Stereo Fm Transmitter

[vlahokel]

Hi everyone,
I am a y11 student in Victoria Australia and have recently made a stereo FM transmitter kit. Now I know you will all post replys about staying legal and the law etc etc but can sombody please tell me if it is at all possible to build a small amplifier to boost the output power. It only goes about 20 meters to a normal radio and Because my Nokia 7250 has a weak FM tuner in it it has alot of noise when trying to listen to it on my phone (even when close).
Can you please reply.
I am allready running the device at its maximum voltage 5.0v (its a one chip device, with other devices as well obviously).
If needed I can post the Circuit diagram but not now (my scanner is not plugged in )
Thank you and BTW this Forum is awsome (except that i cant find an answer)

 

[magickaldan]

According to the schematic you should be running it from 9-16 v but if you run it from 6v you remove a couple of things. According to the note on the schematic. Unless Thats not the schematic.

 

[vlahokel]

The maximum power that the IC can take is 7v i am running the circuit on a 9v battery through the regulator

 

[magickaldan]

Well First I need to know a little about your antenna Like what it's made of and how long it is. Maybe you need a better one. You could try adding a power amplifier to the antenna but that might cause distortion and stuff. But first try and get a good antenna. I've got a simple transistor FM transmitter to transmit upto 100 feet. But the quality was bad because it was simple microphone toy.

 

[magickaldan]

A Operational Amplifier Increases the power output to I hooked a op AMp to my transmitter and it went from 48 microWatts to 350 miliWatts. Thats 656% increase and a gain of 11. according to my sheets. I used a 272 Dual -OP AMP IC which has specs of this:
Supply Voltage:+3v to +16v
Supply Current:1.4 mA (3.2mA Max)
Output Current:+ or - 30 mA (Typical 10mA)
Maximum Voltage Gain: 23,000 typical (5,000) at Room Temperature And When powered by +5 Volts
Gain = 1 + (R2/R1)

So Find a OP Amp IC and Schematic and Hook it up to the antenna.
http://discovercircuits.com/A/a-operational.htm is a page with OP amps Schematics. Copy and Paste into Address bar.
Hope this helps
PS: I havn't tried to see how far because I have to get a new transmitter mine has something wrong with it I overpowered it awhile back and burned some things out. It transmitts only static at this point But Transmitts.

 

[stevez]

My comments:

Find out what the regulations are for your country/area. Determine how compliant (or not) your transmitter is in it's present state. The IC manufacturer should be able to advise on the output power-most likely it will be into a 50 or 75 ohm load. Regulations may be in terms of output power or input power to the final amplifier and there may be limitations on the antenna. Sometimes the regulations are such that distance or property lines are the governing factors.

In order to get the most out of the power you have the antenna must be matched to the transmitter. Amateur radio publications cover this nicely. A dipole antenna can be adjusted to the right length to match so that maximum power is radiated. There are various vertical, yagi, and other antennas that use techniques to focus or direct the power so that what little you have goes where you want it to go.

Before adding an amplifier take a closer look at your schematic. There is a change that can be made however you might need to be more certain that the antenna presents a proper load to the transmitter. The schematic may also be the way it is because of regulations.