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I am currently looking to design a single frequency radio transmitter. The transmitter needs to have a maximum range of about 120 feet. Would a design similar to the one included be able to transmit that far? Additionally, is there a method to estimating the range of a transmitter prior to building?

enter image description here

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  • \$\begingroup\$ I expect it will work, I found several transmitters with the same circuit on the web and they all profess to work. I will take a SWAG; and say it will go 120 feet but can you receive it, that depends on the probacation factors involved, antennas, receivers etc. \$\endgroup\$
    – Gil
    Commented Jun 28, 2021 at 0:19
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    \$\begingroup\$ @gil that really isn't an argument. The internet is full of badly copied and dysfunctional circuits. And this circuit is called "FM transmitter", but it doesn't frequency modulate at all; it's just a bad implementation of an oscillator with an unspecified antenna. \$\endgroup\$
    – mmmm
    Commented Jun 28, 2021 at 0:34
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    \$\begingroup\$ I mean. You can even see that the website this schematic is from just style it somewhere else, and slapped a logo on it, even hiding crucial info. The owner of the website didn't care - most likely, they simply even didn't understand what they were copying. \$\endgroup\$
    – mmmm
    Commented Jun 28, 2021 at 0:36
  • \$\begingroup\$ On top of what mmmm said, the battery is upside down too. \$\endgroup\$
    – Hearth
    Commented Jun 28, 2021 at 1:05

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Radio waves travel infinitely far; therefore a transmitter doesn't have a "range". A full transmission system, including both the transmitter and receiver, can have a range, because the transmit power, with all the loss the transmission has, which scales quadratically with distance, must be higher than the receiver's sensitivity.

If you want to estimate the range of such a system, you hence need to know the sensitivity of the received and the power your transmitter outputs.

The transmitter needs to have a maximum range of about 120 feet.

Operating this circuit is illegal, basically everywhere: it's not stable or precise enough to guarantee you stay in a band where you don't need a license, and it's potentially more powerful than you're allowed to be without a license, too. This circuit simply is just a source of interference to multiple bands and should never be connected to an antenna!

That being said, your circuit is also not a good transmitter, in many ways. You won't be very happy with this. This literally just makes a tone, and not a stable or clean one. Detecting this at your receiver will be error-prone, just because your signal isn't possible to tell apart from e.g. noise that power supplies or fridges cause.

I'd recommend looking either into proper RF integrated circuitry - for example, there's many microcontrollers and microcontroller boards that simply implement a reliable, legal, easy to use transmission system in an ISM band - or into first building a clean crystal or SAW based oscillator, which you can then modulate to transport information (like "I'm transmitter number 1 and on, and not just noise that you pick up").

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