[Elecraft] KPA-1500 Power Output Variation

[Jim Brown]

On 8/24/2019 2:40 PM, Wes wrote:
> Jim I don't have a KPA1500, but do have a KPA500. I'm not particularly 
> surprised at your observations based on mine with the '500.
>
> After a recent thread about amplifier efficiency I ran some tests on 
> mine, using a well characterized dummy load and an LP-100A wattmeter, 
> at rated output.

I've had my KPA1500 for only a few months, and 99% of my operation has 
been on 6M. I also have an LP100A inline, calibrated at their factory. 
On 6M, with the tuner bypassed, The LP100A regularly reads about 100W 
(0.3 dB) lower than KPA1500 on keydown transmissions, and the LP100A 
reads about 1.8:1 SWR. The KPA1500 indicates 1.3:1 - 1.1:1 SWR 
(depending on the weather). If I engage the tuner, output power as read 
by the LP100A drops by 100W or so, indicating loss on the order of 0.3 
dB in the tuner.

How could this be? Simple. There are a relay, two UHF connector 
junctions, and 6-8 ft of RG400 (0.2 dB @ 50 MHz) between the LP100A and 
the output of the KPA1500. The connectors each contribute a bit of 
mismatch at 50 MHz, and the length of coax is transforming the load that 
the amp sees, so the LP100A and the KPA1500 are measuring both power and 
VSWR at different points.

Power accuracy of the LP100A is 5% (0.21 dB) worst case, 3% (0.13 dB) 
typical, and the power reading compensates for mismatch. It can read 
average, peak, and peak hold.

The point of all this is that, when you're trying to see small 
differences, there are a lot of little things that can add errors or 
make you think something is wrong when there isn't. :)

73, Jim K9YC