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Author Topic: 40 meter single-ended 4-FETs  (Read 17179 times)
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ka1tdq
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« on: December 30, 2017, 07:57:17 PM »

Ok, construction has started.  The plan is for 4 parallel FETs in single-ended configuration.  I'll be using a drain choke in the output network.  The picture shows the start of the transmitter.  It'll be mounted on a 1/4" steel plate elevated above the cabinet.  That gives a convenient space for the DDS VFO and the meters.  Inside the cabinet will be the power supply, Heising modulator and audio amp.  The audio section is already done and I just need to mount it.

The goal is 200 watts carrier.  Assuming 90% efficiency, I believe the heat sink is capable of handling the 20 watts of heat.

Jon


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ka1tdq
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« Reply #1 on: December 31, 2017, 02:31:08 PM »

Playtime is over for today.  Got most of the inside cabinet done.  I'll start working on the RF deck when I get some more hardware. 

Jon


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KD6VXI
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« Reply #2 on: December 31, 2017, 03:53:27 PM »

200 watts carrier at 90 pct is 20 watts.

At 1kw pep, that's a hundred watts.

Heat sink capable of that?  Or is the carrier where the most inefficiency is at?

I never thought about it, all my class E and solid state is fan cooled.....

--Shane
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ka1tdq
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« Reply #3 on: December 31, 2017, 04:06:17 PM »

I’m not sure either. At this point though, I’m just hoping to get a class E waveform on 40 meters with this. I’m going to make the FET and driver section as neat and compact as I can.

I’m not sure my modulator has enough umph for 200 watts carrier, but I’ll modulate as much as I can. I can always dial back on carrier.

Jon
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steve_qix
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« Reply #4 on: January 01, 2018, 02:22:50 PM »

If there is any possibility of using the standard single ended push pull design rather than just single ended, it will probably work better.  The harmonic content presented to the output network in a single ended design is extremely high, and I have also found better efficiency with single ended push pull as opposed to just plain single ended.

Along with the drain power dissipation, the heat sink has to be capable of dissipating all of the power consumed in the gates  (which can be a lot), and you need to account for modulation which will increase the heat produced in the drains by 30% to 50% depending on the nature of the modulation.

With analog drive, 100% of the drive power is turned into heat in the gates.  With heat-sink mounted driver ICs and digital drive, 100% of the power fed to the drivers is turned into heat.

Just some suggestions.

Regards,  Steve

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ka1tdq
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« Reply #5 on: January 01, 2018, 02:57:16 PM »

Last year I attempted to build a push-pull 40 meter transmitter using just a single FET in each phase. I couldn’t get it to behave at that high frequency, but it worked great on 75 meters with larger tank components. If I couldn’t get two FETs to work in push-pull, I’m doubting my skill with four FETs.

If sacrificing a little efficiency and having increased harmonics is the trade off for a decent power level 40 meter deck (that works), then I’m game, especially since I use a tuner for 40 meters.

I’m only assuming that single-ended is “easier” to get working since my first class E transmitter was single-ended, using a single FET for 40 meters. I’d just be adding 3 FETs in parallel.

As for heat dissipation, I’ll be using digital drive. My rough calculation puts that at about 15 watts. My heat sink weighs almost as much as a bowling ball so my confidence interval is high.

Jon
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« Reply #6 on: January 01, 2018, 03:07:54 PM »

I'm thinking of a duplexer on the output to deal with harmonics.  Otherwise, that's a ton of energy to have to short to ground.

The solid state amp guys are now moving to duplexes with dummy loads for harmonic energy.  Helps with imd, helps with keeping things cool and helps with efficiency.

--Shane
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ka1tdq
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« Reply #7 on: January 01, 2018, 03:32:15 PM »

We’re missing the big picture here... 14 MHz won’t propagate at night for several years.

Jon
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« Reply #8 on: January 01, 2018, 07:03:31 PM »

We’re missing the big picture here... 14 MHz won’t propagate at night for several years.

Jon

You lost me here.

I was saying, to deal with your increased harmonic content of a single ended amp, use a high pass filter dumping everything above 7 mhz into a dummy load instead of using low pass filters.

The ldmos amps are utilizing this now.

--Shane
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steve_qix
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« Reply #9 on: January 01, 2018, 07:31:21 PM »

Hmm.. when you say push pull, do you mean "real" push pull (with a single output transformer and both devices connected to it), or single ended push pull like I was talking about?
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ka1tdq
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« Reply #10 on: January 01, 2018, 09:54:59 PM »

Gosh, Steve, you know... when you're right, you're right.  These rigs are all 'single-ends' fed out of phase and combined in a transformer.  So, right!

Luckily I didn't drill the holes for the heat sink yet.  Let - me - just - move - that - back - and - stick - in - a - transformer ... yeah.

Ok, so 2x2 it is!

Jon


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ka1tdq
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« Reply #11 on: January 04, 2018, 01:18:04 AM »

I’m just bored and posting pictures, sort of like Facebook.

I did find out that I can login to AmFone.net with facial recognition on my phone.

Jon


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« Reply #12 on: January 04, 2018, 11:32:57 AM »

I like the coil cradle in the pvc pipe section.  Neat idea!

--Shane
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ka1tdq
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« Reply #13 on: January 05, 2018, 01:17:50 PM »

Getting close!  I'm waiting for a few more parts to arrive and then I can start building the heat sink assembly.  Everything else is done except for the two shunts to measure drain currents.  I'm going to mount an SMA connector for each phase very close to the gate drivers.  That'll help minimize lead length.  And for good measure I'm going to build a ground wall between the two phases using a small strip of copper.  I've planned everything else to minimize other lead lengths as well.

Jon



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ka1tdq
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« Reply #14 on: January 06, 2018, 08:05:43 AM »

Everything is done except for the SMA connectors and resistors. The connectors and the current shunts are coming on the slow boat from China. Once they arrive I can begin testing.

Jon


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ka1tdq
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« Reply #15 on: January 13, 2018, 06:46:00 PM »

All done except for the two SMA to BNC short jumper cables. I just need to screw those on and I can begin testing.

Jon


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ka1tdq
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« Reply #16 on: January 14, 2018, 05:56:41 PM »

Change of plans.  I'm allowed.

I ordered a BitX transceiver that I want to use for 20 meters SSB.  Output power is 10 watts so this RF deck platform will be great for a single 811A amplifier.  I can use the existing tuning capacitor for the plate capacitor if I put another fixed capacitor in series. 

Large steel plate - baby linear. 

Jon
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