Steve,
Perhaps it would be more accurate to state "I-squared-R loss attributed to
SWR," rather than "Additional loss due to SWR?" The caveat that addresses the
effect seen on short lines is still needed.
Not sure if this link was provided earlier, but VK1OD presents an excellent
analysis of a relatively recent QST article to illustrate his point.
http://vk1od.net/transmissionline/VSWR/aldv.htm
I think the QST article still offers an excellent explanation for beginners
even when he refers to the "additional loss due to SWR" graph without further
clarification. By the way, there's a gross error on one of the graphs! Looks
like the artist didn't do a good job of lining up the X and Y axis and it
wasn't caught during proofs.
Still, anyone who can even get to that level of understanding without the
caveat is more knowledge on the subject than the vast majority ops. For
example, eHam is still publishing articles from authors who proclaim that only
single-band, resonant antennas can achieve any semblance of high efficiency and
that ATUs do nothing more than "make our transmitters happy."
Paul, W9AC
----- Original Message -----
From: Steve Hunt
To: Paul Christensen
Cc: towertalk@contesting.com
Sent: Wednesday, December 01, 2010 9:52 AM
Subject: Re: [TowerTalk] tuners and power rating
Paul,
I'm not sure it helps to distinguish between "SWR losses" and "I-squared-R
losses" - at HF, all the losses are predominantly "I-squared-R losses".
It may help to picture qualitatively the current profile over a short length
of feedline at the load end - in all cases delivering the same power to the
load:
* If we have a matched load, the current is constant along the line and the
loss-per-unit-length will therefore also constant along the line. Cumulative
losses increase linearly with length.
* If we have a load with a moderately high resistive component the current at
the load will be lower, and therefore loss-per-unit-length will be lower. But
slightly back from the load the current will have increased due to the standing
wave pattern, and therefore the loss-per-unit-length will be higher; eventually
it exceeds the matched case loss-per-unit-length, and even further back the
cumulative losses exceed those of the matched case.
* If we now have a load with a *very* high resistive component, the current
at the load will be very low and the loss-per-unit-length will be even lower
than in the previous case. However, moving back from the load, the rate of
change of current with distance is higher because of the increased ISWR, and it
may be that we reach the "break even" point sooner, despite the
loss-per-unit-length adjacent to the load being lower.
Incidentally, there will be a load value which maximises the distance from
the load of the "break even" point.
So, all the losses are "I-squared-R losses" - it's just that the current
profile (and therefore the cumulative loss profile) changes if the ISWR is not
unity.
73,
Steve G3TXQ
On 01/12/2010 13:44, Paul Christensen wrote:
The additional loss attributed to a mismatch is still relevant once SWR
becomes part of the loss. In the 10 ft. examples we've been using, that
occurs when the line get to roughly 40 degrees in length. When we approach
1/4 wave, loss due to SWR becomes equal in loss to I-squared-R loss. I
think that's was one of Steve's points in that the additional loss
attributed to SWR needs some clarification in the footnotes o be completely
accurate.
Paul, W9AC
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