Topband: EZNEC 5.0 + return loss and another question

[Tom W8JI]

Chuck,

> Next question: in the manual the ALT IMPEDANCE explantion uses a 4:1
> transformer example, which I get.  I've been toggling between RG213 50 ohm
> and rg6/u 75 ohm to observe differences, reasoning that if I use a 75 ohm
> feeder and get 2:0:1 or lower SWR  values, the 50 ohm "system" would start
> (end) at the rig instead of the antenna feed point.  I noted that the SWR
> bumped up a bit when I added a 120ft transmission line, which I would
> expect.

I'm not sure what you are saying there, but it seems the usual "stuff" about 
SWR that trips people up may be confusing you. Unless you want to get into 
complex wave theory that takes a chapter of text to explain a simple 
concept, it is best to look at this a different way.

If we were transmitting video or had very high speed digital systems, we 
would have to consider reflected waves. We have steady-state systems where 
levels change over many RF cycles, not pulse or extreme bandwidth systems.

The real reason system performance changes with SWR is the impedance changes 
throughout the system. This impedance change causes voltages and currents 
throughout the system to change, and this causes losses to change. SWR or 
mismatch can change the loadline seen by the PA device(s), changing how the 
PA device works.

You probably don't need to know any of that to answer your question.

If you can ask a specific question, it should be easy to answer the question 
without confusing you.

Are you asking how to best model 75 ohm transmission lines in EZNEC?  If 
not, what are you trying to do with the model??

73 Tom