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Re: [TowerTalk] tuners and power rating

To: Paul Christensen <>
Subject: Re: [TowerTalk] tuners and power rating
From: Steve Hunt <>
Date: Wed, 01 Dec 2010 14:52:09 +0000
List-post: <">>

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.

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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