RF430FRL152H: RF430FRL15xH Optimum Coil Inductance

RF430FRL15xH

What is the optimum or target coil inductance to add to a RF430 (TI RF430FRL15xH) to achieve maximum performance / range?

The on-chip resonance capacitor (CINT) has a typical value of 35 pF, which without any additional parallel tuning capacitor would require an antenna coil with an inductance of circa 3.9uH.

But I could arbitrarily design a range of coils (and inductance) to resonate with the RF430 by adding the necessary external resonance capacitor parallel to CINT.  A CINT of zero would require 3.9uH (the maximum inductance), but the minimum has no theoretical limit, e.g. 50pF with a 2.7uH, 100pF with a 1.4uH, 1nF with a 137pH, 2nF with a 680pH and so on.

I thought increasing the coupling of the reader antenna to the tag antenna should increase performance and, simplistically, increasing the turns on the tag antenna should increase coupling (assuming the same tag area), so as I reduce the tag coil inductance and reduce the number of turns, but increase the external resonance capacitor to compensate, I will reduce the coupling.

But there must be an optimum, or a target value which optimises performance, but I cannot find any recommendation in any RF430 application note.  SLOA217A states "5.2 External Antenna Inductance Range Without External Capacitor, Theoretically, the antenna inductance can have any value within the logical range of the L/C ratio", which implies simplistically, (a) choose a coil design, (b) measure its inductance, then (c) tune. 

For the TRF79xx reader, SLOA241C (Antenna Design Guide for the TRF79xxA) states "....it is ideal to target for a coil inductance in the range of 1 to 1.5 μH” (but it doesn’t say why).

The RFID Handbook from Klaus Finkenzeller, page 75, suggests that there is an optimum tag antenna (circa 1uH).

Do you have a recommendations, my project requires maximum range with minimum tag size.