Topband: 50 ohm direct burial coax cable‏

[Paul Christensen]

> "A quarter wavelength 75 ohm  coax working into a 50 ohm load, transforms 
> the 50 ohm load to 112.5 ohms, non-reactive, as it appears at the end of 
> the coax next to the transmitter, as previously discussed.

Good so far.

> "If we place a 50-ohm SWR meter at the near end of the coax, between it 
> and the rf source (the transmitter), the meter will "see" 112.5 ohms, not 
> 50 ohms nor 75 ohms. It will read 2.25:1 SWR. But the actual SWR on the 
> coax line remains 1.5:1."

In your example, SWR at any point on a lossless 1/4 wave, 70-ohm line 
section is indeed 1.5:1.  The Z at the input to the 1/4 section is 112.5+j0. 
However, the SWR at any distance from *this point* to the transmitter is not 
just "apparently" 2.25:1.  It is in fact 2.25:1.  The 1/4 wave section not 
only transformed the Z, but the Z transformation set up a change in SWR on 
any subsequent length of 50-ohm line between the transformer and 
transmitter.   The SWR at any point on the 50 ohm section [SWR (50)] between 
the transmitter and 1/4 wave section is 2.25:1 (although the Z is now 
changing along the line) -- and the SWR at any point on the 70-ohm 
transformer section [SWR(70)] is 1.5:1.

The moment the characteristic Z changes along a line, the line SWR also 
changes.  When we hear that "SWR doesn't change along a line," that is true 
when the characteristic Z is uniform along the entire length of the line and 
the line is loss-less.

Paul, W9AC