rectenna, diode selection
Most probably it’s the reverse transition (or recovery) time of the diode. The 1N4148 is exceptionally fast (in the order of a few nano seconds) and, this means that within a few nano seconds of the AC signal reversing polarity, the diode is fully blocking and therefore acting as a proper rectifier should when reversed.

Devices like the 1N400x series can have reverse transition times of tens of microseconds and the impact of using one of these is that it just won’t block high frequency signals.

If you are trying to detect 2.45 GHz, the 1N4148 is starting to run out of steam so I would consider changing technology especially if you are trying to harvest power. The 1N400x is totally dead (and sunk to the bottom) in the water at microwaves.

For FR207, reverse recovery time is quoted at 500 ns so, it also began sinking somewhere above 100 kHz.