Posted by
Ron D'Eau Claire-2 on
Jul 25, 2006; 4:07am
URL: http://elecraft.85.s1.nabble.com/Funny-power-measurements-tp392406p392414.html
Sure, Fran, an O'scope is a good way to measure the voltage. I'd suggest a
good 10:1 probe on that scope instead of driving the vertical input
directly. Many scopes have a 50-ohm termination at their input that can
throw the readings way off since the rig would see 25 ohms looking into both
of the scope and dummy load in parallel. Also, the scope termination is not
designed to dissipate any power and you'd likely destroy it with any
significant power. The 10:1 probe isolates the scope from the dummy load.
Divide the Vp-p reading you measure with the scope by 2.82 to get Vrms and
go from there: (Vrms)^2/50 = watts.
Ron AC7AC
-----Original Message-----
Okay, I guess I left out some critical details from my original post.
I am using a 50 ohm load that is intended to be used to convert test
equipment from hi-impedence input to 50 ohms. It is labled 50 ohms and
measures 50 ohms with my DMM. It should be purely resistive and I do not
have any equipment to prove that it is.
The need to load it properly using the KAT2 was only because I wanted the
KAT2 in-circuit for improved forward power measurement within the K2. I
knew that it would need to load things to cancel itself out.
Based upon what Ron, Don and Jack have said, I guess that I should remove
the KAT2 and RF probe and try something similar to what I tried last time I
attempted to understand how well this radio tracks against the published
specifications.
Would it be reasonable to just place a short piece of coax between the load
and an O'Scope, measure the peak-to-peak voltage, divide by 2*SQRT(2) and go
from there?
Would this require knowing the coax velocity factor and dealing with prioper
lengths of coax based upon wavelength (half or quarter) at the different
frequencies?
I ask these questions because last time I tried using my Scope to make these
measurements, the measurements were obviously incorrect because they
indicated that my rig was way more than 100% efficient.
What I expect is that I can make apropriate measurements at 3 points on each
band and see that I get no more that 10% variation across said band and from
band-to-band. I expect that I have a problem on 10 Meters because I am
getting a HiCur indication at 3 amps when set for 10 watts, so something
needs changing there. However, until I have trustable measurements on the
other bands, I expect that untangling the issues will be impossible.
Help from the experts is needed on how to make such measurements without
fancy RF power meters. Obviously a purely resistive 50 ohm load is the
first requirement, and I believe that I have that.
73,
Fran
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