RF feedback in the intercom system

[cobra]

EMI suppressor for VHF frequencies

After months of problems with my radio and intercom I traced the problem to the following artical by MGL Avioncs - thanks guys

http://www.mglavionics.co.za/Docs/EMI%2 ... encies.pdf

RF feedback in the intercom system may result in distorted transmissions, echo (bathroom sound), chirping (birdlike sounds) and annoying, loud tones during transmission.

[ZULU1]

Well done Siasd

You are correct in installing these and they work well, a good idea is to loop them through the radio feed cable "twice" so if installing cut the radio to intercom cable as short as possible after you have looped the cable thro it. Otherwise the cable will act as a large antenna !!

Trick no 2 on all applications is to face the antenna leaning forward, on the Aquilla it is very simple as you turn the antenna base so the actual antenna may be adjusted leaning forward. This reduces reflected RF.

Trick No3 with the Icom ICA5/23, 6/24 and Vertex Standard Pilot3 is to reduce the Side tone to maybe 2 or 3; with the older intercoms this creates a feedback "loop" and is not an rf whistle but more an audio feedback.

With the ICA 6/24 they have been giving problems on some systems, even when used with the seperate headset adaptor. The 4 wire pin configuration of the external connection is as follows:

Ground
Audio in
PTT
Mic line; Voice one way (AC) and 4,9 Vdc to feed the mic element.

This is the cause in this radio of the "feedback loop", to prevent this and we build a specially designed chatterbox wired as follows.
Isolate the Radio mic supply and feed the mic from a seperate battery, reducing the voltage to a low 1,5 volts. In the process disconnect the audio line from the radio and derive the Side Tone function from the Intercom.

The radio ICA6/24 does work exceptionally well though.

I have traded a lot of old "Brick type" units in which I may modify in the new year to the above configuration for those that still wish to use them.

Demon has some clip on ferrite beads available also which always help. I also stock them. I think Rainiers would be a better price as mine came in two weeks ago with the "new" undesirable exchange rate !!

Hope this helps.

eish ZULU1

[cobra]

Thanks ZULU1 for the advise much appreciated :D , must say the radio was driving me crazy good comms is a must.

[Junkie]

Bloody GPS (196) is interfering with my Radio again (Yaesu) - never had an iota of shit with the GPS3

It used to whine on 124.8 and a few others, except 131.1 when plugged into external power source, no problem when GPS running only on battery power - I installed a separate Power source from batterey (Pos & Neg) to GPs and Voila sorted - Happy Camper.. until...

I started having difficulties with GPS reception and decided to install the remote Aerial for the 196 - put on back of trike above motor (on the Gemini's fairing) so it was far away from RT antenna.. and now the bloody whine is back and on - you guessed it - 131.1 !! but not on any other frequencies and this on External or Battery to GPS
)(&*&(*^*&^986 pissed off !!

[ZULU1]

Hiya Junkie

I have heard from customers experiencing interference with Buzzcom systems and GPS 296 devices but have personally not witnessed this. I can only suggest that a RF bead in the radio intercom cable will fix this.

The Buzzcom units have a very long radio to intercom cable, although shielded its not desirable as they behave as an antenna.

Maybe try purchasing a RF bead from Rainier as you are in the Cape, dissect the intercom box and cut the radio-intercom cable as short as possible, then loop the cable through the bead as close as possible to the intercom box. This should work. If not then you may have to go to the the headsets as it may be coming in there...unsure of your intercom but a 470pf cap across the mic also fixes some rf ingress on the Buzzcom. I have even inserted beads in the headset cable inside the cups, this does also help. Unsure how close the GPS antenna is to your radio antenna ??

Our Chatterboxes have never had a problem as we mount on the boards small rf beads which "fixes" it and the area between the mic amplifier and the output is SMD this also helps.

Unfortunately RF suppression is almost a Black art, I have had years in industry playing with welding suppression and this is even worse.

Some of the new GPS devices are causing havoc in some installations.

This solution should work, even reducing the cable length will help. Our new units have wireless PTTs which eliminates even more cables.

If you dont come right with Rainier, I can supply you, I have also sent some up to Aviation Engines yesterday.

I can see that these problems are not going to improve as Pilots require better toys and more electronic information systems, so just got to find solutions !! And fly faster !!

eish zulu1

[Junkie]

Thanks ZULU1 -going to to try this - will let u know when done