A litle tuning help...(plese)

[Someone Electro]

I found this simple transmiter and i want to tone it to 90 MHz.
Vhat inductor do i nead to do it ?

If i mace the inductor aircore out of wire from a motor(those first electric kit,the rotor whith a I shaped core)

 

[Nigel Goodwin]

There are too many variables in the circuit to accurately calculate the exact tuning values required (most of them down to your exact construction) - that's why the variable capacitor is there (and why the coil is designed to be adjustable).

Simply build it, find where it is on an FM radio, then adjust the capacitor (or the coil spacing) to move it where you want.

Be aware that all these kinds of circuits are pretty unstable, and likely to drift about somewhat.

 

[Someone Electro]

If you dont have the right wire its hard to get it even in to the FM band.

I want eatleat to get it on the fm band.

 

[k7elp60]

There is a formula that will get you close and at least see if you are in the ball park. But a Nigel says you may have to play with the parts to get it on frequency. The formula is: LC=(25330/f x f), where L is in uH, C is in pF and f is in mHz. Using your frequency of 90 mHz, LC=(25330/8100).
LC=3.13. Choosing the value of 0.1uH, C = 3.13/0.1 or 31.3 pF. The circuit should work.

 

[Nigel Goodwin]

If you dont have the right wire its hard to get it even in to the FM band.

I've built a number of these small FM bugs over the years, the wire isn't critical at all - some I've done using insulated solid core wire like the circuit suggests, others I've used thicker uninsulated copper wire (the inner core out of TV coax cable) with the windings spaced slightly apart.

For the feedback capacitor (collector to emitter) I've even used just a couple of short pieces of insulated wire twisted together! - it's a lot easier than it looks!.

 

[panic mode]

This is just the most basic oscillator which is frequently used in
simple FM transmitters. I've built hundreds/tousands of them
(I used to make and sell them in big quantities...).
To make it work in standard 88-108MHz, simply get some
solid wire with diameter 0.5-1.0mm (insulated wire like from
relays or transformers).
Make 5 turns of wire on something round and 5mm in diameter.
Strip the wire ends (make sure they are very short) and remove
the body that was used to wind the coil (coil is just wire in the air
with no ferrite core or similar).
I used to put fixed 15pF capacitor instead of variable capacitor
as shown in your schematic (it's cheaper and I had to make money...).
To change frequency, I would slightly stretch or compress coil so it's
lenght was in range of ca. 4-8mm. Of course this way you cannot
adjust frequency very precise but who cares, you just want to
find spot not occupied by some radio station.

 

[panic mode]

Note that whole circuit should be built as small as possible
with all leads as short as possible (use small PCB with all
components sitting right on the board). Don't forget to
bring audio signal through capacitor (ca 22-100nF) to the
base of the transistor or the transmitter might not work.
In my bestselling design (two transistors, coil and bunch of
capacitors and resistors) the circuit size was ca 3x2cm.

 

[Someone Electro]

Il then make a woden stand and a scrw to compres the inductor. The scrw sod higher inductance but it shod have surtenly have the low enugh range