Reg Edwards wrote:
"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



Okay, Reg, then go read the material referenced by footnote 13. That's one
reason I included it. Maybe that way we won't need to rely on your memory.

John

John - KD5YI wrote:
....

Okay, Reg, then go read the material referenced by footnote 13.
That's one
reason I included it. Maybe that way we won't need to rely on your
memory.

The confusion comes (in the quoting of the texts) because of a failure
to consider that there are two different mechanisms that can limit the
power handling capability of the line. One is power dissipation
(temperature rise), and the other is voltage breakdown. Clearly the
minimum power dissipation for a given input power and matched line
occurs where the line attenuation is minimum. But if you make the
inner conductor slightly larger, it may be able to get rid of heat
enough better (for a given line construction) that the inner conductor
temperature rise is slightly lower, even though the power dissipation
is slightly higher. I would expect, though, that the optimal
construction in most circumstances would result in an impedance only
marginally lower than the minimum attenuation case, and the improvement
would be a very small one. You'd have to convince me it was really
important to get me to worry about it beyond just minimizing
attenuation.

If it's voltage breakdown that limits the line power handling
capability, the air-insulated impedance of the line will be at a D/d
that results in about 30 ohms impedance for air-dielectric line, and
..66 times as much for solid polyethylene line.

And if it's maximum voltage-handling you want, the D/d results in
somewhere around 50 ohms with air-insulated line, about 33 ohms with
solid poly, if memory serves. I could look it up if it's really
important.

Generally in ham applications, (reasonably well matched) lines will be
power dissipation limited, not voltage limited.

Cheers,
Tom

From LF to VHF it is ALWAYS power dissipated in conductor resistance
which limits the power handling capability of the line. Voltage has
nothing to do with it. Above VHF dielectric loss becomes be the
limitation.

Consideration of ambient temperature is vital. Are you located in
Alaska at midnight in mid-winter? Or are you in the New Mexico desert
in July, at noon. It makes hell of a difference?

With coax everything depends on the temperture softening point of
polyethylene and on the the longer-term temperature deterioration
(hardening, cracking, brittleness) of the PVC sheath.

Is the cable embedded in an asbestos insulated brick wall or is it
suspended in free air with a breeze in the shade? Or in sunlight?

The power rating data provided by manufacturers for amateur grade
coaxial cables is useless nonsense. From inspection of manufacturers'
tables (watts) it can be deduced their ratings are based on the
melting point of polyethylene. Salesmen's blurbs, no doubt plagiarised
in ARRL publications, sound very good in order to sell the stuff.

To gain an elementary understanding of what it's all about, download
in a few seconds, easy to use, practical application, small program
"COAXRATE" from website below and run immediately. (Not zipped up).

Program "COAXRATE".
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Reg Edwards wrote:
From LF to VHF it is ALWAYS power dissipated in conductor resistance
which limits the power handling capability of the line. Voltage has
nothing to do with it.

What if it arcs?
--
73, Cecil http://www.qsl.net/w5dxp

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What if it arcs?
--
=================

It shows the voltage rating has been exceeded.

Reg Edwards wrote:
What if it arcs?

It shows the voltage rating has been exceeded.

But, but, but, Reg, you said "voltage has nothing
to do with it." :-)
--
73, Cecil http://www.qsl.net/w5dxp


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What if it arcs?

It shows the voltage rating has been exceeded.

But, but, but, Reg, you said "voltage has nothing
to do with it." :-)

==============================

I'm very sorry Cec, but in future I shall have to make a modest charge
for answering your questions. In advance if you wouldn't mind.

Reg wrote:

"From LF to VHF it is ALWAYS power dissipated in conductor resistance
which limits the power handling capability of the line. Voltage has
nothing to do with it. Above VHF dielectric loss becomes be the
limitation. "

Always? Hardly. Transmission of pulses with low duty cycle will get
you to voltage-limited operation pretty quickly. Transmission of power
to a high-resistance load where the line is a very small fraction of a
wavelength long may get you into voltage-limited operation. Those
perhaps aren't typical ham applications, but they do happen in
practice. Also, though the cable itself may not have trouble with the
applied voltage, the connectors at the ends may. They're generally
rated for much lower voltage than the line itself.

Also, for the small-diameter (nom. RG-58 size) cables I've been using
lately, conductor loss exceeds dielectric loss out past 10GHz. YYMV.

Cheers,
Tom

K7ITM wrote:
Also, though the cable itself may not have trouble with the
applied voltage, the connectors at the ends may. They're generally
rated for much lower voltage than the line itself.

Yep, during duststorms and thunderstorms, the connectors
arc. Arcing at the coax connectors of my IC-745, IC-725,
IC-706, and IC-756PRO has never seemed to injure any of
them. Is that just luck or are they that well protected?
--
73, Cecil http://www.qsl.net/w5dxp


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