Topband: EZNEC 5.0 +
charlie-cunningham at nc.rr.com
Fri Dec 5 19:30:18 EST 2014
Well, I would respectfully disagree with Tom, that "Return Loss" is
confusing or ,misleading. It's just another way of looking at reflections
that often makes more sense of is more useful. For example, many filters
etc. are specified in terms of their input return loss, usually in a 50 ohm
system,and engineers working in the lab with vector network analyzers
measure most all input matches in terms of return loss.
On a Smith Chart, return-loss is a radially scaled parameter, as is VSWR.
The origin of the chart, at a perfect match would be the 1:1 VSWR point,or
the infinite return loss point. Conversely, the perimeter of the chart
corresponds to 0 return loss or infinite VSWR.
EZNEC and network analyzers tend to express things in terms of return-loss,
although VSWR is also available. For modeling in EZNEC and tuning or
sweeping your antenna models,you want to MAXIMIZE return loss and MINIMIZE
VSWR- it's the same thing.
As an example to illustrate Tom's point. IF we fed a resonant 1/2 Wave
dipole at its center, with 600 ohm open-wire line, the VSWR on the feedline
would approach 10:1, but there would be almost no loss in the feedline and
virtually 100% of the incident power would be radiated by the antenna. The
challenge would be to match the transmitter to whatever values of R+JX
presented themselves at the sending end of the line.
Have fun with EZNEC!
From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of Tom W8JI
Sent: Friday, December 05, 2014 4:40 PM
To: Charles Yahrling; topband at contesting.com
Subject: Re: Topband: EZNEC 5.0 +
Return loss is just another misleading confusing way to express SWR.
Return loss, like percentage reflected power, does not indicate any type of
loss. It just expresses SWR in a different form.
We can have 10:1 SWR, which would be a 1.743 dB return or "mismatch" loss
or 67% reflected power, and still have virtually no loss. We can have 67%
reflected power and still have nearly 100% of transmitter power getting into
the antenna and being radiated.
The best advice is to ignore it all, and just use SWR for now. :)
----- Original Message -----
From: "Charles Yahrling" <cfytech24x7 at gmail.com>
To: <topband at contesting.com>
Sent: Friday, December 05, 2014 3:29 PM
Subject: Topband: EZNEC 5.0 +
> Just getting started modelling and looking for answers to questions not
> found in manual so far. For example, what exactly is included in the
> Return Loss figure shown in the SWR window? Just ground reflection loss,
> total system loss, something else? Trying to understand why Return Loss
> greater for lower SWR curve values. e.g see this when toggling between std
> and alt impedance. What is this suggesting, go with lower return loss or
> lower swr curve?
> An incomplete grasp of the fundamentals is admittedly likely here <g>.
> 73, chuck
> de AB1VL
> NAQCC #6799
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