What is "tunable ferrite"?

[Shahriar]

Hi
I want to build this Little transmitter. In descriptions,is written:

" L1 = 9 turns, .15mm wire on 4.5mm diameter tunable ferrite coil former "

1- What is "tunable ferrite"? 

L2=15 turns, .5mm wire, 5mm diameter
L3=3.5 turns, .5mm wire, 5mm diameter

2- Can I replace L2 and L3 with Equivalant RFCs? If yes, How to calculate it?

ThanX
Shahriar

[Guest Decade]

Hi Shahriar!

I think a "tunable ferrite" coil former is an ordinary slug tuned coil which is part of the matching network in the schematic.
L1 and L2 are coils with small number of turns and easy to wind why use RFCC,s?
If you use RFC,s you wont be able to change the tap and you might screw up the matching network too.
To calculate L1 & L2 (which is actually one coil with a tap that match the antenna impedance to the oscillator) you'll have to calculate both the impedance of the transistor and the antenna which is not quite that simple I think.
I think also that just to touch the antenna will load the oscillator, there should be a buffer before the antenna.
Well. that wasn't much but hope it helps a little :)

[audioguru]

What frequency is this transmitter?
It looks similar to the circuit in the transmitter of a cheap RC toy car but its low tuning parts values might tune it to a harmonic of the 27MHz crystal.