Ed G wrote in
. 192.196:



Richard, getting clear details is like drawing teeth. I suspect the
RV might be fibreglass, and so about as RF transparent as you might
get. Further, something hints to me that the grounded ATU is on top
of the ladder, so it isn't really grounded.

But in the absense of clear details of a single scenario, I can't
help.


In my previous experience in newsgroups, I'll agree that getting
details from those who pose questions here is difficult, and I guess
I'm as guilty as the rest. However, my original question was
regarding details of common mode choke filter design and has
apparently morphed into my RV setup.

The RV is a 31' class C, all fiberglass shell, welded aluminum
frame, on a Ford E450 chassis and cab up front. Although I realize
there are many aspects that could cause RFI in the cab area of the
motorhome, I merely wanted to use a choke filter to see what
improvement might take place.

The SG-237 tuner does not use a control line, so no filtering
needed
here. The DC will be pulled off the back of the motorhome close by,
and that source, as I previously mentioned, will also be routed
through a common mode filter.

The antenna feedpoint / loaded base area is about three feet down
from
the top of the metal ladder. The metal ladder vertical rails is one
piece construction, and is bonded at bottom of both rails with 2"
tinned copper braid to the main chassis rails with pieces running
about 22" length. All in all, about as good a ground as can be
obtained under the circumstance.

Ed,

Some thoughts...

In the broad context of a sparse bonded metal frame work, and with your
somewhat elevated feedpoint (though not fully described), it would be no
surprise if most parts of the structure weren't at elevated RF voltages
(as per Richard's summary of the antenna system).

The first steps to a solution might be to try to minimise the potential
differences between the radio interfaces, the nearby structure, and the
operator. I would be trying to bring the feed line down the ladder, along
the chassis, and up the framework to the radio, and to bond it to the
conducting structure at 1, 2, or 3 points along the way to try to ensure
that the radio and operator are at similar potential to the adjacent
conducting structure.

The effect of a choke is almost the opposite, it is to facilitate
potential difference across the choke.

I am not saying that there might not be a need for chokes in the
solution, but just chucking chokes at it until it works doesn't seem to
work systematically through identifying the problem and then designing
solutions to the problem.

Many modern radios are particularly sensitive to RF on the DC and
microphone interfaces. If you split the radio to use a remote head, that
exposes another couple of interfaces to RF.

The loss in 10m of RG58C/U at 14MHz with a load end VSWR of 1.5 is around
0.6dB. Why do you need RG8X, it must be much harder to deal with in a
vehicle than RG58?

Owen