Measuring power is not as easy as one might think. The lab where I had
my HP-436A calibrated told me they thought it would be safe to advertise
an LP-100 calibrated against it at 0.1 dB accuracy. That works out to
~2.3%. If I recall correctly, their reference had a certificate that
said something between 0.01 and 0.02 dB compared to an NIST primary
reference. They have their equipment re-certified once a year, but at
those levels, even temperature and humidity can make a difference.
I don't feel comfortable claiming a 2% accuracy and so rate the LP-100
at 5%, but several owners with access to NIST traceable lab
instrumentation have verified a 2-3% accuracy.
One thing to remember about the LP-100 is that I calibrate the meter at
the center of each band to within 0.1% of my HP-436A, and of course the
correction values for each band are saved and recalled based on the
reading of the internal frequency counter in the meter. Because of this,
the relative band-to-band error of the LP-100 should be very close to
that of the HP-436A.
73,
Larry N8LP
Siu Johnny wrote:
Hi Don,
Thanks a million. You have just hit the nail on its head. Yes,
correct, the difference in power output readings between K2 and LP100
is within 10%. It is wonderful to have you in this reflector. Bearing
in mind, I am not a radio man by profession and can easily head to the
wrong direction.
Could please advise how I can use antenna analyser to achieve better
power output with the LPF? Do you mean feeding the output of the
antenna analyser to the antenna jack with W1 terminated with 50 ohm load?
Thanks a lot indeed. Playing with K2 is really another kind of fun
which cannot be obtained from factory made transceivers. Regrettably,
there are not many hams in Hong Kong exploring this area.
TNX & 73,
Johnny Siu VR2XMC
From: "Don Wilhelm" <[EMAIL PROTECTED]>
Reply-To: <[EMAIL PROTECTED]>
To: "Siu Johnny" <[EMAIL PROTECTED]>,<[email protected]>
Subject: RE: [Elecraft] KAT2 power calibration
Date: Tue, 7 Nov 2006 00:09:59 -0500
Johnny,
Things are not always perfect, so you can expect some variation.
The 1N5711s in the KAT2 are not entirely frequency independent, and the
detector in the LP100 is similarly frequency dependent. The exact results
will vary with the particular 1N5711 diode that is in your KAT2 and the
particular diode detector in your LP100.
OTOH, you have to believe something. Take into consideration the
potential
errors of everything in the calibration chain:
My previous emails with Larry have revealed that he uses an HP836A for
his
reference - consider that the HP836A is good to 2% (that is optimistic) -
then, your LP100 may be good to 5% of the actual reading, and the KAT2
may
also be good to 5% of the actual reading, the total window for error is
12% - you said only that the KAT2 reading is higher than that of the
LP100,
but you failed to say by how much. I would expect that it should be well
within my optimistic error of 12% (1.2 watts at a 10 watt level).
This is a lesson in calibration practices - every potential error in the
calibration system must be accounted for. If you want to obtain
labratory
standard practice, the standard should be 10 times more accurate than the
device being calibrated. I doubt you will find that level acceptable
in any
wattmeters available today. You may just have to be satisfied with some
reasonable discrepancy. Alas, it is not a perfect world, especially
not in
the wattmeter arena.
All in all, I have found that the KAT2 agrees with my 100 MHz 'scope (and
150 MHz probes) and and along woth my precision dummy loads I can
achieve a
KAT2 calibration within 5% across all bands. I believe the LP100 is
rated
at 5% too. So if you are no more than 10% different between the two, you
are doing pretty good. That is a whole lot better than the 'run of mill'
amateur grade wattmeter which is speced at 20% of full scale.
Your output power 'tweaking' is not related to the power output
reported by
the KAT2, but I am glad that you achieved good results by using Gary's
advice. If you have an antenna analyzer, you can 'tweak' the LPF for the
best SWR across the bands and obtain even better results than those
obtained
by 'tweaking' for maximun power output. If you attempt that, be certain
that you terminate the LPF in 50 ohms - which means removing the W1
jumper
and inserting a 50 ohm load (this can be done easily at the K160RX
header if
you have that option installed).
73,
Don W3FPR
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