With the figures you just gave, I would suggest you to connect the voltage
from the RF sensor circuit straight to the base of an ordinary npn
transistor, connect the negative terminal of your 10mA meter to the
collector of this transistor, with the positive terminal of the meter going
to a suitable positive supply (12v?). The emitter of the transistor goes to
ground through a 220 ohm resistor. The voltage of the supply isn't
To fine-tune the sensitivity you could make the 220ohm resistor adjustable.
As Dave says, there will be a slight 'knee' in the characteristic, so the
meter will read low if you run low power, and perhaps even not register at
all if the power is less than 1.75 watts, but if you will not normally
operate that low in power, you will probably never notice this.