[Elecraft] K3 SWR Accuracy - reprise

[Wes Stewart]

--- On Wed, 11/4/09, Steve Ellington <n4lq at carolina.rr.com> wrote:

The first statement is correct. Length of coax will transform impedance and 
cause SWR meters to read differently.

Absent loss, if the cable Zo matches the SWR bridge design impedance (or vice versa), then the SWR reading should be unaffected by the cable length.

I've seen this Smith Chart reference before and it makes no sense. 

Of course it does, it makes perfect sense.  If the chart is normalized to the line Zo, and the load Z is mapped accordingly, then changing line length moves the load Z around the chart.  Yes, the impedance changes, but the SWR does not.  A properly designed instrument, matched to the line Zo should read the same SWR regardless of line length.

These devices aren't impedance bridges, they are (supposedly) reflection coefficient measurement instruments.  As such, they haven't a clue what the impedance is, nor should they.

You can certainly use your feeder to "match" your antenna. Of course, if SWR meters didn't care what the impedance is then yes, it wouldn't matter where you put it along the line but such is not the case.

Well, as I said above, it should be the case.

Wes  N7WS

Steve
N4LQ
N4LQ at carolina.rr.com
----- Original Message ----- 
From: "Phil & Debbie Salas" <dpsalas at tx.rr.com>
To: <elecraft at mailman.qth.net>
Sent: Wednesday, November 04, 2009 5:53 PM
Subject: Re: [Elecraft] K3 SWR Accuracy - reprise


> "How long is the coax between the LP100 sensor and the K3? This can have
> a significant impedance transformation impact as you go to higher
> frequencies, leading to a different indicated SWR at each end."
>
> The impedance changes, but not the SWR.  Looking at this on a Smith Chart,
> you can see that you just rotate around a constant SWR circle.  Minus coax
> losses of course, which are negligable when we're talking a few feet at 
> HF.
>
> Phil - AD5X
>