I’ve seen lots of really poor 8.33Khz radio installations where the new radio is positively worse than the older legacy sets. Given I am taking out my avionics loom and making a new one, it is timely for me to ask what makes a crisp clear jet-like radio installation? I would assume:
In practical terms what else is there that makes an aircraft have really crisp clear transmissions and good reception? I was looking at a picture of a loom on Instagram and they had a grounding wire on the hood of each connector also. What else makes it really good in the air?
Good grounding.
All audio cables screened strictly in accordance with the manufacturers instructions.
Headset sockets isolated from airframe ground and not using the screen of a multi-conductor cable as the earth return.
Firstly, it’s likely some radios are better than others. There have been countless reports of the old King KX radios being much clearer than the ones in the GNS GTN or IFD navigators. Perhaps not surprising when you open up a KX165A and see how much stuff is crammed inside, and what trouble they went to to shield one section from another. It could of course be bad installers; it seems clear that general expertise is constantly heading downhill, especially in Europe, as the customer base is shrinking, and ex air force technicians who make up the bulk of the European avionics business (some of whom were good but an equal number were not) retire.
Use shielded cable for practically everything except (a) power and (b) what in avionics is called “discretes” which are basically status signals, but even many of those are from high impedance sources and if you run one next to an unshielded strobe wire you may get some “fun” because it will pick up the muck and deliver it somewhere else…
Shielding sources of radiation is very important. The reason we often hear the whistling of somebody’s strobes when they transmit is because their rental has the strobes done with unshielded wire which radiates the muck from the inverter all around the aircraft. Socata did that well: all strobe wiring is shielded, plus they run the cables in the cavity along the trailing edge of the wing
Signal grounds have to go to the airframe somewhere. Then you get the debate whether connecting both ends of a shielded cable to the airframe is ok. Generally it is desirable to do this if the shield is just a shield and not acting as a return for the signals – because the shield works better if there is a connection to a good AC ground at both ends.
The mistake is when people ground both ends of a shield which is used as a return for a low level signal. For example one installer I know well ended up with a problem which he could not solve, where using the cockpit loudspeaker started a nasty feedback loop. It turned out he used the airframe as the return for the speaker, and the current involved (perhaps a few hundred mA at audio frequencies) was creating a significant voltage drop which was obviously adding itself onto all the other signals which used the airframe as the ground. Wiring up the headphone sockets and either (a) using the airframe as the return or (b) using the cable shield as the return and grounding both ends of it, would produce a bad result, but that’s true for a lot of other signals.
I’d say the very best practice is to connect the shield to the airframe at both ends (and nowhere else) and use the two conductors inside as the signal and the return.
Most metal planes use the airframe as the return for the starter motor which puts a few hundred amps and a lot of muck through the airframe, but it’s doing that only briefly.
RF stuff is another thing and needs different practices. Use really good quality coax. Many previous threads e.g. search for RG400. Bad coax radiates all over the place. Even when this gets picked up on signals which are well “out of band” on VHF (e.g. audio) it can still affect the circuitry. An op-amp which does absolutely nothing useful at 120MHz will still become near-useless when it gets a bit of 120MHz superimposed on the incoming audio. RG400 is expensive at about £5/m and most installers don’t want to use it; I have had to free issue it on every occassion because they simply don’t use it; they use some cheap stuff. One old post is here. Socata, despite their generally good practices, dropped the ball badly in this area and it took a lot of sorting out to stop GPS interference – here.
Antenna installations need to be done really well. Often they aren’t especially on composite surfaces. I had some fun here. A bad antenna job will make a radio work badly, possibly appearing to have a very short range.
Peter wrote:
Bad coax radiates all over the place
Good coax will also radiate if the antenna install is not done correctly, as the outer coax shield will act as part of the antenna and can bring RF back into the cabin.
Indeed, a search like this digs up some good reading material.
Whether a bad antenna installation produces poor quality audio is interesting. It certainly could, due to 2nd order (amplifier out of band overload) effects. But the main thing you get is short radio range.
You also get short range with static wicks not working; that’s another big thing and a huge problem on “plastic” planes. The static building up on the airframe and shedding itself via discharges messes up radio wuite well.
Peter wrote:
Whether a bad antenna installation produces poor quality audio is interesting
If RF is coming back down the coax shield, it can do.
Given we’re using AM, and AM is stupidly easy to demodulate (basically, any semiconductor junction can act as an AM radio receiver if the field strength is strong enough), if you get strong RF in the cabin and it gets into the audio inputs, you can get feedback which sounds pretty much exactly like putting a microphone too close to a speaker.
When I first owned the Auster we had this problem, and you could tell it was to do with RF getting radiated in the cabin because the feedback could still occur if you plugged a handheld radio with a headset adapter into the coax (due to no panel space, our radio is in the cabin roof and the connectors are easy to get to).
Thanks for all the useful replies, I’ve noted the main points of each post and will check them off the tech who is making my harness and then with the guys installing it.
Best thing to do to all the audio wiring, including wiring going to the jack plugs is to wrap the harnesses in insulating shield, like EMI (electromagnetic interference) tape. I had an issue with 2 Tecnams which had the wiring passing close to a power cable and by doing this, it solved all background noise coming from the alternator.
The rest of the power,data wiring can stay out of the insulating shield
That would be an impressive solution indeed!
Presumably you would connect the tape to the airframe.
Some of those tapes are quite heavy.
