Your antenna is the problem. First work on that and see if you get enough power to broadcast through the house. Commercial AM radios use small guage magnetic wire wound around a ferrit rod about 4 inches long and about 3/8 inch in diameter. Wind 240 feet on this rod. You can also just salvage an antenna from an old radio which you might find at the Salvation Army. I do this for antennas, transformers and variable capacitors.

As for rf amplifiers they just amplify rf and they come in many types including the Common Emitter amplifier used in your oscillator. As for your needing another stage of rf gain, I don't think so now. You need a proper antenna.

 

If you are interested in generic RF designs, look for Ham Radio designs on Google.

If you include 'QRP' (= Low Power) in the search, that will give mostly simpler designs.

Examples - general designs (transmitters at the bottom):
http://www.qsl.net/yo5ofh/projects/ham%20radio%20projects.htm

QRP gear:
http://www.dxzone.com/catalog/Technical_Reference/QRP_Projects/

 

Hello,

Most QRP projects are for CW (morse) or SSB.
On the techical reference page of AC6V you will find a lot of links:
http://ac6v.com/techref.htm

You could also take a look in the RF related link thread I started some time ago:
http://forum.allaboutcircuits.com/showthread.php?t=22011

Bertus

 

Your antenna is the problem. First work on that and see if you get enough power to broadcast through the house. Commercial AM radios use small guage magnetic wire wound around a ferrit rod about 4 inches long and about 3/8 inch in diameter. Wind 240 feet on this rod.

I actually do have a ferrite rod and magnetic wire. I didn't realize an antenna could be wrapped around as an inductor. Would it work if it was air core too?

 

I actually do have a ferrite rod and magnetic wire. I didn't realize an antenna could be wrapped around as an inductor. Would it work if it was air core too?

BlackCow, I honestly don't know the answer to that. My guess is that an aircore would also work (so another toilet paper roll?) but it would not have as much impedance as a ferrite core. I want to experiment with this question myself. I'm learning rf techniques, too. I have a fairly good lab in my home and this is just the kind of stuff that interests me. So if you build an antenna on an air core, be sure to let me know what happens.

Like you, I'm in need of help with antennas. In my mind's eye a good antenna would be a 1/4 wavelength winding, as rjenkins said, with one end plugged into a genuine ground -- a copper rod driven into the Earth or a cold water pipe -- with the other end hooked to the output of your amplifier. Maximizing your rf intensity would necessitate connecting your circuit's common to the earth and when you do this you need to be careful with the test equipment you use. I'll probably catch hell from rjenkins for this.

A word of caution. I once blew up a tube AM radio because I forgot that its chassis was not genuine Earth ground and was actually dependent on how you oriented the power plug when you put it into the wall socket. My cousin blew up a TV doing the same thing with it. Modern equipment gets around that by having one blade of the plug fatter than the other.

But to get back to antennas, one way match the input of your radio front end with the antenna is to construct the antenna as described above, with a 1/4 wavelength of magnetic wire on a core, then to put another winding on top of that one such as you create the secondary of a transformer. This means the impedance of the primary reflects through to the secondary as the square of winding ratio. If 75 ohms of primary, as expected, then a ratio of 1:1 puts 75 ohms at the input of your circuit. But if you step up the voltage 1:2, by winding twice the number of turns on the secondary as the primary then you get 75*2^2=300 ohms at the input. By playing with the windings ratio you have room for maneuver.

The antenna primary has an AC reactance of 75 ohms. This is not measurable by putting an ohm meter across the winding. That is the dc resistance of the wire. The AC reactance of 75 ohms comes about from the inductance of the winding and is given by XL=2*pi*f*L which is in series with the winding resistance.

 

BlackCow, I honestly don't know the answer to that. My guess is that an aircore would also work (so another toilet paper roll?) but it would not have as much impedance as a ferrite core. I want to experiment with this question myself. I'm learning rf techniques, too. I have a fairly good lab in my home and this is just the kind of stuff that interests me. So if you build an antenna on an air core, be sure to let me know what happens.

Like you, I'm in need of help with antennas. In my mind's eye a good antenna would be a 1/4 wavelength winding, as rjenkins said, with one end plugged into a genuine ground -- a copper rod driven into the Earth or a cold water pipe -- with the other end hooked to the output of your amplifier. Maximizing your rf intensity would necessitate connecting your circuit's common to the earth and when you do this you need to be careful with the test equipment you use. I'll probably catch hell from rjenkins for this.

A word of caution. I once blew up a tube AM radio because I forgot that its chassis was not genuine Earth ground and was actually dependent on how you oriented the power plug when you put it into the wall socket. My cousin blew up a TV doing the same thing with it. Modern equipment gets around that by having one blade of the plug fatter than the other.

But to get back to antennas, one way match the input of your radio front end with the antenna is to construct the antenna as described above, with a 1/4 wavelength of magnetic wire on a core, then to put another winding on top of that one such as you create the secondary of a transformer. This means the impedance of the primary reflects through to the secondary as the square of winding ratio. If 75 ohms of primary, as expected, then a ratio of 1:1 puts 75 ohms at the input of your circuit. But if you step up the voltage 1:2, by winding twice the number of turns on the secondary as the primary then you get 75*2^2=300 ohms at the input. By playing with the windings ratio you have room for maneuver.

The antenna primary has an AC reactance of 75 ohms. This is not measurable by putting an ohm meter across the winding. That is the dc resistance of the wire. The AC reactance of 75 ohms comes about from the inductance of the winding and is given by XL=2*pi*f*L which is in series with the winding resistance.

I'll experiment with it and let you know. I need to buy some more enameled copper wire, I don't have enough for a 1/4 wavelength antenna at 1 MHz (234 ft).

That or I could build something with higher frequency. I'm determined to get my AM transmitter to work with reasonable range first though. I'm not planning to launch a pirate radio station or anything like that XD

 

I'll experiment with it and let you know. I need to buy some more enameled copper wire, I don't have enough for a 1/4 wavelength antenna at 1 MHz (234 ft).

Radio Shack sells a three pack of magnetic wire on spools. The small wire would be the best for the antenna.

That or I could build something with higher frequency. I'm determined to get my AM transmitter to work with reasonable range first though. I'm not planning to launch a pirate radio station or anything like that XD

I wonder what a Commercial License for the AM band costs? I'd like to start a true public radio station for nonprofit. I envision great music and lectures by local professors and teachers on a volunteer basis. Speaches by local politicians, poetry reading by local bards and so on. And the cost? A license and great big transmitter on Ahtanum Ridge. Am I a dreamer?

 

A quarter wavelength antenna is driven at one end and the other end is open circuit - as in theoretically infinite impedance and high voltages. This is why the antenna (or the free end at least) must be clear of anything conductive.

Depending on the frequency, either the antenna must be mounted over a ground plane (like a whip antenna on a car roof), or the source must be well grounded .

A few tens of watts into a properly matched and tuned antenna can be enough to produce corona discharge due to the high voltages generated - the tip glows blueish.

Even a walkie talkie or CB producing a few watts can light a fluorescent tube if the end of the whip antenna is held near it, due to the intense electrical field.

A short wire antenna (on MW band) does little as the change in electric field intensity along it's length is tiny.

A ferrite rod antenna uses the magnetic component of the electromagnetic wave, so works better as a compact antenna.

You can also use a loop antenna, like a ring of heavy copper wire or copper pipe, with a tuning cap across the gap. This also works with some level of magnetic coupling.

If you add or remove the ferrite rod from a coil, you will have to drastically change the number of turns to get back to the same inductance so it will tune on the same frequency.

If you are seriously interested in building and experimenting with radio gear, do look at getting a ham licence!

ps. Once you start winding coils, the length of wire used is nothing to do with wavelengths. An antenna coupling coil over your existing tuned coil would probably be somewhere in the range 8 to 12 turns, just a few feet of wire.
One end to ground, one end to the antenna.

If you wanted to try a ferrite antenna, you would need to replace your existing coil & cap with the antenna and matched tuing cap from a scrap medium wave (AM) radio. A ferrite antenna can only work as a tuned coil, so adding that seperately would need two tuning caps and both would have to be set correctly to get any signal.
Replacing the existing coil is far easier.

 

rjenkins, thanks. I'm wondering the same thing BlackCow is. So what is your advice with regard to making a simple and practical antenna for medium frequency and capable of projecting to a radio in the next room? Are there any options other than a wound antenna?

 

You could use dipole.,

Actually there are many antenna options, styles actually, but you still have to match the waves length. usualy 1/4.

 

Why are you using the very old, very poor quality AM broadcast band? Its frequencies are so low that the antenna must be very long.
Does anybody listen to the interference and muffled sounds on the AM broadcast band anymore?

 

Why are you using the very old, very poor quality AM broadcast band? Its frequencies are so low that the antenna must be very long.
Does anybody listen to the interference and muffled sounds on the AM broadcast band anymore?

Yes, I think it's mainly because low frequency broadcast like to travel far, especially at night. Talk shows don't need to care about sound quality as much as reaching a large audience.

Not sure why I decided to mess around with AM transmitting first, I think just because I built a simple AM receiver so the next step I thought would be building an AM transmitter.

I'll probably start playing with higher frequency stuff.

 

ive built numerous am, fm transmitters. the secret is matching the output impedance of the final stage to an antenna. ive gone 3 miles on fm with one general purpose transistor circuit with a 2 foot antenna. I estimated the power to be 62 milliwatts.
on am band i transmitted several miles as well with 1 watt. Its not how much wire you string out there for the antenna. I used to do that but never could go over a few blocks away. then one day i figured it out. my several mile antenna had a vertical wire about 10-15 feet in length but it was tuned by a coli and a variable capacitor at the base of it. What you can do is wrap say no more than 20 turns of wire , same direction as your final tank coil, over your tank coil. this will go a coax receptacle. cable tv coax will work. say 50 feet. The other end of the coax will go to your antenna system. the antenna system is a coil of about 240uh that you can build yourself. it can be of small #28 enamled wire or bigger diameter. make a tap on the coil at 11 turns and 20 just to have a choice because the center of the coax will go there. so one end where you started winding the coil will be your ground and will go to the viax shield. the center of the coax to rither tap 11 turns or tap 20 turns which ever works best. the end of the coil where you finished wrapping goes to the antenna wire vertical. it can be any size from say 2 feet to 30 feet long. i used it vertical because a car antenna is vertical so the wave is vertically polarized. now there is a 365 pf variable capacitor or similar you will attach across the antenna coil from ground side to antenna side. thats what will make it work for you. you may have to experiment and try the variable capacitor between the coil and antenna. if that is too complicated make or buy a 240uh adjustable coil and a 365 pf fixed or variable capacitor. on your circuit where it says antenna, solder a .01 capacitor where it says antenna and then attach to one side of it to the variable capacitor, or fixed capacitor. the other side of the variable capacitor goes to the 240uh coil then the other side of the coil goes to your antenna wire of several feet. you will need a field strength meter or make your own circuit built in to your transmitter. that way you can tune it to maximum power. I suppose i should just make one to show people how its done. it all boils down to a tuned antenna circuit. where the antenna is actually part of the resonant circuit. to tune it you just move the adjustable coil if using a fixed capacitor or adjust the variable capacitor if using a fixed 240uh coil. why these values? because it will cover the am band.

 

Nobody replied to this old thread for over 10 years until yesterday!