RF Guys - Interference Question - Page 1
Forum section: electronics / projects

author: funkyant
author_rank: Unknown
timestamp: June 17, 2015, 09:54:56 am
content: I have recently been getting into building Multirotor aircraft as a hobby, and have noticed that people on the RC forums seem to have concluded that having a high powered 5.8GHz transmitter for video and telemetry (eg. over 600mW) will cause interference of the 2.4GHz receiver for the control signal. The two are typically mounted around 10cm apart on the aircraft.I am thinking that surely the 2.4GHz receiver should have some kind of bandpass filtering which would prevent this?It just doesn't make sense to me that two completely different frequency bands could interfere with each other, regardless of power, unless maybe there was some kind of harmonic interference that is possible?And if the cross band interference is indeed possible, would using oppositely polarized antennas resolve this?My gut tells me that the RC community is misinformed about this, but I want to get some facts before I start suggesting this is a myth.

author: VK5RC
author_rank: Unknown
timestamp: June 17, 2015, 10:13:03 am
content: I haven't done the sums but the band pass receiver on 2.4GHz would need to be at least 70 +dB down at 5GHz, most cheap receivers aren't that good, the higher frequency would probably block the lower receiver. The higher transmitter could also emit some lower frequency spurii. (intermodulation products).

author: funkyant
author_rank: Unknown
timestamp: June 17, 2015, 10:18:45 am
content: I'm guessing the filtering would be done inside the receiver chip package? Time to "tear it apart"I guess I could just hook the PPM output of the receiver up to the scope and see if it is affected at all by the video Tx.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 17, 2015, 10:29:09 am
content: Keep in mind that any electronics can be influenced by any transmitter depending on the field strength, it does not necessary have to be the receiver (front end) that is having problems with the 5.8 GHz transmitter.

author: Psi
author_rank: Super Contributor
timestamp: June 17, 2015, 10:33:19 am
content: Sadly if you put enough power right next to a receiver you will swamp it.With exactly the same RC gear...on an aircraft (with VTX on one wing and control RX on the other around 50cm apart)vson a multirotor 10cm apartThe range difference is like 3+km for the plane and ~500m for the multirotor600mW really is plenty of TX power for a typical multirotor.Unless you have a huge 600cm hexcopter then yeah, you can put more power on if you need.For more range use directional antennas instead of more TX power, it works better.If you need omni-directional coverage use multiple direction antennas in a diversity setup

author: funkyant
author_rank: Unknown
timestamp: June 17, 2015, 10:38:48 am
content: Quote from: Psi on June 17, 2015, 10:33:19 amThat much of a difference!?OK, so now I'm thinking to use the carbon fiber body of the aircraft as an advantage would be a good idea, as it is already there and will help as shielding to isolate the two. 2.4GHz diversity antenna on the Rx under the aircraft, and a 5.8GHz circular polarised antenna for the Tx on the top. There would then be 3 plates of carbon fiber in between.Time for a rebuild!

author: Psi
author_rank: Super Contributor
timestamp: June 17, 2015, 10:41:47 am
content: 50cm was probably a bit conservative for the plane, the TX/RX are more like 90cm apart on my plane, which is what i was using for the comparison.But yeah, a small-medium size quad usually gets 500-1000m and the same gear on a plane usually gets well over 2kmHere's my carbon fiber quad.

author: funkyant
author_rank: Unknown
timestamp: June 17, 2015, 10:49:58 am
content: Nice one! I have a similar 250, and am in the process of building up this new frame:http://www.minihquad.com/frames/mini-spider-hex/blackout-mini-spider-hexAs somebody coming from the world of professional video, being an electronics enthusiast, and new to the hobby of multirotor flying, I was just a little disappointed at the quality of the analog video feed, after buying what is supposed to be the best kit out there in the hobby. I want to eek every bit of performance out of it that I canI have a 200mW transmitter, and have ordered a 600mW and a 1200mW to test, just because.Flying FPV through the Dubai desert is really, really fun. It's like an out of body experience!

author: KJDS
author_rank: Super Contributor
timestamp: June 17, 2015, 10:55:37 am
content: There are rarely band roofing filters on this sort of set up. It would make sense to put a narrowband notch to reduce the 5.8GHz signal on the 2.4GHz receiver. It should be possible just to use an open circuit stub that's a quarter wavelength long at 5.8GHz. The other important thing is antenna separation.

author: funkyant
author_rank: Unknown
timestamp: June 17, 2015, 10:58:23 am
content: Quote from: KJDS on June 17, 2015, 10:55:37 amI think this is my first point to address.

author: ConKbot
author_rank: Super Contributor
timestamp: June 17, 2015, 12:53:00 pm
content: Even if there was a bandpass filter, chances are it doesn't attenuate enough. The stop band of filters that I see typically from minicircuits and the like is only -25 to -40db. With 30db of isolation between antennas that is 70db down. That makes for about -42dbm of your video signal in your control rx. Not great. Might not be enough to totally overdrive the front end, but will certainly desensitize it some. That and remember that cheap flexible coax will be leaky compared to a semiflexible coax and another source of rf bleeding into the rx.

author: richard.cs
author_rank: Super Contributor
timestamp: June 17, 2015, 03:17:54 pm
content: Orthogonal polarisation will help but you'll only gain 10dB or so. I'd suggest the following:0) Have the minimum video TX power you can get away with.1) Antennas as far apart as possible, orthogonal polarisation, basically just try to minimise the coupling between them.2) If you can get away with it (doesn't make the video drop out at inconvenient times then try to create a null in the pattern of the TX antenna that lines up with your RX antenna. Look at "field patterns" for typical antenna designs and arrays on google image search. This is really just minimising coupling again.3) Add filters, start by adding them to your TX side, but you can add them to both if you need to. Any cabling on the non-antenna side of the filter needs to be good quality shielded coax with proper connectors. I'm assuming that you'll buy in conectorised filter lumps from somewhere like minicircuits but you can make your own or use PCB mount filters if you know what you're doing and that would be lighter.4) Finally, after you've tried everything else, wind up the power on your control transmitter.

author: dom0
author_rank: Super Contributor
timestamp: June 17, 2015, 04:19:26 pm
content: Quote from: richard.cs on June 17, 2015, 03:17:54 pmAt least technically it should be possible to actually have a control loop here: The video RX monitors the quality level of the video feed and adjusts (over the control channel) the video TX power. This would allow for minimal video TX power, if implemented properly.

author: eneuro
author_rank: Super Contributor
timestamp: June 17, 2015, 07:18:25 pm
content: Quote from: Psi on June 17, 2015, 10:41:47 amHolly crap-I can't understad how those XXXcopters are used by paparazzi, while they must be heard hundreds of meters away from someones houseStar Wars era will begin soon, and those XXXcopters will be... shot from the sky using laser guns for sure, but it is hopeless most of them haven't got any parachute and can fall from the sky like meteor since they have no wings

author: funkyant
author_rank: Unknown
timestamp: June 17, 2015, 07:22:45 pm
content: Quote from: richard.cs on June 17, 2015, 03:17:54 pmActually, I want the reverse of this. Currently I have no issues with the control, and a lot of issues with video signal strength. Lots of static, dropouts and artifacting - which is equally as bad as control loss when you have VR goggles on and no line of site to the aircraft.

author: KJDS
author_rank: Super Contributor
timestamp: June 17, 2015, 07:25:06 pm
content: Quote from: funkyant on June 17, 2015, 07:22:45 pmCan you use a bigger aerial to receive the video?

author: funkyant
author_rank: Unknown
timestamp: June 17, 2015, 07:26:51 pm
content: Yes, that would be possible. I could just run some coax out to something on a tripod

author: Psi
author_rank: Super Contributor
timestamp: June 20, 2015, 09:03:43 am
content: Did some test flights 2 weeks ago testing some directional 5.8ghz patch antennasUsed a 10mW video transmitter on plane and 14db patch ant on the ground, it got 1km without much probs.You don't need massive power. Just a good setup.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: June 20, 2015, 11:18:31 am
content: but surely you cant use strongly directional antennae on a quadcopter to base station.

author: Psi
author_rank: Super Contributor
timestamp: June 20, 2015, 11:57:13 am
content: Youcan...you can put a antenna tracker on the quadcopter that tracks home position, But yeah, its not the best idea, very few people do that.A directional antenna can go on the TX or the RX, so if you use them you have one on your ground video RX and/or ground control TX.You can do diversity with the RX so many directional antennas are combined to give 360deg high gain coverage.But obviously you can't do TX diversity unless every TX is on a different channel and the quadcopter has a receiver for each as well.So antenna trackers are a better option.

author: hendorog
author_rank: Super Contributor
timestamp: June 20, 2015, 11:16:28 pm
content: QuoteSounds like you would benefit from diversity on the video receive side. Pretty simple and cheap to do.

author: G7PSK
author_rank: Super Contributor
timestamp: June 21, 2015, 08:05:43 am
content: Very basic but are all the control and transmission circuits completely screened along with all the wires.I have seen breakthrough problems on repeaters that were caused by a 6mm hole in a casing looking straight at a computer turn the box around and the problem went.