MGL Avionics User Forum

Hi,

I purchased a Kolb Mark III with a MGL V6 radio on it. The previous owner never got to make it transmit, and after I brought it to the local Avionics shop, they recommended to change the antenna. Now, with a CI-122 antenna and a metallic base plate of 12" x 27", I can hear the transmissions very well on the airport pattern even when the airplane is 7 miles away, folded and tucked in inside a cargo trailer. And airplanes can hear me too, Yay!

However, this only works when only the radio is on.

As soon as I turn on the EIS 2000, or the tailBeaconX/AV-30 MFD combo I installed, disruptive electric noise comes into the headset that prevents me from listening to anything at all, let alone talk.

It sounds like a bad case of Ground Loop, and to that effect I ensured that the positive and negative cables to the radio are independent, even connecting them directly to the battery just to make sure. Still, the noise comes up when any other device is on.

I followed the recommendation from https://homebuilthelp.com/Tip_of_Week/Tip_81.html on Ground Loops to no avail.

Does this sound like I still need to chase bad connectivity that causes a Ground Loop condition? Or this is configuration on the MGL device that I've yet to do right?

BTW, taking the radio to the Avionics shop for a complete resolution will mean to wait 4-6 months and $$$, so I'd like to troubleshoot and fix this as much as I can.

Any advice will be very appreciated.

Respectfully,
-Guillermo

Your radio is likely innocent here. It's doing its job - receiving whatever signal it gets on it's antenna port.

If the noise goes away when you disconnect the antenna it confirms it. You need to eliminate the noise at the source and also ensure that your antenna cable is the correct type (RG58 or compatible) and most importantly that your antenna is tuned properly. You mention a base plate. That needs to be a minimum of 1/4 wavelength - thats some two feet in each direction from the base of the antenna. Normally not an issue if your aircraft has a metal skin. If not - you need to mimic the antenna balance using wires (4 wires of 1/4 wavelength or longer from base of antenna pointing roughly at 20 to 50 degrees downwards (possibly following inside curvature of a non metal fuselage) tends to do the trick - but use a SWR meter to confirm.

Having a well tuned antenna ensures that the antenna cable does not play antenna for you - and that cable goes likely close to contaminated cables (like power supplies or grounds to other digital equipment).

NEVER bundle your antenna cable with anything else unless you know it's "dead". It might look nice but causes trouble.