I saw this circuit in an old
issue of "73 Radio Electronics". I built it, but it didn't work well, so
I changed a few components to get better sensitivity.
This is a very
straightforward circuit. The first stage acts as a crystal receiver. Use
a germanium detector diode (like 1N34, but AA119 is much more common in
Europe), a silicon one won't do. The frequency is determined by L and C.
For the FM band and VHF, wind a coil 5mm in diameter, 6-8 turns of
coated wire 1mm thick. You can always vary the frequency by spacing the
turns a bit looser or tighter. C is much less critical. Something around
100p is preferable, though.
The second stage is based
around the versatile 2N3819 JFET high-impedance amplifier. With the 470k
potentiometer you can adjust sensitivity of the circuit. The trimmer is
used to zero the meter. Use any old 50mA or slightly smaller ammeter
from the junk box.
You can't expect great
performance from such a simple detector-based meter. Sensitivity is just
adequate enough to get a basic idea of the power that your transmitter
is capable of.
Use the field-strength meter
to find out when a transmitter is operating at optimal power. It can be
very handy when aligning stages (like in case of the 4W transmitter) or
experimenting with different antennas.