Practical_Antenna_Handbook_0071639586

C h a p t e r 1 4 : r e c e i v i n g A n t e n n a s f o r H i g h F r e q u e n c y 355
back coupling loop (L 3 ). The degree of feedback is controlled by the coupling between
L 1 and L 3 . The coupling is varied by changing both how close the two coils are and their
relative orientation with respect to each other. Certain other circuits use a series potentiometer
in the L 3 side that controls the amount of feedback.
The Q-multiplier is adjusted to the point that the circuit is just on the verge of oscillating.
As the feedback is backed away from the threshold of oscillation—but not too
far—the bandwidth narrows and the sensitivity increases. It takes some skill to operate
a Q-multiplier, but it is easy to use once you get the hang of it and is a terrific accessory
for any loop antenna.
Loop Amplifier
Figure 14.16 shows the circuit for a practical loop amplifier that can be used with either
shielded or unshielded loop antennas. It is based on junction field-effect transistors (JFETs)
connected in cascade. The standard common-drain configuration is used for each transistor,
so the signals are taken from the source terminals. The drain terminals are connected
together and powered from the +12V dc power supply. A 2.2-F bypass
capacitor is used to put the drain terminals of Q 1 and Q 2 at ground potential for ac signals
while keeping the dc voltage from being shorted out.
The two output signals are applied to the primary of a transformer, the centertap of
which is grounded. To keep the dc on the source terminals from being shorted through
the transformer winding, a pair of blocking capacitors (C 4 , C 5 ) is used.
Input signals are applied to the gate terminals of Q 1 and Q 2 through dc blocking
capacitors C 2 and C 3 . A pair of diodes (D 1 , D 2 ) keeps high-amplitude noise transients
from affecting the operation of the amplifier. They are connected back to back in order
to snub out both polarities of signal.
Tuning capacitor C 1 is used in lieu of a capacitor in the loop; it resonates the loop at
a specific frequency. Its value can be found from the equation given earlier.
The transistors used for the push-pull amplifier (Q 1 , Q 2 ) can be nearly any generalpurpose
JFET device (MPF-102, MPF-104, etc.). A practical approach for many people is
to use transistors from service replacement lines, such as the NTE-312 and NTE-316
devices.
Special Problem for VLF/LF Loops
A capacitance is formed whenever two conductors are side by side. A coil exhibits capacitance
as well as inductance because the turns are side by side. Unfortunately, with
large multiturn loops, this capacitance can be quite large. The distributed capacitance of
the loop self-resonates with the inductance. Loop antennas do not work well at frequencies
above their self-resonant frequency, so it is sometimes important to raise the selfresonance
to where it does not affect operation at the desired frequencies.
Figure 14.17 shows one way to raise the self-resonant point. The turns on the loop
are broken into two or more groups separated by spaces. This method reduces the effective
capacitance by placing the capacitances of each group of wires in series with the
others. The effective capacitance of a series string of capacitors is always less than the
value of the smallest capacitor.
1 = 1 + 1 1
+ ... +
C C C C
TOTAL
1 2
N