Which wattmeter do I believe?

[John Pfeifer-2]

Just finished building KX-1 #1207 with 30m board and ATU.  Nice little rig!

After adjusting C9 on the KXAT-1 per the manual, I decided to compare the
power output measurements obtained via the internal power sensor, a DL-1
dummy load + DMM, and using my OHR WM-2 QRP wattmeter.  BTW, the supply
voltage is 12.5v and I substituted a 33-ohm resistor for R30.  Output before
I changed R30 was about 1-watt lower across the board.  Here are the final
measurements, as built:

Band     DL-1+DMM     Internal      WM-2
20m        2.6           3          3.7
30m        3.7           4          4.7
40m        3.4           4.1        4.3

Which wattmeter should I believe?  I tend to think the DL-1 plus my Fluke
digital multimeter would give me the most accurate reading, but....

Also, no matter how I fiddle with the spacing on L3, 20m output is always
1-watt less than on 30 and 40m.  I guess this is in line with what other
folks have reported.

tnx & 73,
John Pfeifer - WL7M
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[N8LP]

I assume you are using the DL-1 as the load for all three tests, in
which case I would have to go with the DL-1 plus DMM. The impedance
error should be less than 5%, and the voltage measurement error less
than that.

Larry N8LP



John Pfeifer wrote:

>Just finished building KX-1 #1207 with 30m board and ATU.  Nice little rig!
>
>After adjusting C9 on the KXAT-1 per the manual, I decided to compare the
>power output measurements obtained via the internal power sensor, a DL-1
>dummy load + DMM, and using my OHR WM-2 QRP wattmeter.  BTW, the supply
>voltage is 12.5v and I substituted a 33-ohm resistor for R30.  Output before
>I changed R30 was about 1-watt lower across the board.  Here are the final
>measurements, as built:
>
>Band     DL-1+DMM     Internal      WM-2
>20m        2.6           3          3.7
>30m        3.7           4          4.7
>40m        3.4           4.1        4.3
>
>Which wattmeter should I believe?  I tend to think the DL-1 plus my Fluke
>digital multimeter would give me the most accurate reading, but....
>
>Also, no matter how I fiddle with the spacing on L3, 20m output is always
>1-watt less than on 30 and 40m.  I guess this is in line with what other
>folks have reported.
>
>tnx & 73,
>John Pfeifer - WL7M
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>
>
>  
>

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[Don Wilhelm-3]

John,

IMHO, believe the DL-1/DMM combination first.  If you have an opportunity,
check the DL-1 with an antenna analyzer to see how close it actually is to
50 ohms non-reactive - that should increase your confidence level.

The WM-2 when properly calibrated is speced for 5% of full scale, so if you
were reading on the 10 watt scale, its reading could be off by as much as
0.5 watt, if not calibrated, it could be off even more.  I find my W7EL
wattmeter (predecessor to the WM-2) is optimistic in its readings (reads
high) even when calibrated by the 'proper' instructions - one day I will do
a better job of matching the diodes and straighten it out, but in the
meantime I trust my known 50 ohm non-reactive dummy load and my 'scope or RF
probe a lot more.

73,
Don W3FPR

> -----Original Message-----
>
> Just finished building KX-1 #1207 with 30m board and ATU.  Nice
> little rig!
>
> After adjusting C9 on the KXAT-1 per the manual, I decided to compare the
> power output measurements obtained via the internal power sensor, a DL-1
> dummy load + DMM, and using my OHR WM-2 QRP wattmeter.  BTW, the supply
> voltage is 12.5v and I substituted a 33-ohm resistor for R30.
> Output before
> I changed R30 was about 1-watt lower across the board.  Here are
> the final
> measurements, as built:
>
> Band     DL-1+DMM     Internal      WM-2
> 20m        2.6           3          3.7
> 30m        3.7           4          4.7
> 40m        3.4           4.1        4.3
>
> Which wattmeter should I believe?  I tend to think the DL-1 plus my Fluke
> digital multimeter would give me the most accurate reading, but....
>
>

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[Paul Gates]

John, you will have a lot of fun with the KX1. I am getting ready to go on
the Auto Train next Sunday to Fla and getting my KX1 ready to go. I have
26ft of #26 Silky wire for a antenna and 32 ft wire for counterpoise... Also
have the Super Antennas MP1 Vert. that I will take with me.
Paul Gates
K1  #0231
KX1 #1186
XG1
[hidden email]


----- Original Message -----
From: "John Pfeifer" <[hidden email]>
To: <[hidden email]>
Sent: Sunday, July 17, 2005 9:00 PM
Subject: [Elecraft] Which wattmeter do I believe?

> Just finished building KX-1 #1207 with 30m board and ATU.  Nice little
> rig!
>
> After adjusting C9 on the KXAT-1 per the manual, I decided to compare the
> power output measurements obtained via the internal power sensor, a DL-1
> dummy load + DMM, and using my OHR WM-2 QRP wattmeter.  BTW, the supply
> voltage is 12.5v and I substituted a 33-ohm resistor for R30.  Output
> before
> I changed R30 was about 1-watt lower across the board.  Here are the final
> measurements, as built:
>
> Band     DL-1+DMM     Internal      WM-2
> 20m        2.6           3          3.7
> 30m        3.7           4          4.7
> 40m        3.4           4.1        4.3
>
> Which wattmeter should I believe?  I tend to think the DL-1 plus my Fluke
> digital multimeter would give me the most accurate reading, but....
>
> Also, no matter how I fiddle with the spacing on L3, 20m output is always
> 1-watt less than on 30 and 40m.  I guess this is in line with what other
> folks have reported.
>
> tnx & 73,
> John Pfeifer - WL7M
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
>

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