How to Build an Fm Transmitter Out of a Clock Radio
Here’s a fun and interesting experiment in electronics, or if you want a different kind of science project.
First you need an older radio (like from the 1980’s), as they tend to give better results. You can use either a “tube” type or a “transistor ” type. For those of of us over 30, you should remember vacuum tubes. For most of you teenagers, you probably never heard of them.
I discovered this project quite by accident while playing with an old Curtis Mathis am/fm stereo (it was called hi-fi back then). What happened was at the time – a mystery. I had my alarm clock radio on and when I was moving the tuning knob for the tube radio; the clock radio went silent!
I thought this was kind of strange, but I had some idea of what was happening, just not why or how. The tube radio was transmitting to the clock radio. Now if you have some basic knowledge of electronics you’ll know that can sometimes be referred to as oscillating. (Waves are going back and forth).
I started to tap on the tube radio, and I could here it on the clock radio. It sounded like when you tap on the tone arm of a record player (man am I old). Now I knew if I could break this “oscillation,” I could transmit what I wanted.
I only know a little vacuum tube theory, so I’ll be basic. If you are not familiar with tubes DO NOT TRY THIS! The input device (microphone, cd player, ect.) would go between the plate and the ground. (Usually the plate voltage would be the highest out of the rest of the tube pins (around 400 to 600). Then as far as tuning , you simply turn on an fm radio close by and set it at a weak or empty station. Then move the tuning knob on the transmitter until you hear your input device.
This next method is with an old transistor radio. I recommend finding a few old ones to play with because you get different results and in some cases NO results.(That’s why they call it an experiment.
First, turn on the radio you wish to use as a receiver. Make sure it has decent reception. Tune it to a spot with no signal (103 to 108mhz). Also, get as close as you can with the radio you are going to transmit with then later you can play with the signal strength. Turn on the “project” radio and slowly move the tuning up and down the dial until the static on the receiver goes silent (you’ll want to keep the “project” radio volume down while doing this). This may not always wor the first time, so try a few different radios until you get one of them to work.
Once you find the radio that works, take it apart. You’ll most likely see some shiney copper coils (sometimes covered in wax) somewhere close to the tuning capacitor (or variable capacitor). Carefully pull one of them up until it is in the shape of a loop, and make sure you have enough length to cut it in the middle and have two wires to use as connection points. These wires are coated with an enamel that you will need to scrape off carefully (they’re thin) without breaking them. this is where you hook up your audio source.
I strongly stress that all of the stuff you use here, be things you’re willing to part with- not some high dollar mixing board or cd player. USE THE CHEAP STUFF. I’ve done this experiment quite a few times and I still end up trashing stuff once in a while. When you get it working right and get “the bugs” out, then move up to the higher quality stuff.
Now that you have your audio source hooked up, turn everything on (make sure you can be close to the receiver, or in the same room). Tune your “transmitter ” until you hear your audio source on the receiver. It’s better to use a line level source such as a cd or cassette player. DO NOT use a high powered source like a speaker output. (Can you say smoke?).
If you don’t get any results the first time, try using a different coil (there’s usually more than one. Just remember to connect the coil you were using back together.
Once you get it working, you’ll need an antena. This is best done by using a solid, BARE copper wire (around #6 gauge works well). Put the antena in a high location, making sure it doesn’t touch anything grounded (you’ll lose your signal). It only needs to be about 10 feet high, of course unless you want to get crazy. The wire coming from the antena should of course be insulated. You will have to again experiment with the best place to connect it. I have found that connecting it to one of the coils seems to work in a lot of cases, while not at all in others. Also, you can experiment with using a bipolar coiled antena, or if you’re up to it you can calculate the wavelength and make a 1/4 or 1/2 wave antena.
I have been able to go up to a mile with this type of transmitter, maybe you will have better results. What you are demonstrating are two things:Resonant tuning and “reversability in nature.” For instance, a speaker can be a microphone, or a motor can be a generator and so on. Nonetheless this is a fun science project, and it got me honorable mention in my high school science fair (1985), and it landed me my first job in radio when I was 18. Have fun with it.
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29 Responses to “How to Build an Fm Transmitter Out of a Clock Radio”
On April 20, 2007 at 3:30 pm
Does this really work?
On April 28, 2008 at 12:01 am
You lost me somewhere in the beginning! I guess I’m not as wired up as I thought, doh!
On May 6, 2008 at 2:25 pm
Hi
You are quite right .I had a semilar experience back in the days of my school time. I did noticed much better what you had said too.
you are quite right.
On May 17, 2008 at 11:26 pm
i just tried it and the taping worked but not the music
On May 18, 2008 at 7:35 am
This project tends to work better with older radios. If the radio you are using has more than one set of coils, try putting your signal across another set. Keep in mind also, the signal you are using should not be too strong. It should only be millivolts. You will get different results with different radios. Good luck, have fun with it, and thanks for reading my article.
On May 29, 2008 at 4:49 pm
How can i transmitt picture to a nearby tv .can any one help ?
On May 30, 2008 at 11:58 am
can one transmit picture Fm frequency and get it on vhf or uhf channel on tv?
Thanks
On August 28, 2008 at 6:22 pm
It is possible to transmit a TV picture a small distance if you have a VCR or some other device with an RF output. The RF (radio frequency) signal is the same type of signal as what is on the air. Most VCRs can either give the RF signal in channel 3 or 4. The signal they give, however is very week. If it can somehow be amplified, then it could be fed into a transmitting antenna and broadcasted. All you have to do is tune a TV set to the channel that your broadcasting. I was able to send a TV picture about 4 feet with the unamplified RF signal from my VCR. I was not able to amplify the signal, though, because I think that my amplifiers can handle such a high frequency signal. Good luck with experimenting!
On October 8, 2008 at 6:24 am
Hi I just built a small fm transmitter and want to extend it up to two or three kilometer .How can i connect it to an amplifier to to be able to run in a farer distance .I have three domestic amplifiers not power amplifiers.
Can any one help me .
Thanks
On October 8, 2008 at 12:44 pm
Jabbi job, a lot depends on your antenna, and it’s height, and wavelength. The higher, the better, and a 1/4 wavelength seems to work best. It involves some math, because you must calculate the wavelength based on the frequency you are transmitting.
On October 9, 2008 at 11:44 am
well so how do i add the domistic amplifire .when ever i plug the out put of the transmitter to the in put of the amplifire .i will still get the weve withing the same hundred meter range.
I connect the antenna to my tv receiver antenna .i need your more explananation .
thanks
Jabbi job
On February 27, 2009 at 3:09 pm
In building a Dipole antenna for fm transmitter ..What are the right measurment for the two wire for the antenna to be arranged vertically on the pole ?if my transmitter ranges on 93.6fm
Thanks
Jabbi
On February 28, 2009 at 7:03 am
The formula for calculating wavelength is: Speed of light/frequency in Hertz. In your case: 186,000/93,600,000= .0019871 miles, or 10.491888 feet. This is a “Full wavelength.” A 1/4,1/2, or 5/8 wavelength generally works best, and is smaller and easier to handle. I would go with 2.622972 feet.
On March 1, 2009 at 10:13 am
One thing that surprises me is that mine radio is a big fm radion it has a single antenna .It receives the signals very clear .and very powerful on the set …
But comparing the Dipole ..it has two wire connection .why must it have a two wire connection and the radio set have only one antenna reception. Why canyt the antenna have only single wire connection on the transmitter
Jabbi Thanks
On March 1, 2009 at 1:28 pm
Unfortunately, I’m not smart enough to be able to translate this but it seems very useful if I could actually use it!
On March 1, 2009 at 6:18 pm
Great article! Glad I stopped by.
On March 2, 2009 at 3:02 pm
Can any one tell me the reason why Electrolyte condensers are not used in the building of oscillator circuits ….
what will happen if one uses electrolyte condenser to build an oscillator circuits Instade of a ceramic capacitors.
can any one give me good reason..
MILLENARIAN thanks
On March 2, 2009 at 5:55 pm
Electrolytic capacitors are, (in fact) used to build oscillator circuits/RC circuits. Typically, the resistor (or inductor in some cases), will act as the device which controls how fast the electrolytic capacitor discharges. The main thing with electrolytic capacitors, is to observe the proper polarity. Also, the electrolytic caps can be used to cut a sine wave, and therefore act as a noise filter.
On March 3, 2009 at 4:51 am
Ok that is cool idea for me . this is a great serendipity for my research work
On March 3, 2009 at 4:59 am
The formula for calculating wavelength is: Speed of light/frequency in Hertz. In your case: 186,000/93,600,000= .0019871 miles, or 10.491888 feet. This is a “Full wavelength.” A 1/4,1/2, or 5/8 wavelength generally works best, and is smaller and easier to handle. I would go with 2.622972 feet.
Dear Keyboardologist:if the length of the will be 2meters length for 93.6 mgh.should the two dipole wire be two feets each?
or divide the two feet into two?
Thanks
JABBI JOBE
On March 3, 2009 at 5:02 am
Dear Keyboardologist:if the length of the wire will be 2 meters length for 93.6 mgh.should the two dipole wire be two feets each?
or divide the two feet into two?
Thanks
JABBI JOBE
On March 7, 2009 at 8:56 am
Here is a link to the video series I am starting: http://www.youtube.com/watch?v=yWJFgFaPmnE
On March 8, 2009 at 1:03 pm
Dear Keyboardologist .Thanks for your new devlopments …I tried to have viewed the Youtube .but my systems wouldnt take the video ..
I am so curious for it is it possible for you to place the information on our regular blog here .
Many thanks
JJ kc
On March 10, 2009 at 8:54 am
Hi Guys .can any one help me .is it possible to ues the video RF out put to send or transmit movie to a nearby televison?
get back to me
mike
On March 11, 2009 at 7:26 pm
hola, tu mal epanol? jk
i saw your video’s on youtube, and as of right now there isnt a part 3 yet, so i got anxious and lo and behold i found the blog you speak of, this is clear enough i dont know how people could be confused after reading your write up here. Thanks for the information, it’s great to spread the knowledge, especially about the more ‘primitive’ technologies as this important kind of knowledge is fading
On March 15, 2009 at 11:29 am
wow cool you should check out my article on how to construct a solar powered rc plane.
On March 29, 2009 at 4:48 pm
Dear Friend
Thanks you for the experiment you just showed on the utube …Quite interesting…LIke i said in responce to your posting i had a semilar experience back in the time of my school days …But byou use Amplifire and a clock radion i used two radions Sliver big radio…. But on my case i never broadcasted dure to the restriction in my my are in those days … But i was able to jam other atation with my two radios..
Last two years During my experiments i converted my computer cpu to an fm transmitter which was only transmitting withing my room …
But i dont know how to put it on the Youtube …
I enjoyed yours many thanks
MILLENARIAN
On March 29, 2009 at 4:56 pm
DEar Friend
Thank you for the experiments you just sent to the youtube ..Quit interesting .
Like i said in responce to your posting last time :I had a similar experience too :But the different in yours is that you used An amplifire and a clock radio …But mine i used two big radio silver radion of early 70s ..secondly i did not broadcast any message .due to the restriction in my area then .But i could pick my siginal from the the bigger radio …
Last two years during my experiment i converted my cpu computer pentium one to an fm transmitter which was reaching only withing my room ….
But i dont know how to send it to the Youtube
Many thanks I did enjoyed yours
MIllenarian
On April 5, 2009 at 1:47 pm
HELLO EVERY ONE
PLEASE WHERE CAN I GET A UHF CRYSTAL OSCILLATOR …FOR TV TRANSMITTERS …
PLEASE IF ANY IDEA GET BACK TO ME ..AM DOING RESERCH ON UHF PROGRAMM…
MY EMAILL:nkaromiller1@yahoo.com
thanks
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