possible emi from p leads

jimgreen · #1 06-28-2010, 10:05 PM

Due to really bad planning my mag switch and EI ignition switch end fairly close to my headset jacks. The p leads (shielded) will pass about 1 1/2" from the shielded mic. and audio wires. They will cross at right angles.
Will this cause a problem?

Brantel · #2 06-29-2010, 05:32 AM

Most likely not if they are just crossing over at 90?...

elippse · #3 06-29-2010, 03:16 PM

Here's a consideration on "P"-leads. This lead and its ground return are electrically connected across the points. At high manifold pressure, the voltage across the points can be over 100V peak with very high frequency components due to its fast rise time. A shielded wire is electrically a capacitor, so any pulse is coupled from the center conductor to the shield. The current resulting from this coupling must return to the magneto only by way of the shield. If the shield is connected to the aircraft metal at the ignition switch, some current will return via the shield but some will also return through the airframe to the engine and the magneto. A current on this path, especially with its very high frequency components, can get into the radio through conduction on the radio ground as well as radiated energy into the antenna. A "P"-lead should always be in a shield, the shield connected only at the magneto, and never have a so-called filter installed in the "P"-lead.

jimgreen · #4 06-30-2010, 12:02 AM

Thanks for the replies.
My understanding of electricity is pretty superficial. I take your point about only grounding the shield at the mag. If the two wires are at 90 degrees will there be a pulse induced in the headset wire?

On a related theme, I'm trying to find an acceptable location in my panel for a whisky compass. My preferred spot is above my SL40, but I thought it would get too much influence from the radio. Sitting right on top of the radio there was about a 15 deg. swing during transmission. If I sat it on a piece of 1/2" plywood the swing was reduced to 2-3 degrees. Pretty surprising.
So now I'm wondering if, in the p lead case, 1 1/2 or 2" separation might be enough. I know I'm comparing apples to pomegranates.

Daniel Hooper · #5 06-30-2010, 02:22 AM

Yes, it will induce voltage in the audio circuit, but it's doubtful you would ever notice without an oscilloscope, given what you described.

elippse · #6 06-30-2010, 08:09 PM

Quote:

Originally Posted by jimgreen

Thanks for the replies.
My understanding of electricity is pretty superficial. I take your point about only grounding the shield at the mag. If the two wires are at 90 degrees will there be a pulse induced in the headset wire?

On a related theme, I'm trying to find an acceptable location in my panel for a whisky compass. My preferred spot is above my SL40, but I thought it would get too much influence from the radio. Sitting right on top of the radio there was about a 15 deg. swing during transmission. If I sat it on a piece of 1/2" plywood the swing was reduced to 2-3 degrees. Pretty surprising.
So now I'm wondering if, in the p lead case, 1 1/2 or 2" separation might be enough. I know I'm comparing apples to pomegranates.

Usually it is best to keep high-level signals separated from low-level signals, even with 90 deg crossing.
I, too, had a problem with my compass from the large ground return lead running across the bottom of my instrument panel, even though it was at least 7 " or 8" below the compass. I had to do the crossing on the floor to keep the interference to a minimum.