Am I right that this is a very short length of coax in a vehicle, and
the VSWR will be low due to the ATU at the feedpoint, and that the ATU
ground / coax braid will be connected to the vehicle metal framework?
So why use thick coax? Why the choke?

If you need a choke, you could get some cores that are a neat fit over
RG58, at 10MHz appropriate type 31 cores has a Z of 115/inch. That
doesn't address the issue of whether the power / control connection
needs protection.

Actually, I am not sure why you are doing this anyway, is it just in
case or is there a demonstrated need?

Owen


Well, to be honest, I was primarily planning on using the choke for a
base station application.... will have some sort of balanced dipole or
loop fed by an SG-237 tuner. I wanted the choke for the coax feeding the
tuner.

On the RV, I have a 23' whip on the back (ladder) which I had planned
on feeding with another SG-237. ( Currently it is fed with a home-brew
base loading coil assy, on which I must manually change frequency taps.)
In either case, I thought it might be beneficial to put a common mode
choke on the RG8X feeding that antenna, too, since I do have RF causing
various issues inside the coach while I am transmitting.

On your RV grounding question, yes, the antenna feedpoint and all
related ground areas are very well bonded to the main vehicle chassis
with rather large tinned braid, (2" wide).

Ed

On 03 Sep 2007 03:26:50 GMT, Ed G
wrote:

loop fed by an SG-237 tuner. I wanted the choke for the coax feeding the
tuner.

Hi Ed,

You will have to choke the control lines just as much.

I do have RF causing
various issues inside the coach while I am transmitting.

That could be as easily through the air, or "ground currents" in the
chassis. All points of the chassis, doors, hood, etc. are not usually
bonded very well. Even the exhaust system can be "floating" to
provide a path from ground, right up into the engine compartment.

On your RV grounding question, yes, the antenna feedpoint and all
related ground areas are very well bonded to the main vehicle chassis
with rather large tinned braid, (2" wide).

Re my paragraph above, does this include doors? A lot may be taken
for granted and hinged points are sometimes insulated, allowing large
panels (capacitive plates) to couple RF inside what is considered to
be an otherwise tight box.

73's
Richard Clark, KB7QHC

Richard Clark wrote in
:

On 03 Sep 2007 03:26:50 GMT, Ed G
wrote:

loop fed by an SG-237 tuner. I wanted the choke for the coax feeding
the tuner.

Hi Ed,

You will have to choke the control lines just as much.

I do have RF causing
various issues inside the coach while I am transmitting.

That could be as easily through the air, or "ground currents" in the
chassis. All points of the chassis, doors, hood, etc. are not usually
bonded very well. Even the exhaust system can be "floating" to
provide a path from ground, right up into the engine compartment.

On your RV grounding question, yes, the antenna feedpoint and
all
related ground areas are very well bonded to the main vehicle chassis
with rather large tinned braid, (2" wide).

Re my paragraph above, does this include doors? A lot may be taken
for granted and hinged points are sometimes insulated, allowing large
panels (capacitive plates) to couple RF inside what is considered to
be an otherwise tight box.

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.

Owen

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