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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