[TowerTalk] tuners and power rating

[Paul Christensen]

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 at 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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