[Elecraft] K2 power output level fluctuation

[Don Wilhelm]

The KXPA100 will work with the K2, but just about the same as it will 
with any other non-Elecraft QRP transceiver.
Of course, the KXPA100 integrates very nicely with the KX3.

In other words, the K2 wit KPA100 works like a 100 watt class transceiver.
The KX3 with the KXPA100 operates like a 100 watt transceiver.
The KXPA100 with any QRP transceiver driving it (K2 included) operates 
like a transceiver followed by an RF sensed 100 watt amplifier.

73,
Don W3FPR

On 4/23/2015 10:55 PM, Phil Wheeler wrote:
> 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?
>
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