Howdy Tom

The two short pieces of coax with center conductor connected to either
side
of the ladder line is perhaps the best solution. It was covered in QST/The
Doctor is In, June 2008. In the article it suggests using the best quality
coax on hand, keeping the run to the absolute minimum and the coax shields
should be tired at both ends and connected to the station RF earth. This
configuration introduces a small section 100ohm nominal impedance
transmission line into the antenna system which should have minimal
losses,
but most important is that it is still part of the balance transmission
line. The losses for say RG213 at 4mtr would be almost nothing on 80mtr
and
perhaps up to 2db on 10mtr.
Obviously the lengths of coax need to be exactly the same length.

If you would like a copy of the article drop me email.

Peter VK6YSF

http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm

Hi, Peter.
I read the QST article and thought it was crazy and so I tried
replacing the 5 ft length of ladder line from my antenna tuner up the
wall to the two feed through insulators going through the wall to the
600 ohm feed line outside.

With the 5 ft ladder line I could tune the antenna, a 160 meter lazy
quad loop, to all bands through 15 meters. With the 5 ft section of
double coax, just like the doctor said, the antenna would not tune for
20 meters or any higher band. I put the old ladder line section back.

Remember what all the discussions say about keeping the feed line away
from metal objects? Here we are introducing a 5 ft length of metal
just 1/4 inch from the feed line. The extra capacitance just kills the
feed line. Yes, I was using RG-213.

So, the doctor in nuts, as far as I can tell.

Paul, KD7HB


Paul

That's interesting and noted as I was intending to use that approach to get
the balanced line into my new shack, which is a metal shed. I guess it may
be back the old drawing board. Any suggestions?

Peter VK6YSF

http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm

" wrote in
:
....
Remember what all the discussions say about keeping the feed line away
from metal objects? Here we are introducing a 5 ft length of metal
just 1/4 inch from the feed line. The extra capacitance just kills the
feed line. Yes, I was using RG-213.

Paul,

The "5 ft length of metal just 1/4 inch from the feed line" is not a good
method of analysing the behaviour.

The treatment of the shield ends is very important to the operation of
these things.

If the shield ends at each end are tied together, and each end
effectively grounded (and that is a big ask), then any common mode
current on the incoming ladder line flows to ground via the outboard
ground connection, and the section of dual coax transforms the impedance
seen looking into the ladder line in a quite predictable way, and with
some loss. The transformed impedance might not be within the range of
your tuner.

If the outboard shield ends are not effectively grounded, then the
configuration does essentially nothing to reduce common mode current
entering the shack, so the system is a fails it its primary objective.

Owen

"Peter" wrote in
. au:

....
The two short pieces of coax with center conductor connected to either
side of the ladder line is perhaps the best solution. It was covered
in QST/The Doctor is In, June 2008. In the article it suggests using

Thanks Peter, I have had a quick look at the article.

The "Doctor" describes the configuration in his Figure 4. I have posted a
copy of the figure to assist discussion, it is at
http://www.vk1od.net/lost/ShieldedTwin.png .

The treatment of the shield ends is important detail in understanding how
this works.

If the ground connection on the antenna side of the shielded line has a
very low impedance to ground, almost all common mode current on the open
wire line will flow to ground via that connection, and very little common
mode current will flow into the shack.

For example, lets consider a G5RV at that the transition to shielded twin
is at the bottom of the so-called "matching" section. Using a coax pair
was comprised 5m of RG213, the impedance looking into the open wire line
on 80m would be somewhere around 10-j340.

The transformation and loss in the twin RG213 can be calculated using
TLLC (http://www.vk1od.net/calc/tl/tllc.php) with a load of 5-j170 on
RG213. The result is 1.9dB of loss in the 5m of cable, 35% of the power
into the twin RG213 is lost as heat. The impedance looking into the twin
RG213 would be 2-j80, a very challenging load for efficient
transformation to 50 ohms.

Now, this is a pretty awful case, and they won't always be this bad,
though some will be worse. This shows a method of analysing the
configuration.

The "Doctor" says in his article "Whilst it is critical for proper
operation for the shields of the two coaxes be tied together at each end,
whether they are tied to "ground" is not important for operation, but can
make a big improvement in lightning protection."

I disagree with the last statement, the treatment of the ends of the
shields wrt ground is very important to analysis, the configuration
behaves differently for different ground configurations. For example, if
the antenna end of the shields is left disconnected, the common mode
current on each side of the antenna end of the shielded twin is
essentially equal and common mode current flows on the outside of the
shielded twin. The shielding does not eliminate external fields from
common mode current.

Owen