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

[N2EY@aol.com]

In a message dated 1/11/03 12:53:33 AM Eastern Standard Time, 
kevinluxford@optushome.com.au 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