Re: K3 SWR Accuracy - reprise

Posted by Alan Bloom on Nov 05, 2009; 12:16am
URL: http://elecraft.85.s1.nabble.com/K3-SWR-Accuracy-reprise-tp3943810p3949277.html

The SWR definitely does not change with line length.

However the SWR _READING_ often does because of the inaccuracy of
inexpensive SWR meters.  The SWR reading should depend only on the
relative magnitudes of the forward and reflected power and not on the
phase angle between them.  But with inexpensive SWR meters that is often
not the case.

Al N1AL


On Wed, 2009-11-04 at 18:33 -0500, Don Wilhelm wrote:

> 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
>
>
> Eric Swartz -WA6HHQ, Elecraft wrote:
> > That assumes there is no RF on the exterior of the coax and the SWR
> > sensors are ideal. Neither is the case and from personal experience I
> > can tell you that the swr meters will change readings based on coax length.
> >
> > 73, Eric
> >
> >
> > Phil & Debbie Salas wrote:
> >  
> >> "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
> >>    
> >
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