[TowerTalk] Elevated vertical and transmission line basics

[Bob Alexander]

Good morning guys,

The elevated vertical thread is covering what I have been lecturing about
for 15 years.  Several times a year I do a presentation at Hamfests or club
meetings I call 

"Transmission Line Basics".

 

The essential points I try to make are:

 

1.       There are two impedance values on every transmission line.  One is
the Characteristic impedance which never changes unless the cable has been
damaged or in rare cases has been designed to change characteristic
Impedance along its length.

The other is the impedance of the signal on the line.

 

2.       A line terminated in a value equal to its Zo will have a signal Z
all along its length equal to the load Z  with the only changes due to line
losses.

 

3.       Since the signal Z is constant on the matched line, a signal
applied to the line will have the same voltage, current and phase  all along
its length.  Therefore no SWR

 

4.       It's when a line is not terminated in a value equal to its Zo that
things get interesting

 

5.       A mismatched line is an impedance transformer.  There is no point
on the line where the signal Z will equal the line Zo.  It may get close due
to losses but never the same.

 

6.       Since the Z constantly changes, the  signal voltage and current
change along the line length.  The relationships of the changing voltage or
current is SWR. 

 

7.       The Z mismatch seen by a transmitter is the value at the line
input, not at the load.

 

 

Sorry for repeating things several people have already said on this thread
but like I said, I like to talk about transmission lines.

 

73, Bob, W5AH