Borders Gliding Club

JAD's radio reception has been poor for a while and I spent some time this week trying to diagnose the cause.

Two issues had been reported, an excessive level of interference, and poor sensitivity unless the transmitting radio was close by. Some pilots have reported distorted or unintelligible reception, but it is difficult to be sure this is actually a fault in JAD's radio rather than transmission problems with tug radios, for example. Also, the launchpoint handheld radios have lower transmission power than the airborne radios and are prone to picking up wind noise.

The excessive interference was the most obvious and repeatable symptom. The interference is nearly all coming from the Flarm unit, conducting along its power wires and coupling into the radio's antenna cable. The power cables and the antenna cable all run down the channel on the left, below the canopies. The data cables for a Flarm display and a vario also run down the same channel, there are a lot of cables tightly bunched which is a good recipe for coupling RF interference between them. Unfortunately, it is difficult to see an alternative path for some of the cables.

I removed the Flarm and examined it at home. I found no obvious failure but made a small modification to (hopefully) reduce the interference.

Reinstalling it today, the radio's performance does seem improved. With squelch set to 3, there was no interference and I could hear transmissions clearly from the launch point and from aircraft in circuit. Please try it and let me know how it is in the air.

I have some ideas how to improve it further, if necessary, and I would be happy to explain in more detail what I did and how I came to conclude the Flarm is the problem.

Finally, a note about squelch setting. The squelch level sets a threshold for the minimum received signal level that will wake up the radio. Setting the squelch to a higher number makes the radio less sensitive, both to interference and to radio calls you want to hear. The best setting is the lowest number that just avoids interference. In other words, turn the squelch down until interference starts to be a problem, then turn it up one level. Hopefully, squelch 3 should be OK now but, if not, try squelch 4.

Let me know how you get on with it.

And if you read this far, well done!

Barry L

Barry,

I am no expert in these matters, but, had similar problems with reception/transmission in my aircraft, that was until Roger completely re-wired the whole system with proper, white, aircraft grade wiring (which I am guessing is shielded) following a new instrument panel fitting by him. It (the aircraft) had poorer quality wiring in previous to this work being done. Would an in line suppressor on the flarm circuit help?

Trevor D.

Hi Trevor,

Thanks for your interest and comments.

There are already 2 ferrite suppressors fitted to the Flarm power feed. To get a bit more technical, the interference is being generated by the switching power converter in the Flarm. I noticed that the +ve power wire seemed to be 'hotter' (i.e. more RF voltage on it) than the -ve, which implies that the RF interference is differential between the power wires rather than common mode. Clip-on ferrites are only effective against common mode RF and even then the results are usually disappointing because of the high impedance of the coupling path.

I soldered a ceramic chip capacitor directly on to the pads where the power wires land on the PCB to reduce or eliminate the differential voltage, which seems on the ground to have been useful.

Roger's panel wiring is indeed a joy to behold, but the white aircraft grade wires are not generally shielded except for microphone leads etc. Electrically, there is no functional difference to ordinary PVC sheathed wiring, just the white wires withstand higher temperatures and don't give off noxious fumes when they get hot.

Barry

Barry,

Thanks for your comprehensive reply, some of which is definitely beyond me!! Though I have gained some knowledge from it - thank you.

I also agree with you, Roger's work is indeed first class and always very neatly undertaken.

Hope you get the solution for the problem soon.

There were several flights in JAD on Sunday 12th. The radio worked fine all day, Stuart receiving Tx readability 5 from the launch point handheld when over 4,000 ft.
Squelch is still set to 3, which is acceptable, although it would be nice to tame the interference completely and wind the squelch back to 2, which is the minimum. (Squelch 1 is open, i.e. no squelch.)

As before, let me know of any further problems or observations.

Oh, and I repaired the broken mic gooseneck in the rear cockpit. Please try not to pull the mic to your face, rather move your face nearer the mic.

Barry