Transceiver Interference

[barrieslinger]

After a short first flight with my PAW, we got very bad  interference over the radio(Set to 122.6) at a level that could not be squelched out. Interference was in the form of regular pulses. Intensity appeared to vary, there was a lot of radio traffic and sensitivity of radio changed if it was receiving a carrier prior to someone talking but in quiet periods noise pulses were regular.
To confirm that it was the PAW we removed power from it and all was good, added power and the interference return when the PAW had booted. All worked fine as just an ADS-B receiver, there is no ARF connected to the PAW. We are powering from one of the recommended USB battery. I suspect the interference is coming from wifi. Are there any suggestions on how this interference can be corrected?
Thanks
Barrie

[scsirob]

Wifi operates on very different frequencies (~2400 MHz and ~5000MHz), it will not easily interfere with our radios at 118-136MHz.

What will interfere is the Raspberry Pi computer itself. Bare computer boards are notorious for causing interference spikes at random frequencies. Even a change in software can make the spike disappear.

There are a few things you can do to suppress this interference
- Use a grounded all-metal housing for the RPi.
- Use shielded cables in and out of the RPi.
- Add ferrite RF suppressors on cables in and out of the RPi.
- Create distance between the RPi and the COM antenna.

[JCurtis]

After a short first flight with my PAW, we got very bad  interference over the radio(Set to 122.6) at a level that could not be squelched out. Interference was in the form of regular pulses. Intensity appeared to vary, there was a lot of radio traffic and sensitivity of radio changed if it was receiving a carrier prior to someone talking but in quiet periods noise pulses were regular.
To confirm that it was the PAW we removed power from it and all was good, added power and the interference return when the PAW had booted. All worked fine as just an ADS-B receiver, there is no ARF connected to the PAW. We are powering from one of the recommended USB battery. I suspect the interference is coming from wifi. Are there any suggestions on how this interference can be corrected?
Thanks
Barrie

Can you try plugging something else into the battery pack so see if it's the PAW or the power unit?

Some battery packs use boost/buck converters to adjust the LiPo power from the battery (normally around 4v) up to the 5v needed, some of those are pretty effective noise generators. 

[Admin]

Hi Barrie,

Its interesting you mention 'regular pulses', I had seen this previously with an ARF, but you also mention there is no ARF attached.
What is the period of the 'regular pulses' ?

I have not see any intereference issues without an ARF attached, and even with the ARF moving a short distance from the transceiver, semed to cure the problem.

Thx
Lee

[rollingcircle]

After a short first flight with my PAW, we got very bad  interference over the radio(Set to 122.6)

Was this at White Waltham? - if so, was the interference observed on the ground or when airborne - I've noticed some wierd interference of the type you mention in the Yak at certain parts of the airfield, nothing to do with a PAW.  Very odd, and quite localised (it could of course be some bizarre new Russian electrical quirk).

[barrieslinger]

Hi SCS, JC, Lee and RC

Will try and plug alternative load into battery.
Distance RPi to com aerial about 3 ft.
Will time pulses when I visit airfield tomorrow.
Will also borrow hand held transceiver to check out interference.
Plane was from Sherburn flying near Selby.
Ferrite beads seem a useful thing to have so will get some.
Will not attempt metal package until I have ARF in final form.
Thank you all

Barrie

[SteveHutt]

For anyone looking to develop a metal housing for the PAW, this may be of interest......

https://www.youtube.com/watch?v=-DmaHUBhR_w&feature=youtu.be

http://skaarhoj.com/free-stuff/schaeffer-scripts/



https://www.youtube.com/watch?v=Xnk0urpyfpg - this video shows cut-outs and pcb mounting.

I have used Front Panel Express and Schaeffer for a number of 2D panels (e.g. switch and circuit breaker panels for my a/c instrument panel) and very good they are too.

I have not used this new Skaarhoj scripting for enclosures / housings, but it looks pretty comprehensive.

Steve

[rg]

It doesn't necessarily need to be metal to shield the noise.

[scsirob]

Well, plastic ain't gonna stop much RF at 122MHz. You could use copper paint on the inside perhaps, but for a little more money you can have a very, very nice full metal housing. Fits like a glove and looks really professional.
http://lukse.lt/uzrasai/2014-10-raspberry-pi-b-case-is-now-available/

It does take a bit of modification if you plan to use an ARF shield (old or new alike).

[Andy Fell]

The RPis can emit quite a lot of EMC, so this doesn't surprise me.

Best approach would be to put it all in a grounded metal box and use ferrites on the power supply lines to the RPi, as suggested above.

[The Westmorland Flyer]

Questions that do not appear to have been asked: Does the interference only appear on 122.6MHz? Does the nature of the interference change/disappear on adjacent/distant channels? If it is RF-borne interference then a bit of tuning around can reveal much about the interference source and how (indeed if) it is getting into your radio.

Other questions that might help to pin down what is causing the problem include:

* Are you sure that the interference was being picked up as RF by the radio and not by induction into audio lines? That can happen very easily with unshielded audio lines close to noise sources in a small cockpit.

* What was the nature of the interference? Regular pulses (how frequent?) or random crud?

I spend a lot of my time as an air traffic engineer trying to track down EMC issues like these and they can often be quite intractable. The two starter questions are always "what exactly is generating the interference signal" and "where is it getting into my equipment". The answers are seldom simple! Often a new EMC problem turns out to be an issue with the equipment suffering the interference rather than the putative source of the interference.

[barrieslinger]

It has taken a while to get back in the Yorkshire air.
 Been for a 1.5 hr flight with much to my surprise no sign of interference.
Relieved but baffled.
Squelch setting on radio is rather critical in clearing noise but when optimized system is fairly silent.
When troubled by noise previously the RX indicator on the screen of the Garmin 530 flickered with the noise.
Sorry I am not adding much to the information pool.
Will investigate further.

[biggles1998]

not an answer to the root cause of interefernce but I did see some postings the other day where people have created some cavity filters for ADSB, and have given the PCB files, I was thinking of printing some as they are only PCB layered and I expect pretty small put this in line to the antenna and RT dongle..

https://www.reddit.com/r/RTLSDR/comments/1ce9p8/filter_for_adsb_reception/

[scsirob]

Don't overlook the material specs. "The material needs to be 2 sided 62 mil FR4". If you use anything else, the filter will resonate on a different frequency.

[DaveStyles]

interestingly, at the bottom of the comments there's an explanation that they are going for ranges far in excess of what PilotAware needs.

>100 miles is common without a filter, (far in excess of what is useful to us) it looks like they are seeing how far they can get out to the theoretical 480 mile edge.

I'm not sure we need such a filter for the operation of PilotAware. ;-)