[Elecraft] K2 power output tests at low supply voltages

[Wayne Burdick]

I just completed some preliminary tests to see what the K2 could do with really
low supply voltages.

On 20 meters, both K2s I tested could easily put out 10 watts in CW mode down to
as low as 9.5 volts, and 5 watts down to as low as 9.0 volts. In both cases,
there was absolutely no hint of frequency shift or other artifacts, and no
change in the CW keying waveform (as observed on a scope). I also listened on a
separate receiver, and went back and forth between 9 volts and 14 volts while
keying at 5 watts. There was no change in quality or signal strength. 

On the lowest and highest bands I found I had to reduce power slightly below
these levels. 

Based on these tests, here are my (conservative) recommended maximum power
settings vs. DC input voltage:


        DC VOLTAGE     POWER OUTPUT
        ----------     ------------
           9.0 V           2.0 W
           9.5 V           5.0 W
          10.0 V           7.0 W
          10.5 V          10.0 W

These figures should be reassuring to anyone who is planning to operate from
batteries. The signal will be perfectly stable at 5 watts well below the
recommended usable voltage for a depleted 12-V battery. Even a 9.6-volt, 8-cell
NiMH battery pack would work in a pinch. (I'd go with C cells, personally.)
We'll be adding this table to the K2 performance comparisons page soon.

There are some caveats to keep in mind with low voltage operation:

- I was using a 95SQ015 at D10 on the RF board (reverse polarity protection
diode), rather than the original SB530. The 95SQ015 type drops about 0.2 to 0.3
volts less on transmit. This diode will be included in the rev A to rev B
upgrade kit we're working on. 

- To ensure accuracy when testing at low voltages, the voltage must be measured
right at the DC input jack, in transmit mode. Power cables can drop considerable
voltage themselves if they're long and/or use small-gauge wire. To minimize this
effect, I use at least #18 wire, no longer than 3-5 feet, when possible.

- The KBT2 and other 12-V Gel-Cell batteries should not be used below 10 volts
for any length of time, as they can become permanently damaged. (The K2 will
give you a LO BATT warning as a reminder--see the owner's manual for more
details.) Check your supply voltage as needed using the K2's voltage/current
display mode (by tapping DISPLAY).

- If you want to use the maximum possible power output at a given voltage,
rather than the level in the table above, go for it--but listen to the signal on
another receiver first to make sure the keying is still crisp and clean. 

73,
Wayne
N6KR