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Author Topic: 1 kW in 55 cubic inches, 87% efficiency  (Read 3940 times)

Posts: 161

« on: September 06, 2003, 05:52:44 PM »

This was a Cal Tech project to build a class E HF amplifier:

The designers didn't try this, but it looks amenable to having the drain voltage modulated. If that works, you could build a linear amplifier around it.

Looks like it needs a bit more harmonic suppression to be legal. Second harmonic is only 33dB down.

This could be a really fun project. The highest voltage I've ever worked with, though, was on house wiring. Is safe amplifier construction something it's possible to learn from books, or does it require an Elmer at the workbench poised to shriek "NO!" when necessary?

Posts: 124

« Reply #1 on: October 13, 2003, 07:06:20 PM »

IMHO, class E amps are still a little out-of-reach for DIY for the average ham. Reason is it's relatively complicated stuff and will most likely require a solid set of measuring equipment to make sure the unit is working according to the standards.

I would really not recommend anyone starting an amp project without having the 'right' skill level. And I would strongly recommend reading all of Rich Measures articles on RF amps and many more before even thinking of taking a soldering iron into one's hands.

This definitely calls for an Elmer supporting you and preventing you from facing a 'sudden death' (literally!) event.
This is no kidding: these voltages are lethal. Which means: they put you to death right away if you are not taking care of them well enough.

I would like to recommend the following:

- read the amp literature over and over again.
- start with a ready-built amp to get used to the handling and tuning.
- if you then still have the desire to build one yourself: find an Elmer who is supporting you and who keeps you alive.

73 and take care,

Klaus, DK3QN

Posts: 173

« Reply #2 on: November 14, 2003, 11:00:25 PM »

At the low power level of 1kW, the complications of class E/F are hard to justify.

1)  1.5kW, 85% efficiency, average transmit time 33% (CW operation), means average power dissipation is 87 watts.  Change this to 70% efficiency, (properly tuned tube type amplifier) and you're now dissipating 194 watts.  Add 36 watts for the filaments (assuming metal-ceramic tubes) and you're still using less power than your station computer with 17" CRT monitor.

2)  You can not use a class E/F amplifier on the air without modulating the power supply.  Even on CW.  If you key the input wave form on and off, the output has a horrible "snap on" and "snap off" that generate very wide key clicks.  And, unless the power supply is EXTREMELY well regulated, there is waveform overshoot, which rings widely on the band as well, on the onset of the "dit" or "dah".  VERY un-neighborly, and quite in violation of FCC rules.

3)  The circuit that will modulate the power supply is, itself, going to be about 85% efficient.  So now, you're back to about the 70% efficiency level anyway.  So you added complexity and you have no more "overall" efficiency than if you just used tubes.  Plus, a class E/F amplifier loses efficiency in a major way when the SWR changes, such as the antenna blowing in the wind.  So you can not use the added efficiency to use smaller FETs, because you'll then blow them the moment the pine tree blows against the dipole.

4)  Don't even THINK of using SSB unless you're prepared for a major engineering development effort.  To transmit SSB, you need to have a demodulator strip your SSB signal into the ampliftude and phase components, then use the phase data to phase modulate the drive signal to the amp, then use the amplitude data to modulate the power supply.  PLUS, you can not modulate a class E/F amplifier "downward" very far, without adjusting the drive.  From full power down to about 10dB down is all you get without significant envelope distortion.

So you end up with the final PA stage at 85%, the power supply at 85%, then you have a linear amplitude modulator to modulate the input waveform (at 100W drive, that's a class AB modulator, running about 50% efficiency) and a phase modulator to boot.

In the end, the whole thing probably requires twice the power as your venerable old tube amp.

There have been successful class D/E/F envelope removal and restoration systems designed, but the energy they saved was not much, when everything was taken into account.

It DOES work well for AM, and most AM radio stations use some sort of very high efficiency PA stage, with a power supply that is switchmode-modulated.  Overall efficiency can reach 70%.  For AM, this is great - the typical plate-modulated AM broadcast rig only ran about 30% efficiency, when you consider the class A audio modulator that was involved.

But for hams, who tend to want the maximum distance per watt or dollar, it isn't very practical.

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