To: | "Rob Atkinson, K5UJ" <k5uj@hotmail.com> |
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Subject: | Re: [Amps] Centurion cooling improvement ideas for AM service |
From: | Gary Schafer <garyschafer@comcast.net> |
Reply-to: | garyschafer@comcast.net |
Date: | Thu, 26 Aug 2004 21:32:27 -0400 |
List-post: | <mailto:amps@contesting.com> |
Hi Rob, The old QST article kind of uses voltage and power as one and the same in the discussion. He also fails to mention that the power "INPUT" to the amp also increases as well as the power out with modulation. If the power INPUT did not increase with modulation then amp dissipation would probably go down with modulation. But power input does NOT stay constant. Even though the plate meter may say it does. When he says "the carrier is at half level the output is one-fourth the modulation peak output", he really should be saying that the carrier VOLTAGE output is at half level and that the power output is one-fourth peak envelope power out with modulation. You have to be careful reading some of the old AM stuff as the mindset was that what you saw on a scope when looking at an AM signal was exactly what was going out on the air. I.e. it looked like the carrier went down to zero and up to twice the carrier on mod peaks. While this is true for the composite AM signal (the sum of the voltages in the carrier and the sidebands) it is not what the power actually does. Looking at the power spectrum on a spectrum analyzer you can see that the carrier output stays constant whether it is modulated or not. The additional power from modulation appears in the side bands which are independent from the carrier. Put an old thermal couple RF amp meter in the RF output of the AM amps antenna line. You will see that the RF current increases with modulation from what it was with just carrier. The average power out increases. You will not see that on a bird watt meter. It will remain relatively constant. The same thing is happening to the input power of the amp that you can't see on the plate meter. The amp is only tuned for maximum efficiency at the very peaks of the peak envelope power. (same is true for SSB) At any power level under that the efficiency is lower. The bulk of the power (average) which causes all the heat is nearer to carrier power levels and at lower efficiencies. Look at the power comparisons again that I posted before and you can see the increase in dissipation with modulation even though the efficiency increases. It is not a linear increase though. 73 Gary K4FMX Rob Atkinson, K5UJ wrote: What's your source Gary for this?
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