Practical_Antenna_Handbook_0071639586
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Practical_Antenna_Handbook_0071639586

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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

356 Figure 14.16 Practical loop amplifier.

356<br />

Figure 14.16 <strong>Practical</strong> loop amplifier.

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