Line Mismatch -- was -- Re: [BC] Problems that make you want to scream!

[Cowboy]

On Friday 14 October 2005 08:14 pm, DANA PUOPOLO wrote:
>You are not changing the VSWR.

 At the load, I would agree, nor along the line proper.

>What you are doing is changing the position of 
>the voltage peak. 
>See, most VSWR detectors read voltage. By adding to (or taking away) line, you
>are moving the voltage peak away from the voltage reading detector (think sine
>wave here).
>
>The VSWR does not change, only the meter's reading of it.

 By itself, in the absence of current, you would be correct.
 With the current present, and 90 degrees shifted from the voltage,
 there is more at play.

 We're not changing the voltage standing wave ratio along the line,
 but we are certainly changing the Z as seen looking into the line.
 The CB trick was a who cares, as long as the transmitter sees 50 ohms ?
 The line cares, as line losses are not reduced even though the tx can
 couple more power into it.
 On a matched line, the voltage, current, and VSWR is the same at any
 point along its entire length.
 This is not the case on a mismatched line.

 On the other hand, if the section added is the correct length AND Z,
 then you are, in fact, transforming the complex Z at one end of the
 matching section to something else at the other, but that was not the
 case presented here.
 ( and its understood that the VSWR *in* the matching
 section can be quite high )

 A mismatched line has a complex Z that changes from a minimum below
 the characteristic Z of the line to a maximum above the characteristic,
 every 90 degrees along its length.
 If you happen to hit a point on the line close to 50 ohms, a 50 ohm
 transmitter will like it if it can handle the reactance.
 The point of this example as originally presented by Bill ( I think ) was
 that the line was mismatched, and that by changing its length by some
 distance that is not 180 degrees, the apparent VSWR will change.

 We had a similar discussion some weeks ago about a differential VSWR
 set-up consisting of two Bird's spaced 1/4 wave apart.
 If they both read flat, the reading is real and accurate ( apologies to RF
 but for all general intents and purposes ) but if they differ, then neither
 can be taken to be correct, and there is a certainty of mismatch somewhere,
 and danger of a burn-out not necessarily indicated by a single instrument.
 It is possible for the transmitter to see a good load, and continue delivering
 power into a very real fire whos Z has been transformed to look good
 to the transmitter, by virtue of the line length between the transmitter
 measuring device and the fire.

 Knowing something about Z along a mismatched line, my speculation was
 that two Birds would not be necessary, but only two fairly reliable indicators
 spaced some distance not 1/2 wave apart, one of which might even be
 the final amplifier loading. The other could be the tx VSWR meter, provided
 it is not 1/2 wave electricly from the final amp, or a Bird somewhere after
 the transmitter output terminals. A spacing of 1/4 wave would provide the
 greatest differential in readings in the mismatched condition, and therefore
 the greatest sensitivity to mismatch, but any distance not 1/2 wave will work.

-- 
Cowboy

http://cowboys.homeip.net

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