Get out one preamplifier stage and lower your gain as much as possible but still enough to detect the phenomenon, while cut sky/ground effect.
I will give you one idea. Add several different oscillator stages besides original 8MHz, for example 1MHz, 20Mhz, 100 MHz, and mix very small oscillator's output amplitude on the base of 1st transistor. On this way you will have very wide band detection.Amplitudes must be very low to avoid oversaturation of preamplifier, interferation and slope eadge modulation.Also good narrow band pass filter should be added after amplitude detector.
Interesting project but need a lot of mods.
All credits go to Franco Italy.
and remember:
NOISE IS YOUR FRIEND becuase can help you to find a lot of gold coins. 