So my question is: does the driver care (or even know) the difference between a
flat 50 ohm line to an amplifier or one that is running at a 2:1 SWR, but whose
length is such that the driver "sees" an impedance of 50+j0 Ohms.
I say no.
Vic 4X6GP
On 11 Jun 2017, at 16:58, Paul Christensen <w9ac@arrl.net> wrote:
>> "Like Bill said, changing the coax length does not change the swr so there
> is very little "tuning" going on. Using 1/4 wave or less will not fix the
> swr problem so something more is needed. Changing from 50 to 75 ohm coax
> will cause some tuning to take place. Good luck finding a happy length
> though. 73, Gerald K5GW"
>
> It's not only about SWR. Any line-to-load mismatch, no matter how small,
> will cause the complex impedance to change at the input end of the line with
> changing line length, even though the line SWR does not change with length.
> For example, if there's a 2:1 SWR load mismatch, then changing the line
> length may affect the operating performance of the driving stage by creating
> say...a 50+j0 or some other input Z, even though the line SWR hasn't
> changed. The input Z changed, the line SWR didn't.
>
> In any line mismatch condition, SWR on the line remains the same provided
> the characteristic Z remains the same and the line is low-loss. If the line
> characteristic Z changes (e.g., RG-11 to RG-213 transition), then the SWR on
> each portion of the line is different. Changing the line length on one
> section of the transition will: (1) affect the SWR on the other section; and
> (2) cause the complex Z measured on each end to change.
>
> Paul, W9AC
>
>
> In a message dated 6/11/2017 7:22:17 A.M. Central Daylight Time,
> donroden@hiwaay.net writes:
>
> If you change the length of the feed coax, you have created a "coax tuner".
> 1/4 wave or less is all that is required.
>
> Don W4DNR
>
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