l wish to design a voice activated FM transmitter design which transmits at 160.7 MHz. l intend to implement my design using two or three transistors that is oneor two for amplifying and the other as part of the colpitt's oscillator. the transmitter has a range of 100m hence transmitts less power.

the transmitter must transmit only when someone speaks, the microphone, in this case an electrect is to be used as the transducer otherwise it must be in an idle state

 

This sounds like a Bug. Is it?
If you're desirous of keying power to the transmitter circuit by virtue of VOX, then the Mic amplifier circuit must be powered continuously to do this.

 

To design the circuit you should know the method/principle of operation you want to apply for the specific transmitter, specific microphone to use, if to transmit the audio from the same vox activating microphone or from other source, availability of parts, their data sheets, and how to put them together, calibrate, test... in general, you need to learn electronics.

Copying/building an existing circuit is not designing. If that is what you are after, here is one of many on the net :
http://electroschematics.com/354/vox-circuit-schematic/
http://www.zen22142.zen.co.uk/Circuits/rf/davidtx.htm
http://www.zen22142.zen.co.uk/Circuits/rf/2trtx.htm