another FM transmitter project

[Willen]

Hi Willen,

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

Can I use there PN2222 as a replacement of 2n2222? Are they same?

 

[audioguru]

Can I use there PN2222 as a replacement of 2n2222? Are they same?

Compare their spec's on their datasheets.
The 2N2222 is very old so its detailed datasheet might not be available.
I think a PN2222 is the chip of an old 2N2222 in a modern plastic case.

 

[Willen]

The 2N2222 is very old so its detailed datasheet might not be available.
I think a PN2222 is the chip of an old 2N2222 in a modern plastic case.

I found, you are saying several times that ''some old component's datasheet is almost not available.'' But I am in confuse, why are saying that? Because I can download or browse easily all these old component's datasheet on the internet. Like spec's datasheet at americanmicrosemi.com and etc. Are they not valid datasheet?

 

[audioguru]

I found, you are saying several times that ''some old component's datasheet is almost not available.'' But I am in confuse, why are saying that? Because I can download or browse easily all these old component's datasheet on the internet. Like spec's datasheet at americanmicrosemi.com and etc. Are they not valid datasheet?

I get datasheets from www.datasheetarchive.com where they show thousands of manufacturers of a transistor.
They show hundreds of manufacturers for the old 2N2222 mostly in India and there is no detailed datasheet, just a list of similar transistors wirh a few spec's.
MCC, ST Micro and ON Semi have detailed datasheets.
Americanmicrosemi transistors are not available at normal electronic parts distributors since they have their own store. They say "Did you Know...
American Micro specializes in providing obsolete parts that have been discontinued by other manufacturers."
Their website does not have detailed datasheets, just a list of a few spec's.

 

[Willen]

guru, what he is saying?
www.electro-tech-online.com/threads/simple-vhf-amplifier.112922/
How 1.5 watt Tx can transmit 240km?

 

[Willen]

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.

www.electronics-diy.com/tx200.php
It is same as your but saying 200mW! Is it fake?

 

[audioguru]

It is different from mine.
1) It does not have a tuned LC tank at its output. Then its output voltage swings less than half as much as mine.
2) Its oscillator output is from the high impedance collector, mine is from the low impedance emitter. Then the output transistor might kill the oscillator signal.

Its old BF199 output transistor has low maximum allowed collector current and very low current gain. The output power of this circuit might be only 10mW.

 

[Willen]

If i input 300 or 400 or 500mw RF source to 1 watt 2n3866 rf amplifier, may this amplifier lost its biasing due to over rf input? But between source and amplifier, there are some trimmers and coils in series at rf line and also some are parallel with ground. Hope that, these will maintain the impedance and overload. Here is no actual circuit becouse i am going to use almost 300mW-500mW Tx as a RF source and 2n3866 based rf amplifier which has 200mW input terminal (written in circuit). Between last transistor of source Tx and 2n3866 of amplifier, there can find 1 trimmer, 2 turn coil and 15 pf are in series in rf line as a coupler and 2 trimmer and one 10uH choke (base-ground of 2n3866) are in parallel to ground from rf. I think these maintain all bad things between Source and amplifier.
I think It won't be too bad.

 

[Willen]

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.

¤ What is the output impedance of this Tx? May I use 75 ohm coax and 75 ohm straight dipole without Balun?
¤ In the circuit, there are two coupler capacitor 0.33uF. Does it critical? May I use 104 or 22n instead of 0.33?

 

[trash]

For low power circuits it doesn't make a lot of difference. There's about a 1dB insertion loss from the impedance mismatch between 50 and 75 ohms.
So yes, for a 1 watt transmitter this will work ok.

If you wanted to be correct, you would run 50 ohm coax and use a 1/4 wave balun made from a piece of 75 ohm coax.
this doesn't address the minor problem of having an unbalanced feed to a balanced antenna.

My 6m yagi is a simple open dipole driven element. It is fed directly with 50 ohm coax. Again, I'm wearing the 1dB mismatch.
This does not pose a problem even for the higher transmit power because I also have a higher insertion loss (hence a lower inherent return loss) because
of 25 metres of RG213. At the antenna is 6 turns of RG58 which performs the function of a 1:1 balun to dampen reflections back down the shield of the coax and helping keep the radiation pattern centered.

 

[Willen]

If you wanted to be correct, you would run 50 ohm coax and use a 1/4 wave balun made from a piece of 75 ohm coax.

'To match 50 ohm Tx to 75 ohm antenna, should have to use 1/4 balun'??? What !!!?? How?

 

[trash]

There are more 50:75 baluns than I care to remember.
A 1/4 wave piece of 75 ohm coax is just one of them. I can't seem to find any pictures of one being used.
This is a PCB version of the same thing, but the stripline is balanced.
https://martybugs.net/wireless/images/coax_dipole.jpg

There are 1/4 matching stubs, ferrite baluns and slots.
Ah, here's another good example. https://electroschematics.com/wp-content/uploads/2008/04/dipole-half-wave-antenna1.gif

Each has advantages and disadvantages. The trade off is usually something like insertion loss vs bandwidth or radiation pattern.

 

[Willen]

Thanx!

Please go back to 17 page and read #169th post (by me) and suggest me ''may I replace 104 or 223 instead of 330n capacitor?''

 

[audioguru]

¤ What is the output impedance of this Tx?

My FM transmitter has low output power because its output transistor has a fairly high output impedance and does not match the 75 ohms impedance of its antenna.

May I use 75 ohm coax and 75 ohm straight dipole without Balun?

yes.

In the circuit, there are two coupler capacitor 0.33uF. Does it critical? May I use 104 or 22n instead of 0.33?

The value of the coupling capacitors were calculated for good audio low frequency response.
The 330nf capacitor from the electret mic feeds about 22k ohms from about 2k ohms so it has a response which is -3dB at 22Hz.

The 330nF capacitor between the audio preamp transistor and the RF oscillator transistor feeds about 23k ohms from 10k ohms so it has a response which is -3dB at 15Hz. Both capacitors produce audiop which is -3dB at about 40Hz.

You can calculate the different low audio frequency response produced by your different value coupling capacitors. There will be no bass frequencies which will sound thin and tinny.

 

[Willen]

Hi

audioguru,

Among many miniature transmitters, I found your is the best one! Please upgrade your Tx with almost 100mW power by adding easily available transistor like 2n2219. It won't be illegal because I think- desining such a circuit is not illegal but transmitting is illegal. Please.....

 

[audioguru]

My FM transmitter works very well so I will not boost its output power.
There are many high power FM transmitter circuits on the internet. Simply add the output circuit of one.

 

[Willen]

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.

¤ I heared that putting two or more inductors near just like parallel, performs bad because of parasite coupling between each another. It should be in 90 degree or far from each another. But you've put it near and like parallel. Won't it face parasite coupling?
¤ I am designing a PCB layout of your Tx. It is good or bad to keep grounding area under an inductor? (like inductor as a bridge and a grounding copper hub of PCB as a river.) Or better to leave blank?

 

[audioguru]

¤ I heared that putting two or more inductors near just like parallel, performs bad because of parasite coupling between each another. It should be in 90 degree or far from each another. But you've put it near and like parallel. Won't it face parasite coupling?

Each parallel track is very short so the coupling is almost nothing. That is why I cut each track to the minimum length required.

I am designing a PCB layout of your Tx. It is good or bad to keep grounding area under an inductor? (like inductor as a bridge and a grounding copper hub of PCB as a river.) Or better to leave blank?

It doesn't matter because I spaced my inductors 3mm above the circuit board.

 

[Willen]

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.

Ok, finally I designed a PCB layout for your Tx. I've put power LED and external audio input section additionally. Please chek it and and leave some comments if any mistake.

 

[audioguru]

I did not check all the connections.
C14 looks odd. What is "VC"?

You should not use an ordinary 7805 with a 9V battery because when the battery voltage drops as it is used then the regulator "drops out" and does not regulate anymore. Instead use a Low-Dropout regulator like I did.