Thread: Two coax as substitute for open line
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Old February 24th 10, 07:28 PM posted to rec.radio.amateur.antenna
Owen Duffy Owen Duffy is offline
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Default Two coax as substitute for open line
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
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