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I have a working FM transmitter on a raspberry pi2. I'm running my FM raido at 103.3MHz, and the antenna that I connected to the GPIO4 is a 70cm copper wire.

More data:(checked with a multimeter between GPIO4 and pin6-ground)

  • DC voltage: 1.2V
  • AC voltage: 1.8V
  • Current: 18.4mA

update: I manually checked the distance by tunning cell phone fm to 103.3MHz. it was around 20 meter.

My question:

1.How far is my FM transmitter signal going?(Is there a formula?)

2.How do i increase the transmitting distance?(amplify the signal)

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  • $\begingroup$ I am assuming that you are wanting to know how far your signal will reach in air, without anything in between? (It is impossible to answer otherwise.) Also, what do you mean by "how far"? Whether or not a signal can be received depends on a lot of factors including any amplification at the receiver. $\endgroup$ – hazzey Feb 3 '16 at 22:53
  • $\begingroup$ yes, in the air. what factor will be affecting fm signal to reduce it's power? $\endgroup$ – editinit Feb 4 '16 at 9:07
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The main factor limiting FM signal reach is often the height of transmitter and receiver antennas over ground, not the power. The Earth is curved and antennas must be in (or very close to) the zone of direct visibility (you should be able to see one antenna from the top of another).

Longer waves can bend over the Earth curvature, and short waves (the source talks about 3MHz and somewhat above) can reflect from the ionosphere to increase the distance. However 103.3 MHz you talk about is too high to benefit significantly from any of these two.

More can be found here.

Assuming that Earth curvature is not a problem, the distance depends on both the power of transmitter and sensitivity of the receiver (and sensitivity of the receiving part largely depends on antenna construction and quality). Hence it is also difficult to say without knowing these details.

Finally, if there is any material like building walls between transmitter and receiver, the limiting factor most likely will be the thickness and properties of this material (wood? bricks? iron?), not the distance.

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