illrrrrrrW

situ ated on a PC board (Fig. 4) directly
betw een the ends of the loop (Fig. 5) so
that we do not have the usual separation
between active element and tank circ uit .
This, too, enhances stability and sensitivity.
The inside diameter of the nearly complete
circle of copper tubing is 12 in.
Approximately 2 in. of space is allowed
between the ends. This space is occupied
by th e PC board containing the remaining
parts of the loop oscillator.
The use of a high-b eta tran sistor and
Mylar capacito rs results in a sure-fire and
stable oscillator in th e 4 50 kHz region,
despite the extremely low impeda nce of
th e single-turn ta nk circuit. The tubing
sho uld be pro tecte d by some type of
sleeving ( vspaghett!" or heat-shrinkable
tubing).
Fig. 4. Loop oscilletor (circu it side).
'-----f
_ 2 3 / 8 in.- -
J
Fig. 5. Mounting block, showing associat ion with
loop oscilla tor an d handle.
R eference oscillator - in sid e vie w with cover
plat e removed,
12
Oscill ator
The referen ce oscillator utilizes transistor
Q2 in a Colpitts circuit similar to
tha t of the loop oscillator. A quality air
variable capacitor is provid ed for frequen cy
adjustment. Su ch a capacitor is greatly
superior in mechanical and electrical stability
to the mo vable-core indu ctors commonly
used in metal loc ators. Howe ver ,
relatively small tuning-range results from
its use. This is actually de sirable fro m the
standpoin t of stability. Because the lar ge
tu nin g range associated with a movable-slug
inductor is not obtained, it is mandatory
that the prescribed components, or their
equivalents, be selected both for the reference
osc illato r and the loop oscillator. On
th e other hand, the re producibility of the
loop oscillator renders it unn ecessary to
incorporate a wide tuning range for the
reference oscillator. The refe rence oscillator
cir cuit board (Fig. 6) is situated on
th e handle, adja cent to the transistor radio.
A single 9V battery po wers both oscillat
ors. As wit h the lo op oscillator, it is very
imp ortant that all components be securely
73 MAGAZIN E