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On 05/06/2014 12:50 PM, Troy Maynor wrote:
> <wingnut54@charter.net>
>
> I tried all the things suggested except the SWR check...
Well this is important. While you can get away for a light bulb with
just checking for proper connection and shorts, for RF things are more
complicated and not measurable with standard multimeters. You can't
measure moisture that has creeped into the coax spongy insulation, but
it would ruin the SWR and cause the coax to act as an antenna rather
than just a transporter. The same with kinks or other damages. Or
metal objects that are just a bit too close to the antenna and causing
the antenna to refuse to radiate and instead bouncing the energy back
into the coax cable.
Bottomline is that your problem looks like an SWR problem and if you
don't measure this, you won't get nearer to a solution.
> I am planning to replace the RG58 with RG400 from the radio rack to
> the Bob Archer antenna on the port bulkhead aft.
RG400 is a waste of money so is the Bob Archer antenna. Just stick to
the RG58 and a standard dipole antenna. Fancy cables are intended for
losless transmission of frequencies in the GHz range over longer
distances, neither property is valid for your aircraft. For aircraft
purposes, nothing beats the good old dipole antenna (unless there is
not enough space for it but luckily in our Europa's a dipole fits just
fine in the sternpost and this is also the maximum distance away from
our sensitive equipment in the cockpit). No antenna can produce "gain"
without taking energy away from certain directions. But in your
airplane you don't want to favour certain bearings at the cost of
other bearings so a dipole is the ultimate radiator for your ship.
Think of a radiator as a light bulb. Sure there are lenses and
reflectors and you can produce "gain" with it, but is this really what
you want in an airplane?
> So my questions now are: 1. Should I put ferrite torroids on both
> my electric attitude ind. and turn coord. power wires?
Probably not. These devices were designed for use in an airplane, and
virtually every airplane has a radio. It is unlikely the manufacturer
of your attitude indicator has "forgotten" to incorporate ferrite
torroids in their cables or even specs. If your system works as
intended, torroids are not needed anywhere else than on the end of
your coax cable. (They were included in the Europa radio plan to adapt
the asymmetric coax cable to the symmetric dipole).
> 2. Should I run a separate ground wire from the radio to the
> battery itself.
No. With a properly working antenna no RF is transported than via the
coax cable. The ground wire will only transport DC. But of course, if
your antenna (or the cable connecting it to your radio) is
malfunctioning the RF energy is desparate to find a way out and will
radiate over the ground wire, causing all kinds of interesting effects
in adjacent equipment which is now fed by RF rather than by DC.
> 3. The Sigtronics intercom wires weren't shielded. Do they need to
> be wrapped up in some kind of shield foil or braid?
Consult their manual. These things are designed for use in an airplane
with radio's. If the manufacturer doesn't specify this, it should not
be needed in a properly working system.
> I am also trying to find a new 90 degree coax fitting that fits the
> ICOM rack.
This rings bells. Why do you need a new one? Or did you bend the coax
instead? You are probably not aware, but the distance between the
center of the coax and the outer shield is a crucial property of the
cable. Bend the cable and the center conductor will get closer to the
outer shield, ruining the impedance of the cable and causing all sorts
of problems.
One other test you can do: obtain a dummy load. This is just a fairly
big resistor of 50 ohms (It needs to be manufactured for RF use, other
resistors are just a wire coiled up but this is not suitable for RF).
It converts the radio energy into heat instead of radiation. Connect
it directly to the radio and transmit. Most likely the problem is
gone. Connect it to your coax cable instead of the antenna and test
again. Is the problem still gone? Then your antenna is the problem.
Does the problem reoccur? Then the cable is faulty.
Succes with troubleshooting!
Frans
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