[Elecraft] KX3/KXPA100 Power Output Difference

[Bob McGraw K4TAX]

Don's points are very good.   Add to that the resistance or load value 
to which the power indicator was calibrated.  Ideally it is 50 ohms R 
+j0.  However, being a voltage sensing circuit, any deviation from 50 
ohms will impact the indication.  Thus a 55 ohm load, showing a SWR of 
1.1:1 can cause a 10% error as a higher power indication.   And with a 
45 ohm load, showing a SWR of 1.1:1, will show a 10% lower power 
reading.  Many ham watt meters are only accurate to about 20% of full 
scale largely to the above as well as circuit components being frequency 
dependent.

73

Bob, K4TAX


On 12/1/2017 10:23 PM, Don Wilhelm wrote:
> Jim,
>
> Amateur wattmeters are notoriously "famous" for their inaccuracies. 
> Errors of up to 20% are not unusual, and in many cases it depends on 
> the band because the voltage produced by the measurement diodes are 
> frequency dependent.
>
> So bottom line, don't worry about the 10 watt differential.  You are 
> like the man with two watches who does not really know what time it is!
>
> On the KX3 Yahoo Group question, I receive individual emails (not the 
> digest), but something has happened, and I cannot post to any of the 
> Yahoo groups that I am subscribed to for the past 2 weeks. I think 
> Yahoo has "gone to the dogs".
>
> 73,
> Don W3FPR
>
> On 12/1/2017 9:33 PM, Jim Ruff via Elecraft wrote:
>> I using a KX3/KXPA100 and I'm noticing that when I'm in TX the RF 
>> meter on the rig matches the set power of the rig, but on the KXP100 
>> meter, it shows 10 or more watts less output.  Some examples:Rig   
>> Amp50w  40w60w  50w70w 60w80w  70w
>> Here is what i've checked so far.  The Powerwerx PS is showing 13.8v 
>> when receiving and dips to 12.9v when in TX, so I don't believe I 
>> have a voltage problem.  I've moved the rig's power connection  back 
>> and forth from the front of the Powerwerx  PS to the top power pole 
>> connection on the back of the KXPA100 with no change.I've also 
>> re-tuned all bands from 10-40 meters with my LDG100 remote tuner and 
>> the highest SWR I receive is 1.4:1 on 30 and 40 meters.  All of the 
>> other bands are 1:1 to 1.2:1, so I don't think I'm getting any power 
>> foldback because of high SWR..  Does anyone have any thoughts on 
>> what's causing this differential, and if two power output meters are 
>> never going to be in sync, which meter is the most accurate?
>> Also, as an aside. Is there something wrong with the KX3 group from 
>> Yahoo? I haven't recieved a digest in days and when I try to get into 
>> the group I get an error message
>> Thanks,Jim W7JHR
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