 video FPV than the other way round. A lot of FPV flyers in this country are moving to 5.8gHz video down link and using CP (circular polorised) Tx and Rx aerials to overcome the drop out/multi pathing problems.

Out of interest, as I mentioned the 459mHz band previously, there are several FPV flyers developing a sytem to opperate on this band. They are using an open source UHF LRS RC set (from abroad) that opperates on the 70cm band and rewriting the souce code. So far it is working well. As the Tx unit comes in a Futabe TX modul case and costs $79 with a RX costing $59 this may well bring the 459mHz band back to life.

[GONZO]

HERE IS A LINK to just one of the forum threads I have been following on developing circularly polarised antenna for FPV.

[Peter Beeney]

When we were using 27 MHz we heard all the stories about the interference so we did quite a lot of experiments trying to shoot models down with CB radios, these were generally high flying thermal soarers. On of the members then was a radio boffin that worked for the MOD with access to the right sort of kit. We never had the slightest success, you could listen to the tx stick tones on the monitor running up and down, the CB interference would then completely wipe this out but the model would continue serenely on it way, not deviating for a second. The rx simply did not see the interfering signal. However, if you switched on another tx on the same frequency the model would go berserk. I don’t think we were ever able to get the CB frequencies exactly on the model frequencies. In all the soaring I did at the time I never saw anything that I could have said was definitely external interference.

Also with 35 MHz, I’ve never seen any external influences, and I also use to scan the channels for a long while but eventually gave it up because I never heard any unknown signals.

It would be very interesting if someone could take a 2.4 receiver near a high powered transmitter and find out exactly what does happen when they try to operate in monitored conditions. I can’t help feeling that if these transmissions were able to cause so much havoc then some of the many, many other users of the ISM band would also be affected in some way. Maybe this is in fact happening, but I’m sure I’ve not read anything about it, at least not so far.

We have an FPV man, he just watches through the goggles whilst someone else flys the model. He has the problem in reverse, the model is on 35, as normal, the FPV on 2.4. This as never interfered with a model, but if his base station is too close to a 2.4 transmitter it corrupts the FPV signal so he just moves it away from the strip a short distance.

With the greatest respect, I’m just wondering what sort of transmission powers are involved here, such that a FPV transmission can cause trouble with a 2.4 model control radio, and I’m also now confused as to how a 2.4 radio can glitch the servos. I thought this was virtually impossible but maybe not. I have to say that at our site, after some careful investigation any problems with 2.4 have proven to be very much a fault on the individual set as opposed to any external interference or any other condition, and once we’ve fixed that then there are no more recurrences.

There’s always something new to ponder…

PB

[Daithi O Buitigh]

Perhaps it should be pointed out that operation on 70 cM is illegal unless you have an amateur license (and even then we aren't permitted airborne operation here)

[Peter Smith 12]

Posted by Ben B on 25/08/2011 21:46:15:

We fly next to a hospital (lots of lovely bleeps and other RIF noise), a mobile phone mast (chucking out lots of lovely 2.4Ghz (ish) noise), and a prison (with a mobile phone jammer). And just to add to the fun there's a mahusive mains cable running under our site which supplies most of north London.

 

So have lots of fun Still, you get used to the glitches after a while.

 

The main source of problems is the person who turns up with the FPV 2.4Ghz rig....

I have just tried a rough and ready bench test with my Futaba 2.4G radio by
placing a sweeping 1.8Ghz milliwatt signal about 6" from a receiver then
walking away 5 yards with the transmitter on low power and it
does block the receiver most definitely as you might expect. However moving the 1.8G source a further 6" away and controls are restored. A pretty severe test though. I would expect similar results with a 2.1GHz signal too

[Wingman]

In this photo taken at the Scottish Nationals you can clearly see the phone masts directly across the flight line from the pilots stance - obviously no problem there.