On Feb 24, 11:28*am, Owen Duffy wrote:
ve2pid wrote in news:cd290c79-c0c7-4308-af41-
:

Let's suppose that we use to identical length of coax, and on each
one, we connect the shield to the inner conductor at each of its ends.
If we use these two independant (?) sections as a substitute for an
open-wire line, could we use the usual formula Z=276*log(2S/D) to
compute the impedance of that line?

In that case, the characteristic impedance Zo of the coax could be
neglected. And what about the matched losses in dB/100' of the
resultant line? Twice the one of the coax?

Pierre,

The formula you give is an approximation which is not good for low Zo,
and it ignores the effect of the dielectric (coax jacket).

I have seen discussion of this recently in another place.

Yes, you can fabricate a two wire open line like this. You need to find a
way to bind the two cables for consistent physical spacing, if the jacket
is PVC you are using a lossy dielectric, the braided conductor is lossier
at RF than an equivalent solid copper tube. So an expensive low Zo line,
not a low as you might think, with poor performance.

Why would you do this?

Owen

I certainly agree with Owen that it's not a particularly good way to
get a low-loss two-wire balanced line. To me, the attraction of a two-
wire balanced line is the low loss when it's properly implemented at
relatively higher impedance. That said, however, one can buy "Siamese
twin" cables with two pieces of coax in the same jacket, arranged to
make it easy to separate them. The web of jacket between the two
lines is relatively thin. Such line is common in CATV installations
in homes.

At low frequencies (HF, especially lower HF frequencies), the
dielectric loss may not be too bad, especially for the low impedance
involved. It may be a cheap way to get an approximately 100 ohm
balanced line. Even though the braided conductors will have higher
loss than equivalent diameter smooth conductors, the copper loss for
typical RG-6-type line used that way (about 5mm OD outer conductor)
will almost certainly be less than that using, say, 2mm diameter solid
conductors. You may be able to find surplus (e.g. reel-ends) of this
sort of line at very attractive prices. Even new, it's not too bad--a
lot cheaper than equivalent diameter solid copper!

Cheers,
Tom