FM circuit help

[devil6600]

hi,

i have build the attached circuit, can any one tell me the use of the first inductor (L1) present near the antenna? is the first inductor (L1) a filter?
if i remove the first inductor (L1) and connect the antenna to the tank circuit will it work?

thanks

 

[audioguru]

L1 provides a little amount of isolation of things moving near the antenna which changes its capacitance from changing the frequency of the oscillator. But the frequency still changes when something moves toward or moves away from the antenna. Removing L1 will make the frequency change worse.
It is a hopeless circuit because its frequency also changes as the battery voltage runs down. it needs a voltage regulator.
The transmission will sound muffled when heard on a normal FM radio because it is missing pre-emphasis (treble boost) that all FM radio stations have and all FM radios have de-emphasis (treble cut).

 

[MikeMl]

It is a "loading coil", which turns an otherwise non-resonant short whip into a resonant one.

Wiki's definition:

A (mobile) radio antenna, shorter than a quarter wavelength for practical reasons, presents capacitive reactance to a transmission line. This can be canceled by inserting an equal and opposite (inductive) reactance in series, by means of a loading coil typically at the base or center of the antenna. Consequently the antenna presents a resistance (desirable) to the transmission line.

 

[mbarazeen]

hi,

i have build the attached circuit, can any one tell me the use of the first inductor (L1) present near the antenna? is the first inductor (L1) a filter?
if i remove the first inductor (L1) and connect the antenna to the tank circuit will it work?

thanks

it wil work, but you have to again tune the circuit to the required frequency, a capacitor instead of L will work too.

 

[audioguru]

You tune the simple circuit simply by getting closer or farther away from the antenna. You also tune it with the decreasing battery voltage.