"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