I seem to have these problems pretty well solved. Initially tried shielded
cables
(shield grounded to A/C ground point near inst panel - unconnected at other
end) with lots of toroids to the ELT and Magnetometer. Also made the
interconnect
between the Primary Flight Display and the MFD a shielded cable. Had little
or no effect. Then tried just shielded cable (no toroids) to ELT and wrapped
the entire ELT in aluminum foil. Foil is connected to the cable shield and
ground
is as before. That solved the ELT problem. From other forums I learned that
there are D Sub filters which have capacitors built in bypassing each connection
to the shell of the connector. Acquired Spectrum Control Series 100 filters,
5600 pf (from Newark Electronics) for the Magnetometer and the PFD (main
display). At the Magnetometer I connected the cable shield to the D Sub shell
since the shell is floating inside the Mag and I needed a return for the filter.
Found the PFD filter didn't have any effect but the Mag helped some. Finally
wrapped the Mag in aluminum foil which is connected to the shield and that
seemed to solve the problems. I think there is so much RF from the com antenna
and no shielding because of the composite, that it overwhelms the ELT and
Magnetometer.
I also found a local ham with an antenna analyzer (MFJ Model 269) and had a look
at my com antenna. Found it had very low SWR but the resistance of the antenna
changed if I walked near the antenna and the resistance was not constant or
nearly so across the 118 to 136 Mhz range. We both suspected the toroid balun
just didn't perform properly. So we decided to make a balun from coax as the
ham radio books have shown for years. It is very much like the one on the Aero
Electric website except the length of the balun section is an electrical half
wavelength, so you must take into account the velocity factor of the cable.
Using
RG400 (vf=.695), the balun section is 16 1/4" long. Took off the toroids
and installed the new balun. Had to trim the antenna to get the SWR low near the
center of the band, but now the SWR is less than 2:1 across the entire band,
the resistance stays constant and walking near the antenna makes no difference.
Hope this helps someone else.
Jim Butcher
Europa N241BW
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