ve2pid wrote in news:e44076bb-dfdc-4aa4-8c13-
:

In the ARRL's Antenna Book 21st ed page 24-21, we see that if we
connect the two shields of the coax cables together, we obtain
'Shielded parallel Lines' . In that case, the resultant impedance is
simply the sum of the characteristic impedances of each coax.

So, there is quite a difference between the two independent coax I
mentioned in my first message (we connect the shield to the inner
conductor at each of its ends) (A) and the 'Shielded Parallel Lines'
case (B).

I am trying to understand why and it is the reason I posted my first
message...

That is not quite what you asked.

However, in the case you described in your first post, the shield of each
cable carries the differential current on the outside of the shield.

In the case of B above, the differential current is carried entirely
inside the shield, no differential current flows on the outside of the
shield.

So, the field structures, and therefore Zo are different in the two
cases.

In (A), the Z=276*log(2S/D) applies, so the Zo of each coax does not
matter.. but in (B), Z=Zo1+Zo2, so the value of each Zo matters.

Am I right? And how to compute matched line loss in case (A) and in
case (B) ?

I opine that you will have to measure MLL for case A, mainly because the
effective RF resistance of the braid is not easy to estimate.

(B) can be found from tables, or better still,
http://www.vk1od.net/calc/tl/tllc.php , it is the same as the single
cable used to fabricate the pair... think about it.

Owen