[Elecraft] [K2] SWR meter reads 'funny' on just 20 meters

[Don Wilhelm]

Steve,

Yes, the weather has been great here in the Triangle area.

Generally OCF dipoles can generate a lot of common mode RF and bring it 
into the shack unless measures are taken to kill that aspect.
However, your results on 20 meters with the dummy load do deserve a lot 
of careful consideration.

First a statement that the KAT2 is not band dependent when in CAL P or 
CAL S ATU menu state.
The response of the KAT2 diodes may vary a bit with frequency, but not 
to the extent you have stated, and certainly not on any one band.

You say the KAT2 is not "in line".  The menu ATU settings that take the 
KAT2 "off line" are CAL n, CAL P and CAL S.  If you were using CAL P, 
the L and C solution for 20 meters was still present, but the display 
during a TUNE would show the power output with that tuning solution present.

I would suggest that you continue to test into the dummy load, and set 
the KAT2 to Auto, then do a TUNE on 20 meters to see if anything changes.

If things remain the same, I would suspect that there is a relay in the 
KAT2 that is not working properly and leaving some residual L or C in 
the circuit.  Normally, that would affect all bands, but there is an off 
chance that it would only be noticed on one band.  The relays for the 
capacitors are relatively easy to check - go to CAL P or CAL S in the 
ATU menu and then check the resistance to ground for both legs of the 
capacitor - all should measure infinite resistance.

Checking the inductors is not as easy (unless you measure the inductance 
from input to output of the KAT2 with an inductance meter).  The normal 
procedure is to lift one end of each of the inductors and make certain 
that there is still continuity from the input center conductor to the 
output center conductor.  If that continuity test fails, find the relay 
that is open and once found determine why it is not operating to short 
out the inductor.

OK, so much for that kind of theory stuff.  Let's simplify a bit.
If you have excessive L or C in the KAT2, the wattmeter bridge will not 
balance.  So the first quick check is to set the ATU menu to CAL n and 
check the wattmeter for a good null on C55.
If you do achieve a good null, then do the CAL P and CAL S calibration 
and check the SWR indication on all bands again.

If you cannot achieve a good null, it could be that something has 
changed in the wattmeter section, or (as above) it could be a stuck 
relay).   To check whether it is a wattmeter problem or an L/C or relay 
problem, set to ANT1, and then separate the KAT2 LC board from the 
Control Board, then put a long jumper between J4 pin 2 and J5 pin 15 
(one pin in from the ends of those two headers).
Again try the null procedure and the calibration.  If all is OK with 
that test, the problem is on the L/C board, but if you still cannot 
achieve a good null, blame the wattmeter components.
Should it be the wattmeter at fault, any of the components C54, C55, R3, 
R4, D1 and D2 are suspect.
First check the diodes with an ohmmeter preferably set for diode test.  
If OK, proceed to the capacitors - replacement is most certain test 
because they may behave differently at RF than at the low test frequency 
used in a capacitance meter like the AADE.  For the resistors, check the 
soldering first.  If you have to measure the values, lift one end to 
make the measurement.

Good luck with it.

Lastly Steve, if you are using your garage mounted high power dummy 
load, I suggest that you use another dummy load at the workbench (or 
shack) with a short length of coax as a check.  There is a possibility 
that the particular length of coax has somehow changed characteristics 
(maybe loose connectors) and is causing problems on 20 meters due to its 
length.

That exhausts all my current suggestions.

73,
Don W3FPR



On 5/6/2014 9:21 PM, inventor61 . wrote:
> Don (and list)
>
>
> I just replaced my 'back up shack' antenna today (it was a 45/90 OCF 
> experiment) with a 138' open wire fed doublet.  The center is up 50' 
> and the elements are more-or-less installed.  They will get more 
> installed later.
>
> I used the 'Ladder-Snap' pieces to make the feedline.  In 35 years of 
> being licensed and active, I have never had an open wire fed antenna.  
> They look cool, kinda retro, sort of old (like me, I am 53 today), a 
> real classic.
>
> Anyway, I fired up the old K2 and hooded it up to the new aerial.  The 
> K2 (the same one since 1999) has the KAT2 installed, which works well, 
> but the ATU portion isn't in line for this setup.  Instead, I have a 
> DC-phantom-powered SGC-239 out at the feedpoint.
>
> Inside, I have an A-B switch that can steer the rig's output to a lab 
> grade 50 ohm load.
>
> On all bands but 20 meters, the outside tuner quickly found a 
> 'solution' for the new doublet, none worse a match that 1.5:1.  Nice.  
> However, on 20, the K2 said it was 3.0:1  ...
>
> Hmm..
>
> I flipped to the dummy load.  All bands show 1:1 on the K2 display 
> except for 20, where it reads 1.8:1 ... into 50 ohms.
>
> I switched the K2 display to read FWD & REV, instead of SWR, and then 
> the display showed 5.7W fwd and 0.7W reverse into the dummy load.
>
> So, what do you think the problem might be?
>
> Thanks in advance for the opinion.  Hope you're enjoying the terrific 
> weather.
>
>
> Steve KZ1X
> Chapel Hill