David,
Your question about "what is good enough" is an informative one.
When I was doing repair/alignment work for others, it was important to
me to have tools that had traceable calibration info. I had some good
50 ohm dummy loads that were good to 1 GHz, and an LP-100A that was
calibrated to NIST standards (which I used as a sanity check for other
power measurement instruments). Having those calibrated tools were
important to maintain my customer's trust.
In amateur practice, such care is not necessarily needed, but the
K3/KX2/KX3 TX gain calibration does require a good 50 ohm load up to 50
MHz. That is easily checked by sweeping it with an antenna analyzer.
As for the power measurement, once you have a good 50 ohm dummy load, it
is easy to use a 'scope (with a 10X probe) connected across that dummy
load (use a TEE adapter to gain access to the coax center conductor).
Then you can measure the RF voltage (peak to peak shown on the 'scope),
and use the formula Vp-p squared then divided by 400 to quickly obtain
an accurate calculation of the power. Nothing really expensive except
for the 'scope (which can be borrowed in many cases).
So what you need first is a good 50 ohm dummy load, and secondly, access
to a 'scope having reasonable calibration for the vertical deflection at
the frequency of interest.
With that combination, you can check the error in other in-line power
measurement equipment.
In most cases a 10% or 20% error is not going to make a big difference
in on-the-air performance - anywhere in the ballpark is usually OK.
You are correct that we have become complacent because of digital
displays. Take a 4 digit display in an instrument having accuracy of
10% - the difference between a display of 10.41 and 9.37 or 11.45 is
meaningless given the 10% accuracy of the instrument itself. You should
round that reading to 10 volts. Analog readouts automatically do that
compensation because you cannot read the meter scale with 4 digit
accuracy. Even my 12 inch slide rule in college was limited to 3
significant digits.
There is a difference between doing it "good enough" and with greater
accuracy depends on the purposes of your measurements.
73,
Don W3FPR
On 7/23/2019 11:38 AM, CUTTER DAVID via Elecraft wrote:
I agree with all that has been said and suspect a lot of responders on this
list are or were engineers who are used to using precision equipment. I think
it can be misleading to look at some instruments built into a transceiver for
instance, but it never seemed to matter, until Elecraft came along. I did my
calibration with a Bird power meter and a Bird 500W load but I have no idea
when they were last calibrated, so, what value do I put on the measurements?
Well, it's good enough for me. But calibration is a special case and I'd never
heard of amateurs calibrating for power before Elecraft. Accurate power meters
are costly to buy and maintain if you are in business, but for me now I'm
retired it doesn't matter so much because I'm not doing it for a paying
customer. I think we are misled by the easy precision of the digital
multimeter which for a few £ or $ is ten times as good as what we previously
thought were good analogue instruments. In short, I think technology can cause
us to bite our finger nails unnecessarily. 10% or 20% power error is hardly
noticeable to the guy on the other side of the world.
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