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walid

Audioguru FM Tx

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Hi Sssaaa,
The circuit you posted uses a crystal in an odd oscillator circuit. Since a crystal cannot be frequency modulated very much then the transmitter must be narrow-band for a ham radio receiver. It also doesn't have pre-emphasis like radio station FM transmitters use.

250mW is probably its input power, not its output power.

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250mW is probably its input power, not its output power.

input power??
this is written on its web
"This Narrow Band FM VHF transmitter will output approximately 250mW of RF power using a 2N2222 and can operate between 75 and 146MHz"

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This Andy guy says his FM transmitter has an RF output of 250mW. The other guy says that my FM transmitter that has nearly the same output circuit has only 10mW output. I think it is much more.

I suspect that the output stage is operating in class-B or class-C and therefore has a pretty high output power which is about half the power supplied by the battery. The coupling capacitor charges by the base-emitter junction in the output transistor rectifying the signal from the oscillator. Therefore the base voltage is a little bit negative and the transistor swings widely between saturation and well into cutoff. The tuned circuit at the output helps the wide swing and filters out some of the harmonics.

Who are we going to believe?

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I tested my transmitter with a new 9V alkaline battery and it went far. After about 1 hour the voltage was about 8V and the range was less.
It should have more power output with an 11V supply but I don't know how much. The output transistor might be OK or might get too hot and melt.

The drive level to the output transistor is fixed so there is a minimum supply voltage where increasing it won't increase the power output.

The 2N3904 is a general purpose audio transitor, therefore its gain at 100MHz is a max of only about 3. A real RF transistor would have much more gain, give a much higher output power, require an emitter resistor to limit the power and require a heatsink.

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The 2N3904 is a general purpose audio transitor, therefore its gain at 100MHz is a max of only about 3. A real RF transistor would have much more gain, give a much higher output power, require an emitter resistor to limit the power and require a heatsink.

you know any transistor which i use in the place of 2N3904 which will increse the power?

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i found many high power transmitter on net but they are all drift fequency or very big circuits (a lot of parts and coils), high voltage and very complicated.
i want small & stable circuit like yours but incease power like 500mW or 1W.
thanks.
plz think about mod-5 ;)

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This circuit was posted on the other board

For all those that were asking for more power, well here is a design. This uses the output of the last circuit I posted at about 100mW. The power transistor can be a 2N3866 or 2N4427 and both work well. The transistor runs quite cool and needs only a small push-on heat sink. The values in brackets are my settings to obtain maximum output. This is quite close to my initial design values. On both 0.7W and 1W out the 2nd harmonic is 20dB below the carrier. This may need more filtering not to cause interference. Mount the 3.3k resistor directly over the 1uH choke. Use responsibly!

post-9230-14279143096221_thumb.jpg

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Here is the follow up posted.

Also tested with a BFS22A transistor. Work ok, but output slightly lower (~830mW) as this transistor needs a bit more drive being a 4W device. Overall the 2N4427 is the best choice, giving me 1.3W @ 12V and 0.8W @ 9V with about the same harmonic performance.

post-9230-14279143096427_thumb.jpg

post-9230-14279143102643_thumb.jpg

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Or get you own island

Hee, hee.  ;D
I don't know about international laws, but I think the RF cops would even arrest you on your own island if you cause interference to radio stations.
Your island would need to be in your own country. Where can you buy a country???

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Harry's circuit has an extremly low level of 2nd and 3rd harmonics at 0.1% (-60dB). The oscillator and driver are emitter-followers and therefore produce a low output, too low to drive a class-C output transistor, so the output transistor has some bias to operate in class-B at a faily low level.

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Yes that might be the reason.

I was searching for prices on the 2n3866 and 2n4427 and found Mouser have them at $1.60 and $2.00. Unable to find any sale of the Bfs22a. The only ones are the component brokers that wants you to ask them for a quote. I tried them before and boy they want you to spend minimum $200 or more. I hate that!

Even better from JDR! $1.00 http://www.jdr.com/interact/item.asp?itemno=2N3866

Also found some 2n3866A.  What will be the difference  from a 2n3866?

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