REFLECTOR: Intercom noise

Thu Jan 6 15:03:57 CST 2011

Brian is right on. That's the reason I bought a spool of wire just for my sensors. The wire comes with 3 sets of twisted/shielded pairs (6 wires total) them all 3 sets are shielded. All are very high quality wire that is teflex insulated. That keeps the cross section of the wire much smaller than cheaper wire. 

I haven't noticed any interference problems... yet. 

Kurt 

-----Original Message-----
From: Brian Michalk <michalk at awpi.com>
To: sbjknox <sbjknox at earthlink.net>; Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
Sent: Thu, Jan 6, 2011 12:14 pm
Subject: Re: REFLECTOR: Intercom noise

I forgot to ask about your spark plug wire routes. 

Since the automotive wires are not shielded, one should keep sensor wires away from them, and avoid running them through current loops. 

A current loop is the area bounded by current running in a wire. Picture a piece of lamp cord, the kind that's easy to unzip into two single wires. Imagine connecting two leads at one end to a noisy source like a badly oscillating DC/DC converter, and twist the two leads at the other end together. 
If one were to try to sense the magnetic field generated, it would be very small. The current loop area is the distance between the two wires. Now, unzip the cord so that it's a big circle. The current loop is much larger, and the magnetic fields do not cancel each other out as they did before. 

On the engine, the spark current goes through a wire to the plug, across the spark gap, into the engine block, to the other spark plug(from the same coil pack, since it's wasted spark), across that spark gap, then through the plug wire, eventually returning to the coil pack. Picture this as a large current loop. Try to keep the plug wires as close to each other as possible, as well as close to the engine block. Also try not to run any sensor wires through this loop, or near the plug wires. 

But, as you mentioned, you disconnected the EI, so this shouldn't be your problem. 

This is the reason why I twist all of my high current circuits in the ducts. The EI power is twisted, making current loops as small as possible. Shielded wire only protects against capacitive coupling, which is rarely a problem with low frequency, low impedance circuits like these. 

On 1/6/2011 11:56 AM, sbjknox wrote: 
> I have been following this thread and also am interested to know what > you find. 
> 
>> On 01/05/2011 06:30 PM, aminetech at bluefrog.com wrote: 
>> I'm still trying to eliminate noise on my intercom. It occurs when >> the engine is running, 
>> even while starting. 
> 
> As I understand the current situation based on the discussions between > you and Brian, the noise is *not* caused by: 
> 1) Radiation or conduction from the primary side of either the mag or > the EI. 
> > I've tried turning off the mag, and the EI. I even disconnected the > power to the EI. 
> > I disconnected the ground and P lead to the mag. 
> 2) Alternator 
> > I removed the alternator belt. 
> 
> That does not leave much... What remains is: 
> 1) Secondary ignition (high voltage to plugs). When you turn off one > of the ignitions to listen to the other, does the characteristic of > the noise change (ie., does one sound different than the other)? If > the sound is different, then perhaps there is a problem with plug > wires (?) 
> 2) Some other source... something else running that is generating noise. 
> Does the frequency of the noise follow engine rpm? 
> 
>> Should I consider a mag filter? (links to two filters, one for >> P-lead, one for power bus). 
> 
> Since you have disconnected the P-lead at the mag for no improvement, > the P-lead filter would probably do nothing for you. The power line > filter 'might' help if there is something on the power bus that is > generating the noise. On the other hand, if there is something (#2 > above), it might make more sense to fix the noise source. 
> 
> How confident are you that the intercom wiring is correct... proper > wire type (shielded) and shields connected where they should be, etc... 
> 
> One thing you said "It occurs when the engine is running, even while > starting." that seems odd to me. I think it is bad practice to have > the avionics powered-up while cranking the engine. I'm not suggesting > that this is related to your intercom noise... just a side observation. 
> 
> Please keep us informed of your efforts. 
> 
> Sid Knox 
> Oklahoma 
> 
> Velocity 173 RG N199RS 
> Starduster N666SK 
> KR2 N24TC 
> W7QJQ 
> 
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