The diode detector's sensitivity can be improved by the following:
1. Match the input impedance [1]
2. Change topology from doubler to single. This will improve the sensitivity for input power below -10 dBm [2].
3. use a zero bias Schottky [3]
Although some have commented on the 1N5711's C@0V < 2pF, but IMHO, it is NOT the most critical weakness of the leaded package because, replacing it with a plastic packaged SMD Schottky intended for microwave mixer/detector such as HSMS-282x will only marginally improve C@0V to < 1.5 pF. On the other hand, parasitic inductance is a greater difference as a leaded package has ~ 8nH [4], whereas a SOT-323 has ~1.1 nH [5]. Nevertheless, if impedance matching is used, the parasitics can be mitigated by absorbing them into the network.
References:
[1] Avago AN1089, "Designing Detectors for RF/ID Tags"
[2] Boaventura & et al, "Optimum behavior: Wireless power transmission system design through behavioral models and efficient synthesis techniques", IEEE Microwave Magazine 14(2):26-35, Mar. 2013
[3] Avago AN969, "The Zero Bias Schottky Detector Diode"
[4] R. W. Rhea, Oscillator design & computer simulation, section 1.10 "Component parasitics".
[5] Avago AN1124, "Linear Models for Diode Surface Mount Packages"