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California Dreaminí




 
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ka1tdq
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« on: July 05, 2020, 09:16:13 PM »

Iím living in California now so I guess I need to start building cool transmitters again. Iím going to do a 2-FET 75 meter class E rig that is self contained except for the modulator and power supply. Iíll use a simplified version of Steveís VFO since I donít need 160 meter capability. Iím departing from my usual open frame construction in favor of a polished aluminum chassis that matches nicely with my plate modulated tube rig. Nothing fancy in the power class, just a basic 100 watt carrier E rig.

My intention at first was to build a single 3-500 amp for the tube rig, and I had accumulated all the parts. But, my god, itís like driving a Sherman tank for a grocery run! This E rig will be about the same power and more practical.

Anyway, hereís the start. The Radio Shack audio amp will be the heart of the modulator.

Jon


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K1JJ
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« Reply #1 on: July 06, 2020, 02:26:57 AM »


Anyway, hereís the start. The Radio Shack audio amp will be the heart of the modulator.

Jon

Hola Jon,

Interesting about using the audio amp to modulate the class E rig.  Could you post a schematic how you will do this?

I'm looking at designs and ideas for a 25-30 watt little rig and open to any suggestions at this point.

I'll be watching your project as it evolves.

T
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We die three times; when our body expires,  when we are buried and when our name is uttered for the last time.  All my dogs are named "Yaz."
ka1tdq
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« Reply #2 on: July 06, 2020, 09:26:04 AM »

Hey Tom,

Everything on an E rig is sequenced. Iíll just discuss the modulator portion here. K1 closes first and RF carrier flows to the antenna. K2 is open which is isolating and protecting the audio amp. R2 is a big 100 ohm resistor. Itís value is small enough to provide a short time constant for C1 to charge to drain voltage while still large enough not to absorb too much power from the audio amp. If K2 and R2 werenít there, C1 would charge through the audio amp with a power spike and possibly damage the amp.

When switching from transmit to receive, the process is reversed. K2 opens first and K1 opens last.

The modulator is modified Heising using the combination of L1 and C1. Their values are based on the audio response desired and modulation resistance (Drain Voltage/Drain Current). The values Iím using are 160uF and 100mH. I forgot the formula to derive those. Stu discussed them on another thread.

D1 and R1 allow for L1 to discharge when K1 opens. It also provides a basic form of negative peak limiting. My modulation resistance is around 18 ohms (45 volts divided by 2.5 amps). This is based on a 100 watt rig operating at 90% efficiency. The value for my R1 will be 200 ohms (about 10x modulation resistance).

This is the simplest way to modulate a solid state transmitter using only passive components. Audio response is amazing.

Jon



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K1JJ
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« Reply #3 on: July 06, 2020, 01:29:38 PM »

Very cool, Jon!

Thanks for the description.  What jumps out at me is needing only 100 MH for the audio choke - the sign of a very low impedance circuit. (compared to tubes)

I already have my Dayton APA-150 (150 watt) audio amp modulating the 4D32 rig thru the Hammond xfmr, so it could be easily toggled over to feed a small e-rig as you've shown.

As you progress, please post a schematic of your refined class E rig with component values and I may join you building one in the 30 watt area, or whatever works out to be a minimum power level to fit my need for a little rig.  I've built my share of 24 FET e-rigs, so should be able to pull this off without too much trouble.   It would be my only homebrew fully solid state AM rig in the shack, so a worthwhile goal.

Keep posting your ideas and don't be shy...  Grin

T

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We die three times; when our body expires,  when we are buried and when our name is uttered for the last time.  All my dogs are named "Yaz."
ka1tdq
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« Reply #4 on: July 06, 2020, 01:38:03 PM »

Yes, I actually used twice that (200mH) in a 50 watt rig a while ago. I have three 100mH chokes, so Iíll play around with the values.

Iím temporarily living in an apartment so Iím limited on blasting and drilling for now. I can solder though so first up is the frequency divider/duty cycle adjuster circuit board.

Iím even limited in my operating activities. HF from the car and only 2 meters (CB) from the apartment. Keying that though causes my FM stereo to go into fault mode and turns on my Bluetooth headphones.

Anyway, Iíll post progress.

Jon
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km6sn
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« Reply #5 on: July 06, 2020, 04:10:28 PM »

Hi,
There is a simple approach: run a class D single FET at approx 24v and a drain load of 12.5 ohms.

To make a 12.5 ohm RF deck, use 2 conductors bifilar wound on a ferrite core, wired as 4:1 impedance transformation,
then a 50 ohm LPF network.

See the attached tech note and spread sheet for PWM modulation method.

I am in process of building it now.

73,
Rod
see attached files

* tech-note.pdf (329.83 KB - downloaded 40 times.)
* levels.pdf (14.75 KB - downloaded 33 times.)
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« Reply #6 on: July 06, 2020, 07:05:09 PM »

Jon,

Where about in California?  I'm on the central coast.....  Arroyo Grande / Santa Maria....

--Shane
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ka1tdq
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« Reply #7 on: July 06, 2020, 07:33:09 PM »

San Diego. Iím working for the military again at Miramar.

Jon
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« Reply #8 on: July 07, 2020, 01:28:19 PM »

That Radio Shack amp also makes a great amp for PC speakers.
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« Reply #9 on: July 07, 2020, 09:59:38 PM »

San Diego. Iím working for the military again at Miramar.

Jon

Small world.  When my oldest son was born I lived maybe 5 minutes from MCAS Miramar.  Worked on Arjons drive.

As my Filipino coworkers used to say....  You live in manilla Mesa bud....  Lol

Good deal, weather there is usually great.

--Shane
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ka1tdq
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« Reply #10 on: July 11, 2020, 02:18:41 PM »

Here's the frequency divider, duty cycle adjust, and frequency counter connect. I'm still waiting for my JK flippy-floppy to arrive.

The IXDD's that are used to drive the IXDD's (seems weird but makes sense) will be mounted to the chassis just off the board.

Jon


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ka1tdq
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« Reply #11 on: July 18, 2020, 11:45:30 AM »

I'm changing gears a little and I'm going to make this a 200 watt carrier transmitter rather than 100. It makes sense as it will only require $20 more in parts (2 FETs and two drivers).

My problem is that I can't build at this location. Remarkably, I live in a very quiet apartment complex. Eerily quiet. I just got the parts to build a portable building desk that I can disassemble and set up anywhere. It's just a 2'x4' piece of plywood that'll be wing nutted down to two collapsable steel sawhorses.

The problem is where can I setup without attracting too much attention.

Jon


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WD5JKO
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« Reply #12 on: July 18, 2020, 12:23:08 PM »

The problem is where can I setup without attracting too much attention.
Jon

  Hi Jon,

  I always enjoy your building projects. You have gotten really good at putting complicated things together, and doing so from scratch.

My son lived in San Diego area for many years while working at Spaywar. They had a whole fleet of fully autonomous Humvee's running around off road with difficult terrain to navigate through. Project suddenly lost funding, and the employees flew the coup. As a Gov contractor, no charge number, you don't last long.

Anyway, my son lived in a Condo located in Miramar. The HOA had roving mobs of old ladies patrolling the complex. An infraction such as a paint scuff on the front door would result in a biting HOA citation. He misses Ca, but not that part.

Are you worried about an HOA problem with your work bench, or are you worried about keeping your kids isolated from Dad's work area?

Remember your old enough now to need better lighting, so a light with a magnified lens would be helpful.

What about the antenna? Might consider an indoor magnetic loop, and limit the power to 20 watts.

Jim
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ka1tdq
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« Reply #13 on: July 18, 2020, 12:44:31 PM »

No, I'm worried about making noise and disturbing the neighbors. I'm temporarily in this apartment until my house sells in Phoenix and we buy something new here. That all takes time.

But actually I have figured out my operating situation here in the meantime. I'm now on HF and VHF using stealth antennas. I actually had an armchair rag chew on 40 meters CW the other night with Texas. My CQ call was heard fairly easily in New Zealand too, according to the reverse beacon.

Jon


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