GA/Homebuilt. RF interference and wiring

[blue up]

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Well, I've tried the internet and various homebuilder books but am still not 100% convinced of the correct methods for wiring a comms system into a wooden airframe.

The comms box (VALCOM 760 channel, tray mounted) works just fine until the fuel pump comes on. Now, I screwed up first time round by not using shielded Tefzel wiring since the original glider fitment used automotive grade PVC wires. I also didn't have a separate bus bar for the comms.
I now have a stock of shielded wire but am a little uncertain as to which wires need shielding and which ends need to be earthed.

1. Mic and phones. I believe that they need to be shieded and then only 'grounded' through the shielding on the 'radio' end. Is that right?

2. PTT. Some websites suggest the PTT also needs the same treatment as above whilst others say not. Which is it?

3. Can such wires be grounded to the chassis of the radio tray?

4. I use the ubiquitous Facet "Cube shape" pump which has a metal casing and unshielded wires. Is it worth trying a shield made from braid and slipping it over the wires? If so, do I ground it at one or at two ends?

5. I use a VW conversion with dual electronic ignition (Leburg) that uses carbon leads. Should they also be shielded or is the 10K ohms enough to dampen out the RFI?

Lastly, is there anone in the UK that I could call upon (e-mail or phone) with any other avionics questions?

Thanks, guys.

Rob ( and G-BMMF)

Light Aircraft Association : G-BMMF (http://www.laa-archive.org.uk/results.php?q=G-BMMF&fields=reg&page=0&sort=reg_a_z&ft=&limit=10)

[john_tullamarine]

Moved here as the answers will more likely be expansive.

[blue up]

Thanks, John.:O

[muduckace]

Wow, do not know the standard practices for a wooden airframe and grounding. Does it even have or need static wicks (not much of a GA guy).

It would not be a bad idea to shield any wire that carries a signal, also do your best to seperate wires that carry current from those that are signal carriers.

I suppose grounding shields to the rack would be a good idea but what is the rack grounded to?

On the aircraft I am familiar with grnd is a big conductive alluminum tube and wings with static dischargers that play a big part in keeping radio transmissions clear.

Ok, for wooden airframes all grounds are wired to the engine. I would ground to the tray and have a decent bonding strap going from the tray to the engine.

[AvionicsGuy]

ive worked on composite aircraft, and its standard in my case to see i ground bus some where under the inst panel, with a dirty big wire going to the battery negative.

make sure you only ground your shields at 1 end, (are you useing twisted pairs, if so that will also help). also see if you can bond the rack too ground.

also make sure you intercom/ audio wiring is shielded,

would also look into static discharge... dont know if it would be a problem, but some things i imagine would build up static? and i wouldnt no how to stop it on a wood A/C

[Hasherucf]

Interesting question. Seen many wooden gliders and none have static wicks .

[happybiker]

Hi Blue up.

As well as ensuring that the sheilding to the mic and tel wires are properly grounded (one end only) ensure that the co-ax cable to the aerial is properly terminated at both ends. The wires for the radio should be routed away from any wires and components that may radiate interference, fuel pump etc. Also the aerial will need an effective ground plane to maximise its gain and provide a strong signal to the receiver.

[Bus429]

Does a two-wire systems require fuses in both lines? Seen radio problems on a Spitfire (metal airctaft) where relatively modern avionics gear (KI 201?) showed FSD constantly due blown negative fuse on the radio bus (suppose ground seeking through meter coil).

[piggybank]

Not sure if it will be what you are looking for, but if you go to casa.gov.au
you can find a PDF file on aircraft bonding. Its about 6 MB

[Saintsman]

Have you thought of fitting a supressor to the fuel pump?

As has been mentioned, make sure that the radio wires are isolated from the other wires. ie in a different loom run alll together.

You also need to check equipment is earthed / bonded. Just because they are connected it does not mean that there is an electrical path. I've seen equipment that have been treated or painted so that you cannot get an electrical path. I've even seen earths fitted to plastic boxes!

[ionagh]

You have answered your own question. The interference is only there when the fuel pump isn running. The Facet has a DC motor pump and this is causing the RF noise. IIRC the cube is an automotive product and has no EMI filtering.

At a rough guess a 470nF non polarised capacitor of at least 50Volts across the power feed to the pump (as close to the pump as possible) would make a significant improvement.

For general reading: http://www.fightingrobots.co.uk/documents/emi.pdf

[SeldomFixit]

Would about half a pico farad cap do the job?

[blue up]

I've decided to ditch the pump completely. There is enough gravity flow to supply my needs and have got a ram-air system to assist in keeping pressure above the fuel in the tank.
Another owner has suggested 1uf capacitor over the pump if I decided to keep it.

Thanks for the help, guys.

http://i82.photobucket.com/albums/j279/foggythomas/P1000542.jpg