The electronic circuitry is inside a diecast aluminum box of 120 x 95
x 61 mm. The antenna is tuned by capacitors C1, C2 and C3. C2
and C3 were selected so that C1 can tune the antenna to the centre
of the band. It then happened to be at its minimum capacitance
as one of the photographs shows. R4 was added to widen the frequency
response so that it is sufficiently flat over the range of interest.
D1 and D2 protect the meter when used near a live transmitter.
T1 amplifies the signal without loading the antenna circuit. It
is important in a direct conversion receiver that the signal from
the local oscillator cannot reach the antenna circuit. The rf
amplifier, including C7, is therefore mounted on a separate piece
of copper clad circuit board (not etched, I never use PCB's).
Where insulated tie points are required I add small pieces of
the same material to the board with instant glue.
Field Strength meter PA0SE
Schematic is on the next page. The values of both L1
and L2 should be mH and not uH.
C15 should be 220 pF not nF.
The LED is shown upside down.
. The resistor feeding bias to the BB104:s has no value. However 100k is a good
choice which I have used in some other projects