Medium to high power FM transmitter circuit and more

[florinanghel]

First, the context. I'm trying to make an FM transmitter circuit that is capable of transmitting on medium distances (clear signal up to 500 meters/1500 feet), so that I can listen the music on my PC wherever I am around my place. I'd also like to use it as a private radio station for me and my friends.

Now I'd like, if possible, to start from the following medium power circuit that can transmit up to 100 meters (and adapt it for a computer stereo speaker output):

This raises a series of questions:

• Can I amplify the signal from the above circuit instead of using another one?
• If yes, are there any losses in sound quality and such?
• How do I know the exact frequency I'm transmitting on (check C6 on the diagram)? [ANSWERED]
• Can I simply plug the microphone wires from the circuit to the stereo speaker jack? [ANSWERED]
• How do I prevent my computer's sound card from being toasted (by the additional 12V power supply)? [ANSWERED]
• Do I have to add a resistor to bring the power lower? (the 12V from the power supply + the few volts from the stereo speaker output add up to more than necessary) [ANSWERED]
• Will the above circuit transmit the signal as stereo, or will it be converted to mono? [ANSWERED]
• How am I supposed to ground the circuit? [ANSWERED]
• Can I power the above circuit from AC without using a rectifier? [ANSWERED]

Additional questions (added after initial post):

• Would it be hard to encrypt/decrypt data somehow before sending/after receiving it (even better if the encrypted data sounds like background radiation)?

That's all for now, but I'll probably come up with more in time. Thanks in advance!

 

[davenn]

some answers amongst your Q's


This raises a series of questions:

...Can I amplify the signal from the above circuit instead of using another one?

As in RF amplify ? more RF power out ? if you did so, be aware of licencing requirements in your country

....How do I know the exact frequency I'm transmitting on (check C6 on the diagram)?

A frequency counter will tell you that accurately

....Can I simply plug the microphone wires from the circuit to the stereo speaker jack?

No... the circuit is designed for the very low level of a microphone

....How do I prevent my computer's sound card from being toasted (by the additional 12V power supply)?

the audio path between the soundcard line output and the transmitter input only needs to be capacitively decoupled ... ie a series capacitor say ~ 4.7 - 10uF

....Do I have to add a resistor to bring the power lower? (the 12V from the power supply + the few volts from the stereo speaker output add up to more than necessary)

the transmitter is running off 9V not 12V and you are only using audio signal from the computer/stereo see previous decoupling comment

...Will the above circuit transmit the signal as stereo, or will it be converted to mono?

it is mono only, a lot of work would have to be done to make it a stereo transmitter

....How am I supposed to ground the circuit?

probably wouldnt need to be "grounded" as such would be more important for low
broadcast freq's 500kc to 30MHz rather than usual FM TX freq's of ~ 88-108MHz

....Can I power the above circuit from AC without using a rectifier?

NO, you will kill it you need +9VDC as said on the circuit diag.


cheers
Dave

 

[florinanghel]

Thanks for answering, Dave!

...Can I amplify the signal from the above circuit instead of using another one?
As in RF amplify ? more RF power out ? if you did so, be aware of licencing requirements in your country

As in increasing the transmission range. And yes, I am aware of the licensing requirements, as I read almost the entire law on that. I would probably need a license even for transmitting 1 feet away.

the transmitter is running off 9V not 12V

You're right, my bad.

it is mono only, a lot of work would have to be done to make it a stereo transmitter

Oh, snap! House music on mono is just as bad as it sounds.

 

[davenn]

Thanks for answering, Dave!

As in increasing the transmission range. And yes, I am aware of the licensing requirements, as I read almost the entire law on that. I would probably need a license even for transmitting 1 feet away.

You will know your local laws better than me but as an example, here in Australia
we have a low power device class, from memory, up to 0.5 W (1/2 W) before a licence is required. 0.5W on the FM band will get you a decent distance distance... many km's
under ideal conditions. It would easily be heard all around your and your neighbour's properties.
You may have seen the devices that you can plug into your MP3 player and it transmits a signal in the FM band that you can pick up on any 88 - 108MHz FM receiver. They are stereo and have good audio quality.

cheers
Dave

 

[florinanghel]

As long as it can be transmitted on a long distance and it can be picked up by a standard FM radio receiver, I don't really care about the frequency/power and all that stuff (maybe that's because I'm not even sure what you mean by low power device class). From what I read in the Romanian law about this stuff, they didn't specify any legal no-license FM band range or something like that.

But how exactly does the power influence it? Is it range-related, frequency-related, both?

 

[NickS]

All of the commercially available personalstereo FM transmitters that I have modified(3 different makers) use a chip from this series LINK I think their max power output is 0dBm.

I just use mine in my car but is runs over any broadcast I want and its delivering +5dBm max to a 30" whip antenna.

 

[florinanghel]

You're loosing me.

 

[davenn]

All of the commercially available personalstereo FM transmitters that I have modified(3 different makers) use a chip from this series LINK I think their max power output is 0dBm.
I just use mine in my car but is runs over any broadcast I want and its delivering +5dBm max to a 30" whip antenna.

That link goes to a SMD 3 terminal V- reg, Nick maybe the wrong link pasted ??

Dave

 

[NickS]

Sorry here is what I meant LINK