Brings up an interesting question.
My recollection from numerous amps that they may read OK (within 10
-15%) near full power, at low power they typically were way off. I
suspect whatever calibration was done, if any, was at the high end of
the scale. The load was probably pure resistive and near 50 ohms.
By way off, I mean perhaps 50% not a factor of 3 like being reported.
I wonder if anybody has any hard data on this. If it is true, why?
In the old days with a non-digital meter, nobody could tell....
73 de Brian/K3KO
On 9/10/2017 21:32 PM, Don Wilhelm wrote:
Bill,
I would say something is not calibrated correctly - either the K3
wattmeter or the power determining device in the amp. It is difficult
to say which one without doing some measurements.
I would connect the K3 to a dummy load for the 1st test. Get a 'scope
out with a 10x probe. The 'scope and probe should have a frequency
rating of at least 3 times the operating frequency.
A coax TEE adapter inserted in-line will give you access to the coax
center conductor so you can read the RF voltage peak to peak value.
That RF voltage should be used to compute the actual power developed by
the K3. Compare that to the power reading shown on the K3 - you can use
the formula {V(p-p) squared} divided by 400 to obtain the power in watts
to a 50 ohm load. Derivation of that formula is "left to the student",
but if you don't believe it, you can always convert to RMS voltage and
calculate from there. Formula reduction is required before plugging in
values.
Once you know the actual power when the K3 reads 10 watts, remove the
dummy load and connect the amp (leaving the TEE adapter in-line). Now
send what the K3 indicates as 10 watts, and again measure the RF
voltage. That will tell you whether the K3 power indication is correct
or the amplifier indication is correct.
73,
Don W3FPR
On 9/10/2017 5:10 PM, Bill Frantz wrote:
I have just set up an Expert 1.3K-FA amplifier with my K3 (with most
of the upgrades to become a K3S). My question is, when I drive it with
10W set on the K3 output power, it shows a drive level of about 30W
and and output level of 950W. What is going on?
Details: The two boxes are connected with about 6' of RG-58 for the RF
and a custom cable made by KC5PCB, Dan Tassell for the rest of the
signals. The output is into a dummy load, although very similar
measurements occur with my trapped vertical antenna. Tests were in the
CW portion of the 20M band.
The Amp is powered from a dedicated 120V circuit which shows a 7 volt
drop at 12.7A during CW key down transmission. (Voltage is measured by
a Fluke 87 DVM and current by a Kill-A-Watt PA4400.) An external YS-60
power meter shows 800W, which seems reasonable agreement given the
accuracy of RF power measurements.
Why does the K3 show 10W and the amp show 30W? Is there something
screwy here, or is it normal?
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