[Elecraft] Accurate QRP Wattmeters?

[Larry Phipps]

Jay, the Tek 492 will do the job IF you have an accurate way of 
calibrating it and IF you know exactly the attenuation at all 
frequencies of interest of the attenuator you will be using. I'm not 
familiar with the '492... I have a 7L13, and on the 2dB scale the 
divisions are .4dB.. which are pretty big steps.

Don is right about the dummy load method, but first you have to 
determine the actual Z of the dummy load at all frequencies if you want 
to be within a couple percent. The resistors in the Elecraft dummy load 
are 5% metal film, so statistically, the average Z will probably be 
better than 5%, but unless you can measure it, you won't know. You also 
have to take into account the diode drop in the Elecraft to wring out 
the last bit of accuracy.

Anything better than 5% absolute power accuracy, BTW, is wishful 
thinking on the part of manufacturers. Be advised too that even the 
Birds are rated at 5% of FULL SCALE... meaning ~ 10% of half-scale if 
that's where your reading happens to be... and that's best case with a 
purely resistive load.

In develping my LP-100 wattmeter kit (more on my website), I learned a 
lot about power accuracy. It's a very elusive thing. My meter is 
accurate within a couple % of the calibrating standard at any power 
level from .1 milliwatt to 2500 watts... but ultimately, the accuracy of 
the standard determines the overall accuracy of the meter.

In my shop, I can measure power to better than .001dB on a relative (not 
absolute) basis with my HP VNA, but it still all comes down what you're 
measuring against. One thing I can do, and have done for my own use is 
create Z vs. freq charts for all my precision loads (and Cantenna and 
other non-precision loads as well). These charts are accurate to better 
than 1%.

As part of my LP-100 project, I plan to offer a 50W precision dummy load 
using thick film metal power resistors mounted on a heatsink to allow 
easy calibration of the meter. It would be a nice addition to any shack. 
I will provide a custom chart of Z vs. frequency for each one. It will 
be similar to the Elecraft DL1 in operation, but will use precision 
parts and come with the custom charts. I am also thinking of making a 
version of the dummy load with digital readout for power that comes 
already calibrated. It would be considerably simpler than the LP-100, 
and would not be a thru-line device, so would not measure SWR, etc.

In the meantime, if you have a DL1 and want me to run a custom chart for 
you, send it with return postage and I will chart it for you.

Larry N8LP
www.telepostinc.com/n8lp.html

J. Coote wrote:

>Hello List,
>I've posted my question on QRP-L and here as well- since I will be building
>a KX-1.
>I am looking for good ways to measure power output from my KX-1 while I am
>building it.
>What first came to mind was to set up my Tek 492 analyzer on a 2 dB scale
>and then use
>a 30 or 40 watt (high power) attenuator to measure CW power.  I have several
>Bird 43's
>but have never seen Bird slugs for QRP LF, MF or HF.  I think a competing
>company may
>have a through wattmeter with HF but I forget the lowest scale of that one.
>I have read
>the E-Ham reviews for the Oak Hills QRP wattmeter-  300 Khz to 54 MHz and
>100mW, 1W and 10W
>scales- still thinking over the reviews.
>
>Comments on the accuracy of using the Tek 492 method, and on other accurate
>wattmeter options?
>
>73
>Jay
>W6CJ
>AAR9QM
>
>
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