[Elecraft] K2 power output level fluctuation

[Phil Wheeler]

Don,

Since I have the KPA/KAT100 I've not really given 
it much thought, but how about using a KXPA100 
with the K2?  Would they integrate well?

Phil W7OX

On 4/23/15 7:21 PM, Don Wilhelm wrote:
> George,
>
> That is a relatively small change in power 
> output.  I do not think you will have a problem 
> with it in normal operation.
> Keep in mind that the K2 power control resets 
> after a band change or a change in the requested 
> power setting - the output starts out low and 
> then increases to the requested setting.  That 
> is done purposefully to prevent overshoot.
>
> The initial increase you see is likely a result 
> of starting out at a lower power level and then 
> increasing.  It takes a couple dit times to come 
> up to the requested power.
>
> I have no suggestions on the homebrew amplifier 
> other than to say that if you include a closed 
> loop power control as is done in the KPA100, you 
> may find it easier to simply add the real KPA100 
> - the KPA100 wattmeter circuit is scaled for 
> high power operation and eliminates the scaling 
> for low power operation - all that is controlled 
> by the KPA100 firmware.  I doubt your homebrew 
> amplifier can achieve that level of integration 
> with the K2 at a price less than the KPA100.
>
> 73,
> Don W3FPR
>
> On 4/23/2015 8:43 PM, George Kuehn wrote:
>> I know from the forum that a K2 will show a 
>> power increase over a given setting if left in 
>> tune for an extended time as a consequence of 
>> the final transistors heating up.  That makes 
>> sense from what I’m seeing here.  The effect is 
>> greater at higher power settings although not 
>> proportionally higher as my set level 
>> increases.  I’m just wondering if the amount of 
>> shift I’m seeing is normal.
>>
>> Here’s what I’m seeing:  With my K2 operating 
>> into a zero SWR 80 watt (nominal) dummy, the 
>> observed increase both on the K2 display as 
>> well as on my computer monitored W2 wattmeter 
>> tends to be 1.5 watts or less over a ten second 
>> period.  The initial increase happens quite 
>> quickly and then slows.   It is difficult to 
>> confirm, as I don’t want to leave the rig in 
>> tune for an extended period, but it seems like 
>> the system stabilizes or at least increases 
>> very slowly once it passes the initial 
>> increase. Operating CW at 12-15 wpm into the 
>> dummy, the power output fluctuates but doesn’t 
>> exhibit a tendency to rise more than noted 
>> above, given a 50/50% sending/receiving behavior.
>>
>> My observations were made at 7.100 MHz with 
>> power settings at 2, 5 and 10 watts.  The K2 
>> was connected by short length coax leads 
>> through the 200 watt W2 Wattmeter  Directional 
>> Coupler into a dummy consisting of a set of 4 
>> series-parallel 50 ohm Caddock MP820 thick film 
>> non-inductive resistors mounted on a fan-cooled 
>> heatsink (nominal 80 watt rating) that shows 
>> zero SWR from 160 meters through 24 MHz and 1.1 
>> SWR thereafter though 6 meters as measured with 
>> an MFJ 259B Antenna Analyzer.  The resistors 
>> remain cool to the touch even after extensive 
>> testing.
>>
>> So, do I need to be concerned about the power 
>> increase behavior I’m seeing?
>>
>> I do wish to operate digital modes such as 
>> PSK31, Hellschreiber, and similar modes where 
>> the duty cycle is higher than casual CW or SSB 
>> phone.  Assuming normal power level control in 
>> the K2, is there a power level setting 
>> recommended to protect the final in this type 
>> of operation?
>>
>> In the future, I plan to homebrew a linear 
>> amplifier to give me PEP output in the 80 to 
>> 100 watt range primarily for SSB.  I was hoping 
>> not to have to create gain control back into 
>> the K2 from the amp as is done in the K2/100 
>> setup.  Any recommendations from those who run 
>> a homebrew linear?