[Elecraft] k3 SWR readings seem generous

[Don Wilhelm]

Mike,

I have to accept your observations, but must ask - have you calibrated 
the K3 wattmeter?  Neglecting to do that can cause the SWR readings to 
be off by a fair amount.

That being said, consider the purpose of the K3 having an SWR indication 
at all - that is to determine the minimum SWR during a TUNE cycle.  The 
*minimum* SWR is the important parameter to indicate the best tuning 
point rather than any absolute SWR reading.

Long ago when using manual tuners, my elmer told me to simply tune for 
the lowest reflected power possible, and that was the correct tuning 
point.  The KAT3 tuner algorithm does the same thing - it seeks the 
minimum reflected power - the absolute SWR is not important, although it 
may be of interest.

Even so, the K3 SWR indication on your K3 does seem to be lower than it 
should be.  Again, if you have not done the Wattmeter Calibration, that 
would be a step in the right direction.

73,
Don W3FPR

Mike Scott wrote:
> I have been working on antennas today and have a puzzle. I am finding that
> the K3 SWR indications are unbelievable generous. 
>
> With my 40M inverted V fed with RG8X (50 ohm) coax I see that the K3 seems
> generous in the SWR indication on tune with ATU bypassed. I have noticed a
> slight change in SWR with K3 power setting so the following was taken with
> K3 tune power set to 100 watts. If I set it to 10 watts the discrepancy is
> greater...
>               SWR     SWR     SWR
> Freq.         K3      VA1     Daiwa
> 7.00 MHz     1.3      1.92    2.0
> 7.05 MHz     1.2      1.92    1.9
> 7.1 MHz      1.2      1.98    1.95
> 7.15 MHz     1.3      2.26    2.1
> 7.2 MHz      1.9      2.60    2.3
> 7.25 MHz     1.5      2.88    2.6
> 7.3 MHz      1.6      3.32    2.7
>
> The Daiwa is the CN-101L cross needle meter. The VA1 is the Autek Rx Vector
> antenna analyzer.  The VA1 does not have a tuned input so it is possible
> that it can be fooled by local broadcast stations. I don't suspect this
> issue here as there aren't any close by broadcast stations that I am aware
> of. The Daiwa and the VA1 tend to agree pretty well anyway. The K3
> measurements are the outliers. This antenna on a "Tenna Dipper" shows
> minimal SWR at 7.040 MHz, just where I tuned it a few years ago. The LED
> does not extinguish indicating higher than 1:1 SWR at the minimum. If you
> look at the numbers, I guess I don't believe the K3, Do you think my antenna
> is below 1.6 to 1 across the entire 40M band?
>
>
> So now on to 20M dipole fed with RG58 (50 Ohm Coax)...
> The Tenna Dipper fully extinguishes at 13.805 MHz indicating that my antenna
> is bit too long but is close to 1:1 at that frequency.
>
> Freq        K3     VA1     Daiwa
> 13.81       --     1.12    --
> 14.0 MHz    1.1    1.24    1.31
> 14.1 MHz    1.1    1.30    1.70
> 14.2 MHz    1.1    1.55    1.85
> 14.3 MHz    1.2    1.69    1.85
>  
> RG58 may have some losses on this band so perhaps all of the SWR indications
> are generous, some more than others. The dipole is a low one and probably
> isn't 72 ohms at the feed point.
>   
>