Posted by
Don Wilhelm-3 on
Jul 25, 2006; 5:40am
URL: http://elecraft.85.s1.nabble.com/Funny-power-measurements-tp392406p392410.html
Fran,
You are correct in leaving the KAT2 out of the picture for that
measurement - unless the KAT2 is in a bypass state. The addition of extra
tuning elements (the tuner L and C values) can create a tuning peak that is
unaccounted for in the relationship that you are trying to verify. Too many
variables cause un-needed confusion.
If you have a 10X probe for the 'scope you can make accurate RF Voltage
measurements. Check the probe and 'scope calibration first, most 'scopes
have a built-in calibrator that can allow you to check that a nice clean
square wave is displayed (if not adjust the compensating cap in the probe),
and also check that the vertical deflection is correct based on the internal
calibrator.
Apply the 10X probe at the dummy load location rather than at the signal
generator (K2) end for the better accuracy.
Be aware of the frequencies that you will be testing and the specified
frequency limits of your 'scope and probe. With my 100 mHz 'scope and my
150 MHz rated probes, I find the 'scope displayed peak to peak voltage is
correct through 15 meters and it drops off noticably at 10 meters. This is
a frequency factor of approximately 5 (100 MHz to 20 MHz) where I can depend
on the displayed voltage being accurate. Check your equipment before simply
believing that it is telling you what you are seeing - instruments sometimes
'tell falsehoods' - know your equipment, and trust it only to the extent
that it has proven itself.
Once you know that you have good RF Voltage measurements, it is a simple
matter to compute the power. The standard method of first converting to RMS
voltage and then calculating will work, but you can also use formula
reduction to do it all in one 'fell swoop' - the reduced formula for power
from resistance and peak to peak voltage is: Power = (Vp-p)squared/8R, so
for a 50 ohm load, it is a easy matter on most calculators to obtain the
square of the peak to peak voltage and divide by 400. A 40 volt peak to
peak RF Voltage will be 4 watts of power into a 50 ohm load. The derivation
of this relationship is an "exercise left to the student" <G>.
You may want to do calculation of the potential error values too. You should
be aware that the potential percentage error of your power calculation will
be proportional to 2 times your ability to correctly read the peak to peak
voltage and also directly proportional to the tolerance of your dummy load.
The net of all that is to say you will have to read the RF voltage
carefully. A reading of 40 volts p-p will be 4 watts, but if your reading
is in error by 1 volt (2.5%), the error (at that power level) will be 0.2
watts (or 5%) - the percentages will hold true for all power levels, the
actual error value will have to be computed for any particular level.
All that is not to say that such measurements are bad - indeed quite the
opposite, for I consider this method to be the best available for the
equipment I have. It is just valuable to be able to quantify how far off
the mark your readings and results can be due to whatever variables are
beyond our control or abilities - that is why it is important to understand
the limits of whatever equipment may be used in the measurements - we then
know whether any 'problems' are real or are the result of measurement
tolerances.
If your goal is to have less than 10% power output variation across each
band, I would expect that you could achieve that for all bands except for 10
meters (10 meters is quite broad). The K2's power controlling ALC will also
be a factor, and can contribute to a raw variation of up to 10% in addition
to your potential measurement error. You can minimize the K2 power ALC
error by taking many readings at the same frequency (I would say averaging
10 readings should be sufficient). If you do not average, and you read the
RF voltage to within 1 volt, you could state that a 15% variation in
readings would indicate that the 10% spec is met.
73,
Don W3FPR
> -----Original Message-----
>
> 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
>
_______________________________________________
Elecraft mailing list
Post to:
[hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help:
http://mailman.qth.net/subscribers.htmElecraft web page:
http://www.elecraft.com