Fred Klein wrote:
> I find in the Garmin Installation Manual for their 320A transponder a
> max. length for the RG-400 coax to be 8.8 feet, 12.5 feet for RG304 (a
> coax which I've yet to see for sale anywhere, and 17.0 feet for "ECS
> Type 311601 sold by Electronic Cable Specialists in Wisconsin (again, a
> product which I haven't seen in any of our wiring discussions). This may
> be a moot point as I expect to use the Narco 165 (Value Series, to save
> a few bucks [unless the consensus might be that doing so would be
> pennywise/pound foolish].)
This is one of the reasons why I choosed a transponder where the actual
transmitter and the panel part are separate. There is just a very thin
and light weight data cable in between; I will mount the antenna
directly onto the transmitter part, which will be located somewhere in
the tail section. Another reason for this is of course that mode-S is
mandatory here.
You might consider doing the same, especially if mode S requirements are
foreseen in the near future. It saves you some headaches now and maybe
in the future if mode S is coming your way ultimately.
If you choose to go for the coax routing, keep in mind that the 8.8 foot
is an arbitrairy value. You won't notice much difference between 8.5
foot of cable or 9 foot of cable. The whole point is that the high
frequencies attenuate very fast in coax cable, so the shorter the
better. In fact, a less optimal antenna location but with a shorter
cable is preferable over a better antenna location but with a much
longer cable. All this has nothing to do with the transponder
manufacturer but with the signals used by the transponder, and is equal
for Garmins and Narco's.
Thicker, heavier cables have less attenuation than standard size coax,
so these bulky cables can be used for longer distances. However, more
length of a cable which is also heavier by itself, comes with a
substantial weight penalty, and these cables are difficult to route.
Bends need a large radius; a little bit to tight and the attenuation
comes up again spoiling the advantages of the expensive heavy cable. Oh,
and they often forget to tell you, but these cables "age" and losses
build up year after year. A tiny amount of moisture in the cable kills
the signals. All with all, I think I wouldn't go for low loss cables.
An option would be to mount the transponder antenna under the radiator
cowling. There is already metal there that can double duty as a ground
plane, and as a shield/reflector, so the signals go to the ground
instead of to your head and nearby avionics. ;-) And this allows for a
very short cable run.
And of course, it all depends about how much you care about transponder
performance. ;-)
--
Frans Veldman
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