how to calculate rfid range

Hi, i've a question for which need your help!
I've simulated a microstrip antenna on ground plane for an UHF RFID tag with HFSS 10 and i've obtained a good matching for Zin and a good G(7dB).
After using Friis formula for reader range (r=(λ/4π)√(PtGtGrτ )Pth) where PtGt is EIRP power of the transmitter, Gr is the gain of tag, τ is the transmission coefficient of the tag (about 0,9) and Pth the power delivered to the load i've calculate theoretical reader range(about 3,3m).
After i've done an experimental measure with the tag and i've obtained a reader range that was half than that theoretical.(with theoretical formula i will be read to 3,3m but with experimental measure i'm reading at maximum distance of about 1,7m).
The measure was done not in an anechoic room but in a laboratory,i've used a low power transmitted for limit reflection with the wall (about 20mw to transmitter antenna), i've repeat the measure at different height from the floor but i've also a great difference in reader range.
I've seen that if height from the floor is bigger than reader range increased but more than 2 meters i can't go because after i'm too near the ceiling and the measure will be influenced from reflection. At the distance which i can read the tag i'm in far zone and for that Friis formulas is valid.
What can will be the reason for which i've that great difference in reader range?
The problem can will be reflection? or simulating parameter can suffer of error?
Please help me! from few days i must write result in my work of thesis.

hi, i also do the similar project. i am working on simulation work now. maybe you can share the simulation experience with me. which kind of antenna do you work for? i tried meander line dipole for 868mhz, but i can't got good results until now. later, i have to fabricate it and measure it. what method did you ues for measurement? could you please share some materials with me ?

thank you very much. i hope other people in this forum can help you.

hi, Nataliezhang
for measurements you can measure S parameter using a network analyzer or if you want you can also measure Gain but is more difficult because you must have a reference antenna.
You can also do a measure of reader range and for that obviously you must connect ASIC to the antenna and see how is the distance in reference with the power transmitting which you are using.
There are many type of structure which you can use for obtain the same result.
I don't known which type of measure you need to do.
If you can explain your problem i can try to give you a better answer.
Bye

thank you very much.

i have no experience on measurement at all. my tutor and i plan to measure the gain, input impedance and read range of the antenna. but i have no idea on it. i have got the chip and i will connect it with antenna for measurement.

how long did you take for the measurement? i have to finish all the work before august. i don't know if i have enough time for measurement and report.

Due to effect of multi-path , you can't get the real data to match theoretical data.

Because the Friis formula is used in free-space .

The higher height for antenna make it near free-space (but is't near ceilling)

, so you can get large range .

If you want to get the data near theoretical data, please measure the data in chamber.