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4W FM Transmitter


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Connect 2 in parallel, and connect a 4.7ohm emitter resistor in series with each of them, and a heatsink would also be a good idea.

The 2N2222A is also rated for 1.8W at <25OC

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Well here it says 1.8W for Tc of <25oC and 0.5W for Tamb <25oC so am I rightly assuming Tc means case temperature?

I don't know about soldering to the case as it would exceed it's storage temperature.


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Down Under?
D'day mate! Are you near Oz? How do you like it to be upside down all the time? With kangaroos and wallabees running around all over the place! I have snow when you are sweating.

Go to www.farnell.com and click on your flag. Farnell has nearly everything except refrigerators. Dick Smith has them.
The clip-on heatsink is very important for Q2.  ;D

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Farnell is in at least 23 countries including Australia. I clicked on your flag and went here:
They don't have any 2N2219 transistors because their supplier doesn't make it anymore.
They have thousands here in Canada.
They don't have a 2N3866 transistor either. Sorry.

Try paralleling two 2N2222 transistors as Alun said. Keep the power down with a lower supply voltage. Australia's power limit without a licence is only 10mW.

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ok thanx ill try using 2 2n2222 in parallel
thanx a lot

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For both of the initial questions.
T1 can be 2n2222, 2n2222a, 2n2219, 2n2219a, bsx20 maybe 2n3904 and other simillar transistor, slide variations for the C7 capacitor's value to achive oscillation at the desire frequency.
T2 is better to be 2N4427, 2N3866, 2N3924, 2N3553.

For the coils use 1mm or even 0,8mm wire.

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Made a heatsink out of a section of thick (3mm) copper tubing lower section fits over T2  6 vertical cuts made from top  20 mm long,    pieces bent 90 degrees to tubing.
small fan from cpu wired to top of heatsink

Using rundown 9v battery for power while tuning
Difficult,  trimmers small even with plastic tuning tool
presence of hand affects tuning.

oscillator var cap replaced  with one from FM radio
Using probe made from 2 diodes 2 capacitors to measure output

Output power drops dramatically at higher frquencies
soldering rather messy,  component placement could be better,
Is this a possible cause of this problem?


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Hi Radiopirate,
The case of the transistor is connected to its collector. The stray capacitance of the heatsink to its surroundings kills high frequencies at the transistor's collector.

Can you peak the output with the trimmer capacitors? If they run off the end of their adjustment then their associated coils need their length changed.  ;D

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Hi Guys,
I measured the range of my mod4 FM transmitter: about 1km before it was drown by a real station on the same RF frequency. The signal faded when I walked or drove with houses in beween and might travel 5km or more line-of-sight if I had a clear station frequency.

It transmits with low distortion and wideband audio. For its range I had it transmitting the sound of my son's alarm clock beeping. Then I had it transmitting the sound it picked-up from my souped-up clock radio that was playing another FM station's music. Its reception on my Walkman, car radio and home stereo sounded exactly the same as the station directly!  ;D Except it was in mono, not stereo.  :'(

The range dropped sharply when I reduced the length of its 30" antenna.
It seems to eat batteries. After a couple of hours, its brand new alkaline 9V battery measures 8.25V and its current dropped to about 39mA.
I took it from my 21 degrees C home to 13 degrees C outside and didn't notice any drift in its RF frequency. I guess the tempco of the RF caps I used luckily reverse-matched the tempco of the oscillator transistor and my coils.
With its low-dropout voltage regulator powering its mic, preamp and RF oscillator nothing much changed when the battery voltage reduced. Its range probably reduced but I didn't re-measure it.

Google has 137,000 links for FM transmitters in my city (I wanted to find the weakest one the farthest from me, to use its RF frequency). Subtracting the 100 real FM stations, that leaves about 136,899 copies of my project here!  ;D ;D 

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Hi Sasi,
Your very nice ground-plane antenna is strong enough to withstand a hurricane (monsoon?).
Are the lengths 1/4 wavelength?
I might be incorrect, but I figure that a dipole has 3dB gain over a whip antenna and your ground-plane has 3dB gain over a dipole and is omni-directional. Therefore yor ground-plane antenna has double the range of a whip. Correct?

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Referring to my previous post.  Yes Audioguru, you was right.  I was reffering to a wrong schematic, I was talking about http://www.electronics-lab.com/projects/rf/001/index.html.  I don't know how do I did such a reference mistake.

The transmitter you are talking about (4W with two transistors 2N2219) has certainly overheating problems and could never produce 4w of RF power, at least with those transistors.
By replacing T1 with 2N4427 and T2 with 2N4427, 2N3866, 2N3553, 2N3924, BFS23A, you can achieve a power increase compare to 2N2219 and a good heatsink would be sufficient for cooling them down.  2N2219 cannot dissipate such power levels. (easyly at least)

In case you increase R3 from 47 to anything between 51 to 100 Ohms, the transistor will run cooler but the power would be lower, still high for a good range.  I suggest T2 be a real RF transistor like as suggested above.  They have higher gain and efficiency at the desired frequencies.


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  • 1 month later...

Replaced T2 with another transistor Blw29
T2 gets hot but does not need a heatsink
getting 1- 1 1/2 watts out of it

I find this difficult to tune.
could the tuning capacitor be replaced with one of a smaller value
perhaps in parallel with another capicitor
I want to run this at 88Mhz. What values should I use?

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