Kitfox List <kitfox@lists.csn.net>
/Two questions occur to me.
/I have heard that length of coax is critical
/to performance but when I mention this to techs they scoff
/and brush me off with the comment that this is old data
/and no longer applies?!
Actually, it NEVER applied . . . at least not for any
good reason. Coax cable does have some losses that get
worse as you go up in frequency . . . DME, Transponder,
and GPS antenna coaxes should be MINIMIZED but no
antenna coax requires OPTOMIZATION for length.
/Also, which antenna presents the least drag while still giving
/good performance.
The differences in drag for the different brands and styles
of antenna for the same task are calculable but hardly
an issue for anyone except folk who are trying to win races.
Performance of all antennas for an application is about the
same too . . . as long as they don't screw up the design
with too many "matching" devices that tend to make VSWR
look really good and efficiency look really bad.
/Is it worthwhile to replace coax when replacing radios?
If the coax is more than 10 years old or has been subject
to hyrocarbon contamination (fuel, hydraulic fluid, oil,
lots of guckem from exhaust gasses leaking through
belly skins, etc.) it's probably allright.
/How do you determine if your existing coax is up to snuff?
VERY good question and the answer is IT AIN'T EASY. It's
unfortunate that many of the contaminants that cause a
coax feedline to degrade in terms of losses from one end
to the other will make the coax look BETTER when you
measure VSWR. The best rule of thumb is if you're doing
other work in the area, the coax is ten years old and
replacing it isn't a really big deal, why not? Otherwise
it takes some pretty sophisticated test equipment that
very vew avionics shops have.
Regards,
Bob . . .
AeroElectric Connection
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