Specifications

CIRCUIT IDEAS
contain some residual carrier. This is
known as carrier leakage. By adjusting
the 100-ohm preset (VR2) and trimmer
(C7) you can anull the scarier leakage.
To receive DSB signals you need a
beat frequency oscillator to reinsert the
missing carrier. If you don’t have a beat
frequency oscillator, or want to trans-
mit only AM signal, adjust preset VR2
to leak some carrier so that you can
receive the signals on any ordinary ra-
dio receiver. In AM mode 100% modu-
lation can be attained by adjusting pre-
sets VR1 and VR2.
The DSBSC signal available at the
output of the balanced modulator is am-
plified by two stages of RF linear am-
plifiers. Transistor 2N2222A (T3) is used
as an RF amplifier, which provides
enough signal amplification to drive the
final power amplifier around transistor
SL100B. The output of the final power
amplifier is connected to the antenna.
All coils are to be wound on ferrite
balun core (same as used in TV balun
transformer of size 1.4 cm x 0.6 cm)
T
he circuit presented here pro-
vides the simplest way to mea-
sure earth conductivity. Using
this, radio hobbyists can choose a suit-
able site for the erection of antennae
that require good grounding; for ex-
ample, a
vertical an-
tenna that
uses radial
ground
plane. This
circuit is
also useful
for selecting
a suitable
site to carry
out the
earthing of
industrial sheds or housing. With a
little ingenuity and experimentation,
one can even predict the availability of
underground water at a site. It should
be noted that geologists employ a simi-
lar probe, but in place of 50Hz AC
mains used here, they employ RF for
this purpose.
Though electronics hobbyists hav-
ing a flair for gardening can measure
the salinity or the fertility of soil to be
used for their special plants, using the
method described here, it however
needs a lot of experimentation and
study.
The circuit presented here employs
a simple AC measurement technique.
The efficiency of the circuit and results
improve as the frequency of AC in-
creases. With regular mains voltage
source of 230 volts, 5-amp current ca-
pacity, and 50Hz frequency available,
the measurements result in 25 per cent
PRASAD J.
GROUND CONDUCTIVITY
MEASUREMENT
S.C. DWIVEDI
using 24SWG enameled copper wire.
Proper heat-sink should be provided for
SL100B transistor used as final power
amplifier.
Range of the order of a few
kilometres can be easily achieved by
proper choice of site, type of antenna
(such as a resonant half-wave dipole of
length 20 metres for 7.05 MHz fre-
quency) and proper matching of trans-
mitter to the antanna. Use good-qual-
ity shielded wire of short length to con-
nect the crystals.
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