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Low power SW Transmitter

Posted: Thu Aug 15, 2019 8:58 pm
by Gigahertz
Came across a post about the corsette SW transmitter on another forum. The guy seemed to get it working in the end so for abit of fun decided to give it ago!

Only problem with these older designs are some of the components are getting hard to get hold of but you can find them if you look hard enough. The crystal was the hardest to locate but I was lucky to find a ebay seller who had crystals and some were in the 48M band.

The transmitter can be built in an few hours and with 12v mine produced just under a watt, add a few more components and you can make the 10w version using IRF510 (will try this later).

Information might be handy for someone on here who wants to give SW transmitter a try?

http://freeradiotx.blogspot.com/2009/06 ... itter.html

Re: Low power SW Transmitter

Posted: Fri Aug 16, 2019 8:26 am
by projuicer
When are you going to fire it up? :-)

Wouldnt mind a link to the other forum either.

Re: Low power SW Transmitter

Posted: Fri Aug 16, 2019 11:17 am
by Albert H
The problem with that little Shortwave Rig will be audio distortion. If you amplify the output of the two stage low power effort, you'll have to get lots of linearity in the final stage if you want undistorted modulation. This implies very low PA efficiency, as it's going to have to be biased towards Class A (the most inefficient amplification class).

If you're going to use this kind of modulation scheme, you need to introduce "Envelope Feedback" to correct the inherent distortion. Fortunately, the LM386 IC that's used as the modulator has two inputs - non-inverting (that's used at the moment for the incoming audio) and inverting (that's just grounded at the moment. If you put a resistor between pin 2 (the inverting input) and ground, you can use that as the injection point for the recovered audio that's going to be used for the envelope feedback. If you take a little of the output signal through a low value capacitor, then a diode demodulator and an audio amplifier with a low-pass filter to send the demodulated audio (in the correct sense!) to the inverting input of the '386.

When you get the amount of envelope feedback right, it's a revelation - the nasty, crunchy modulation cleans up to almost hi-fi quality!

Remember - your audio is going to need plenty of compression and bandwidth limiting.

Re: Low power SW Transmitter

Posted: Fri Aug 16, 2019 12:21 pm
by SamTheDog
Superb! Interesting Build-Style!..Collector modulated with a choke?..

Re: Low power SW Transmitter

Posted: Fri Aug 16, 2019 5:19 pm
by davemartin
Sam you dipstick, they are my circuits on those pages. Get on for over 20 years old. Dozens have been built and in use across the world. The 30 watt carrier version is what WNKR used for SW from 2000 on.

Dave

Re: Low power SW Transmitter

Posted: Fri Aug 16, 2019 8:10 pm
by Gigahertz
Albert I only built this for an experiment to make sure I could get it to work and once I have the chance am going to make the haigher output version.

The other forum is more decided to shortwave HFUNDERGROUND

Thanks for sharing your design Dave and looking around the web as you say it’s been reproduced over and over.

Re: Low power SW Transmitter

Posted: Fri Aug 16, 2019 8:21 pm
by Albert H
Dave - did you try envelope feedback? The first time I tried it was way back in the early 80s when I tried gate modulation of a 2SK135 (a wonderful FET for medium wave!). I ended up with a rig that only needed milliwatts of modulation, and the PA was close to 85% efficient, and the audio was fabulous. I never managed to blow up a 2SK135, despite really trying hard!

The rig was the usual three IC synthesiser, then with an LM311 comparator as the modulator, effectively pulse-width modulating the carrier into the gate of a '135. The output was "sniffed" inductively after the output filter, rectified to recover the audio and put into one input of an op-amp (configured for a low pass characteristic), with the other input getting the incoming mod. The output of the op-amp fed into the comparator, to modulate the gate of the final. It sounded really weird to listen to the the output of the op-amp - it was distorted to hell, but gave clean audio when used to drive the modulator.

I found that I could get around 25 Watts carrier / 100 Watts peak out of a single FET and slightly more than twice those figures with a pair of them. I used to get them for around £2 each. The only expensive components in those rigs were the power supply parts! I used a supply rail of about 50V, and the FET didn't mind since it was rated up to 180V. The only time I came close to blowing one up was when the aerial blew down in a storm. I went to investigate the poor signal and found a red-hot box!

Re: Low power SW Transmitter

Posted: Sat Aug 17, 2019 1:31 pm
by SamTheDog
davemartin wrote:
Fri Aug 16, 2019 5:19 pm
Sam you dipstick, they are my circuits on those pages. Get on for over 20 years old. Dozens have been built and in use across the world. The 30 watt carrier version is what WNKR used for SW from 2000 on.

Dave
Kin-Ell... All I said was interesting build style... and asked about the Mod-Method... Buy me a beer you old git.... Pages..?

Re: RE: Re: Low power SW Transmitter

Posted: Mon Aug 19, 2019 7:34 pm
by davemartin
SamTheDog wrote:
davemartin wrote:
Fri Aug 16, 2019 5:19 pm
Sam you dipstick, they are my circuits on those pages. Get on for over 20 years old. Dozens have been built and in use across the world. The 30 watt carrier version is what WNKR used for SW from 2000 on.

Dave
Kin-Ell... All I said was interesting build style... and asked about the Mod-Method... Buy me a beer you old git.... Pages..?
Love it!! Will have to meet for a beer soon.

Sent from my SM-J610FN using Tapatalk


Re: RE: Re: Low power SW Transmitter

Posted: Mon Aug 19, 2019 8:02 pm
by davemartin
Albert H wrote:The problem with that little Shortwave Rig will be audio distortion. If you amplify the output of the two stage low power effort, you'll have to get lots of linearity in the final stage if you want undistorted modulation. This implies very low PA efficiency, as it's going to have to be biased towards Class A (the most inefficient amplification class).

If you're going to use this kind of modulation scheme, you need to introduce "Envelope Feedback" to correct the inherent distortion. Fortunately, the LM386 IC that's used as the modulator has two inputs - non-inverting (that's used at the moment for the incoming audio) and inverting (that's just grounded at the moment. If you put a resistor between pin 2 (the inverting input) and ground, you can use that as the injection point for the recovered audio that's going to be used for the envelope feedback. If you take a little of the output signal through a low value capacitor, then a diode demodulator and an audio amplifier with a low-pass filter to send the demodulated audio (in the correct sense!) to the inverting input of the '386.

When you get the amount of envelope feedback right, it's a revelation - the nasty, crunchy modulation cleans up to almost hi-fi quality!

Remember - your audio is going to need plenty of compression and bandwidth limiting.
Albert you are missing the point of the design. It was purely to get people building and experimenting themselves rather than the usual "can you build it for me?" That idea only works if we follow the KISS principle. Many that built these basic circuits have progressed to much more advanced circuits and not just RF. My circuits whet their appetites and gave them the confidence to go their own directions. I am proud of that because it was the intention. I dont actively maintain these circuits today, but still answer questions from those beginners who wish to try building their own.


Dave Martin
Founder and manager of WNKR 
Designer of the Commando and Corsair AM transmitters, developed to promote easy to build short wave transmitters for the hobby LBP.


Re: Low power SW Transmitter

Posted: Mon Aug 19, 2019 8:57 pm
by SamTheDog
You've got some nice simple designs on that site barney. NICE.. Do you remenber when we went to Bex-Heath library and drew some circuits?...

I build one based on that dedign, with four IRF fets in parallel. It's OK, But I'm modding the supply rail in the old way through a transformer.

The cossette circuit is nice and simple to get started with. Mod on it looks odd though. All juice for the final from the audio-amp via L3...

Re: Low power SW Transmitter

Posted: Mon Aug 19, 2019 10:02 pm
by Albert H
My "simple" SW rig uses a FET Colpitts voltage-tuned oscillator and two transistor buffer to give a couple of volts p-p at the output frequency. I take a sample from the output of the buffer into an MC145106 PLL IC, and compare with a standard (cheap) crystal. The stability is as good as the rock, and odd frequencies are no problem. My design (usually) goes in 10 kHz steps. The VCO / PLL block runs from its own 78L12 regulator off the main supply.

The next stage is a medium power driver - I've used 4049 CMOS chips (with the gates in parallel), and I've used medium power transistors (like BFY51 or BD131) - you just need anything that can charge and discharge the gate capacitance of the output FET(s) fast enough! This runs at 15V.

The output is (usually) a pair (or four) cheap IRF-series FETs. They're supplied from either a simple series modulator with a chunky pass transistor (2N3055 or 3773) or (if we're going for a bit more complexity and improved efficiency) a PWM modulator. The main output supply rail is 48V, idling at about 23.7V. The output filter is usually three toroid coils with capacitors to deck and turns over at about 1.3 times the output frequency - so that it's way down by the second harmonic.

The whole thing costs <£20 to build (if you go for the no-frills version), and is in fairly widespread use in the USA, Central and South America and in parts of Asia. The "commercial" versions of this rig go in a 2U 19" case, and the most expensive parts are the case, transformer and heatsink! They're simple, cheap, frequency-agile and robust. They're really difficult to blow up! They're usually supplied with a Transmatch unit that allows them to match into almost anything! The capacitors in the Transmatch are another expensive problem!

The biggest problem I've had was trying to use a Switched-Mode PSU in an effort to give it "universal" power input - try as I might, I could never find one that was stable, clean and robust enough to handle the rigs at a reasonable price!

I might put the circuit up here to inspire some more construction.....

Re: RE: Re: Low power SW Transmitter

Posted: Tue Aug 20, 2019 6:09 am
by davemartin
SamTheDog wrote:You've got some nice simple designs on that site barney. NICE.. Do you remenber when we went to Bex-Heath library and drew some circuits?...

I build one based on that dedign, with four IRF fets in parallel. It's OK, But I'm modding the supply rail in the old way through a transformer.

The cossette circuit is nice and simple to get started with. Mod on it looks odd though. All juice for the final from the audio-amp via L3...
It's just a series modulator, the output of a LM386 sits at half supply volts. It is all supposed to be simple and to run off a vehicle battery.

Dave Martin
Founder and manager of WNKR 
Designer of the Commando and Corsair AM transmitters, developed to promote easy to build short wave transmitters for the hobby LBP.


Re: Low power SW Transmitter

Posted: Wed Aug 21, 2019 2:18 am
by Albert H
I found the 386 to be rather underpowered for that kind of use. I used the LM1875 or the TDA2030 (etc) "power op-amps". They had plenty of grunt, and thermal shutdown for those times when the aerial blows down......

Re: RE: Re: Low power SW Transmitter

Posted: Wed Aug 21, 2019 6:09 am
by davemartin
Albert H wrote:I found the 386 to be rather underpowered for that kind of use. I used the LM1875 or the TDA2030 (etc) "power op-amps". They had plenty of grunt, and thermal shutdown for those times when the aerial blows down......
The LM386 proved more than adequate for the circuit it was in and of course it could always be bypassed.


Dave Martin
Founder and manager of WNKR 
Designer of the Commando and Corsair AM transmitters, developed to promote easy to build short wave transmitters for the hobby LBP.