[Elecraft] [KPA500] K3S/KPA500/KAT500 - Adjusting TX Power

[Jack Brindle]

The three units are taking widely different measurements. The KPA500 measures the signals it sees going into the KAT500,
while the KAT500 measures what it sees going into the choke. Finally, the P3TXMON’s coupler measures what it
sees going into the coax going to the SG-235. It is quite probable they will actually see different things. To make matters
worse, you are trying to take measurements with a varying signal.

You really want to step back and make the measurements with a steady signal (CW, key down) to see what is really happening.
But when you do, be aware of what is happening in the SG-235. While the SGC literature says it is rated for 500W SSB, the important
line says only 200W continuous. That means at 500W there is quite  about of heating occurring in the toroid cores in the tuner. They
won’t have much of any time to cool down between transmissions (SSB has very low peak to off times). Much of this and you will
see the P3TXMon and KAT500 reading skyrocket and then will send the SGC in for repairs.

If you intend to keep this setup, dial the K3S power down so that the SGC really only sees 200 watts, test everything at that level, then
don’t change power levels when you switch to the digital mode. Also, you might want to move the choke to the output side of the
P3TXMon pickup unit.The choke is designed to clear up currents on the outside of the coax shield (which you don’t want for
reasons others will most likely explain). You are exposing the P3TXMON to these currents and probably adversely affecting
its measurements. Make that change and the P3TXMON and KAT500 readings should become a bit closer.

And remember to do all this at CW. Taking measurements with a varying signal can introduce interesting issues in
the measurements themselves. The issue has to do with when the two signals (forward and reflected voltage) are actually
measured. Unless they are simultaneously measured, then you won’t get precise values unless the signal is steady-state.
I suspect you are running into a bit of this also.

- Jack, W6FB


> On Sep 4, 2017, at 5:06 AM, MaverickNH <cbjesseeNH at comcast.net> wrote:
> 
> I'm learning about a new KPA500/KAT500 installed with my K3S. When I have the
> K3S power knob set, in this case, to show 420-440W peak-hold as read on the
> KPA500 LED screen, I see the KPA500 SWR showing 1-3 LEDs lit (it may stay at
> 1 LED or climb from 1 to 3 LEDs over 15sec TX, always below 1.5), the KAT500
> with SWR 1.2 LED lit, and P3TXMON indicating SWR 1.00 (it might start at
> 1.15 and settle back to 1.00 over 15sec TX). But PEP on the P3TXMON can
> range from 10-15W up to ~400W on successive 15sec periods of TX using WSJT-X
> FT8 (going for very distant CQs). Adjusting the K3S power knob down may
> bring P3TXMON displayed power up if its 10-15W. If the P3TXMON shows 200W
> adjusting the K3S power knob up may bring the P3TXMON power shown down.
> 
> Is that how it's supposed to work? 
> 
> I have a P3TXMON 2000W 1.8-54MHz unit installed on my feedline at the rig as
> follows:  KAT500 to Common Mode Choke to P3TXMON to Bias-T to SG-235 remote
> ATU at antenna base to 43ft Vertical antenna, with some Lightning GDTs
> between at antenna base and home feedline entrance. My tuning practice is to
> change Frequency, turn KAT500 to BYP, tune the remote ATU, then set KAT500
> to AUTO and let it adjust on 1st TX.
> 
> Bret/N4SRN
> 
> 
> 
> --
> Sent from: http://elecraft.365791.n2.nabble.com/
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