Paul,
I checked that page of Owen's earlier for a chart which shows the losses
at various distances from a mismatched load; it was the epitomy of "a
picture paints a thousand words"! Unfortunately Owen seems to have
removed it as part of his recent site re-vamp.
73,
Steve G3TXQ
On 01/12/2010 15:30, Paul Christensen wrote:
> 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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