[Elecraft] K2 power output level fluctuation

Fri Apr 24 07:43:43 EDT 2015

Matt is correct.
Adding an Amplifier Keying Circuit to the K2 is necessary and quite easy.
Take a look at http://www.mmccs.com/mmarc/n0ss/index_k2.html for a good 
amp T-R Keying circuit.
Note that this is the preserved website of Tom Hammond N0SS (SK). The 
kits mentioned are *not* available, but the circuit is easy enough to 
wire on perfboard - Tom provided the layout.  Of course, if you etch 
your own boards, you can do that too.

73,
Don W3FPR

On 4/24/2015 4:56 AM, Matt VK2RQ wrote:
> RF-sensed in that it detects what frequency you are transmitting on, and selects the appropriate bandpass filter, antenna and tuning solution from memory. The keying is not RF-sensing, but is based on a key-out line from the K2 -- you may need to add one, if your K2 doesn't have one already. I find that the K2 with KXPA100 is a FB combination, no fans, silent keying.
>
> 73, Matt VK2RQ
>
>> On 24 Apr 2015, at 2:22 pm, Don Wilhelm <w3fpr at embarqmail.com> wrote:
>>
>> 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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