[Elecraft] Re: I^2*R Losses and Coax Impedance (Was Random Wire)

Sat Jan 11 19:25:06 2003

Jim and Stuart,

Many thanks for taking the time to reply to me on this one.

It is a case of back to the books for me so that I have a good grip on this
topic.

73
Kevin
VK3DAP

----- Original Message -----
From: <[email protected]>
To: <[email protected]>; <[email protected]>;
<[email protected]>; <[email protected]>;
<[email protected]>
Sent: Sunday, January 12, 2003 7:06 AM
Subject: Re: [Elecraft] Re: I^2*R Losses and Coax Impedance (Was Random
Wire)

> In a message dated 1/11/03 12:53:33 AM Eastern Standard Time,
> [email protected] writes:
>
> > I have just purchased Kraus Antennas and Kraus Electromagnetics and have
not
> >  got far with them as yet, however, I question Stuart's assertion that
with
> >  respect to coax feeders, "... the higher the impedance the lower the I
> >  squared R losses of a feeder."
> >
> >  Surely the I^2*R losses have to do with the resistive losses of  the
copper
> >  conductor and the parallel conductance of the dielectric.  At low
> >  frequencies these are practically negligible unless you have a very
long
> >  cable and are running high powers.  A coax cable would still have a
> >  characteristic impedance even if the conductors had zero resistance.
> >
> >  Somebody with more knowledge will no doubt correct me for which I would
be
> >  grateful.
> >
> Nothing to correct, from what I can see.
>
> Several factors to remember:
>
> - skin effect as frequency increases raises loss
> - need for more insulation in most coax designs
> - low Z parallel lines require very close spacing of large conductors,
which
> becomes impractical (try to design 50 ohm parallel line with air
insulation)
> - high Z coax requires a tiny center conductor and enormous outer
conductor,
> which becomes impractical (try to design 600 ohm coax with poly
insulation)
> - Loss is not the only consideration - there's also power handling
> capability, limited by sparkover and heating from dielectric and I2R loss.
>
> End result is that the optimum values are about what we have.
>
> The following may be an urban legend, but I post it here for info and
> confirmation/disproof:
>
> Why 75 and 50 ohm coax? Because back in WW2, it was determined that for a
> given amount of solid poly insulation and copper per unit length in
flexible
> coax, 50 ohms gave the highest power handling capacity and 75 ohms gave
the
> lowest loss.
>
> 73 de Jim, N2EY
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