Posted by
Dunc Carter - W5DC on
Nov 04, 2009; 11:23pm
URL: http://elecraft.85.s1.nabble.com/K3-SWR-Accuracy-reprise-tp3943810p3949061.html
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
>
> _
.
Generally, simple swr indicators are much less sensitive to impedances
on the low impedance side of the smith chart. For example, I compared
my K3 with an elderly Drake W4 using my 30 meter short vertical dipole
which has a feed point resistance of of about 33 ohms using a six foot
piece of RG-213 and then adding an additional 16 feet of coax.
Surprisingly, the K3 and the R4 agreed within 1 per cent on forward
power and had the same variation in swr. The antenna feed point
resistance was measured with an elderly General Radio model 1606A HF
impedance bridge. Of course, with only six feet of cable, it's
difficult to get excited about a little swr. The reason that the feed
line is so short is that the feed point is inside the shack. I have
permission from my hoa to have an outside antenna provided that it's
unobtrusive and mine is almost stealth.
http://vibrotek.com/w5dc/w5dcant.htmlI'm currently at 95 dxcc countries since summer, up 33 from a few weeks
ago when I replaced my 1975 vintage, very modified, FT-101E with the K3.
One problem that I do have is that the K3 output seems low on 30 meters,
varying from 85 to 90 watts with both the antenna and with my Heath
Cantenna. Other bands show 110 watts.
One thing to keep in mind is that a lot of coax that's sold as 50 ohms
is really more like 60 ohms. The typical "50 ohm" foam coax actually
has a characteristic impedance of about 59.5 ohms so that an expected
1.1 to 1 swr could really be 1.44 and vice versa.
73, Dunc, W5DC (sometimes professional antenna designer - read qrz.com bio)
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