[Elecraft] KPA1500 wattmeter adjustment

[Ronnie Hull]

One in every crowd… why do we have to throw rocks at each other

Sent from my iPhone

> On Feb 6, 2022, at 1:15 PM, Bob McGraw <rmcgraw at benlomand.net> wrote:
> 
> 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
>> 
>>    ______________________________________________________________
>>    Elecraft mailing list
>>    Home: http://mailman.qth.net/mailman/listinfo/elecraft
>>    Help: http://mailman.qth.net/mmfaq.htm
>>    Post: mailto:Elecraft at mailman.qth.net
>> 
>>    This list hosted by: http://www.qsl.net
>>    Please help support this email list: http://www.qsl.net/donate.html
>>    Message delivered to marklgoldberg at gmail.com 
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Elecraft at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to w5sum at comcast.net