HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: Antennae on January 09, 2014, 1855 UTC
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I was looking at the schematic on makeRF's website.
http://makerf.com/posts/so_you_want_to_be_a_shortwave_pirate
The modulator/amplifier taps right into the Michigan Mighty Mite transmitter. Can one take the modulator/amplifier from this and tap into any CW transmitter (hopefully one with more power)?
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Long answer yes, short answer maybe. It depends on the architecture of said CW transmitter. Most are setup for class C amplification, which means any low level AM is going to be stripped off (mostly) during the amplification process. Thats why most low power rigs (The commando comes to mind) use high level modulation of the final output device. Because of intrinsic feedthrough capacitance, the drive to the final usually has to be modulated to some extent as well to assure good modulation depth (-100% modulation). Other rigs, like the corsair, use low level modulation followed by a "linear" amplifier. I may have screwed up some details, but that's the gist of it.
+-RH
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Thanks redhat. I don't know much about electronics so I had to let your response percolate. So I think I understood your part about the class C amplification. But the rest was Latin.
Then I looked up what kind of modulation methods there are. So I learned you can modulate a signal in the low power domain and then amplify it for transmission . Or it gets modulated in the high power domain of the transmitted signal. (wikipedia) And I figure this is what you are talking about with the commando and corsair.
I just found this website about adding modulation to a Pixie 2 transmitter. After that its called a Talking Pixie 2. 8)
http://www.indianapolis.net/QRPp-I/talking_pixie2.htm
I don't think the makeRF's "amplifier/modulator" would work with the Pixie 2 because makeRF's is both a modulator and amplifier.
Conclusion:
Cool. So it is possible but you say most are setup for class C amplification. I learned that class C is more efficient and it makes sense to use that class for CW with such little power.
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You can high-level modulate the final amplifier. Drop the DC input to about 75% of that used for CW for continuous AM use.
If the final is a transistor though, watch the Vcc or Vdd rating on modulation peaks, lest you let the magic smoke out!
Peace!
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Back in the hollow state days, you could build a cathode modulator that you could plug in to the key socket, but they were a pain in the ass to adjust and notorious for negative modulation.
I remember my Uncle, who was a tv repairman, trying to build one for an old CW rig he'd picked up somewhere. He wanted to be a "Big Gun" on the CB bands, but he was as cheap as they come. He worked on that thing for weeks before he said the Hell with it and built a linear amp for his CB transceiver.
There's a fellow who's designed a very nice variant of the Talking Pixie called the "Big Talker". I can't remember the name of his website but you can type "The Big Talker" in your favorite search engine and see what comes up. One caveat, it's designed for Part-15 broadcasting.
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cmradio:
Yeah, I don't want to let any magic smoke out thats not supposed to be letting out. But it seems like a worthy notch to put in your belt as one of life's accomplishments. I'm looking at transistor things because the tubey things would suck up battery power.
Pigmeat:
-Thats a cool concept to plug it right into the key socket.
-The Big Talker... I found the Big Talker and its cool that you can change the power output settings. Part-15... I was fiddling around with an old Ramsey FM-10 transmitter (Part-15) for a whole day and it didn't go far enough to sate my thirst. So the Part-15 leaves a bad taste in my mouth. It only made me want more. Its audio suuuuuuuucked too.
-I'm cheap too. The affordable mobile things I can find are too weak and CW. Or too expensive (mobile transceivers).
Come to think about it, I invested all that time in the FM-10 that I should go hang it in a tree and give it one last try.
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Mighty Mite transmitter -- try different transistors i don't remember what transistor i ended up using
but it ran on 18 volt did 7 watts or so carrier did high level modulation thing
transistor came from a old cb radio
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Cool, 7 watts. I bet the transistor had a heat sink on it? I'll read up on the Mighty Mite.
There's a Ramsey QAMP 20 kit that gives 20w with scary reviews on eham.net.
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My FM-10 is good for about a 1/3 of a mile. I bought it built at a hamfest from a real character. He may have modified it. ;) He's a weird old coot, but he always has interesting stuff under the table for his regulars.
It still drifts a bit until it warms up, but it was heck of a deal for 15 bucks.
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So you have a supplier for the good stuff at the hamfest you frequent? Thats fun, I've never been to one, they sound cool.
I did a 16 foot experiment with my FM-10 today. The antenna is a vertical dipole hooked right into the antenna jack. Length is cut to the transmitting frequency.
5' off ground = .1 mi rough at .3
12' off ground = .3 - .4mi
16' off ground = .6 mi
With my trusty portable in the right spot I got it. The hills around here make the radius inconsistent depending on the direction.
PS: I realized the audio sucks because I wired the headphone jack wrong! Only one channel was working correctly. Now to be scientific I need to rewire it and test again...
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I was looking at the schematic on makeRF's website.
The modulator/amplifier taps right into the Michigan Mighty Mite transmitter. Can one take the modulator/amplifier from this and tap into any CW transmitter (hopefully one with more power)?
Hi Antennae. I have been down with the flu for a few days here so I haven't jumped in to the discussion. You might find the 30 below article about modulating a carrier (in this case, from the LULU transmitter) will help you better understand how to do AM:
http://hfunderground.com/blog/?p=25
I hope that helps...
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Thanks ff, are you sure it was the flu? Maybe the fflulu? (Just a corny joke.)
So first off I would guess that LULU uses high level modulation because the "mod tranny" is so big and beefy.
*a period of percolation...
I'm getting it, I had to use my brain. So the first section makes the oscillations for the transmitting frequency and transmits it. But it needs some audio. The second section gets the audio, amplifies it and then modulates it. This goes into the first section to get transmitted.
Toroid winding: It looks scary. But recently, after learning that men invented knitting (men can knit too), and seeing knitting projects take months to finish, I think this wouldn't be nearly as difficult. I think we were made to do menial repetitive tasks (grooming, weaving, knitting, ...) Some months ago I used knitting to calm myself down after a hard day's work. Its very therapeutic. So toroid winding probably falls in that category.
I've seen a wound toroid thing here in this speaker system. Its big and has lots of windings. Scroll 2/5ths downs the page to see it.
http://forums.logitech.com/t5/Speakers/Logitech-Z-2300-The-Last-of-the-Titans-Review/td-p/500830
Can anybody recommend a good transmitter book for beginners?
Edit: I'll look into the ARRL handbook that redhat recommends.
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Join the yahoo homebrew pirate radio group and look at the schematics there. Channel Z's rev 2 is very simple and only has one obscure part which you can get on ebay occasionally. Hell, I'll send you one since I have a couple cases. It is a fine transmitter which with a good antenna and good conditions will get you coverage of 50% of the states. 5-10 AM watts in a fist size box. Be sure to slap a computer fan on the innards or the transistors will smoke early.
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So the first section makes the oscillations for the transmitting frequency and transmits it. But it needs some audio. The second section gets the audio, amplifies it and then modulates it. This goes into the first section to get transmitted.
You're getting closer. The oscillator drives that final amp on and off at the (radio) frequency of operation. This generates the carrier which can be turned on and off with a code key to make CW. If instead this carrier is turned on all the time, it can be modulated with audio. The modulator is actually the power supply for the final stage in high level modulation. The voltage to that IRF510 (or whatever) final amp is fed through the modulator. That voltage is modulated (made to change) at the rate of the inputted audio. Through the magic of inductance (remember that scary toroid?), the modulated voltage (of a fully modulated carrier) swings from 0 volts to TWICE the supply voltage, at this audio rate. In other words, at an instantaneous modulation peak the supply voltage of a 12 volt transmitter final is 24 volts, and at an instantaneous modulation valley, the voltage supplied to that final is ZERO... no voltage at all at that instant in time. This is why you hear us radio geeks talk about carrier power and peak envelope power (PEP). The carrier power is the generated power without modulation. On a fully modulated carrier, the PEP is FOUR times that amount. Why? Because twice the voltage will force the final amp to draw twice the current. But that PEP is only on the peaks. That's a simplified, ball park kinda explanation, but it should suffice. BTW - don't apologize for the corny joke. I'm so corny - I enjoyed it! ;D