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# Simple FM transmitter question/s + help

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I build the circuit here: http://www.electronics-lab.com/projects/rf/030/index.html

replaced the 5.6pF with 5pF
wound my own air-inductor(4 turns wound on the inside of a pen... shows near 1uH)
operating at 3V 2AA batteries

just finished lately, and I'm having difficulty in tuning it into my radio(no sound even beside the tX)
don't know whats wrong... the inductor, wrong formula/area? Maybe a soldering damage(overheat??)?

I have a multitester(Volts Ohms Amps only)
I'll appreciate if someone will guide me on this. :'( :'( :'(

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It won't make any difference if you use 5pF rather than 5.6pF the capacitor probably has a 10% tolerance anyway.

How have you built the circuit?

Is it on a bread board?

Does it have long wires?

It should be on a stripboard or PCB and the connections should be as compact as possible.

The inductor value is wrong with the capacitor set to 6pF the frequency will be 65MHz, in pratcise it'll be a little lower because of the parasitic capacitance and inductance.

F = 1/(2π√(LC)) = 1/(2π√(10-6

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I built it on a 2in PCB, no enclosure yet.
Im feeding the mic with an mp4 player input. (mp4 speaker beside the mic. for detecting purposes only)
The antenna is a #22 awg coil(2 layers of 1in long)

I appreciate the 150nH suggestion... but is there a way to narrow the frequency(temporarily) to 90-100Mhz?

just to be sure, I'll wind up the 150nH coil later today. Thanks

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If you use the coil winding formulae yto calculate the number of turns required for 150nH you'll discover that all you need to do is remove some turns.

Indeed you could do it experimentally but it's easier just to calculate it.

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I just figured it out lately... came up with 2 turns, same radius (3mm), 5mm long, but only at 0.379uH(379nH). need some estimates to arrive at 150nH... haven't soldered it yet.

here are some pics if you dont mind

the coil here is my antenna

the whole thing
[img width=680 height=510]' alt='' class='ipsImage' >
back parts
[img width=680 height=510]' alt='' class='ipsImage' >

view these on a digital camera, its brighter

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My FM transmitter uses a tuning coil with 9 turns, not just 2 turns.
My antenna is a straight wire that is 80cm long, not a little coil that will not work.

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You've made a mistake with the calculations.

I use the following formula to calculate inductance which is the same as the one on Wikipedia but converted to metric and multiplied by 1000 to give the answer in nH rather than

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This is the one that Im using

from http://en.wikipedia.org/wiki/Inductor#Inductance_formula

or in simple: L=Mr(N^2*A/l)k
k=1.26E-6
Mr=1(for air coils)
N=turns
A=area (4/3pi*r^2*l)

meters, so for mm, I use the suffix E-3 (ex. 1mm; 1E-3)

EDIT:
A= pi*r^2*l
no 4/3

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You've got the area calculation wrong.

It's the cross-sectional area = πr2

I make it 213nH for  N = 6, r = 3 and l = 6

L = (1.2566

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The stray capacitance of the wiring on the pcb will make the frequency much lower than is calculated.
Also the capacitance of each transistor is different.

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I'll post again here after I wind up the new coils...

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well ok, the circuit works! I can hear something when I rub the mic with my finger and I can hear my voice... I just can't make it stable enough to work smoothly. seems like every movement does something on the whole circuit... (a bad PCB board layout?)
when I leave it without touching anything, nothing, like its off.
when I press on the board, sometimes its working...

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It sounds like it could be a loose connection to me.

Another explanation is that moving your hands near subtly alters the properties of parts the circuit (i.e. capacitance); this can be solved by putting it in a screened enclosure.

You might also have a problem with the air coils being microphonic, i.e. when they vibrate their inductance fluctuates which could cause the vibrations to become superimposed on the signal.

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well ok, the circuit works! I can hear something when I rub the mic with my finger and I can hear my voice... I just can't make it stable enough to work smoothly. seems like every movement does something on the whole circuit... (a bad PCB board layout?)
when I leave it without touching anything, nothing, like its off.
when I press on the board, sometimes its working...

The project is titled, "Simple" which means poor performance.
Its antenna is connected directly to its tuned circuit so its frequency changes when something moves toward the antenna which increases the capacitance and when something moves away from the antenna which reduces the capacitance. It should have a buffer transistor between the oscillator and the antenna to isolate them from each other.
Its frequency also changes as the battery voltage runs down because it is missing a voltage regulator.
It is also missing pre-emphasis (treble boost) that all FM radio stations have so it will sound muffled without high audio frequencies when played on an FM radio.

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