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Analog modulating a 1000 watt carrier class E rig




 
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Author Topic: Analog modulating a 1000 watt carrier class E rig  (Read 8300 times)
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ka1tdq
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« on: November 24, 2018, 01:45:55 PM »

I'm interested again in building a 24 FET class E rig for 75 meters. The schematic is already out there, but I've been planning the mechanics of the build. Attached is what I'm going to do. We've discussed this before, but rather than elongate the transformer, I'm going to wrap it on itself as shown in the picture.

Even though it is relatively cheap to build, it will still take money, but I do have a plan. I know it is cheaper to modulate this via PWM, but I still prefer analog Heising modulation. I've done the numbers and here are the components that would be required:

10mH at 25 amps, minimum

2600uF at 450vdc, minimum

The numbers work, and I do have a spare megawatt, single-channel car audio amp.

Is this practical?

Jon

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« Reply #1 on: November 24, 2018, 10:13:19 PM »

Rather think this post belongs in the Class E section.
Steve WA1QIX has modulated these rigs by assorted means.
Including a class AB amplification section.

My reaction to the car amp idea is definitely maybe.
The first thing you will need is a pretty serious DC supply, as the amp is
going to be a lot less efficient than 100%. If it is a class D car amp, I'd be
cautious... they probably don't like "weird" reactance/phase angles. But,
maybe.

So napkin design says for 1000 watts at 10 volts supply = 100amps!
A bit less at 13.8vdc. But you need more taking into account the efficiency
of the car amp...

Now if you had a Crown Macrotech 2400 run in mono... Cheesy

Don't forget that your modulator's voltage will need to swing whatever the peak
of the RF section puts out, or more (for over 100% positive).

Steve sells the PWM modulator board for pretty low $$, with parts, fyi.

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ka1tdq
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« Reply #2 on: November 24, 2018, 10:46:46 PM »

I do have a class D car amp. Iíll have some resistance in parallel anyway to damper weird reactance, but efficiency will be depleted further. My solution to a beefy 12 volt supply: a car battery.

When the time comes, I might just go PWM. Why push a Prius with a 450 cubic inch 4 barrel carb. Iím not a car guy, but I think that made sense.

Jon

**But in this case, this is a pretty mother, badass Prius.


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« Reply #3 on: November 25, 2018, 12:09:02 AM »

I can only add one comment at this point: Copper heat spreader plate
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ka1tdq
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« Reply #4 on: November 25, 2018, 03:04:54 AM »

Yeah, throw that in the box too!  Grin

Jon
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ka1tdq
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« Reply #5 on: November 25, 2018, 07:46:06 AM »

...actually, thereís an industrial metal store not too far from my house. I can get very large blocks of aluminum fairly cheap. The block by itsel is a good enough heat sink, but I may put fins on it. 1000 watts at 90% efficiency is still a lot of power to dissipate. Fortunately the antenna dissipates most.

Jon
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« Reply #6 on: November 25, 2018, 10:03:01 AM »

Imho, you are not going to get much on-air time with a car battery.
Also, if you draw at those currents, you're going to create a boatload of hydrogen gas...
VERY DANGEROUS in the house!

Also at this point it is no longer necessary, afaik, to use 24 fets... newer cost effective devices.

Aluminum heatsinking is likely sufficient.
For heatsinking MASS will work up to the point where it becomes "saturated", then it becomes
an inefficient radiator of heat. Fins are required, or a fan, or a fan + fins (depending on the
actual situation). Getting rid of 100watts of HEAT requires a significant amount of heatsink.
I do this all the time in audio.

Basically you want a big heatsink with 2.5" or better fins, wider spaced is better for natural
convection... big being maybe 8 x 10" or better.

Getting rid of 200 watts of heat, depending on how ur running the proposed transmitter, requires
that much more heatsink. Wakefield's site has the formulas, they're not terribly complex. You only
need an order of magnitude and then add 20% for safety.

In the case of 24 fets, you don't need a copper spreader, since it's already spread!
If you use 8 fets at the same power, MAYBE you need a spreader, since the devices are
small... the thickness of the heatsink's backplate matters...

The transfer between surfaces in heatsinks matters...

Afaik the efficiency of the Class E transmitter is WAY better than 90%, so the transmitters
I've seen generally have relatively modest heatsinking...

As I said this belongs in the Class E section...

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N1BCG
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« Reply #7 on: November 25, 2018, 10:10:42 AM »

Whatever the manufacturer claims your car amplifier is rated for is using absolute best case marketing dept-driven DC input watts, so doing the efficiency math drops your options dramatically. And then there's the unflattering THD at high output.

Why 1kW RF?

btw: Heising means common supply and no transformer, just a choke. Not sure if you meant that or a mod transformer with a reactor.
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ka1tdq
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« Reply #8 on: November 25, 2018, 10:19:29 AM »

Fair enough on the amp and the battery. When the time comes, I'll probably go with PWM. I do need to build a working RF deck though. I'm fairly confident in my ability to do that, but the proof is the wattmeter.

True, 1kw isn't needed. I could simply run it at 500 watts and be happy with that. Then Heising becomes a little more practical. I could probably crank some more out of my current modulator and use that.

I'll continue this thread in the class E section when the time comes. I need to finish my receiver project and then focus on getting the parts for this RF deck.

So, to be continued after tax return time!

Jon
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« Reply #9 on: November 25, 2018, 04:47:39 PM »

Ah, go for the full boat  Cool   If you have an antenna up high enough above the ground, we should be able to hear you out here in good old New England.  That would be cool.
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« Reply #10 on: November 25, 2018, 04:49:48 PM »

Another good reason to go with PWM is you get things like fast overload shutdown for "free".  It's in the design.

Not to mention the smaller size, smaller weight and reasonable power/current requirements.
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« Reply #11 on: November 25, 2018, 05:11:38 PM »

An AGM battery is supposed to be safe to use regard outgassing, unless it is abused but that is any battery. Don't take my word for it though. I ran one in my truck behind an 8-transistor RF amplifier for a few years and never an issue.
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pa0ast
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« Reply #12 on: November 25, 2018, 05:13:30 PM »

Have You considered that most car amps are class D Amps. ? The switching freq is still present in the output if not strong suppessed. Could be a problem if you use it as an modulator. Also the switching  power converters that  convert 12 V dc to + and - 48 Vdc that the amplifiers run with , can give problems in the transmitter. You have to filter it well .
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ka1tdq
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« Reply #13 on: November 26, 2018, 12:05:18 AM »

Yes, weíve spoken on the air with my measly 400 watts carrier before. Iíve also chatted with Tim.

PWM is really the only practical choice at this power level. Iíll save my Crunch car audio amp for blasting Roy Orbison driving down the I-10 in Phoenix.

Jon
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« Reply #14 on: November 26, 2018, 01:01:12 AM »

Practical is in the eye of the beholder...
QIX did go from AB to Class H to PWM. So, they're all doable! Cheesy

Don't wimp out, go for the full bore!
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« Reply #15 on: November 26, 2018, 05:50:30 PM »

Practical is in the eye of the beholder...
QIX did go from AB to Class H to PWM. So, they're all doable! Cheesy

Don't wimp out, go for the full bore!

Never actually used AB - it was class A (below 100 watts).  Then class H (100 to 400 watts), then PWM.  However, now I use PWM for everything above 150 watts.  It's just so much smaller and very reliable.
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ka1tdq
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« Reply #16 on: November 26, 2018, 08:05:25 PM »

I'm actually just going to just key drain voltage to this thing and make it a CW transmitter. Just pass drain voltage supply through a J-38 key.  Make sure though to put a .01 capacitor across the terminals. 

https://www.youtube.com/watch?v=3gQANgjMEog

Kidding...

Jon
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ka1tdq
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« Reply #17 on: November 28, 2018, 06:00:18 PM »

Behold! The plate of power!

Iíll join an aluminum heat sink on the back with a heat transfer compound.

Jon


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« Reply #18 on: November 30, 2018, 10:24:29 AM »

Not a bad looking coil for being made in a Loweís parking lot (not while driving).

Jon


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PA0NVD
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« Reply #19 on: November 30, 2018, 11:24:37 AM »

Inside size of a beer can?
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ka1tdq
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« Reply #20 on: November 30, 2018, 02:51:44 PM »

I wish. An adult toy would be an even better story. Nope, black PVC.
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ka1tdq
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« Reply #21 on: December 04, 2018, 08:43:59 PM »

Cutting copper strips with tin snips leaves jagged edges and a rough look. Cutting with a paper cutter gives a cleaner look.

Jon


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ka1tdq
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« Reply #22 on: April 04, 2019, 10:33:05 PM »

I've started this project up again. I've decided to build the 24-FET deck but just run it at 400 watts carrier and use my existing modulator.

I just got the frame today to mount the components on. It's a piece of 1/8" aluminum sheet that's 16" x 30". It'll look similar in design to my 8-FET transmitter, being open frame and all. The good thing is that I've learned a bunch of stuff since that rig that I'll incorporate into this one.

Jon


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ka1tdq
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« Reply #23 on: April 06, 2019, 10:23:25 AM »

I'm learning to use the markup feature on my iPhone.

I did find a nice block of cherry wood to mount the output transformer to rather than using just teflon.

Jon


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« Reply #24 on: April 15, 2019, 09:37:29 PM »

I have the heat sink mounted to the back of the aluminum block and all the holes drilled and tapped. I'm using a driver for each FET so the next stage is a little pricey. I'm going to build the heat sink assembly with all components then start building the chassis.

A little solid state heavy metal.

Jon


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