Need help fm transmitter...

[healthkart]

Hello,


Guys i am trying to build a spy fm transmiter (bug)

i have found a great resource http://www.talkingelectronics.com/projects/Spy Circuits/SpyCircuits-1.html

but he has written too lenghty /although accurately & am too confused from this resource, i would like to build the lucidscience.com bug to which he has made corrections but the last capacitance value & how to seperate the module stuff has confused me.

he has a voyager bug which am considering but its fr 2 km

my requirements is as such

300 mtrs transmission
can be listened on mobile phone fm reciever
small size(a bug should be a bug)

Help me with my dilemma,
if any of you guys knw of a crkt which has been verified as working with optimal results do share with me.

thank you

 

[BradtheRad]

It will be very difficult to construct a tiny transmitter that can broadcast 300 meters.

To hear it on a mobile phone, you'll need to understand the protocols which go into mobile communications.

A kit would be a good first move. I once purchased an FM transmitter kit from a mail order house. It was a few inches long, and ran off a 9V battery. I could hear my voice on a radio a few rooms away.

 

[Audioguru]

A few things determine the range of an FM transmitter:
1) Output power. High power for a long time needs a huge battery. A "bug" with a huge battery?
2) Length of its antenna. A whip antenna at 100MHz should be 80cm long but that is much too long to hide for a "bug". A short antenna produces a short range.
3) Orientation of its antenna. Best range occurs when the transmitting antenna and receiving antenna are parallel.
4) Sensitivity of the receiver. A huge difference between a cheap radio and an excellent radio. I do not know if the radio in a phone is sensitive, probably not.
5) Receiver antenna length and orientation. The antenna for the radio in a phone is its earphones wires. What about when it is a speakerphone?
6) Barriers like hills, walls etc.

My FM transmitter has pre-emphasis and low distortion so it sounds great with music or voices. It is built compactly on stripboard and is 2" (5cm) long, 1.25" (3cm) wide and is 0.6" (1.4cm) thick.
It operates for a few hours from a 9V alkaline battery. Its whip antenna is 80cm long.
Its range is across the street to an FM radio from The Dollar Store, down the street about 300m to my Sony Walkman radio or is more than 2km to my expensive home stereo or my excellent car radio over a wide river valley.
I have used it for only about 1 hour to test its range since there is no vacant FM channel in my city, so I interfered with a low power foreign language radio station and the RF cops did not catch me.

 

[Venkadesh_M]

I did three years before, but unfortunately i don have both circuit and hardware. I will have a search. but not audible after 20 to 30 mtrs, but its rated for 1km.

To hear it on a mobile phone, you'll need to understand the protocols which go into mobile communications.

He meant like hearing in FM application in mobile phone, thats what we did.

 

[healthkart]

A few things determine the range of an FM transmitter:
1) Output power. High power for a long time needs a huge battery. A "bug" with a huge battery?
2) Length of its antenna. A whip antenna at 100MHz should be 80cm long but that is much too long to hide for a "bug". A short antenna produces a short range.
3) Orientation of its antenna. Best range occurs when the transmitting antenna and receiving antenna are parallel.
4) Sensitivity of the receiver. A huge difference between a cheap radio and an excellent radio. I do not know if the radio in a phone is sensitive, probably not.
5) Receiver antenna length and orientation. The antenna for the radio in a phone is its earphones wires. What about when it is a speakerphone?
6) Barriers like hills, walls etc.

My FM transmitter has pre-emphasis and low distortion so it sounds great with music or voices. It is built compactly on stripboard and is 2" (5cm) long, 1.25" (3cm) wide and is 0.6" (1.4cm) thick.
It operates for a few hours from a 9V alkaline battery. Its whip antenna is 80cm long.
Its range is across the street to an FM radio from The Dollar Store, down the street about 300m to my Sony Walkman radio or is more than 2km to my expensive home stereo or my excellent car radio over a wide river valley.
I have used it for only about 1 hour to test its range since there is no vacant FM channel in my city, so I interfered with a low power foreign language radio station and the RF cops did not catch me.

ok so ur fm transmitter works but do u sell it or are the schematics available?

 

[healthkart]

thanx for your response guys but i was thinking like the mobile has a receiving frequency limit like from 87 to 108 mhz or smthng so why should the protocols & stuff matter if my bug will be transmitting in the same frequency.??

also can one of u please check the link provided & give your view on it.

 

[FvM]

Help me with my dilemma

I think the dilemma is that your intention is illegal in several regards:
- using a public broadcast frequency with a transmitter range more than a few meters
- eavesdropping
I presume the latter action is enforced by more severe penalty in most countries.

 

[Audioguru]

The Voyager FM transmitter will sound awful and work poorly:
1) The 68k resistor value powering the Jfet in the electret mic is too high. It should be 10k ohms for the voltage across the mic and its current to be higher to reduce distortion.
2) The value of the 22nF coupling capacitor from the mic to the preamp transistor should have a value that is much higher to pass low audio frequencies. The preamp transistor has an input impedance of 4k ohms and the mic has an impedance of 2.5k ohms for a total of 6.5k ohms. Then the cutoff frequency of the 22nF capacitor with the 6.5k ohms is 1.1kHz so there will be no low frequencies. If the coupling capacitor is 1uF then audio frequencies down to 25Hz will be passed.
3) There is no pre-emphasis so it will not have high audio frequencies when heard on an FM radio and will sound like an AM radio or a telephone.
4) Its antenna is connected directly to the tuned LC in the oscillator so its frequency stability is poor then its frequency will change if something moves toward or away from its antenna.
5) It does not have a voltage regulator so its frequency will change as its battery voltage runs down.

My FM transmitter uses a low dropout voltage regulator, an RF amplifier between the oscillator and the antenna, it passes all audio frequencies and has pre-emphasis like all FM radio stations have.