K3/KPA500 VSWR on 6m

[wa9fvp]

Does anyone have this problem?

I was having problems on the low end of 6 meters.  VSWR was all over the place!  The K3 would always measure a 1.2 :1, the KPA500 measured 1.5 to one and the KAT500 wouldn't match my 6 meter beam.  I agree that there was a problem with my Hygain 66DX6 but even on my Bird dummy load the SWR measurements were off.  On Sunday, I re-tuned my 6 meter beam.  I was able to match the antenna but the VSWR on the K3 was still all over the map.  

I remember when I was building the KPA500 that there was a 4" twisted wire from the PA board to the RF out jack.  A short twisted wire as a transmission line on the HF bands is probably OK but On 6 meters, the inductance in twisted wire, I'm sure, will exhibit loss.  

I replace it the wire with a 4" length of RG58U.  Now all 3 SWR readings agree and the VSWR is 1.1:1.

Jack WA9FVP

[Don Wilhelm-4]

Jack,

I am sorry to hear that you had problems with the twisted pair wire.

Normally twisted pair makes a very effective and self shielding
transmission line, even at 6 meters.
The characteristic impedance does depend on the diameter of the
conductors and the thickness of the insulation separating the two
conductors.

I am glad that you had success with your RG-58 substitution, but I
believe there was some other condition present that your substitution
corrected.

73,
Don W3FPR

On 11/17/2015 5:55 PM, wa9fvp wrote:
> I remember when I was building the KPA500 that there was a 4" twisted wire
> from the PA board to the RF out jack.  A short twisted wire as a
> transmission line on the HF bands is probably OK but On 6 meters, the
> inductance in twisted wire, I'm sure, will exhibit loss.
>
> I replace it the wire with a 4" length of RG58U.  Now all 3 SWR readings
> agree and the VSWR is 1.1:1.
>
>

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[wa9fvp]

When the KPA500 was keyed up, the K3 VSWR went from 1.2:1 to 1.0:1. Because the input circuit offered a j0 match, I concluded that the problem had to be associated with the amplifiers output.  In bypass mode, the bypass relay connected the input directly to the output.  I was confident that the Elecraft did their homework and the PA, PC board design was correct and offered a j0 match from the input to the output.  I think the problem is on the ground side of the twisted pair. There's enough stray inductance to introduce loss between the PA board ground and the connector ground. There's no direct connection from the wire ground pin to the chassis. If my memory serves me, the only ground points on the PA board are on the corners of the board. On the input connection, there's a mini coax jumper.

Before changing it, I untwisted the wire about 4 turns and the VSWR went from 1.2:1 to 1.4:1.  I twisted the wire tighter and it went back to the original measurement.

When I worked at Argonne National labs, in the Avanced Photon Source RF group, we were working with 350 MHz, 1 megawatt klystrons.  We learned all about ground loops the hard way.

jack WA9FVP

[KD7PY]

I noticed that my vswr on the K3 did not match what the KPA500 showed in the standby mode.
what I found was that I had the 6 mtr ant connected to the ant 3 jack.
I changed it to ant 1 and the vswr was in more in agrement with the K3.
It seems that the distance from the input to ant 3 connector is enough to add strays to the input to output ckt.

I think I'll change the twisted wire to coax and see if it improves

Ed  KD7PY  ex: K7WIA

[KD7PY]

That should read  the antenna connections on the back of the KAT500