[Elecraft] K3 SWR Accuracy - reprise

Wes Stewart n7ws at yahoo.com
Wed Nov 4 19:24:25 EST 2009


You have something terribly wrong.  Two feet of coax at 7 MHz is negligible (~8 electrical degrees for solid dielectrics), even if its impedance is wildly different from 50 ohm.

Furthermore, any loss in the cable should reduce the SWR, not increase it.

--- On Wed, 11/4/09, Don Wilhelm <w3fpr at embarqmail.com> wrote:

From: Don Wilhelm <w3fpr at embarqmail.com>
Subject: Re: [Elecraft] K3 SWR Accuracy - reprise
To: elecraft at mailman.qth.net
Cc: "Phil & Debbie Salas" <dpsalas at tx.rr.com>
Date: Wednesday, November 4, 2009, 4:33 PM

I can vouch for Eric's statement.  I routinely calibrate KPA100 
wattmeters for SWR using a precision 100 ohm dummy load (which should 
produce a 2.0 SWR).  At 40 meters, I get 2.0 if I use a direct 
connection with a male to male adapter, with a 1 foot coax, it shows 
SWR=2.1 and with a 2 foot coax, it indicates SWR=2.2.  The coax length 
is NOT negligible.
BTW - my MFJ-259B shows the same thing with those same cable lengths.

The Smith Chart constant SWR circle is for ideal (theoretical) 
conditions, and the real world conditions of cable loss and RF in places 
the Smith Chart does not consider must be factored in to explain 
phenomenon like this.
Since most instrumentation is balanced/calibrated for 50 ohms, things 
agree when the impedance is 50 ohms resistive, but away from that point, 
other factors come into play.

73,
Don W3FPR





      


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