Pre-selector for 136 kHz
This pre-selector can be used to increase both the gain and the RF
selectivity of an LF receiving system. This circuit is lilely to improve
considerably the reception of LF signals when using one of the modern 'general coverage'
receivers, as such equipment often performs poorly below 500 kHz.
A gain of about
14 dB can be expected.
FIGURE 1 - Circuit diagram of Receive Pre-selector
Figure 1 shows a simple, fixed-tuned circuit for use on 136 kHz. L1 and L2
use old 'long wave' coils mounted on individual 60 mm lengths of ferrite rod.
L3 was wound on a 25 mm OD 3C85 ring core:
36 turns on the primary; 4 turns on the secondary, using wire obtained from stripped-down
internal telephone cable (the wire is plastic coated with a conductor size of 0.020 inch -
25 SWG, or about 24 AWG).
During construction, L1; L2; and L3 were initially resonated to 136 kHz using a
variable capacitor to determine the required value of fixed capacitors. Magnetic
coupling is used between L1 and L2 by mounting the two ferrite rod cores in parallelism,
at a centre-to-centre spacing of 30 mm.
If you don't have any old long wave coils in the junk box (and you can't bear to strip
down that favourite transistor radio from the 1960s), there are lots of other
possibilities. Any coils in the 1 to 3 mH range should resonate just fine. The
coils used for bias oscillators in VCRs and audio tape recorders would work well, and
often have a high Q. The use of capacitive top-coupling (with, say, a 100 pF
capacitor), instead of magnetic coupling, will work fine too.
If you don't have a resonant antenna matched to 50 ohms, you could try connecting a
long wire to the top of L1, instead of using the link coupling.