[Elecraft] K3 SWR Accuracy - reprise

[Steve Ellington]

So here is the bottom line:
1. SWR is the same anywhere along the transmission line per Mr. Smith and 
his Chart.
2. When the transmission line doesn't match the antenna we have an SWR other 
than 1:1.
3. The SWR meter will often read differently at the antenna vs. at 
transmitter.
4. Example: A full wave dipole center fed with 50 ohm coax. SWR reads 
infinite at the antenna but with 1/4 wavelenth of coax, SWR reads low!
5. Why? SWR meters are designed to work with a specific impedance and the 
impedance is obviously different in the example.
6. Putting that SWR meter along various points along the xmission line gives 
different readings.
7. You can fiddle with the xmission line length and fool an SWR meter into 
thinking the SWR is 1:1 when it is really quiet high.
8. SWR meters are good for making sure your transmitter sees 50 ohms unless 
you like climbing.
9. SWR meters are good for antenna matching if they are placed at the 
feedpoint of the antenna. Tower or tree climbing needed unless you have a 
ground mounted vertical.
10 SWR meters will read correctly if the xmission line is a multiple of 1/2 
wavelenth ( A rare occurance).
11. If your line is not matched, you could hook 100 SWR meters in series and 
they would all read something different.

Steve
N4LQ
N4LQ at carolina.rr.com
----- Original Message ----- 
From: "Phil & Debbie Salas" <dpsalas at tx.rr.com>
To: "Steve Ellington" <n4lq at carolina.rr.com>; <elecraft at mailman.qth.net>
Sent: Wednesday, November 04, 2009 7:18 PM
Subject: Re: [Elecraft] K3 SWR Accuracy - reprise


> Transmission line theory (and therefore the Smith Chart info) IS correct.
> Transmission line length DOES transform the impedance, but not SWR.  So 
> you
> may be changing the impedance to something that your tuner can tune when 
> you
> add coax length, but you are not changing the SWR by adding coax - other
> than the change due to coax loss which is negligable for short lengths.
>
> Now I do agree that different SWR meters probably read differently when 
> the
> SWR is the same but the impedances presented to the two SWR meters are
> different.
>
> Phil - AD5X
>
> ----- Original Message ----- 
> From: "Steve Ellington" <n4lq at carolina.rr.com>
> To: "Phil & Debbie Salas" <dpsalas at tx.rr.com>; <elecraft at mailman.qth.net>
> Sent: Wednesday, November 04, 2009 5:59 PM
> Subject: Re: [Elecraft] K3 SWR Accuracy - reprise
>
>
>> The first statement is correct. Length of coax will transform impedance
>> and cause SWR meters to read differently.
>> I've seen this Smith Chart reference before and it makes no sense. 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.
>> Steve
>> N4LQ
>


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