Why does my barometer swing when I transmit?

[blueflyer]

I have a Dynon D100 and a Microair radio. When I transmit on my radio, my barometer increases almost 3", and then goes back to normal after my transmission.

I am guessing that I have a radio wire too close to the remote compass wire. Would you guys agree with that assessment, or should I look for another reason for this issue?

 

[Dynon]

Are you saying the actual baro setting changes? Or altitude goes up? Or do you mean the MAP sensor reading?

 

[blueflyer]

yes, the actual baro setting changes. Example: if barometer is set to 29.92 inches, and then I transmit on the radio, it increases up to about 32.50 inches. Then, when I let go of the push to talk button, the barometer setting goes back to 29.92. I did not happen to notice if the altitude changed also.

 

[Dynon]

Looking into it...

 

[Dynon]

Any chance you can take a video of the behavior?

 

[blueflyer]

yes, I can do that. I'll post it on youtube and put a link to the video in my next post.

 

[Dynon]

And to clarify, is this an EFIS-D100, or a SkyView D1000?

 

[blueflyer]

EFIS D100

 

[blueflyer]

here is the youtube link:

http://www.youtube.com/watch?v=siDqHQcn-20&feature=youtu.be

 

[Dynon]

So first of all, the baro is the indication below the altitude tape. What you're there is your manifold pressure reading from the engine monitoring side of things (you have RPM/MAP configured as your left info item widget.

So on the EMS page, does anything else move when you PTT? Or just MAP?

Is the antenna bundled or passing by the EMS sensor wires by chance? As a first step, if you can easily separate them a bit, that may change things.

 

[blueflyer]

sorry about that. I will have to go back tomorrow and look at the D-120 to see if anything is moving on that screen.

I do not think the radio antenna coax travels by the EMS sensor wires, but I will verify that tomorrow and post what I find.

 

[blueflyer]

The D-120 also swings the MAP reading when transmitting. I ran the radio antenna coax straight up from the floor to the back of the radio (unbundled from other wires). It did improve the MAP swing, down to only a 0.3" swing now.

All the D-120 wires are on the pax side of the plane except for the DSAB wires that connect the D-100.

The remote compass wires are near the radio antenna coax.

 

[Dynon]

One thing to check is what kind of coax your are using. Something like RG-58 doesn't work as well at preventing RF from leaking out as something that has a double braided shield like RG-400. If you switches your antenna cable to something better shielded, that could help.

Another solution is to keep the antenna cable as far away from the EMS wires as possible. Where they do get close or need to cross, have them cross at right angles. RF couples over when wires are parallel, especially when they are nearby.

You could also try using a shielded wire for the MAP EMS wire.

Is you routing the ground wire from the MAP sensor from the sensor to right near the EMS? If you have connected the ground somewhere else on the airframe, the current draw from the radio could be causing the EMS and the sensor's ground to be at different voltages, which would result in different readings. But since moving the antenna coax changes the behavior, this explanation is less likely.

 

[blueflyer]

OK, thanks. I will try these solutions to get my MAP to not swing. thanks again.