On Sun, 02 Sep 2007 00:54:23 -0500, Cecil Moore wrote:
Ed G wrote:
I am having a heck of a time locating a source of good information on
how to make an effective common mode filter using the snap-on type ferrite
beads such as available at RAdio Shack.

Someone needs to characterize those beads.
What is the one-turn choking impedance at 10 MHz?

I'm sure it varies from week-to-week -- depending on which communist
chinese Piece-of-Crap factory it comes from and how high the dog dung
component was in the mix for the day.

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

Ed G wrote:

...
Ed

Clamp 'em on, recheck match/swr/your-chosen-methods-of-tuning (or, with
the gods--for that matter) to antenna ...

It can't explode; well, I don't think it can!

Since the power flow is on the center conductor and the inside shield of
the braid, any impedance of unwanted currents on the outside would,
logically, be of some benefit ...

If it ain't enough, buy more or change to a different type of material.
Knowing the expected values of components, and benefits to expect,
would only put a lot of icing on that cake ...

Regards,
JS

John Smith wrote:
Ed G wrote:

...
Ed

Clamp 'em on, recheck match/swr/your-chosen-methods-of-tuning (or, with
the gods--for that matter) to antenna ...
...

But then, you already guessed that--and what to expect here!

Regards,
JS

On 03 Sep 2007 17:06:26 GMT, Ed G
wrote:

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.

Hi Ed,

Point in fact is that the "antenna" is actually a tuned radial, and it
is your "ground" that is the majority radiator.

73's
Richard Clark, KB7QHC

Ed G wrote:

...
Ed

Oh yeah, Richard is quite correct there--don't choke off the
counterpoise--but, I bet you already suspected that ... and, don't use
the coax as a counterpoise! Well, unless you design for that ... but
then, don't run the counterpoise in the inside of the vehicle ... well,
your vehicle is fiberglass, kinda hard to hide from the
antenna/counterpoise, but again, you already knew that ...

Regards,
JS

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

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.


Actually, that is step one, which is on the project table.
Currently, when operating HF fixed portable, I run the coax out the
coach, on the ground, and back up to the feed point. My plan is to
route it neatly from the radio location, down to the chassis, along the
chassis, out to the ladder and to the feedpoint. I suppose that, alone,
could help in my RF issues. They are not serious issues, but are
annoying at times.

Perhaps I'll reserve my ferrite split beads for a choke filter on my
fixed station antenna when I get to that.



Ed