Simple AM transmitter using only crystal and audio amplifier

Thread Starter

hazim

Joined Jan 3, 2008
434
The title says it all... Here is a very simple circuit, even it's not a circuit, it's only a connection between 1000Ω/8Ω audio amplifier and a 1MHz crystal, with a 9v battery supply. I just want to share this thing with you, and to discuss it. Will this really works? what may be it's power consumption and it's broadcasting range?
Here is it with some explanation:
http://sci-toys.com/scitoys/scitoys/radio/am_transmitter.html

 

Wendy

Joined Mar 24, 2008
22,798
It might work, I see what they are trying to do.

Let us know. :) You get to be a Guinna Pig.

Just be aware of the broadcast requirements where you are. In the USA it is less than 10mw and an antenna length of 10' or less, and you can't interfere with an existing AM station.
 

ELECTRONERD

Joined May 26, 2009
1,147
You could add a power amplifier on the output of the antenna to increase the usable range; I doubt it will go very far under the current conditions. Also, an adequate antenna would increase your distance. Despite all of this, you should check with your RF regulations (FCC in the USA, etc.) first to make sure you don't perform any malfeasance.

Austin
 

Wendy

Joined Mar 24, 2008
22,798
You could add a power amplifier on the output of the antenna to increase the usable range; I doubt it will go very far under the current conditions. Also, an adequate antenna would increase your distance. Despite all of this, you should check with your RF regulations (FCC in the USA, etc.) first to make sure you don't perform any malfeasance.

Austin
This would likely violate the rules I posted. 10mw is not very much power.

Another thought occurred to me, this is likely a digital output. An LC circuit might be a good idea to limit splattering across the band.
 

Wendy

Joined Mar 24, 2008
22,798
Have you built it? The stuff I built when I was a teen went further than that, but I wasn't interested in range, but theory.
 

Thread Starter

hazim

Joined Jan 3, 2008
434
No I didn't build it yet.. when I built it I'll post a small review here...

Just be aware of the broadcast requirements where you are. In the USA it is less than 10mw and an antenna length of 10' or less, and you can't interfere with an existing AM station.
I'm from Lebanon and I live in it.. Lebanon is a small country but is known well... wars, jobbery, political deception, sectarian regime.... no rules.... no fcc :D .... I'm sure that I can use any fm/am/vhf/uhf/transmitter with 10W here :D
Here it's a good place for radio circuits amateurs ;)
 

KL7AJ

Joined Nov 4, 2008
2,229
This should work, but just be aware that you're likely to have simutaneous PHASE modulation as well.. This is unavoidably when you modulate an oscillator's supply voltage.

eric
 

Thread Starter

hazim

Joined Jan 3, 2008
434
What about the diode they talked about at the bottom of the page? where should it be connected?
 

JDT

Joined Feb 12, 2009
657
The trouble with your simple AM transmitter circuit is that your crystal oscillator produces a square wave output. This will produce harmonics up to very high frequencies and is not good!

Secondly, the oscillator is probably designed to be powered from 5V only and will not like being run from the amplitude modulated voltage. You will either damage it - when the voltage gets too high, or it will stop oscillating - when the voltage gets too low. At best you will get a lot of audio distortion and probably more harmonics.

I suggest you add a few more components as in the attached circuit. Here, the oscillator drives a transistor. You will have to experiment with the value of the base resistor, it needs to be just low enough to drive the transistor on. The coil and the capacitor in the collector circuit must be tuned to 1MHz. Do this by adjusting the variable capacitor for the strongest signal on a AM radio some distance away (say 100m away). The coil is 50 turns of insulated or enameled wire on a former. The cardboard former from a loo roll is just right! The capacitor comes from an old AM radio. The antenna is a length of insulated wire strung up on a tree. A good earth connection is important. Connect to a water pipe. The iron cored inductor could be difficult to find. Try the primary winding of a small mains transformer. Use the audio output of an MP3 player or small audio amplifier.

This is, of course slightly illegal (in the UK anyway). But very interesting to do! If you use a small transistor and a good antenna you might get a range of a mile or so. Not much going on on the AM band here these days so probably no one will notice.

Back in the day, I used to make much higher power versions of this using a valve and a 600V power supply. Highly dangerous, very illegal - and a range of 50+ miles! (Name and address withheld).
 

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Thread Starter

hazim

Joined Jan 3, 2008
434
Thanks all.. I bought the audio transformer but the buyer told me that this transformer is used in bird chatter amplifier devices...(used by huntsman), and that it's not labeled as 1000/8 ohm and he don't know anything about that...and even nothing is labeled on this transformer... it's about 1cm x 1cm. What do you think? can I find its input and output resistance? how??
 

nerdegutta

Joined Dec 15, 2009
2,684
Hi.

For this to work, you need to turn the 1000Ohm/8Ohm around, so the three leg-side is pointing towards the oscillator.

I have build this, but the range is not so good. I grounded the oscillator to my coldwater pipe, and I've been experimenting with an antenna, but I can't seem to find the best solutions.

I'm also interested in how to increase the range up to around 200 m.

How di I build
add a power amplifier on the output of the antenna to increase the usable range;
?

n
 

Wendy

Joined Mar 24, 2008
22,798
Simple, since you're in a country that does have rules about such things. You get an amateur radio license (aka Ham license) and build a legal transmitter on a legal frequency. To build something with that kind of range on a commercial channel violates the law a bit otherwise. An easy way to do it is put a resonant antenna out, say about 1.8 miles (4Km?) of wire, this can vary ±400% depending on frequency.

In the USA the antenna limit is 10 feet (around 2.2 meters).
 
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