"John - KD5YI" wrote
Cecil Moore wrote:
Reg Edwards wrote:

Many years back a similar sort of calculation was done for coax.
Coax
does not suffer from proximity effect. It's easier to work out.
The
answer was 75 ohms. That's how 75 ohms became the standard
comunications Ro.


I vaguely remember something about efficiency Vs power
handling capability being the difference in the 75 ohm
standard and the 50 ohm standard. Is that right?
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Page 5-15 of The ARRL UHF/Microwave Experimenter's Manual says:

"Consider that both power handling capability and cable losses vary
with Zo.
It has been shown that cable losses are minimum at a characteristic
impedance on the order of 75 [Ohms], while power handling capability
is
maximum at a Zo of about 30 [Ohms]."

(The book used the Greek symbol rather than [Ohms])

The quoted passage is in a chapter by Dr. Paul Shuch, N6TX,
Professor of
Electronics, Pennsylvania College of Technology. At the end of the
quote, is
an indication to see footnote 13 which is:

"Moreno, Theodore, Microwave Transmission Design Data, Dover
Publications,
1948."

73,
John
============================================

It is unreliable to use ARRL and similar publications as Bibles. They
are written by amateurs for amateurs and tell only a sufficient
fraction of the whole story. Phrases such as "It has been shown that
..... " arise. They also refer to UHF/Microwave when LF and HF are of
interest. At microwave frequencies the dielectric loss cannot be
considered negligible.


For minimum attenuation, air-spaced coax Zo = 75 ohms and D/d = 3.6
For solid polyethylene Zo is smaller.


Confusion about the value of Zo which maximises power handling
capabilty arises because coax cables have different shapes and
materials to support the inner conductor. Even though the dielectric
may be considered lossless its presence affects matched line loss.


If my memory serves me correct, for maximum power handling I think 50
ohms refers to air-spaced coax and 30 ohms or thereabouts refers to
solid polyethylene dielectric. Or it may be the other way about. It
will be different again for a different dielectric permittivity.


For a coax line used as a tuned circuit, eg., when short-circuited,
maximum impedance at resonance occurs when Zo = 132 ohms and D/d ratio
= 9.1


And just to add a little more to the confusion, whether the outer
conductor is solid or braided also makes a small difference.
----
Reg, G4FGQ