MGL Avionics User Forum

Having issues with RF from iEFIS display interfering with ATC on frequencies around 125.5 and 132MHz. Apparently this has been a known issue for years based on these videos on YouTube showing the exact same RF noise at similar frequencies. What’s the fix?

https://www.youtube.com/watch?v=kg8bZ5wZ3bk

https://www.youtube.com/watch?v=8HS3BxZQLdg

Way past the point of super frustrated with beta testing a system that has been in the market for years…

The issue is not your EFIS - the issue is your RF installation is not up to scratch. If you have a properly tuned antenna (correct ground plane is super critical here) and the required distance (FCC requires 2 meters from any person) to the EFIS panel then there will be no nolse.
The problem is that when your antenna is not playing well - you are actually using your antenna cable as antenna - the inner core of the cable simply couples to the outside shield (it's a capacitor). That then acts as open ended or shorted wire (as it is not correctly terminated at the antenna).

The radiated noise comes almost exclusively from the LCD display and is caused by the way these displays work - you have a multitude of "antennas" on the glass surface - metalized very thin electrodes with very fast digital signals. These radiate mostly a magnetic field - easy to pickup if you have a handheld radio as the coiled antennas used here are very good at receiving magnetic fields. Magnetic field strength however dissipates very fast with distance - usually two meters or so is enough to get it it drop to very low levels.

There is one catch - depending on your installation it is possible for the magnetic field to couple onto nearly wires or even metal structures if they have suitable dimensions. In bad cases you can route the tiny amount of energy onto the entire airframe and then that acts as giant transmit antenna. There are many ways to cause this - the all time favorite is to use your airframe as convenient ground for equipment. Never do this on an aircraft - your airframe must be grounded on one point only so no current can flow - and that ground point must be "silent" - usually the battery negative terminal is a good location. Be aware of vampire grounds - with many radios often the antenna base is connected to the airframe and the radios antenna shield is actually connected to power supply ground (usually the case of the radio). If you have that the best way to solve this issue is to route the antenna cable next to conductive airframe members/skin so there is no physical space between cable and the airframe (we want to avoid creating a loop antenna as that loves magnetic fields and in order to create a loop we need physical space between the antenna cable and airframe).

Lastly - you can use ferrites (the clip-on variety works nice for this). Identify the cables (if any) that contain the offending signals - listen to the noise while moving / clamping your hand tightly around the cables at various places to see if you can create a difference in noise pattern or volume - if yes you found a culprit. Have a close look at the routing of the wire and if that cannot be changed use a ferrite. Note that the position of the ferrite is critical as you can actually worsen the problem if you place it in a location where you create tuned antennas (the ferrite blocks RF from traveling on the wire so RF thinks that is the end of the wire). Usually the ferrite should be placed close to the interference source.

Noise from displays is related to both refresh rates of the displays and size (we go as low as we can on refresh rates and on some EFIS systems you actually have a menu setting to influence this). The size of the display plays an obvious role - smaller displays tend to radiate a lot less.