RF range calculation

Mar 26, 2006 3 Replies
S

Hello,



Can anybody tell me the real-world usable range or how to calculate it for a transceiver? (Not looking for answers like "infinity" or "to the end of the known universe")



I'm trying to build an RF link at the board level to replace a serial connection to a computer.



I've been looking at Linx and Amtel RF components on digikey.com to use. e.g.

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For instance, one of the transceivers I looked at has a dBm of +4 at



2.4GHz. Basically I just want to make a serial RF connection to send
96 byte data packets every few minutes. I'm looking at transceivers because the "field unit" will be listening for a request to send data.

Most of the stuff I see has a range of about 300-500 feet. But some of the components I've looked at don't specify the range. I've found TX and RX separate with 3000 foot range.



If I haven't provided enough information plug in the basics and let me know where to start. I think a whip antenna would be 234/Frequency so anywhere from 1 inch to 2 inches depending on if I stick with 2.4GHz or not.



I need to keep the TX power consumption under 40mA as well.



Thanks



Scott



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M

Most calculations are based on free space losses. sounds like your application is not free space

I think your best bet is to take a system you have as a comparison and work from there. Increasing the Tx power by 10x (+10dB) will increase the range by about 3x. Ideally the power spreads by the inverse square law.

Depend a lot on the antennas. Using outdoor elevated antennas (if connected with low loss feedline) can make a very large improvement.

Mark

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R

RF attenuation in free space between isotropic antennas is given as: { 36.6

  • 20 log frequency + 20 log distance } Where frequency is in megacycles and distance is in miles. Calculate the free space loss and subtract that amount from the TX output. The result is the amount of signal appearing at the receiver.

For example: At 2.4 GHz and distance of 500 feet there would be free space loss of 84 dB. From TX power out of +4 dbm the signal at 500 feet would be (+4 - 84 = -80 dbm) at the receiver. Minus 80 dBm should be well within the receiver sensitivity spec of the cheapest 2.4 Ghz receiver. There would be some gain in the 2 antennas above isotropic, minimum of 2 db each if they are dipoles, adding at least another 4 db of signal strength over the path. Be aware, you probably don't have free space conditions, so make tests in the working environment.

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S

Good responses, thanks a lot.

Any good books on RF communication, logic, theory, etc? I wouldn't mind learning a little more than I need to know for what I'm trying to do.

I may need to look into a small RF amp as well in order to increase the dBm.

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