Thoughts on this AM Transmitter

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MiXiN
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Thoughts on this AM Transmitter

Post by MiXiN » Tue Dec 05, 2017 12:04 am

I've just been looking on eBay and found this; https://www.ebay.co.uk/itm/Trasmettitor ... ect=mobile

Am I right in thinking it won't do 1W and will drift due to the fact there's no PLL/Xtal?

MiXiN
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Re: Thoughts on this AM Transmitter

Post by MiXiN » Tue Dec 05, 2017 1:18 am

I was too late to edit my first post, so I'll have to mention it in this one; There's a circuit diagram shown on the eBay listing, and it's in the 2nd image.

Albert H
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Re: Thoughts on this AM Transmitter

Post by Albert H » Tue Dec 05, 2017 10:40 am

It's a free running oscillator, so it will drift. They shouldn't drive the logic output straight into the gate of the output FET - it's NOT the way to do it. Finally - there's no way this will do "1 Watt". It's designed to operate into a short bit of wire as the aerial. It'll give a reasonable signal around your house, but won't go any further than that.

It's easy to generate a PLL-locked carrier for medium wave - which won't drift! There's a standard crystal (4608kHz) that gives the 9kHz steps you need, and three cheap CMOS chips (4060, 40103, 4046) do the job. This will give you a squarewave at the carrier frequency. If you buffer this with a 4049 (another standard CMOS chip) with all the gates paralleled, you can drive the gate of a medium power FET - an IRF640 is a good choice. The drain of the output FET needs a reasonably big choke to its supply. The supply for the final FET should be from the output of an audio power amplifier IC - I use the TDA2030. The audio IC provides half supply without modulation. I use a small RF transformer to convert the low output impedance from the FET up to around 50Ω for the output lowpass filter - wound on a ferrite toroid.

It's also worth including a basic compressor / limiter into the audio input, so that the rig will handle a range of levels without the need to tweak. Audio lowpass filtering is also essential to keep the bandwidth sensible. The circuit I use has a quad op-amp and a couple of audio FETs and keeps the modulation pretty tight.

Medium wave aerials (if they're to be any use) have to be quite long. A quarter-wavelength is best. You also need a good earth - I used to use a huge area of buried chicken-wire!

To keep the impedances sensible, you want a supply of about three to four times the output power: a 10 Watt carrier rig works well with a couple of car batteries in series, and will work all weekend without a recharge! 10 Watts carrier is 40 Watts peak, and will cover quite a big area with a reasonably good aerial and earth in the day time.
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Re: Thoughts on this AM Transmitter

Post by doc retro » Thu Dec 07, 2017 8:33 am

So Albert t h .
Will a 1 watt am rig have a peak to peak of around 4 watts and as for antenna build what's the best 1/4 wave build that can be condensed down to make a smaller foot print but still resonate quite efficiently.?

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Re: Thoughts on this AM Transmitter

Post by geegosh » Sat Dec 09, 2017 10:02 pm

Have just purchased one of his medium wave TX's - will try and report back when I've had a play with it.

Don't get upset about the lack of PLL - engineer's are still turning dials to keep huge medium wave transmitters aligned in some sites to this day, and back in the 1960's on the ships nobody would've dreamt of all this Class E solid state malarky.

I'm interested to see the advertised RF power and quality as the designs are pretty simple, for the price I'd snap one up and see for yourself too

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Re: Thoughts on this AM Transmitter

Post by Analyser » Sun Dec 10, 2017 7:18 am

It might be just that you're operating the amp in a non-linear mode. You can try increasing the voltage on the bias pin of the RA module to see if that helps the issue.

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Re: Thoughts on this AM Transmitter

Post by OgreVorbis » Sun Dec 10, 2017 7:18 pm

I don't really see the point in buying something like that unless it's just for novelty.

If you want a micro AM transmitter, DDS is the best way to go. PLL/VCO technology is still better at FM frequencies because it generates less noise and is more cost effective because DDS is still pretty noisy up there in the higher frequencies. However the AD9850 is a really clean DDS that works at these lower frequencies.

You can find some AD9850 modules on ebay, but those require an external MCU (such as arduino, etc.), but there are also a few that come with all that integrated and an LCD screen for changing frequency. Some only do SSB mode though, so you need to look carefully.

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Re: Thoughts on this AM Transmitter

Post by Albert H » Wed Dec 13, 2017 3:21 am

A PLL MW rig is really easy if you just want a few Watts. The cheap (<£1) IRF FETs do the job easily. If you use an audio power amp IC like the LM1875 or TDA2030, your modulator is trivial.....
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: Thoughts on this AM Transmitter

Post by NOYB » Sun Dec 31, 2017 3:10 pm

Recently bought one of these. I'm hoping that it will sound pretty good with Stereo Tool setup correctly. Interested to see how far it goes with a LOOONG antenna!
https://www.amateurradioshop.nl/webshop ... uwkit.html

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Re: Thoughts on this AM Transmitter

Post by Albert H » Mon Jan 01, 2018 6:50 pm

doc retro wrote:
Thu Dec 07, 2017 8:33 am
So Albert t h .
Will a 1 watt am rig have a peak to peak of around 4 watts and as for antenna build what's the best 1/4 wave build that can be condensed down to make a smaller foot print but still resonate quite efficiently.?
The best aerial is a quarter wave vertical over a huge ground mat! Sadly, it's not usually possible to put up a huge tower. You can't really "condense" a medium wave aerial if you want to have useful radiation efficiency.

I've usually used modified versions of 160m amateur aerial designs, with "Q" reducing measures to broaden their bandwidth a bit. The "Inverted-L" is often quite practical, and has been used very successfully by some pirates for directional MW coverage into a target town or city. In some cases, it was harder to get a good enough earth than it was to put up an effective radiator.

The best clandestine aerial I've ever used on Medium Wave Was the "Tower Block Sloper" - a ¼-wavelength of wire coming down from the roof of a block at 15° - 18° from vertical, tied off at the lower end with fishing line (to provide an insulator) to a tree or lamp post. Using the lightning conductor of the building as the counterpoise, the radiation efficiency was astonishing. A little 8-Watt carrier / 35 Watt peak rig (on a quiet frequency) gave good daytime coverage to London and beyond.

It's trivially easy to build a clean, stable FET output PLL medium wave rig these days. A £1 FET will give you 20W carrier / 80W peak easily. The only expensive parts are the heatsinks! If you have a reasonably full junk box, the only expensive part could be the aerial wire!

It's important to have good, tight audio processing, but this isn't difficult to achieve (though Caroline are making a mess of it on 648kHz). Audio Bandwidth limitation is important too - easily achieved with a couple of stages of filtering.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: Thoughts on this AM Transmitter

Post by MiXiN » Tue Jan 02, 2018 1:41 am

NOYB wrote:
Sun Dec 31, 2017 3:10 pm
Recently bought one of these. I'm hoping that it will sound pretty good with Stereo Tool setup correctly. Interested to see how far it goes with a LOOONG antenna!
https://www.amateurradioshop.nl/webshop ... uwkit.html
I've got that exact one, and I get around 200m using a 6ft long piece of wire if I remember rightly.

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Re: Thoughts on this AM Transmitter

Post by Albert H » Tue Jan 02, 2018 2:08 am

MiXiN wrote:
Tue Jan 02, 2018 1:41 am
NOYB wrote:
Sun Dec 31, 2017 3:10 pm
Recently bought one of these. I'm hoping that it will sound pretty good with Stereo Tool setup correctly. Interested to see how far it goes with a LOOONG antenna!
https://www.amateurradioshop.nl/webshop ... uwkit.html
I've got that exact one, and I get around 200m using a 6ft long piece of wire if I remember rightly.
It's a great little circuit, and works well for "around-the-house" medium wave transmission. It uses pulse width modulation, and the quality is quite remarkable. The synthesiser is similar to the one I use.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: Thoughts on this AM Transmitter

Post by NOYB » Fri Jan 05, 2018 1:16 pm

Ah the old PWM and then filter the fundamental out trick I presume? The same was done in the old Redifon BT1000 transmitters so that they could do "Class D" amplification - ie amplify a square wave which was more efficient than conventional linear amplification.

Image

Quite impressed with myself that the circuit worked first time - it's been years since I built anything! I'll be looking for recommendations of easy ways how to amplify it a bit - nothing excessive, maybe 1 watt or so.

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Re: Thoughts on this AM Transmitter

Post by Hamnchips » Fri Jan 05, 2018 2:08 pm

I've just bought one of these little things (the PWM one). Gonna have lots of fun I think :)

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Re: Thoughts on this AM Transmitter

Post by NOYB » Fri Jan 05, 2018 2:13 pm

Bargain aren't they?!

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Re: Thoughts on this AM Transmitter

Post by Albert H » Sat Jan 06, 2018 2:52 am

If yuo're going to amplify it, you need a truly linear PA. This isn't easy to achieve, but if you're not too worried about efficiency,l a pair of BFY51s driven by a 2N2222 will get you there. I'll put a circuit up here shortly.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: Thoughts on this AM Transmitter

Post by NOYB » Wed Jan 10, 2018 3:08 pm

I'll put a circuit up here shortly
You are a gentleman and a scholar!

Next I'll have to do some research on LC filters!

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Re: Thoughts on this AM Transmitter

Post by hailstorm » Mon Jan 15, 2018 3:43 pm

Just bought the PLL AM kit. Looking forward to getting it put together... Then I saw the Mosquito TX on the 6v6.co.uk website. It's 1 watt carrier - PLL, ready built and £90. It says it's optimized for 50% modulation though (for the Netherlands LPAM) market.

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Re: Thoughts on this AM Transmitter

Post by Albert H » Tue Jan 16, 2018 1:53 am

50% modulation is a waste of time. It'll be too quiet, and disappear into the atmospheric noise much of the time. They also say that it's "optimised for a 3m wire aerial". 3m is about 1% of the wavelength, so will be about as much use as a piece of string. If the transmitter is generating a real Watt, it's probably radiating 10mW at best!

If you want to do this properly, you'll need a proper aerial, significantly more power, and a good earth. I'll put a nice, simple 20 Watt / 80 Watt peak single FET output Class E rig that uses PDM up here later on. This kind of rig has been widely used in Eastern Europe and Africa, can be set up for any medium wave frequency, and can be aligned with a multimeter and a frequency counter (at the bare minimum). To get the very best results requires the use of an oscilloscope, but you'll get good results without one if you take a little care. If you have any proper electronic constructional skills and shop around, you can build one for about £30.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: Thoughts on this AM Transmitter

Post by Gigahertz » Wed Jan 17, 2018 8:12 pm

Albert H wrote:
Tue Jan 16, 2018 1:53 am

My little MW rigs are about the size of a shoebox and come in 10/50 Watt and 20/100 Watt versions (that's Carrier and PEP ratings). The smaller one runs off a single car battery, and the bigger one uses two car batteries (in series). If anyone wants to give it a go, I'll put the circuit diagrams up here, along with some simple suggested wire aerials.



If you want to do this properly, you'll need a proper aerial, significantly more power, and a good earth. I'll put a nice, simple 20 Watt / 80 Watt peak single FET output Class E rig that uses PDM up here later on. This kind of rig has been widely used in Eastern Europe and Africa, can be set up for any medium wave frequency, and can be aligned with a multimeter and a frequency counter (at the bare minimum). To get the very best results requires the use of an oscilloscope, but you'll get good results without one if you take a little care. If you have any proper electronic constructional skills and shop around, you can build one for about £30.
Be nice to see circuits Albert ??

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