# Simple AM Transmitter

## Main Question or Discussion Point

I'm working on a personal project. Going to build a small AM transmitter. The goal is to be able to pick up the audio broadcast from my laptop, at various areas around the house with AM radios.

I have purchased a crystal oscillator from ebay.
Not sure if that link will work.

The crystal oscillator has a frequency of 1.544 mHz.

I'm wondering how I would go about determining the optimum voltage for a crystal of this frequency.
Also how could I determine the range? Are there any simple formulas for calculations like this?

Its going to be a pretty simple circuit but I'm open to suggestions additional components and any other information on how i could make the circuit better. I plan to buy some battery holders, 6 volt and 9 volt probably, a breadboard, some alligator clips, jumper wires for the bread board and I might need some alligator clips too.

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Averagesupernova
Gold Member
This crystal you purchased is a clock module. It is made to specifically generate 1.544 Mhz and that is it. It takes more circuits to make a modulator and transmitter. Why not get an FM module? They sell these to do exactly what you want except receive it in the FM band.

Yea a module would allow me to change the frequency, but this is cheap and its an experiment and learning process. Anyone can go out and buy an FM module.

I understand that it only generates a frequency of 1.544 MHz. I bought it because it lies within the range of 530-1700kHz. Which is what Am radios are capable of tuning too. Are you saying its not going to work at all? From what I understood I would be able to create a simple circuit play some audio through it and tune my radio to AM 1544.

I have been using these guides

http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p024.shtml

http://www.instructables.com/id/Make-a-simple-AM-transmitter/

Why not make an oscillator using a transistor, a coil and a tunable capacitor instead of the crystal? Worked fine for me when I built my transmitter a long time ago.

mgb_phys
Homework Helper
Why not make an oscillator using a transistor, a coil and a tunable capacitor instead of the crystal?
That's an interesting idea. I'm new to this sort of thing so I will have to do some reading. How would one go about making an oscillator that way? Do you know of any online guides or diagrams or circuits using a capacitor, coil and transistor?

Any reason you are building AM rather than FM?
No not really, except I thought that the AM range might not have as much noise from radio stations.

berkeman
Mentor
I understand that it only generates a frequency of 1.544 MHz. I bought it because it lies within the range of 530-1700kHz. Which is what Am radios are capable of tuning too. Are you saying its not going to work at all? From what I understood I would be able to create a simple circuit play some audio through it and tune my radio to AM 1544.

I have been using these guides

http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p024.shtml

http://www.instructables.com/id/Make-a-simple-AM-transmitter/

step 3Connect an antenna to it
Connect the last pin (the pin opposite of the dot) to an antenna. I used a really long telephone wire for my antenna (25 feet or so). I got it to play through my whole nieghborhood. Any sort of insulated wire will work for an antenna.
That is a violation of FCC rules, and can be pretty dangerous, depending on what other bands you end up stepping on with a hobby/experimenter transmitter. Stepping on other AM broadcasts is mostly just obnoxious to other listeners, but stepping on frequencies like Police, Fire, EMS, maritime, etc, can be very bad, and is not taken lightly by the FCC and other radio operators. The reason he had to use a long wire is that the AM band is so low in frequency (around 1MHz), so you need a large antenna to be able to launch an EM wave.

You are allowed to transmit signals below a certain limit (it varies by band), and this low power transmission in the FM band will get you about 10 meters of range. Not quite your whole house, but at least it would be legal. That's how the simple FM transmitter kits can be sold legally -- their output power is limited to stay under the FCC limits.

So an AM band transmitter is not a great project idea, because there is no practical way to make a transmit antenna that can launch a legal, low power signal, without being bigger than your whole house. If you really want to experiment with AM transmitters and receivers, just use coax cable to connect them, so that you are not trying to launch your signal out through the neighborhood.

It's good that you want to experiment and learn about circuits -- that's how many of us started. Be sure to learn about and understand the other issues involved in the circuits that you are building.

But you can use a coil and capacitor to use a short antenna for AM. You can then compensate for the large imaginary part of the impedance...

Here are some examples of how to use transistors to make oscillators:

http://www.tpub.com/neets/book9/35c.htm

Hello;
Is there anyway to make FM or AM transmitter without coils and crystals because I don't know how to make them. Crystal is not availablehere and I can't make a perfect coil.

MATLABdude
Hello;
Is there anyway to make FM or AM transmitter without coils and crystals because I don't know how to make them. Crystal is not availablehere and I can't make a perfect coil.

vk6kro
Coils are just a piece of wire wound into a spiral, so they are not hard to make.
However, any circuit is going to require other components that may also not be available in your country.
You nickname sound Middle Eastern, probably Arabic, so I can understand that you may have serious limits on what you can get. Where are you?

Are you trying to do something like this?
I've never tried this, but it looks like it could work. However, it appears that the modulation transformer is shown backwards. I think the 8 ohm should go to the audio source and the 1000 ohm should go to the oscillator.

update:

Ok, I just noticed that the same circuit is shown in one of your links. The one in your link is correct. The one in my link shows the modulation transformer reversed. I found an 8mhz oscillator in my junk box so I tried it out and it does work. However, I did not have the correct transformer. I could only find an audio isolation transformer. I got a strong carrier but very weak audio. The correct transformer should give you good results. Also, you should use 6 volts and not 9 volts. The oscillator is designed to work at 5 volts. I ran mine at 9 volts and it got a little warm after just a few minutes.

Also, the power input is under 100mw at 6 volts so it should be legal. At 9 volts it was around 125mw.

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But you can use a coil and capacitor to use a short antenna for AM. You can then compensate for the large imaginary part of the impedance...

Here are some examples of how to use transistors to make oscillators:

http://www.tpub.com/neets/book9/35c.htm

Looking at the examples, how would you know what frequency to be on? And what voltage would be necessary to use the circuit?

sophiecentaur
Gold Member
High Power AM is much harder to implement than FM but there is no need for a Modulation Transformer if you are not concerned with efficiency. You can buy an analogue multiplier circuit, put RF on one input and AF + some DC on the other and that will produce an AM signal. Just put this through a linear power amp to give yourself a few mW of AM signal. The best way to serve your whole house, locally, would be to use a 'frame antenna' (just several turns of wire, wrapped around a large (say 3m square) former. If it is mounted horizontally, in the roof, it will not radiate much outside but provide a reasonable signal inside the house if you use a vertical ferrite rod.
It isn't totally trivial to work out how to match this to your transmitter but you could just try a range of matching capacitors in series (? I think) for maximum signal. There must be info on the net about the impedance of large diameter coils with multiple turns which would give you a clue where to start.
Such a system has the advantage that it doesn't require high efficiency so a 'fancy' transmitter / modulator is not needed.

Of course, the usual caveats about the legality of transmitting with illegal equipment apply. But, with such low power and, essentially, an inductively coupled system, you should be fairly safe as long as you choose a 'free' frequency.

Man, this is bringing back some wonderful memories. When I was a kid, I tossed together any number of crystal sets and little colpitts oscillators that you could pick up with the AM radio.

None of the things would reach much beyond the confines of the house, but they were great fun. I'd just get a good diameter pill bottle, stab holes in either end, and start winding. Thanks to Ma Bell, 22 Ga wire was the choice of the day. Just wound a coil that was about 3 inches long, soldered in a variable cap, curtousy of an old radio set, whatever transistor / caps / resistors I could pull out of an old set and went to town.

Only peice of equipment I had was a dirt-cheap volt-ohm meter, but that was good enough, because you could see the overating point shift on the base when you got it oscillating. Surprisingly, it wasn't very difficult to to get it to the band either.

There were also alot of audio transformers in those days, so it wasn't much of a trick to put one in series with the power for oscillator and put a little sound on it.

I'm very encouraged by this line of thought that someone would go for it. Things are far too complicated in this age and trying to reason through one of these developments is probably the shurest way to avoid success. If an 8yr old can do it from an encyclopedia and old radios, success can 't be too far away. :)