AM Transmitter

[chickens1127]

Hi! I'm building an AM transmitter, and I found a really easy link to a design here:

Chapter 4: Radio -- Build a very simple AM radio transmitter


Now, I'm real cheap, and I was reading another article on instructables, and people were taking them out of cheap radio recievers. Would they have one inside them? I really don't wanna pay $8 shipping on a $3 item which will take a week to come.

Second, you need an audio transformer, 1000 ohms to 8 ohms. I'm happy to get that from Radio Shack, but can I salvage it possibly from some cheap electronics with a headphone jack?

Finally, FCC Part 15 says AM transmitters can only use 100 mW of power, which, according to most sites, with a good antenna, you can get 1/4-1 mile. Now, this page claims only 150 feet, so is this rig transmitting 100mW and needs a better antenna? If so, should I just add more coils, or another design of an antenna. (I am a ham operator.) Or, how do I improve this to max out on power. I'm going to make a mixer with 2 or 3 inputs with switches and such and local radio for my community. Thanks!

 

[Hero999]

That will produce lots of harmonics, in fact because the aerial is electrically short you'll probably get more harmonics than the fundamental, i.e. it'll probably probably broadcast a stronger signal on 2MHz, 3MHz and 4Mhz than 1Mhz.

I recommend adding a 1MHz bandpass filter to reduce the harmonics.

 

[chickens1127]

Thanks. Does anyone else have a simple AM transmitter similar to this which may improve on this problem? I saw a $35 Ramsey kit and I'm real tempted. But for now any other ideas?

 

[Hero999]

Why not build an FM transmitter? It will sound much better, is just as hard/easy to build and will cost the same,if not less.

 

[chickens1127]

But FM has lower range, right??? With micro radio, you need as much range as bloody possible.

 

[Hero999]

No, FM goes further than AM for the same amount of power.

The only reason why AM broadcast goes further than the FM broadcast is because it's a low frequency so it bounces of the ionosphere and can travel over the horizon. FM is VHF so penetrates the horizon and shoots off into space.

If you boradcast AM on VHF then it won't go anywhere near as far as FM for the same power level. In your case you won't have enough power to go over the horizon so that isn't a factor.

 

[chickens1127]

So, any plans for a real EASY transmitter, I'll look but then you get 300 sites for mp3 fm car stereo transmitters.

 

[Dragon Tamer]

FM's radio waves are constantly varying, but since it's a higher frequency then there is not as much noise in the circuit. The major advantage of AM, is that the radio waves like to "bounce" off of everything.

 

[chickens1127]

Final Question- can I get one of these oscillators out of a cheap AM radio??? Thanks

 

[audioguru]

The AM broadcast band is at such a low RF frequency that the transmitting antenna needs to be as tall as 15 stories and as big as a football (or soccer) field.
An FM radio station transmitter has a frequency that is 100 times higher so its antenna is 100 times smaller.

 

[Hero999]

Final Question- can I get one of these oscillators out of a cheap AM radio??? Thanks

The answer is yes, you can build an AM transmitter from an old radio.

Goodness knows why you'd do it when you might as well go with FM.

 

[chickens1127]

Well all the plans I see for FM stations are way too complicated for me, with 70+ components

EDIT: I'm a ham radio operator. I know AM is near 80 meters, so you need a big antenna, and FM is between 6m and 2m. But, I'm still intrested in AM becuase if I have an atenna pointing against my brick house, I can broadcast into a huge development down in the valley. FM dosen't reflect like AM. But would it help if I made an antenna 15 feet tall vs the 1 foot antenna in the plans I posted before.

 

[Hero999]

The US AM broadcast band covers 176m to 566m.

Here's a simple FM circuit:
FM Transmitter

 

[transistor495]

Being a ham operator, are you going to make it for 2m/6m VHF bands? What's the actual purpose of this transmission I'm still confused.

 

[chickens1127]

I wanted to start a micro AM station, broadcasting on whatever frequency I can get an oscillator out of a radio for. This way I could aim the antenna against my brick house(I think brick is a reflector) and broadcast into the valley, using Part 15 100mW limit. Now, I'm a ham radio operator, so I know how radio works, and the different bands, so I know that FM can't reflect and the circuits are more complicated. So, I really don't see the problem with the AM circuit except I'm gonna get a little bit of harmonics/

 

[transistor495]

100mW? How long it'll go is a question even if you use the antenna mentioned. Broadcasting down to the valley means you've got a community of listeners down in the valley??
What you're going to broadcast is another question. SSB signals or just music/voice? Listeners got FM receivers or just AM receivers

..or did I ask some stupid questions.

500mW FM transmission is a much better idea if you live in a remote place. Use a dipole antenna to adjust for the clear area coverage.

 

[JimB]

I have thought long and hard before replying to this thread, it is said that if you can't say anything nice don't say anything at all.

However, reading through the post in this thread it is clear that there is a widespread lack of understanding of the principles of radio circuits and radio wave propagation, especially on the part of the one who claims:

I'm a ham radio operator, so I know how radio works

If by that you mean that you have studied for and passed the FCC* exam, I am mortified by the inaccuracy of some of your statements.
Before you build your micro broadcast station, please get a good book on radio and start reading at chapter one.

* I assume that you are in the USA as you refer to FCC "Part 15 regulations".

JimB

 

[chickens1127]

Errmmm... Ya, KC2QVS, passed my liscense 3 years ago, got bored of it, but remember some of the stuff....

EDIT: Sorry, I lost power. Anyways I'm starting the transmitter right now, going through crappy broken AM radios to see if I can find a suitable oscillator. Wish me luck...

 

[mneary]

Perhaps you should research exactly what an "oscillator" is. You probably won't find one in an old broken AM radio, since it will be made from discrete components that might not have an obvious relationship to one another. In some old AM radios, especially when transistors were costly, the mixer and L.O. used the same transistor.

You certainly won't find a neat little silver box like the one in your photo.

See if it gets clearer after you've explored a few of the projects on this site: **broken link removed**

 

[k7elp60]

Hi! I'm building an AM transmitter, and I found a really easy link to a design here:

Chapter 4: Radio -- Build a very simple AM radio transmitter


Now, I'm real cheap, and I was reading another article on instructables, and people were taking them out of cheap radio recievers. Would they have one inside them? I really don't wanna pay $8 shipping on a $3 item which will take a week to come.

Second, you need an audio transformer, 1000 ohms to 8 ohms. I'm happy to get that from Radio Shack, but can I salvage it possibly from some cheap electronics with a headphone jack?

Finally, FCC Part 15 says AM transmitters can only use 100 mW of power, which, according to most sites, with a good antenna, you can get 1/4-1 mile. Now, this page claims only 150 feet, so is this rig transmitting 100mW and needs a better antenna? If so, should I just add more coils, or another design of an antenna. (I am a ham operator.) Or, how do I improve this to max out on power. I'm going to make a mixer with 2 or 3 inputs with switches and such and local radio for my community. Thanks!

I thought you might want to know that the 100mW is the input power to the final stage of the transmitter and there is also a limitation on field strength.
This link has more information about antennas and field strength
**broken link removed**