[AMRadio] League pulls RM-11306


D. Chester k4kyv at charter.net
Mon Apr 30 00:24:33 EDT 2007


> 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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