[Elecraft] 80 meters

[Jim Brown]

On 12/31/2017 10:42 AM, Bob McGraw K4TAX wrote:
> Yes, I've done that.  However, since the radio draws a different 
> amount of current at 100 watts CW into a 50 ohm dummy load depending 
> on band, the IR drop in the DC  line will be slightly different from 
> band to band. 

Most rigs that run on DC are designed to be run at 13.8V. It's important 
to realize that IMD in many rigs, including the K3, increases with power 
output AND with reduced power supply voltage. K6XX is an EE at Elecraft 
who works in manufacturing -- that is, taking designs from the team, 
putting them into production, and preliminary testing for the FCC 
approval process. Bob, a serious contester, is my neighbor, and we are 
both members of the Northern California Contest Club (as is Eric Swartz).

Several years ago, Bob put together a technical talk for NCCC on IMD, to 
which I contributed some on-air measurements and observations on the 
contributions of audio to distortion and splatter. Bob's slides include 
lab measurements documenting the increased IMD resulting from operating 
at reduced power supply voltage and closer to rated output. BTW -- early 
K3 firmware permitted operation to 120W; that was reduced to 110W 
because IMD at the higher power level was judged excessive. The slides 
for that talk are at

http://k9yc.com/K6XXAmpTalk.pdf

I've since done my own measurements showing that CW bandwidth is 
considerably lower at the 30-60W levels needed to drive most power amps 
than at 100W. That data is at

http://k9yc.com/P3_Spectrum_Measurements.pdf

In my own shack, a pair of K3s (for SO2R) runs from a 100Ah 
float-charged LiFePO battery, so terminal voltage stays in the range of 
13-13.6V keydown. About two years ago, I switched to the LiFePO battery 
from a big lead acid battery, because terminal voltage under load was in 
the range of 12.3 - 11.8V.

73, Jim K9YC