MGL Avionics User Forum

Hello,
I have Xtreme+SP6+SP7+RDAC ULP+RDAC XF MAP+MX1
When switching on all, except the MX1 it's working fine.
When I switch on the MX1 it interferes with the radio. The MX1 is working ok and the radio (Trig) is completely seperated from the MGL's.
Only a "noise" on the radio. All data ok.
What can be the cause and what can I do about it?

Marco

It is menu dependend. When in some menu's the interference is gone.

It's an old topic, almost as old as us making EFIS systems.

What you are experiencing is the EFIS display transmitting signals at your tuned VHF frequency. This is normal and caused by harmonics of the many fast digital signal edges - the display itself has a large number of invisible conductors on the surface which act as antennas.
The field created is virtually only magnetic (rather than the electrostatic field of a real antenna) and typically has a range of about two meters (if you use a handheld radio to receive it - these radios tend to have wound antennas to make them shorter and can pickup magnetic fields easier.

Normally this is not an issue - your VHF antenna is likely more than two meters away (or close to that) and tends to be mounted in a position or surface where it is shielded from the various digital systems in your aircraft that can radiate.

These signals can couple onto airframe members or aircraft wiring or be conducted via ground and power or other wiring around your aircraft - if any of this happens to find a resonant wire or length of metal it can start radiating a much larger distance.

Also make very sure your radios antenna installation is actually correct - we have seen many times that an incorrectly installed antenna causes the actual cable to be the antenna - and that of course tends to go pretty close to anything that can radiate. It is important that the cable finds a good impedance match in the antenna - and that means a correctly dimensioned and functional ground plane. Only then will the cable be "dead" and not react to external fields.

Usually you can find radiating cables by a process of elimination - simply remove them one by one temporarily (except power of course). Include the GPS antenna cable here as well - that can also radiate.

Make sure your antenna cable runs on its own and is not bundled together with other cables - might look nice but it asks for trouble. On metal skin aircraft your antenna is grounded at the same point your cable is - at the base of the antenna - make sure that has a good electrical contact. Be suspect of using sealants here.
This arrangement forms a ground loop - that is a very good way of both receiving as well as transmitting a magnetic field. To prevent that make sure you route the antenna cable tightly against the skin of the aircraft - you want to avoid creating a physical space between cable shield and aircraft skin that will create the ground loop. This is the worst case - here you use the entire aircraft as antenna and even the tiniest of signals will become impossible to suppress.

Ferrites made for the VHF band (100Mhz and more) can be used to stop or dampen interfering signals travelling on cables. Use a clamp on style ferrite to find out where this is effective (you can perhaps replace with a more effective barrel type later). Ferrites are not usually needed but sometimes wiring or other space constraints make use of these unavoidable. Install these always close to the source of interference (short cable length to the source).

These kind of interference problems are usually easily fixed if you go about it methodically - this also works for other kinds of interference such as from GPS receivers or anything with an LCD display, processor, strobe lights etc. In general, the larger the LCD display the more signal is emitted. Color displays tend to radiate more than monochrome displays.

Hi,
are you foound a solution for your interfere problem? I have exactly the same with my voyager. the problem appear specificaly for one frequence and seem mitigate when you move away from this frequencie!
Regards
André

Hi,
allow me to revive you. did you find a solution? I'm wondering if I shouldn't completely change my EFIS (which works very well apart from this interference problem).
regards
André