Re: KPA-1500 issues

Posted by Bob McGraw - K4TAX on Nov 30, 2018; 1:22am
URL: http://elecraft.85.s1.nabble.com/KPA-1500-issues-tp7646571p7646653.html

In checking my K3S into an antenna that presents a 2:1 SWR as indicated
on my external Power/SWR meter, my KPA500 in STBY, and my KAT500 in
bypass mode, it does deliver 100 watts.   Although, I prefer not to
operate in that configuration if not necessary.

I think in general we fret too much over values of SWR which are 1.5:1
or less at the station end i.e. between the tuner and the amp or
transceiver.  This is  without regard to the actual SWR on the feedline
between the tuner and the antenna load.  That is where the real power
loss occurs.  Also, in many cases, using a tuner in a feedline where the
SWR is 1.5:1 or less, most likely the tuner introduces more loss than if
the tuner was not in the path.    There is a nice XLS spreadsheet
http://www.dj0ip.de/antenna-matchboxes/matchbox-shoot-out/ showing the
actual loss of many different tuners, different bands, and different
matched and unmatched losses.   Rather eye opening.

Regarding the SWR fault at 1.4:1, is that at 35 ohms or at 70 ohms?  
Significant difference in current demand of the PA between the two
values while SWR appears to remain the same.   This is largely the
reason I often state that SWR only tells 1/2 of the story of antenna
system performance.  A PA correctly designed will monitor the current in
addition to SWR  and not just SWR as most seem to do.

73

Bob, K4TAX


On 11/29/2018 6:16 PM, K9MA wrote:

>
> That certainly is not the case with the KPA1500. An SWR of 1.4:1 can
> cause it to fault well below 1500 W due to either excess current or
> excessive drive power, or to greatly exceed 1500 W output.
>
> I don't think the K3 will deliver anywhere near 100 W at 2:1, either.
>
> 73,
>
> Scott K9MA
>

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