another FM transmitter project

[Nepaliman]

My 2N3904 output transistor has a very simple circuit that has low output power because it does not properly match the fairly high impedance of the transistor to the low impedance of a coax cable and antenna.

-If so, why don't you upgrade your transmitter with perfect match?

-Varactor diode and Trimmer both has a same function on tank circuit. If a tank circuit designed with varactor diode, can we replace this diode with Trimmer? Why they use varactor diode instead of trimmer?

 

[audioguru]

-If so, why don't you upgrade your transmitter with perfect match?

Because a high power radio transmitter on a broadcast band is illegal in most countries.

Varactor diode and Trimmer both has a same function on tank circuit. If a tank circuit designed with varactor diode, can we replace this diode with Trimmer? Why they use varactor diode instead of trimmer?

A varactor has its capacitance changed with a variable voltage. The variable voltage can sweep the frequency or it can modulate the radio signal making FM.

 

[Nepaliman]

Of course, if it is matched with a balun transformer because the transistor and coax cable are unbalanced and a dipole antenna is balanced. My 2N3904 output transistor has a very simple circuit that has low output power because it does not properly match the fairly high impedance of the transistor to the low impedance of a coax cable and antenna.

May I use 'Balun' used on my TV in your transmitter to use dipole antenna? I think it gives more range, isn't it?

 

[audioguru]

The balun in a very old TV (new TVs do not have a balun) works fine at the 100MHz of FM radio stations.
A dipole antenna is directional. It produces a strong signal in two directions and almost no signal in two other directions. Learn about it.

 

[Nepaliman]

Does it produce really 500mW?

It has a general low power transistors! I cannot believe on 500mW output, Does it?
www.electronics-diy.com/tx500.php
(but i like its rf output indicator.)

Connecting this indicator on rf out may decrease max output power or not?

 

[audioguru]

It has a general low power transistors! I cannot believe on 500mW output, Does it?

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.

 

[Nepaliman]

I have a BALUN transformer used on normal 'V' shaped (Rabbit ear) antenna of my TV. This balun has been labeled by ''75 ohm/300 ohm''. Balun has 4 pins, 2 for TV side and 2 for antenna side.

I am going to connect it to your 3904 transmittter but getting confuse! Because I think transmiting and receiving are just opposit process. So confusing to connect this Receivere's (TV's) balun on Transmitter.

Should I have to connect this balun on transmitter same as TV (TV side pin=FM transmitter side and 'V' antenna side pin=FM's dipole side)??? OR just opposit way (TV side pin=FM's dipole side)??

 

[audioguru]

I have a BALUN transformer used on normal 'V' shaped (Rabbit ear) antenna of my TV. This balun has been labeled by ''75 ohm/300 ohm''. Balun has 4 pins, 2 for TV side and 2 for antenna side.

I am going to connect it to your 3904 transmittter but getting confuse! Because I think transmiting and receiving are just opposit process. So confusing to connect this Receivere's (TV's) balun on Transmitter.

Should I have to connect this balun on transmitter same as TV (TV side pin=FM transmitter side and 'V' antenna side pin=FM's dipole side)??? OR just opposit way (TV side pin=FM's dipole side)??

Maybe your TV has a 75 ohm coax cable antenna connector? Then connect it to the output and ground of the transmitter coax cable.
Connect the 300 ohm balanced antenna terminals to a dipole antenna 160cm long.

The antenna for the transmitter and receiver must both have the same direction (vertical or horizontal) and be parallel.

 

[Nepaliman]

1.

You mean- 2 pins of 'V' shaped antenna side terminal of balun are 300 ohm balanced terminal and I should have to connect it to FM's dipole. And TV side 2 pins should have to connect to the coax of Transmitter. OK I WILL DO. Thank you!

2.

Transistors BC549 and PN2222 both are almost same in voltage, power dissipate(625mW). But BC549 has 200mA max current and PN2222 has 600mA max current. So I think, PN2222 is better than BC549 for maximum performance, isn't it?

 

[audioguru]

The BC549 is an audio preamp transistor.
The PN2222 is a general purpose transistor.
Both do not have spec's at FM radio frequencies.

The PN2222 can survive a current of 600mA but its datasheet shows that its performance at such a high current is horrible.

 

[Nepaliman]

Got all answers!

 

[audioguru]

Since you do not look up what a choke is used for then somebody else can teach you the basics of electronics.

 

[Nepaliman]

Got all answers!

 

[audioguru]

Aren't there any electronic chat forums in your language?

 

[Nepaliman]

Got all answers!

 

[Nepaliman]

Got all answers!

 

[Willen]

Maybe your TV has a 75 ohm coax cable antenna connector? Then connect it to the output and ground of the transmitter coax cable.
Connect the 300 ohm balanced antenna terminals to a dipole antenna 160cm long.

The antenna for the transmitter and receiver must both have the same direction (vertical or horizontal) and be parallel.

Having same problem....
You mean- TV's balun should have to use on fm Tx regarding transmitter as a TV and dipole as a rabbit ear antenna? I mean input=input and output=output? like same of TV ?

 

[audioguru]

Having same problem....
You mean- TV's balun should have to use on fm Tx regarding transmitter as a TV and dipole as a rabbit ear antenna? I mean input=input and output=output? like same of TV ?

The input of a modern TV is 75 ohms unbalanced with a coax connector for coax cable. The output of an FM tranjsmitter is also 75 ohms unbalanced with a coax connector for coax cable.

A dipole antenna or rabbit ears are 300 ohms balanced and can be used by a TV with a balun or used by an FM transmitter with a balun.

 

[Willen]

The input of a modern TV is 75 ohms unbalanced with a coax connector for coax cable. The output of an FM tranjsmitter is also 75 ohms unbalanced with a coax connector for coax cable.

A dipole antenna or rabbit ears are 300 ohms balanced and can be used by a TV with a balun or used by an FM transmitter with a balun.

Which order have to follow?
-Transmitter>Balun>Coax>Dipole
-Transmitter>Coax>Balun>Dipole

 

[audioguru]

Which order have to follow?
-Transmitter>Balun>Coax>Dipole
-Transmitter>Coax>Balun>Dipole

"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.