John's biggest problem now is (other than finding time to get on the
air, what with two boys in scouts, school activities, his wife, and
running a successful business!) that in order to keep the output to
1.5kW PEP output, he can't run no more than about 220w of carrier output.
Are you sure -you- know how much PEP your rig is running, without
distorting/flat-topping the peaks?
Use the envelope pattern on a scope. That will tell you right away if your
positive peaks are flat-topping. Flat-topping will cause exactly the same
kind of splatter as overmodulation in the negative direction, and is to be
avoided.
If I run my unmodulated carrier output to 600 watts into my 50-ohm
nonreactive dummy load, and measure power using a Mirage wattmeter that I
calibrated against a Bird 43, at 100% sinewave modulation (both positive and
negative, as indicated on the envelope pattern) the average power increases
to about 735 watts. The p.e.p. setting indicates about 1800 watts. With
100% voice modulation, the pep reading hits about 1100 watts on occasional
voice peaks while the positive modulation on the scope appears hit close to
135%. The plate volt/current meters show a steady DC input reading of about
900 watts.
On the antenna, I just run the rig at 640 watts DC input and make sure the
negative peaks don't break the baseline and that the positive peaks don't
flat-top. I adjust the final for the best modulation linearity with the
trapezoid pattern, and let the positive peaks go where they may to achieve
100% negative without flat-topping in the positive direction. I don't even
try to use the wattmeter in-line because I know the feedline to the real
antenna does not present the transmitter with a 50-ohm nonreactive load.
Instead I rely on my trusted old thermocouple rf ammeter to monitor the
feedline current for the normal value..
HF-300's are supposed to be good for 1 kw DC input 100% modulated. But at
that power level, mine show a noticeable curvature on the sides of the
triangle with the trapezoid pattern, no matter how hard I drive the final
with rf or how much bias I run, and the plate current meter deflects
slightly downwards, indicating a negative carrier shift and poor modulation
linearity. At 640 watts DC input the sides are nearly perfectly straight
and the DC plate meter stands almost still, indicating good modulation
linearity.
I think most hams are overly optimistic on how much power they are getting
out of their transmitter. Theoretically, the class-C tube type final is
supposed to deliver 75% efficiency, but once you consider losses in the tank
circuit, antenna tuner and feedline to the antenna, you are lucky to get as
much as 60% efficiency when comparing DC input to the final to rf input to
the antenna. I would venture to guess typical overall efficiency to be
closer to 50%. With slopbucket through a "leenyar", efficiency is much
less. Class-E rigs are the way to go if you are looking for ultra high
efficiency.
Don k4kyv
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