Referring to Friis Equation, when the frequency increases the pathloss increases. However, the range of the RFID tags in UHF (from 860 Mhz to 960 Mhz depending on the country) is greater (about 10 m) than that of HF (about 13.56 MHz) RFID tags (about 1m). So which factors enable to have such better range in higher frequencies for passive RFID tags?
This is quite interesting but also complicated question.
There are two types of coupling mechanism here for 13.56-MHz ISM band and 860/915/960-MHz/2.4 GHz/etc. bands, respectively. The first one used for 13.56 MHz is called inductive coupling which is also used for Wireless Charging and NFC communications, while the second one is based on backscatter coupling mechanism. Therefore, the first one is not about EM wave propagation, it is more about EM coupling like transformers, consequently, it does not work for long range.
On the other hand, I suppose, one of the reasons why 13.56 MHz RFID adopts Inductive Coupling mechanism, is that, it might be easier to develop high-performance RFID antenna coils in small area ( like a credit-card dimension ) instead of RF antennas at this band (imagine a typical antenna dimension at this frequency band).
Nevertheless, 13.56-MHz got its own advantages such as dealing with human/animal identification [1], moreover, it is insensitive to waters, snow or other liquids even when immersed and read distance,
[1] M. Rata, G. Rata, A. Graur and V. Popa, "The influence of different materials in 13.56 RFID system," 2007 1st Annual RFID Eurasia, Istanbul, 2007, pp. 1-3.
The 13.56MHz system has a wavelength of about 22m. It normally operates at distances much less than a wavelength in the reactive near-field coupling regime. Here the Friss equation does not apply. The 900MHz system has a wavelength of about 0.33m and thus operates at distances of up to several wavelengths. Here the Friss equation can apply. Efficient antennas are much easier to fabricate in a credit card sized device at 900MHz. Also beware the Friss equation written in terms of antenna gains. It implies that free space path loss is frequency dependent when in fact it is simply an inverse square law and independent of frequency. The frequency dependence in the Friss equation is due to the frequency dependent relationship between the gain of the receiving antenna and its receiving aperture. If communications engineers would use aperture rather than gain for the receiving antenna this confusion would not arise.
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