[Elecraft] K3 - SWR Indication

[Steve Ellington]

The method of comparison is flawed. It appears that your coax does not 
exactly match the impedance of your dipole (It rarely does). Take any SWR 
meter and insert it at different locations along the length of the feeder 
and you will get different readings. The only place it is anywhere close to 
correct is right at the antenna or at the end of the feedline if it happens 
to be exactly 1/2 wavelength (rare). Even though your K3's meter and your 
external meters are close together, there is still probably a measurable 
electrical distance between them so there's no way they will read the same. 
I've seen it a hundred times. You are dealing with a reactive circuit here. 
The only way to compare meters is to put them in a resistive circuit, ie. 
dummy load. All 3 meters should read 1:1 into a 50 ohm dummy. Connect 2 
dummies in series for 100 ohms and all meters should read 2:1. That's how 
the ARRL test them.
Trimming that dipole to force your meter to indicate 1:1 does not mean that 
there is no swr on your coax. You may just be moving the low spot around 
until it lands where your meter happens to be connected. Changing your 
feeder length would accomplish the same thing. If you do manage to get a 1:1 
reading, try changing the length of your coax. If you really do have a 1:1 
swr, the length of the coax won't matter and all your meters "should" read 
the same. There are just too many factors that affect the impedance of that 
dipole. The height above ground can change the swr from 1:1 to 3:1 very 
easily. Try the dummy load trick first and see what you get. I bet they a 3 
read 1:1. Just remember, that swr meter in the K3 does not necessarily 
indicate the match between your dipole and your coax. That's why turners are 
so handy.
Sorry for the ramblings. Good luck!
Steve Ellington N4LQ
N4LQ at carolina.rr.com
----- Original Message ----- 
From: "Geoffrey Downs" <geoffrey at downs.globalnet.co.uk>
To: <elecraft at mailman.qth.net>
Sent: Sunday, May 03, 2009 7:12 PM
Subject: [Elecraft] K3 - SWR Indication


> In the past week I've been tuning and pruning an inverted V dipole I've 
> put
> up for 40m, fed with coax. The job isn't finished yet but I'm noticing
> significant differences between SWR readings on my K3 and an external
> LP100A.
>
> For example: At 7.000 MHz the K3 currently shows 1.2:1 and the LP100A
> 2.00:1. At 7.200 MHz the K3 shows 1.0:1 and the LP100A 1.33:1. Both agree
> that it's 1.0:1 at 7.145 MHz. My MFJ 259B shows similar readings to the
> LP100A.
>
> The differences seem more than one would expect from stray impedance in 
> the
> KAT3 as referred to when this subject was brought up on the reflector in
> January this year.  I've not made direct comparisons like this before so I
> don't know if it's a recent phenomenon with my K3 (S/n 266) or has always
> been so.
>
> The readings were taken with PWR at 15w and the LP100A coupler connected 
> to
> ANT1 with a back-to-back PL259 adapter. ATU was of course in bypass mode.
> Firmware 3.11.
>
> What do others see?
>
> 73 to all
>
> Geoff
> G3UCK
>
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