"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
Quoting Bill Turner <dezrat@outlook.com>:
------------ ORIGINAL MESSAGE ------------(may be snipped)
On Sat, 10 Jun 2017 19:57:42 -0500, W7RY wrote:
Tried different lengths of coax between the amp and the radio and
still no better.
REPLY:
The length of coax does not affect SWR unless it is extremely long,
where line loss comes into play. Even then, the effect is to reduce
SWR, not increase it.
It does affect the impedance the transceiver "sees", but not SWR.
73, Bill W6WRT
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