[Elecraft] K2/100 - Power investigation

[David Wilburn]

Understand.  I did pick up the DL1, and check the readings.  They were 
along the lines of what I experienced with the MFJ.  But I could not 
read it at a higher power, as it is only a 20w dummy load.  But it 
jibed with what I was seeing.  I will get further readings with it and 
a voltmeter.


Dave Wilburn
K4DGW
K2/100 - S/N 5982


Don Wilhelm wrote:
> Dave,
> 
> I cannot really give you a quick positive answer - the real answer is 
> "it all depends".  To give a positive answer would require that you 
> operate into a dummy load which is 50 ohms non-reactive at all 
> frequencies in question.  One can measure the dummy load with an antenna 
> analyzer - expect to see 50 + j0 at all frequencies of interest for 
> reliable and repeatable results.
> 
> I do not generally trust any wattmeter (including an uncalibrated Bird) 
> to provide any greater than 20% of full scale accuracy.  If the full 
> scale reading is 100 watts, then the error can be as great as 20 watts 
> at any point on its scale.  A properly calibrated KPA100 wattmeter can 
> do better than that.
> 
> I find that with a good 50 ohm dummy load and an 100 MHz oscilloscope 
> with a 10X probe (probe rated for 100 MHz), I can calibrate the KPA100 
> wattmeter to less than 5% accuracy on bands below 20 meters - at higher 
> frequencies, the 'scope and probe rolloff will indicate lower than 
> expected results.  But I can state that once calibrated, the KPA100 
> power calibration can be trusted to greater accuracy than the 
> run-of-the-mill wattmeters.
> 
> Using a good 50 ohm non-reactive dummy load and measuring the RF voltage 
> across it:
> At 4 watts, the Peak to Peak RF voltage will be 40 volts.
> At 10 watts the p-p RF voltage will be 63.25 volts
> At 40 watts, the p-p RF voltage will be 126.5 volts
> At 100 watts, the p-p RF voltage will be 200 volts.
> 
> My dummy load is a heat-sinked Caddock MP9100-50.0-1% resistor rated at 
> 100 watts with zero length leads that has been measured flat at 50 ohms 
> up to 60 MHz.
> 
> Up to the frequency limits of my oscilloscope, I trust my dummy load and 
> the peak to peak voltage measurement to provide more accurate power 
> measurement than any wattmeter available.  I find that, after 
> calibrating with these tools, the typical KPA100 power indications are 
> within 5% of the actual power output.
> 
> I can also say that the TelePost LP-200 (and I assume the LP100) will 
> also produce power readings which are within 5% of the actual power 
> after proper calibration.  I cannot make such statements about the 
> accuracy of the more common wattmeters which spec their accuracy as a 
> percentage of the full scale reading.  A Bird with a 100 watt slug that 
> has been recently calibrated will have an accuracy of 5% of full scale, 
> which is a 5 watt error anywhere on the scale - so a recently calibrated 
> Bird with a 100 watt slug will indicate anywhere between 5 watts and 15 
> watts for an actual power of 10 watts.  One can do better with a 
> precision 50 ohm load and a 100 MHz oscilloscope.
> 
> OK, that is my wattmeter rant for this month!!!
> 
> 73,
> Don W3FPR
> 
> David Wilburn wrote:
>> After trying unsuccessfully to get email to take data in tabs, I ended 
>> up just putting the information on my website.  Sorry for the trouble.
>>
>> I was playing with an assortment of power meters, and noticed my K2 
>> was reading lower than I expected.  The closes things I had to a 
>> gauge, was an IC-703+ that had just been to the shop, and repaired, 
>> thus I felt it was likely to be putting out 10w.
>>
>> When I compared a Bird 43 with an element I picked up on line, and one 
>> that came with it, it read a bit low.  But when I used an MFJ949e, it 
>> was right on.  Gotta run with what I have.
>>
>> So I checked out the K2/100 at what it said was 10w/53w/111w (Full 
>> Power).  And I got the results, that are displayed here;
>>
>> http://www.k4dgw.com/k2.html
>>
>> The 10w settings seem a little bit more dispersed than I would have 
>> thought.  The 10m, 12m & 20m 53w settings seem a bit low.  Then at 
>> what the rig says is 111w/Full Power, the rig seems even lower.
>>
>> Do I need to go back through and run the power calibration, or should 
>> I start troubleshooting?
>>
>> I have a KAT100, that was powered down, and the power was taken out of 
>> the back of the K2/100.
>