Becker 6201 interference

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Becker 6201 interference

Postby Alan Hall » Fri Jun 01, 2018 9:10 am

We recently replaced the Becker 4201 radio in our Stemme 10 motorglider with a Becker 6201.This is a direct plug-compatible swap. We now have dreadful interference which I’m 99% certain is due to radiated ignition noise (it sounds right and disappears when the antenna is unplugged). The previous radio was not entirely silent but the difference is chalk and cheese. We now struggle to hear a major ground station at 10 miles over the noise.

The engine is a Limbach with single Slick 4330 magneto which has just been serviced (at the same time as the radio swap unfortunately). The HT harness, which is unchanged, at first sight appears to use unshielded automotive cable but is in fact Skytronics Aero-Light” as sold fitted to Slick mag caps by e.g. LAS. This is shielded and bonded to the mag cap, but the shield terminates just before the plug cap, which is an NGK resistive unit.

It is so drastic I had expected to find a simple and severe fault and I’ve tried all the usual things (being an electronics bod) but am unable to significantly improve matters. I’d be grateful for any pointers but in particular, to establish some sort of reference, I’d like to know what is considered “normal” for this type of harness. Surely there must be many people flying with similar harnesses or even ones with no shielding at all? Does this usually work OK? Should I always expect some interference without a fully shielded system? Has anybody swapped to a Becker 6201 and had similar problems?

Several people have suggested replacing the harness with a fully shielded one and I suspect this will “fix” it but I would prefer to avoid the expense and moreover I’d like to understand what’s going on.

TIA
Alan
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Re: Becker 6201 interference

Postby coxargus » Fri Jun 01, 2018 5:35 pm

You may get some ideas by trawling through this-

http://www.supercub.org/forum/showthrea ... t=ignition

Frank cox
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Re: Becker 6201 interference

Postby tnowak » Mon Jun 04, 2018 9:06 am

Alan,

I am not familiar with the Limbach engine but my first thought are that unless you have a fully screened harness (from mag to plug body) you will never get totally rid of mag interference.
If you can't fit screened plugs then a few "low cost" things to try:
1. Is you antenna cable a high quality coax? Some provide better screening than others.
2. Are you mag switch leads screened?
3. Is your radio coax cable routed away from the mag switch harness?
4. Check that the ground plane your antenna is attached to is "good".

I presume the radio inteference stops completely when you do a dead cut mag test?

Tony
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Re: Becker 6201 interference

Postby Alan Hall » Thu Jun 07, 2018 10:02 pm

Thank you Frank and Tony.

I think we have looked at all the issues you suggested. I maybe wasn’t clear that this is a motorglider and indeed there is no noise with the engine stopped!

The antenna is embedded in the fin and I believe is an end-fed sleeve dipole. I have checked it with a SWR/VNA. In the Stemme the engine is behind the cockpit, below the wing centre section, so unfortunately the antenna is probably no further from the engine than in a conventional configuration. Being mainly GRP, although with a tubular steel engine subframe, there is no screening as you might get with a metal engine cowl. Also, the coax inevitably passes close by the engine on its way from cockpit to tail. I suspect none of these factors are crucial, as the interference can also be heard on a nearby handheld.

In the meantime I have had a chance to listen to the radio in a Grob109, recently professionally refitted with a Funke 8.33 radio, and now suffering noise for the first time. It has the same plug leads as I do, and also a Limbach engine / Slick mag.

I have also had further correspondence from other people who tell me they have had an identical experience, and that the problem was fixed by going to fully screened leads and plugs, so I am rather reluctantly coming to the conclusion that may be the pragmatic solution.

However the engineer in me still wants to understand what’s going on here. There does seem to be a common theme of changing to a new 8.33 radio and suffering ignition noise from part shielded leads which were previously satisfactory. I would have thought that if anything the narrower bandwidth would reduce broadband noise. One suggestion, that the new radios are more sensitive, doesn’t convince me as the noise is present on top of strong local stations i.e. it’s the noise that has gone up not the radio sensitivity.

These partially-shielded leads are an openly-sold stock product that must presumably be used by a significant number of people (although I have yet to find any)! My RV6 (with perfect radio) has identical leads, although on a Pmag ignition. I’m sure I have seen plenty of simple VW engine aircraft at fly-ins with either part-shielded leads like mine, or even car leads. Do they not have interference problems, or do they just not have radios?

I have written to Becker, Limbach and Stemme and await replies.

Alan (hoping to hear from SOMEBODY who has this arrangement working)!
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Re: Becker 6201 interference

Postby tnowak » Fri Jun 08, 2018 7:47 am

Hi Alan,

A few extra things you could try, but I still think you will need to go with screened plugs and leads.
1. Does interference stay at the same level if you check with your your old radio?
2. If you have access to a 50 ohm dummy load disconnect your antenna coax and connect the dummy load. Will indicate that, hopefully all received ignition noise is coming in via the antenna. No interference should be present.
3. Borrow (or make) a temporary VHF antenna and use good quality coax to see if moving the antenna around the aircraft makes any difference to received noise levels. Could indicate replacing the coax will help.
4. For ground running only, is there any way to temporarily screen your plugs with tin foil and foil grounded to an appropriate ground point?

I can't think of any reason why a new 8.33 radio shoud be more susceptable to interference than a 25Khz unless the receiver is much more sensitive. Do you have receiver specs. for old and new to compare?

Many years ago my A65 engine used to have one Slick and one unscreened Eisman mag. I could never get rid of ignition interference until I swapped it for a new Slick and went with a shielded harness. Noise went away.
Good luck.
Tony
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Re: Becker 6201 interference

Postby mcfadyeanda » Sat Jun 16, 2018 12:23 pm

Is the mag switch body earthed?
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