New to AM and need recommendation for class e

[AB1GX]

Ian,

I wish I had seen that article before I started simulation.   It would have saved me some time.

What I am doing different is driving the FET with a 0-10V square wave using bipolar transistors.

Also, I'm using homebrew fixed air caps for high-Q at very large RF currents.  They are each 4"x4" and about an inch thick.

And I appear to be using much lower-inductance/higher-capacitance in the output network,

Yeah, the inductance of the leads do matter quite a bit so I made the FET itself part of the tank circuit.

Thanks,

Tom 

[AB1GX]

Thanks again for that article!

The 2nd harmonic trap not only lowered the problem spur but also cleaned up the waveforms in the class e amp.  Push-pull, I'm sure, would have done the same, but now I have a simple one-transistor 1.5KW class e design.

The only thing tricky is that two caps (.005uF) have RF peak currents of 80A and building air caps is a pain!

How's Mica?  MultisectionCeramic?  Anyone know?

[WA1GFZ]

30 dB down from 1500 watts is 1.5W.

[KF1Z]

I've been curious....

1.5kw at carrier??

Or capable of 1.5kw with modulation?


building air caps is probably easier than paying for
something to handle that current.....

[AB1GX]

I've been curious....

1.5kw at carrier??

Or capable of 1.5kw with modulation?

1.5kW CW, which would also be the peak for AM.  Legal power is 1.5kW PEP, so an AM carrier would be less than half of that.

Regarding the high current caps, 1 mil teflon could reduce the size of the caps by 1/2.  I have no clue where I could buy this dialectric.

Tom

[AB1GX]

30 dB down from 1500 watts is 1.5W.

Right, my second harmonic is down 45 dB with the change, so it will be under 100mW (at the amp output) at l500W.

[KF1Z]

I've been curious....

1.5kw at carrier??

Or capable of 1.5kw with modulation?

1.5kW CW, which would also be the peak for AM.  Legal power is 1.5kW PEP, so an AM carrier would be less than half of that.

Regarding the high current caps, 1 mil teflon could reduce the size of the caps by 1/2.  I have no clue where I could buy this dialectric.

Tom

Ok, was just surious if you were designing it as 1.5kw continuous, or for the 375watt /100% mod. thing....


Don't know about teflon.... but Kapton is fairly cheap on ebay....
I think I paid $9. for 3 sq/ft.

That's 2 mil... (other sizes avail... of course)
But, now that I think of it,  that may be too thick to get .005uf easily
You would need 1 mil... still be pretty large surface area..

Maybe Frank GFZ will chime in on that, I know he's used it...

[WA1GFZ]

Kapton is 7000 volts per mil. I use 3 mil myself and have never flashed through it. I did lap the mating surfaces with wet dry sand paper to remove any burs that could cut through it. Also hit all the holes in the heat sink with a over sized drill bit to remove any sharp edges around the holes before and after tapping them. I used 3 by 5 inch copper spreaders to act as one plate of the cap and a common drain connection. Check out my QEX Nov 2005 article. Pictures tell a lot more that words. You can down load it as the sample article for free or let me know and I will send you one if you provide an email address.
My mistake was it was the right size for 40 meters but not big enough for 80 or 160 forcing me to add door knobs to get enough shunt C.
I used the simple cap formula in the handbook using 4.2 as the dielectric constant for Kapton. This formula will tell you the total area needed for the cap plates.  My configuration used the heat spreader as a low L connection to the transformer, a way to balance the temperature of the parallel fets and the shunt cap plate working against the heat sink. gfz

[AB1GX]

Frank,

Nice article!  Kapton looks like a great material.  20 sq in gives me the .005uF.

Tom

[WA1GFZ]

once I subtracted for all the holes I needed to add more C to get back to 5000 pf. Each FET has a nylon insulator to isolate the screw from the heat spreader. This hole was about 1/4 inch. Then the void for the transformer primary tubing. I ended up adding another 2000 pf to my rig to get the shunt so I could do 1200 watts at about 47 volts.