another FM transmitter project

Willen · May 24, 2012

audioguru said:
"Transmitter>Balun>Coax>Dipole" is wrong because the output of a simple transmitter is unbalanced and can directly feed coax cable that is also unbalanced. It is also wrong because coax cable is unbalanced but a dipole antenna is balanced so they do not go together properly.

"Transmitter>Coax>Balun>Dipole" is correct.

Why are you using a dipole antenna? It is directional and the signal will not be received in two directions. Its gain is only a little more than a whip antenna.

I am trying to transmit my 20mW little FM Tx's signal as far as possible as an experiment of antenna. Can't dipole do good job (than whip)? Two directions means like only horizontal or like only east-west? Which types of antenna can transmit as far as possible?

audioguru · May 24, 2012

A Yagi antenna has a lot of gain in one left to right direction. A few Yagi antennas mounted one above the other will have more gain because the signal is concentrated at the horizon instead of going up into space and down to the ground.

Willen · May 25, 2012

I've found several terms of instruction for coil. Generally I've found like this-
-10 turn, 1mm, 5mm diameter (air core).
-10 turn, 1mm, over 5mm former air core
in such case,5mm mean inner or outer? Confusing in such kind of terms. If it is outer diameter, I should've to wind over 3mm diameter rod (for 5mm outer) isn't it?

OH GOD!!! HOW HARD TO FIND INSTRUCTION OF 10uH CHOKE! (turns,core,SWG..). Any idea?

audioguru · May 25, 2012

You wind the coil on a 5mm diameter rod so 5mm is the inner diameter of the coil.
The instructions say to use an AIR core which is nothing.

You should find a project that explains everything and shows photos.

Willen · May 26, 2012

some coil looks like resistor. Are they 'uH' coil or 'mH' coil? This color bands ''Brown+Black+Black+Silver'' represent 10 uH coil, isn't it?

audioguru · May 26, 2012

The coils that look like resistors might self-resonate at a medium radio frequency which makes them no good at very high radio frequencies.
Look up its spec's on its datasheet.

panic mode · May 26, 2012

https://how-to.wikia.com/wiki/How_to_identify_inductor_markings

Nepaliman · May 26, 2012

In a tiny auto scan FM receiver, there is only one little coil and varactor diode (BB910) connected with parallel. One side of coil has been out for antenna. So i guessed this tank circuit is to receive (tune) 88-108 MHz FM frequency. So, can I use this BB910 varactor diode in oscillator of FM Tx?

audioguru · May 26, 2012

Nepaliman said:
So, can I use this BB910 varactor diode in oscillator of FM Tx?

Yes but a few other things also change the tuning of a very simple FM transmitter:
1) The supply voltage.
2) The temperature.
3) If anything conductive (or a person) moves toward or away from the antenna.
4) The length of the antenna.

Willen · May 27, 2012

Can I test the polarity of this kind of varactor diode like general diode?

audioguru · May 27, 2012

EVERY diode changes its capacitance when the reverse voltage across it is changed. Look at the datasheet of an ordinary diode like a 1N4148.
But a varactor diode changes its capacitance more than an ordinary diode.

Of course you can test the polarity of a varactor diode because it is a diode.

Willen · Jun 6, 2012

audioguru said:
This is my FM transmitter circuit. I built it on stripboard and I cut the strips with a drill bit. The strips form half of a pcb.

Here, if I replace your normal oscillator with little high power varactor basaed oscillator, can I get little high power output rf from this Tx than now?

audioguru · Jun 7, 2012

Willen said:
Here, if I replace your normal oscillator with little high power varactor basaed oscillator, can I get little high power output rf from this Tx than now?

No, because my output amplifier circuit is extremely simple and its high impedance does not match the low impedance of the antenna.
Also, its output power drops as the battery voltage runs down.

Willen · Jun 8, 2012

guru,
I've found a circuit almost same as your 3904 tx but little different in output stage (Q3). It has no LC (tank) circuit in Q3 BC547. Instead of tank circuit, it has a singal resistor of 330 ohm and base biasing resistor is 150K from +ve. Oscillator and amplifier is coupled by 10 pf capacitor. I've assembled it and oscillator is doing its job with great quality at 94MHz but when i connect it to last stage through 10 pf capacitor, there is not good anything! Friquency has been almost lost. My radio receives its too much little signal in some 2/3 frequencies but I think it's not exact friquency, it may be harmonics. I adjusted tank circuit to get exact friquency but got same result! Again, when i separate oscillator, it is doing job with good quality.
What may be the problem? Can this last stage change oscilllator's friquency? Without tank circuit, how this last stage is complete?
Should i have to adjust any component of this last stage to solve problem?

audioguru · Jun 9, 2012

The new FM transmitter circuit you found is awful.
I was taught to never bias a transistor like its output transistor because a high gain transistor will be saturated and a low gain transistor will be cutoff.
A tank at the collector of a transistor allows a signal level that is much higher than the power supply voltage. So this circuit will have low output power.

I think that your radio is cheap and is overloaded by the output of this circuit. Move them apart to see if the reception improves.
The harmonics are double (188MHz), triple 282MHz), quadruple (376MHz) and higher. No harmonic is in the FM broadcast band.

Willen · Jun 9, 2012

audioguru said:
www.electronics-diy.com/tx500.php
Its output power might be 500mW into a 50 ohm or 75 ohm antenna if all the tuned circuits are carefully adjusted to one frequency.
The BC549 transistor is a bad choice for Q3 because its max peak collector current is 200mA and it performs poorly above 50mA.
A BC549 transistor can easily be destroyed in that circuit.

Transistors have a range of current gain. Some have low gain then the output power of the transmitter will be low. Some transistors have high current gain and they might be destroyed in that circuit.

The power meter uses a very small amount of output power to activate it because its input coupling capacitor value is almost nothing (0.9pF). The power meter will not work because its LED is upside down.

But looks like an easy to build. BC549 is not available in my city. May I use BC546/47/48 or 2n3904 as a replacement of BC549? I saw datasheet and got more similar. Need your Decision!

(As you said- If Q3 will destroy, i will modify it with little more power transistor like 2n2219)

Willen · Jun 9, 2012

Need a decision for Tx500 mW !!

Tx 500 looks like an easy to build. BC549 is not
available in my city. May I use BC546/47/48
or 2n3904 as a replacement of BC549? I
saw datasheet and got more similar. Which will be the best?

audioguru · Jun 9, 2012

For Q2 and Q3 in that circuit I would use 2M4401 transistors that have a max collector current of 600mA and pretty good gain below 400mA. You can use an old 2N2219 instead if you want.

Willen · Jun 10, 2012

Need your decision in Tx 500mW

....so sad

Boncuk · Jun 10, 2012

Hi Willen,

you might use any of the mentioned transistors plus a BC550.

another FM transmitter project

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