Interference between 2.4GHz RC and other 2.4GHz equipment

[Chris Downing]

Does anyone know of any definite testing that has been done to prove if there is any intereference between 2.4GHz RC systems and other 2.4GHz equipment?

 

There are 2.4GHz radio modems used in airborne vehicles for data transmission and also 2.4GHz video transmitters in use.  Some have single channel transmission, others use frequency hopping. However I have not heard any definite, conclusive reports about the safety of using such systems together.

I contacted Futaba and JR about this and all they could say was that they had tested their own systems and had no interference between many RC transmitters at the same time.

They haven't tested radio modems and video transmitters all at the same time as the 2.4GHz RC gear.  They simply stated that frequency hopping systems avoid/prevent interference, or at least minimise the impact of a simultaneous transmission on the same frequency.

 

My concern is that some radio communication modems send airborne vehicle control data over their link, and the simultaneous transmissions from two systems on the same spot frequency (however infrequent on frequency hopping) can cause momemtary data loss, thereby requiring retransmission and causing delays in control signals being received.

 

 

 

 

Any news or links would be welcome.

Edited By Chris Downing on 18/04/2010 01:58:51

[Olly P]

Given that 2.4 Ghz is a digital transmission system each data packet should be prefaced with a unique identifier, this address means that only the correct reciever should have any reaction to the input.

 

This ip-packetised data transmission is quite commonly found in many digital data systems. I am uncertain as to if ip addressing is used on out systems, but I would expect it to be.

 

Once my new Tx and Module arrive from HK I will try some testing and see what I can find out.

[Olly P]

http://en.wikipedia.org/wiki/Packet_radio

[Chris Downing]

Olly, Thanks for your input here. Sorry I didn't get back to this thread until now- This year has been a really odd one!

I'll take some time to read through the link you sent.

I don't have any conclusive evidence on this myself but in working with some equipment here we have noticed on occasion that usng a 2.4GHz RC system on an aircraft which also has a 2.4GHz radio modem on board seemed to result in the radio modem link dropping packets very often. The modem is only 50mW transmit power.

 

Some other people are reporting that using a more powerful radio modem (1Watt output) the RC system is being reduced in range.

So two seemingly opposite problems are being noticed but I can't verify them.

Rather annoying.

[Steve W-O]

I think what you have noticed is generally accepted as being the case.

 

It is recommended not to fit a 2.4GHz video transmitter to a model using 2.4GHz for control, and many laptop manufacturers state in the "instructions" that using bluetooth and wifi at the same time can lead to problems.

 

Packet loss on an active link (in the example you gave in the first post) would normally not be noticed, as the packets would be resent much quicker than you would notice any loss of control.

 

The big difference of 2.4 is that interference should not result in servos acting in a random fashion, as can happen with analogue transmissions, (many later 35MHz receivers do make sure there is a valid signal before passing info on to the servos, but older ones do not)  if the interference was severe, it should just result in loss of link, as if you had gone out of range. This may not seem much good, but at least the model should continue in a reasonably stable flight path giving chance to re-establish control.

 

I don't think the problems you stated are opposite, they are the same.

 

I don't know what data is being transmitted in the examples you quoted, if it data on the aircraft, then there are semi-duplex systems available for control and data.

 

If it is other data, there are other frequencies available for telemetry.

[Chris Downing]

Thanks  Steve- it's good to get input from others.

Thanks to both you and Olly for reading the post and replying.  

The best approach then is to not rely totally on frequency hopping to prevent all interference but to use a different frequency for a co-existing system, like video transmission or other data link.

 

 

[Tim Mackey]

And thats the problem with the FPV systems - they use 2.4Ghz transmitters, mounted in the model, and therefore if the models RC system is also the now very popular 2.4G equipment, the video sender tx swamps the signal being received by the radio. It seems that GUID is little defence against a much stronger signal swamping it.

When doing some review work on the new flycam transmission system recently, my home wireless router for the internet stopped working altogether when the camera was switched on in another room.

[Steve W-O]

Posted by Tim Mackey - Administrator on 06/09/2010 17:48:42:

And thats the problem with the FPV systems - they use 2.4Ghz transmitters, mounted in the model, and therefore if the models RC system is also the now very popular 2.4G equipment, the video sender tx swamps the signal being received by the radio. It seems that GUID is little defence against a much stronger signal swamping it.

When doing some review work on the new flycam transmission system recently, my home wireless router for the internet stopped working altogether when the camera was switched on in another room.

 

This is the point I made in another thread, and is the main reason for loss of data on 2.4.

The signal does not have to directly interfere with or corrupt the other signal.

 

The answer is to use a 900Mhz transmitter in the plane (I'm not sure where in 900MHz it is, because cellphones use the lower little bit of 900, I'm sure there is a frquency in the 400's that could be used to) There are also some video transmitters higher up as well for home use, whether it is legal for mobile use is another question.

Edited By Steve W-O on 06/09/2010 18:16:03

[Bri.]

Long live 35megs

[David Turner 5]

I can give you some real-world results.

 

I bought a 2Watt video transmitter  (2.4 GHz) and tested it against a Spekky. Try as I might, I could not disrupt the radio-control link.

 

Now, the same 2W video system totally demolished my 35 Meg link.

 

hth

[Myron Beaumont]

In other words ,you're saying -Stay away from modellers who creep up on you with 2 watt systems . Now you know why I fly early morning on my own ! (Both 35 meg & 2.4)

[Chris Bott - Moderator]

I tried a 2.4Ghz video link alongside 2.4Ghz R/C equipment. The R/C was fine, but the video link (analogue I believe) was badly affected by the R/C

[Tim Mackey]

A flycam video transmitting camera ( 2.4Ghz ) completely knocked out my wireless LAN hub when switched on in an adjacent room.

[Chris Bell]

I have a 2.4 video sender at home - you can tell when the kids or the wife is setting up a model with the TX on as the picture and sound goes to hell

 

I recon (although I've never seen FPV kit in the flesh) the problem would be more with the picture than the Controll

 

Chris

[Tim Mackey]

I wouldnt bet on that Chris - after all the models receiver is passive, and could be easily swamped by the transmitted signal of the onboard video transmitter

[Christian Hollyer]

I wouldn't mind some advice here. Aurora 9 bound to a BODA receiver. Elevators driven by two channels. When functioning, after about 7 inputs to the elevator, they go there separate ways momentarily and the rudder (on the other stick and not being touched) deflects to the left momentarily. All this engine off.

Have tried: different tx, rx, power supplies. These functions are taking place at home ( I live in a block of flats). Could domestic wireless from other flats be causing this?

[Frank Skilbeck]

Interference would cause the radio to go into failsafe. It could be a fault with the Tx, but I'd also check that the reciever battery is in a good state of charge, what you could be seeing is the servos drawing the receiver battery voltage down the receiver then shutting down and rebooting as the voltage recovers and it's the defections occur as the receiver is rebooting.

[Peter Christy]

Most 2.4 GHz video downlinks are analogue, not digital. From memory, the regulations stipulate only 10mW of power for any downlink from an airborne model - be it video or telemetry. To use the full 100mW we are permitted for the "command" channel, the transmitters have to operate "politely", either by finding a clear channel before powering up (DSM systems) or by rapidly frequency hopping.

Analogue video downlinks are very wideband - typically 3 to 5 MHz to get decent resolution - and will block out a big chunk of the band. Installing one close to a command receiver is bound to cause issues. To use an analogy, its like trying to hear what someone is saying on the other side of the street whilst stood next to a pneumatic drill! It doesn't matter that its only 10mW - its the proximity that does the damage. Radio follows an inverse square law - if you double the distance between a transmitter and receiver, you QUARTER the received signal. So no matter that the command transmitter has ten times the power - it is more than 3.3 times distant, and will be drowned out by the TV transmitter!

If you use an (illegal!) booster, the problem just gets worse. It is VERY UNWISE to use the same frequency band for a video downlink as you use for the command channel, as many people have already discovered.

Some of the newer downlinks are using the 5GHz band, where more power is permitted anyway (25mW IIRC), but the signal is less robust at these higher frequencies.

And remember that 2.4 GHz is a SHARED BAND, unlike 35 MHz which was ours exclusively. There's all sorts of junk out there on 2.4 GHz. Mostly it doesn't bother us, because we are flying out in the sticks, and the interference is produced in and around homes and factories. But just because our equipment operates "politely", it doesn't mean everything out there does! And for most of the public, cost is a more important factor than not interfering with others.....!

[Simon Chambers]

Posted by Peter Christy on 25/04/2013 19:32:59:

Most 2.4 GHz video downlinks are analogue, not digital. From memory, the regulations stipulate only 10mW of power for any downlink from an airborne model - be it video or telemetry. To use the full 100mW we are permitted for the "command" channel, the transmitters have to operate "politely", either by finding a clear channel before powering up (DSM systems) or by rapidly frequency hopping.

There is no additional restriction on the output power for a telemetry signal compared to the 'command' signal - the max power is still 100mw EIRP, along as you meet the neccessary technical requirements. I can tell you with 100% certainity that the Hitec receivers Telemetry outputs at (almost) 100mW EIRP.

Video transmitters (on 2.4) are restricted to 10mW in the EU due to the fact that they operate at a 100% duty cycle.

How a system co-operates in the 2.4GHz band has changed recently. Previously the requirement was that "mechanism designed to facilitate spectrum sharing with other devices in a wireless network." and that was it. Which part of the problem was that it was completely open to interpretation. Lets give an example. So like you said, DSM picks a channel which was free at power on - however that may not be polite to all systems. As when that channel was free at the time, if the transmitter physically moves it may start interfering with another user - but DSM won't (and can't) change that channel. Now for the most part the bandwidth is pretty small, so most devices (which nowadays are pretty much all frequency hopping or very wide bandwidth) can work around this problem. However it could cause problems for analog video transmitters or other devices running on a fixed channel.

So in the latest standards which will become the currently used standards soon, there is a requirement for Listen Before Talk. Basically before the 2.4GHz device transmit a burst of data, it must check the channel to see if its clear (i.e. no-one transmitting). If there is, it won't transmit on that channel. This is to allow a greater co-operation between other 2.4GHz users.

However if you're transmitting below 10mW, you don't need these co-operation tactics - you can pretty transmit as much as you like - along as you keep within the power output (plus a few other technical requirements). This is why CE certified video transmitters (which are almost all analog) are limited to 10mW output power.

Si.

[Simon Chambers]

Posted by Christian Hollyer on 25/04/2013 18:29:30:

I wouldn't mind some advice here. Aurora 9 bound to a BODA receiver. Elevators driven by two channels. When functioning, after about 7 inputs to the elevator, they go there separate ways momentarily and the rudder (on the other stick and not being touched) deflects to the left momentarily. All this engine off.

Have tried: different tx, rx, power supplies. These functions are taking place at home ( I live in a block of flats). Could domestic wireless from other flats be causing this?

A couple of things.

1. Have you updated your receivers, transmitter and transmitter module to the latest version?
2. Have you accidentally activated scan mode when binding? This is pretty easy to do if you're not aware by pressing and holding the bind button on the module when not in bind mode. So if you momentarily release the bind button when powering on, it will start in normal transmit mode but then when the button is pushed back in for a couple of seconds - the module will switch into scan mode.
   Scan mode will retune the module so that it picks a bunch of new channels. Unfortunately and often this will pick some of the same channels as last time. So the receiver will still pick up on those channels but fail to pick any signal on others.
3. Make sure the transmitter isn't too close to the receiver. Try a minimum distance of 50cm (ideally 1-3m) and see if that fixes it.

Unlike 35MHz, the receiver won't pick up other 2.4GHz signals and interpret that as data to move the servos. All that will happen is that the receiver will reject that data. If it rejects enough data, it will enter failsafe (or servo position hold) - until it can pick a valid signal back up again.

Si.

[Christian Hollyer]

Thank you all. Have checked and tried all different hardware and precautions above with the exception of Simon's concerning binding. Will do that ASAP.