High Power RF Detector Circuit
First of all, that circuit is rubbish. The LED will never light up.

I assume that your "8mm wire" is in fact a quarter-wave monopole antenna. As such, it will have a feed-point impedance of about 36&Omega;. From this, you can calculate the voltage and current available:

$$V_{RMS} = \sqrt{P \cdot R} = 51V$$

$$I_{RMS} = \sqrt{P / R} = 1.4A$$

You're going to have to set up a power divider so that only about 1/3600 of this power goes through the LED.