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I want to build an FM transmitter that will transmit (on an unused frequency) from my laptop (near my video projector) to my receiver (about 20' away on the other side of the room). Then, I thought wouldn't it be cool to transmit music to the bedroom...again about 20' but with a wall in between. And of course, how about the workshop? about 1500sf, completely open.

How do I determine the signal strength that will meet these requirements:

  1. maximum transmission distance perhaps 50-75' sometimes with a wall or two, but mostly without, but
  2. weak enough to fall within FCC regulations for an unregulated xmitter.

I'd hate to solder something together that only transmits 5'.

Given a transmission distance, how do I calculate the power? does the power requirement change with frequency (e.g., 88.1mHz needs less/more power than 107.1mHz to transmit the same distance. Is there a rule of thumb for factoring in walls between trx and rx? or is that negligible?

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  • \$\begingroup\$ The output power required is directly related to the receiver's input sensitivity. So what's the reciever's sensitivity? Also, with the walls in between, you won't be able to find a definite power without doing some RF measurements (or trial and error). \$\endgroup\$ – jippie Mar 16 '13 at 14:36
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    \$\begingroup\$ It's also going to matter how much antenna gain you have --- that is, are you transmitting with a directional antenna or radiating in all directions equally? \$\endgroup\$ – The Photon Mar 16 '13 at 15:53
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It is both complex, and easy in the same time. Easy, because there is a single formula involved, complex because you will not be able to determine some of the variables.

If you had omnidirectional antennas (which you have since there is no predetermined direction of transmission) they are calculated in with 0 dB gain, so it is all about receiver sensitivity, and the transmission loss which you can determine only for the free air transmission. So the electrical power you refer to does not mean to much here, like with cars, you care about power at the wheels, not only the raw engine power at the flywheel :-)

What you ask is called a power budget calculation, and you can learn a bit about it here: http://en.wikipedia.org/wiki/Link_budget

Then of course, you should know how much of your transmitted power was actually handed over to the antenna (it is a matter of matching the transmitter to the antenna resulting in something called SWR - for Standing Wave Ratio). All in all, you should take the maximum allowed by regulations, and hope it would be enough. And yes, make sure you are within some allowed frequency band (some ISM band is safe to use, I assume)

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