[Elecraft] K3 SHUTDOWN.

[Lyle Johnson]

In the end, it is a THERMAL problem.

The K3 PA heatsink+fan has a limited, finite thermal resistance to the 
ambient air, as well as a limited thermal mass of its own to absorb 
heat.  The heat dissipated in the PA transistors has to go somewhere, 
and by design and construction it goes into the heatsink.  The fan uses 
moving ambient air to reduce the thermal resistance of the heat sink (or 
perhaps you can view this as increasing its heat capacity).

Now, if your ambient air is cold and dense, and perhaps a bit moist, it 
has relatively larger heat capacity than if it is hot, thin and dry.  
Put another way, you can run more power in high duty-cycle modes if your 
operating environment is cool and your altitude low than you can in a 
high desert.

To find out how much power you can safely run in a high duty cycle 
mode,,you should run the PA (preferably into an adequate dummy load) and 
monitor its temperature rise, rate of rise and final stable temperature 
for various power levels and bands (since the efficiency typically 
varies from band to band) while noting your ambient air temperature (and 
perhaps humidity).  If you do this, you'll be able to figure out the 
temperature difference between the PA and the ambient air, and how it is 
affected by the power you are running or wanting to run.  Or if you need 
an external fan, or redirect an air conditioning duct towards the radio, 
or...

Then, BEFORE the contest, while your mind is still clear, you can 
determine what power setting you can safely run on each band, and set up 
the K3 for that power level.  You can now run the contest with one more 
piece of station engineering firmly in hand for a competitive edge 
(either by having a bit more power, or by not having the radio shut down 
when working that rare multiplier).

As a bit of encouragement to spend a little time doing this, you may be 
surprised at the PA power dissipation versus power output. And you may 
learn something about thermal design, resistance, and so forth.

73,

Lyle KK7P

> ...I have not seen any caveats either in the Elecraft manual, or in Fred's
> book about not being able to run at 100 watts...