> aol.com!JamesCone@matronics.com wrote:
>
> I have all of my antennas mounted in the rear of my fuselage. VOR/GS, LORAN,
> TRANSPONDER, and COM. My question is, can all of the coax cables be bundled
> to get up to the radios in the cockpit or should they be separated. I'm
> fairly sure that the passive coax cables that come from the VOR/GS and the
> LORAN can be run together but am not sure about the coax cables that transmit
> a signal like the COM and TRANSPONDER. Thanks in advance for a response.
>
>> Roy answers:
>>Coaxial cable consists of a center concudtor which carries the signal and a
braided
>>outer conductor (shield) which is chasis-grounded. This makes coax very
>>resistant
to
>>radiating or receiving electromagnetic energy. You should not have a problem
with
>>"cross-talk" between your coax, especially considering the very low power
>>levels
and
>>frequencies that the avionics transmits.
>>Most important to a good installation is that you don't over-bend, kink, or
smash the
>>coax. This will change the properties of the coax by changing the
>>characteristic
>>impedence (Zo) at the damaged spot which will degrade the performance due to
>>reflections, and that you ensure your coax connectors are correctly built for
the same
>>reason.
>>Roy
>>RV-8 #80096 and x-Navy avionics tech/instructor of advanced electronics.
I agree and would add the following observations: A number of builders
have experienced interaction between items of avioinics that transmit
(com, transponder, dme) and things that receive (com, vor) and indicate
(electronic instrumentation of all types). An almost STANDARD fix offered
by manufacturers of instruments is to replace all antenna feedlines with
double-shielded coax.
In my experience, when coax cables "talk" to each other and to other
systems,
the problem lies with installation. My favorite connector (UG-88/U BNC and
its brothers and sisters) can be VERY frustrating for first time installers.
It's not uncommon for the outer conductor to be improperly captured by the
clamps causing the coax to become just another piece of wire as opposed to
the carefully controlled transmission line. Many people have experienced
successful elimination of interaction with double-shielded coax but I
suspect that the change had more to do with better installation of
connectors
than from change of wire.
Ordinary, garden variety RG-58 coax has been used in tens of thousands of
airplanes for over 50 years. When and if undesireable interaction is
discovered, check the installation before jerking out the wire bundles.
One very common interaction in composite airplanse happens when the (1) comm
antenna is mounted too close to the panel and/or (2) installed over an
inadequate
ground plane. I this case, (1) DIRECT radiation of susceptable systems
occurs
right from the antenna -or- (2) the SWR is so high on the coax feedline that
its shielding qualities are negated.
Regards,
Bob . . .
AeroElectric Connection
////
(o o)
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