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Posted on behalf of Wclarkstill@aol.com:
Guys - I used to be a ham radio operator (K4BMG) working 2 & 6 meters
(above and below the standard aero VHF band) and have been worried by the
wing tip VOR antenna housing arrangement. Generally I've not heard many
problems on LML but maybe few people have actually measured the
sensitivity of their setups - unless there were obvious problems like the
one you describe.
I'm not familiar with the standard wing tip antenna situation but the
winglet one bothers me. In particular, the winglet VOR antennas are nicely
housed in fiberglass on one side but very close to a conductive carbon
surface on the other. Ideally you'd want a conductive ground plane
perpendicular to the main axis of the antenna or 180 degrees away from it
(to make a simple dipole antenna) - but to have it (the carbon top
surface of the winglet) very near the antenna itself would likely degrade
sensitivity (and also likely give the antenna undesirable directional
sensitivity properties). I've been thinking about replacing part of
the upper winglet surface with fiberglass to minimize such antenna shielding.
A couple of tests to see if the environment inside your wingtip is a
problem for your antenna (as previously suggested, I'd first switch to
RG400 & check all connections by careful inspection and for electrical
continuity from the antenna itself to the inner pin of the BNC connector
going into your NAV receiver)
1) Connect up your NAV receiver to your COM antenna (the one in the vert
stab or a maybe a standard belly antenna - ones that are not largely
surrounded by carbon). You will presumably get much better sensitivity and
know your NAV receiver is ok and that the place you've chosen to do your
test doesn't have some problem (like a hangar between you and the ILS
antenna).
2) If possible, remove the winglet (or tip) and attach a simple 17" long
copper wire to the inner conductor of the NAV coax at the end of your wing
in place of the wingtip VOR antenna. Ideally support the wire out and up
away from the wing but prob not necessary. That wire will act as simple
antenna minus all the things in your wingtip or winglet that may be
degrading performance. If this improves your signal strength, then you
need to work on getting conductive material (wiring, fuel vent lines,
carbon fiber) away from the antenna itself.
Please let me know how this works out. Would be interested to know what
sort of experiments on VOR antenna sensitivity have been carried out -
would be really great to arrange a direct comparison of VOR sensitivity
with glass vs the standard carbon winglets.
What does Lancair Avionics have to say about this?
..Clark Still (20% IVP)
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LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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