[Elecraft] KPA1500 wattmeter adjustment

[Bob McGraw]

Power measurements can be accomplished in other ways.  One is to use an 
RF Amp meter and a known value load.   Broadcast stations have done it 
this way for years.  Hence, 4.4 amps into 50 ohms is 1000 watts.  
Another way is to use the voltage across a value of R and calculate 
power.   Thus 223.6 volts across 50 ohms is 1000 watts.  And finally if 
the voltage and current is known, then E x I = P.   The LP-100 series, 
as I understand, uses two methods, one measuring current and one 
measuring voltage and calculates power. The instrument also costs in the 
neighborhood of $500+.

Most ham meters, including the Bird series are voltage sensing devices 
and presume the load to be some value which was used for calibration.  
Thus if the meter is in a true 50 ohm circuit, it will most likely be 
correct within the percentage of full scale accuracy.  However, if the 
meter was originally calibrated for 50 ohms and it is in a 20 ohm 
circuit the error will be significant. Thus most ham type power meters 
are "relative reading" instruments, and not absolute measuring instruments.

RF Amp meters are relatively easy to verify accuracy.  Being 
thermocouple devices, one can apply 60 Hz of some value of voltage and a 
known load and then determine the accuracy.   Again, I = E/R.

Did we forget about how to use Ohms Law?   Seems so.  Or perhaps we 
never knew to start with.

73

Bob, K4TAX




On 2/6/2022 12:47 PM, Mark Goldberg wrote:
> The only way to really know for sure is to have a NIST traceable 
> calibrated wattmeter and have it calibrated on a regular basis. Hams 
> are generally too cheap to do that. The LP100 does come calibrated, so 
> that would be my go to reference, but I'll bet Hams will hardly ever 
> get the calibration checked after purchase. I could not find how much 
> they charge for that service. Even that is only calibrated to 5% with 
> 3% typical. So, if you off 5W while measuring 100W, who knows, it 
> might be just the calibration error.
>
> 73,
>
> Mark
> W7MLG
>
> On Sun, Feb 6, 2022 at 10:11 AM Bob McGraw <rmcgraw at benlomand.net> wrote:
>
>     'Twas said "a man with one watch always knows the time.  A man
>     with 3 or
>     4 watches is never quite sure of the time.".
>
>     I just recently went through power calibrations on my K3S and my
>     KPA500.  All now agree with the power meter reference I used.
>     Also, this
>     was done using a known 50.5 ohm dummy load.  On the antennas, even
>     though SWR is indicated 1.1:1, there will be differences due to
>     difference line complex impedance's.
>
>     73
>
>     Bob, K4TAX
>
>
>     On 2/6/2022 9:51 AM, elecraft-request at mailman.qth.net wrote:
>     > Message: 15
>     > Date: Sat, 5 Feb 2022 14:53:59 -0600
>     > From: Ronnie Hull<w5sum at comcast.net>
>     > To: Dick Dievendorff<dick at elecraft.com>
>     > Cc:Elecraft at mailman.qth.net <mailto:Cc%3AElecraft at mailman.qth.net>
>     > Subject: Re: [Elecraft] KPA1500 wattmeter adjustment
>     > Message-ID:<BEE49DCD-B7EA-4DA5-89E6-35F933166F77 at comcast.net>
>     > Content-Type: text/plain; charset=utf-8
>     >
>     > Lucky your that close! My K3, P:, Lp100 and kpa1500 are all
>     wayyyy different. I accept the LPA100 as being correct!
>     >
>     > Ron W5SUM
>
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