Finished my first project :D(need some help on some fine tuning)

[maor]

Yeah I finished my first project and its (like everyone) a fm transmitter.
it works great even so there is some noise and im no sure about the range of the transmitter.
Also when i removed the coil from the circuit there was no difrence in any way from with the coil and W/O the coil.
I think the noise is being caused by the face im using just random(with the correct values) caps the circuit did say use 1% low noise caps but i don't i have any of those so i just used any kind of cap i had.
I think i didn't do the coil right...
Is the coils purpose to like lower the noise in the transmittion?
oh and this is the circuit
**broken link removed**
and a picture
**broken link removed**

 

[HarveyH42]

Pretty sure all RF stuff uses coils/inductors of some sort. Breadboards aren't good for RF or high frequency, or so I've read some where... I've only had a minimal in RF stuff, mostly because of the coils. For yours to be working at all, the reciever must be very close to the transmitter.

 

[audioguru]

An FM transmitter operates at the very high frequency of about 100MHz. Wires at that frequency have series inductance and parallel capacitance so it won't work on a breadboard. The wires must be very short at such a high frequency.
Use a pcb or make it with very short wires on stripboard.

Your transistors don't look like modern 2N3904 ones, they look like they are in a very old metal case. What part number are they?
I don't see the coil in your picture, it should look like mine.
I also don't see the 4-40pF tuning trimmer capacitor in your picture.
My FM transmitter looks like this:

 

[Nigel Goodwin]

He seems to have ignored any parts he didn't like?

And I agree, you can't make RF circuits on a breadboard, so it doesn't have a prayer of working.

 

[audioguru]

I think he said "it works great". I wonder why.

 

[yngndrw]

He seems to have ignored any parts he didn't like?

Slightly off topic, but still ...
In A Level electronics, we were once building AM radios. The only one we managed to get working was one with just a Ge Diode. No inductors, no Capacitors, No tuner - Just a diode and an arial. :c

 

[audioguru]

Some people pickup AM radio stations from old metal fillings in their teeth.
No diode, no inductor, no capacitor, no arial.

 

[yngndrw]

That's crazy. I hope it's a station they like.

 

[Krumlink]

Some people pickup AM radio stations from old metal fillings in their teeth.
No diode, no inductor, no capacitor, no arial.

Thats a bunch of bolognie (spelling problems yes I know)! Mythbusters tested that one to death! go to www.discoverychannel.com and check it out.

 

[kchriste]

Some people pickup AM radio stations from old metal fillings in their teeth. No diode, no inductor, no capacitor, no arial.

There is an island about 10 Km from here with a commercial AM transmitter on it. I remember years ago sailing over there and anchoring in the bay about 0.5Km from the main tower. The sailboats aluminum mast acted like a large antenna and because there was a loose corroded nut at the base of the mast acting as a combined diode/speaker you could clearly hear the radio station in the mast. Tightening the loose bolts, which was a good idea anyway, silenced it.

 

[audioguru]

I have heard of metal fences "playing" the sounds of a nearby AM radio station transmitter.

 

[maor]

An FM transmitter operates at the very high frequency of about 100MHz. Wires at that frequency have series inductance and parallel capacitance so it won't work on a breadboard. The wires must be very short at such a high frequency.
Use a pcb or make it with very short wires on stripboard.

Your transistors don't look like modern 2N3904 ones, they look like they are in a very old metal case. What part number are they?
I don't see the coil in your picture, it should look like mine.
I also don't see the 4-40pF tuning trimmer capacitor in your picture.
My FM transmitter looks like this:

1.mine trnasmitted on 105.5 Mhz
2.my transsitors are motorola 2n1711 are defently difrent from the site.
but when i checked the currect gain it showed the same as 2n3903(the closest thing i had on my simulation program)
3.since i didn't have a tuning cap i asked on a previous topic if using a fixed cap is ok and you guys sayed yes so i used it
see the cap that is near the trnassitor next to the antena thats where the varriable cap was supposed to be.
4.i built this thing out of spare parts i had in my tool box whitch my brother used i didn't buy any of these parts.
5.i made this circuit exactly as the site showed and all the connections were made correctly
6.if you would read the notes it sayed

Most of the parts values are not critical, so you can try adjusting them to see what happens.

and

The default for the capacitors type is ceramic, preferably the npo 1% (low noise) type or equivalent. But basically nothing critical here. Use any capacitor you have laying around.

 

[bananasiong]

But you said that there is no different with or without the coil, that means the coil is not giving any effect on your circuit. Maybe you receiver the signal luckily caused by the stray capacitance and inductance?
Try moving the circuit onto a PCB, I'm sure it won't work.

 

[Sceadwian]

Audioguru, a fence playing an AM radio station is a far cry from a filling. A fence actually has a prayers chance of being an antenna and receiving some degree of power, although I can't imagine what's acting as a detector. A piece of metal 1/4 inch long is what, 1/50,000th or so of a wavelength at AM frequencies.

 

[audioguru]

The very old 2N1711 transistor has a minimum transition frequency of only 70MHz when it has an optimum high current. It is less at the low current used in this project. For the recommended 2N3904 transistor it is 300MHz. Therefore a minimum 2N1711 cannot oscillate above about 60MHz.

I simulated the preamp transistor. A 2N3904 is incorrectly biased and it becomes cutoff when the battery voltage drops to 7.5V. Then it has horrible distortion and low voltage gain. An old 2N1711 transistor has less current gain so it will be cutoff with a higher battery voltage.

 

[maor]

The very old 2N1711 transistor has a minimum transition frequency of only 70MHz when it has an optimum high current. It is less at the low current used in this project. For the recommended 2N3904 transistor it is 300MHz. Therefore a minimum 2N1711 cannot oscillate above about 60MHz.

I simulated the preamp transistor. A 2N3904 is incorrectly biased and it becomes cutoff when the battery voltage drops to 7.5V. Then it has horrible distortion and low voltage gain. An old 2N1711 transistor has less current gain so it will be cutoff with a higher battery voltage.

1.so the transsitor acts as a noise reducer?
2.if one is wrong how come mine transmittered on 105.5 Mhz?
3.the site also sayed that if you are not using the exact transsitor type you need to lower the voltage in half(4.5v)

 

[audioguru]

The 1st transistor is supposed to be an audio pre-amplifier for the microphone.
But it is incorrectly biased to work properly when the battery voltage drops less than 7.5V.

The 2nd transistor is supposed to be an oscillator that is tuned to about 100MHz.

Look at my simulation of the 1st transistor. With a 2N3904 and a 9V battery voltage then its collector is set correctly at half the supply voltage of about 4.5V.
But when the battery voltage drops to 7.5V then the collector voltage is too high at about 5.4V instead of half the supply at 3.75V.
It is worse if the transistor has less gain or if the battery voltage is lower.

 

[maor]

so what am i supposed to do?

 

[yngndrw]

Change them to suitable transistors ..

Audioguru: What program are you using for the simulations ?

 

[audioguru]

For simulations I use the free program SwCAD III from Linear Technology.

The proper transistors and capacitors should be used and the circuit should be made on a pcb or on stripboard with very short wires.
The preamp can be fixed with a few minor changes like this: