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#14
December 9th 08, 08:00 PM
posted to rec.radio.amateur.antenna
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confusion about path loss calculation for zigbee
On Dec 9, 12:24 pm, Jim Lux wrote:
Richard Harrison wrote: Jeff wrote: "- 96 db seems about right for "free space" path loss." 96 seems abour the right number for a path loss at 2400 MHz at a distance of 500 meters. To nit pick, -96 db loss is a gain. I worked a few years with Pete Saveskie who wrote a book he called "Propagation". It contains a formula for free space loss: 23 db is lost in the first wavelength from the transmitter and after that 6 db loss is added every time the distance is doubled. With Pete`s formula, you would lose the 23 db at a distance of 0.125 meter, and at a distance of 512 meters you would lose a total of 95 db. That`s close enough agreement for me. Best regards, Richard Harrison, KB5WZI Also, a lot of published descriptions of various communications schemes (e.g. Zigbee, 802.16e) might give a maximum data rate and a maximum range, but that doesn't mean you get both at the same time. The former is often related to the system bandwidth, the latter to the minimum data rate and transmitter power. I think you can get a ballpark feel pretty quick.. use the free space path loss (in whatever form you like). Calculate required receiver power as -174 dBm + 10log(datarate in bps)+ 3 + receiver NF. (use 2dB for NF if you like) Add path loss to required receiver power, and that's what you'll need for EIRP from the transmitter. Yes, assumes isotropes, and ignores coding gains, etc. But you'll be within 10dB or so, and that's enough to know if you're even in the ballpark. If you do the calculations and you come up with a required transmit power of +50dBm (100W), and you're thinking small battery powered, you know it ain't gonna work. If you come up with +10dBm, and battery powered is the goal, you're in the ballpark, and THEN you can start thrashing through the more detailed modeling. I'm within 27.5 dBm |
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