Theoretical and experimental reader range in UHF RFID tag

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.

If you can list all the numbers you used to calcuate the reading range, it will be helpful for trouble shooting here.
In your calcuation, did you take the polarization mismatch into the consideration?
Can you access to the rectifier circuit DC voltage output?

Regards,

The following formular is the final equation for calculating Reader range

Hi ThaiHoa
thanks for your interesting.
I known this formula and i've used it but the problem is yet the same.
you known if activaction chip power which in the formula is Plmin is the real lower power which must deliver to the chip? or that power is used only for particular chip operations?

Regards

Added after 13 minutes:

Hi asdfaa
I've used the following number in the formula which is write in ThaiHoa's posts
Gtag=7dB transmission coefficient 0.99 polarization mismatch none because i'm using vertical polarization for tag and transmitting antenna.
Antenna transmitting gain=8 dB power transmitting at the antenna 20mW,activation chip power -14dBm. and with this number i'll obtain an ideal reader range of about 3,3 m but with sperimental measure i read only at 1,7m.

I can only measure the voltage which is at the input pin of the ASIC, i can't measure DC voltage.
You know if activation chip power which manufacturer declare is the lower power which need to be deliver to the chip for read the tag or is the minimum power for chip operation?
Thanks a lot
Regards