Hi,
I am reviewing RFID specifications and all antennas provide a gain (in dBi), a frequency (900Mhz range) as well as a maximum power.
There is no specification on range… However the range if what I’m looking for.
How do you convert dBi, frequency and power into a physical range (line of sight)?
Thanks!
Use the inverse square law (far-field) or inverse cube law (near-field).
Leon
Look up “Link Budget Calculation”, specifically for a UHF RFID application (if you’re using a 900 MHz system), you should be able to find some good webpages discussing calculating your link budget, it’s pretty easy to do if you know the variables (i.e. receive sensitivity, transmit power (not maximum antenna power!), etc) of your system. Take the worst case link budget (it’ll be either from your reader to your tag, or from your tag to your reader) and then you should be able to use the inverse square law to calculate open space, line of sight theoretical range…
Then keep in mind line of sight, is not just a line, and your actual range will be much reduced.
Hopefully that will get you started. 
jeanseb:
Hi,I am reviewing RFID specifications and all antennas provide a gain (in dBi), a frequency (900Mhz range) as well as a maximum power.
There is no specification on range… However the range if what I’m looking for.
How do you convert dBi, frequency and power into a physical range (line of sight)?
Thanks!
Here’s one
20 * Log10 (frequency in MHz) + 20 * Log10 (Distance in Miles) + 36.6
that’s the path loss in dB in a perfectly line-of-sight condition.
Going on…
transmitter’s power in dBm. So 0dBm is 1mW, 10dBm is 10mW, 20dBm is 100mW.
Transmitter power + transmitting antenna gain - path loss + receiving antenna gain = received signal power in dBm. (add in a few dB for coax and connector losses if applicable)
example
Digi Xbee and PCB chip antenna
Tx Power 0dBm = 1mW (XBee PRO is about 50mW)
Ant gain: -2dB inclusive of cables. Negative gain here due to teeny PCB antenna. Rubbery ducky would be about +2dBi
Path loss, use formula above
receiver ant gain: same as trasnsmitter
If using 802.15.4 go for received signal strength of -90dBm or better (more positive). More conservative and allowing for fades: -85dBm. For 802.11, go for 10dB stronger signals
Add to path loss these options:
non-line-of-sight - this is very complex and an art. Or just measured.
Fresnel zone occlusion (google it) if any
derate antenna gain if path between antennas is not on the ideal boresight of the pattern. Even omni antennas have nulls.Antenna gain in dBi is for the impossible perfect sphere pattern of an omni.