problem with vhf fm transmitter

[deepak007]

hello

i have built a small vhf fm transmitter in the size of a sugar cube powered with a 1.5v button cell.i have built it on 1*1 inch perfboard.the problem is only when i touch the antenna or the -ve terminal i get the output from receiver.otherwise no.i have not enclosed it.pls help me.

 

[rogerynt]

It seems the Tx power does no radiate through the antenna. You need to check the followings :

1.) Ensure the Tx power is alright through conductive measurement

2.) The length of the antenna. If a very short antenna is used then you need to match the antenna very carefully


Assume 1.) & 2.) are ok

Check your layout gounding. Very often a transmitter gives good conductive measurement but poor radiative power are caused by poor gounding. You need to short some gnd plates together and check whether there is any improvement.

Rogerynt

 

[waseemsh]

I think you must have a grounding layer at the bottom of your design.

 

[vfone]

At this size on VHF frequencies there is no magic to make the transmitter to radiate in proper conditions using such small antenna.
When you touch the circuit actually your hand becomes the antenna.
Try to use a loop antenna with multiple turns, like the ones used in RFID.

 

[deepak007]

thanks all of u for your support.but what is grounding layer?

 

[rogerynt]

Ground plate is the plate that connects to battery ' -ve '. A poor gnd causes lots of problem. A gnd plagte should be a complete ( target ) piece of copper instead of bits and pieces.