How to modify the circuit to increase the range?

[sudheerelktronics]

Hi!

I have recently seen a post: FM Transmitter.

I felt that this concept is very interesting and the circuit also seems to be easy to implement. I am planning to do this project. But, here it is mentioned that it can work up to 2km. only. i.e. It's range is 2km.

Can anybody suggest me the required modifications to be done in the circuit and also the values to be changed to increase the range up to 15 km?

And also, Is it possible to use this circuit in forests in real life?

Kindly clarify my doubts.

[stj]

Re: How to modify the circuit to increase the range?

forget that design.

if you search a bit, there are some very powerfull transmitter diagrams about.
http://xtronic.org/category/circuit/transmitter/

[Elysarian]

Re: How to modify the circuit to increase the range?

One thing to remember with radio transmitters is the inverse square law.

You're looking to increase the range by a factor of 7.5 so the output power (for the same received signal strength) would need to be increased by a factor of 56.25...

the BC109 can dissipate at most 300mW - so (assuming nothing else changes and there are no additional losses) your output transistor would need to be able to dissipate around 17W.

I built something similar to this as a college project many years ago (we also built an AM receiver)

[momaka]

Re: How to modify the circuit to increase the range?

Another thing is the antenna - bigger is better and the higher you mount it, the better.

Also, I suggest staying away from cheap PLL VCOs such as the ones found in cheap car FM modulators - they tend to "broadcast" (if you can call it that) on multiple frequencies at a time. Their output is kind of "dirty" too.

A simple FM circuit like the one you found is quite simple (I mean that in a good way) and if soldered on a vector board, you can actually get pretty clean and stable output. I built something similar on a breadboard, and it worked okay for the most part. A common problem you may encounter with passive resonators, however, is drift. It probably will take some time for the circuit to settle on a frequency as it "warms up". Once warmed up, it will be stable for the most part. Might need very occasional tune-ups. I was actually planning on building that 2KM FM transmitter too, someday. Just never seem to get to it. I definitely want to try it with a home-made varicap, though. The small ones available from the store tend to be too sensitive (either that, or you can tune your circuit to a certain frequency and then design it so the varicap just "fine-tunes" the frequency).

Lastly, I saw one circuit on that xtronic website above that suggested using 741 op-amp for the audio amplification - that's a big NO-NO in my book. 741 is a terrible for audio amplification. Avoid!