Posted by
K9MA on
Aug 25, 2019; 4:51am
URL: http://elecraft.85.s1.nabble.com/KPA-1500-Power-Output-Variation-tp7654657p7654684.html
Here’s a way to get a worst case estimate of the effect of SWR on power output. Assume the amplifier has constant VOLTAGE gain and an output impedance much smaller than 50 Ohms. A 1.3:1 SWR means the load impedance could be as low as 39 Ohms or as high as 65 Ohms. Let’s say the SWR is 1:1 at the frequency at which the drive is set for 1500 W output. Then you tune to where the SWR is 1.3:1. At that SWR, the output could be anything from 1150 to 1950 W. The range may be less, as the amplifier output impedance may not be all that low compared to 50 Ohms. In any case, this shows that the power output of a solid state amplifier may be very sensitive to load SWR.
This is why the KPA1500 ATU really needs to be able to keep the SWR low. The recently improved autotune firmware does this very well, if you “train” it across the band. It is also why it’s important to use the ATU, even if the antenna SWR is “only”, say, 1.5:1.
I keep hoping Elecraft will come up with a way to automatically adjust drive power to maintain constant output. That would also compensate for change in temperature.
73,
Scott K9MA
----------
Scott Ellington
--- via iPad
> On Aug 24, 2019, at 1:45 PM, Jim McCook <
[hidden email]> wrote:
>
> I'm using my K-3 with the KPA-1500 and have a problem with extreme changes in power output with a given drive level on 40m CW. This happens to a lesser extent on other bands with different antennas. My 40m antenna is tuned to 7.025 where SWR is 1.1:1. On 7.000 it is 1.4:1 and at 7.050 it's 1.3:1. Within that range of frequencies the output _without_ the tuner at a given drive level (30w) varies from 1300w to well over 1700w. Using careful _tuner_ settings it's only slightly better. I have to reduce drive to 24 or 25w to get below 1500w, depending on the temperature of the amplifier (power output varies a lot with temperature changes). Higher temp = lower output.
>
> I have tried to compensate in the segments where output is excessive by using manual settings via the Utility. This has helped slightly, but still isn't adequate. I've resisted using ALC because of concern for generating distortion. Having to ride the drive level from the K-3 is a PIA, especially during contests.
>
> It seems to me a good answer would be to have higher input attenuation so drive requirement would be higher, maybe around 70+ watts. I'd expect changing drive level by the same number of watts (5 or 6) would not affect output as much. I'd also assume the power output would not vary nearly so much with a given drive level.
>
> Is this a possible solution? I've also understood that most transmitters (even the K-3) have cleaner output near highest rated output.
>
> Help!
>
> Jim W6YA
>
>
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