There is good research out there on this topic.  Here are two papers that cover circuit design of the tank circuit and the diodes involved.  A narrow band tank circuit centered around the radiation frequency and sized for the bandwidth should yield the most energy.

http://iopscience.iop.org/article/10.1088/1757-899X/79/1/012011/pdf

https://repository.library.northeastern.edu/files/neu:1339/fulltext.pdf

The Schottky diode model consists of both, linear and nonlinear parts. The non-linear part corresponds to the metal-semiconductor junction, and the linear part contains everything else.

To find the input impedance different approaches are they (Mathematical modeling, equivalent ckt, simulation etc )

germanium is almost a forgotten process and it has generally, very lower junction capacitance. Also the configuration need to be differential voltage doubled across an instrument amplifier. The high impedance of the instrument amplifier input and the gain over a wide band of frequencies probably will work better than Schottky. The reason is the Schottky is designed generally for high current rating switch mode and the rectification efficiency suffers. The tank circuit can be tuned by the instrument amplifier active feedback. Large signal S-parameters for the Diode is helpful. If it configured correctly then full wave voltage scavenging with class B- instrument amplifier set to operate into the diode turn - on knee

First you need choice the frequency operation and the power level range that you go make the energy harvesting. The impedance of diode change with the frequency and with the power level. Zero bias schottky detector diodes are a good choice and the impedance is provided in site of the manufacturer in a file in the format s1p. Best regards

For ambient RF energy harvesting, Schottky diode HSMS-285C is the best option as it has low threshold voltage about 0.15 V.  As the diode is a nonlinear device, therefore, its input impedance varies as a function of frequency and input power. Thus, to calculate the impedance of the diode first select the frequency of operation and input power.  Use the harmonic balance and LSSP analysis in ADS for input impedance calculation.

You can make your own primitive model easily - The best diode will be the one which produces the most DC output at the highest frequency of harvesting signal. This is the RF rectification efficiency. Most harvesting diodes are classified as Small Signal - which means they will not be high current carrying devices. The rectification efficiency is dependent on the value of reverse bias capacitance. The larger the capacitance the bigger the diode and poor rectification efficiency will result. Start here - there are a couple more steps for the model but essentially the small signal characteristics with low junction capacitance will be first priority

hsms 2850 schottkey diode is not available in market. instead of that which diode can be replaced for rf harvesting pls let me know

HSMS2855 is a good alternative to HSMS2850. I have used it instead and it worked well for FM and TV signals detection and harvesting.