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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