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Loop amplifier
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Figure 15-12 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 (JFET) 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 12-V dc power
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314 Small loop receiving antennas
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15-10 Use of a spoiler loop to null an undesired signal
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15-11 A Q-multiplier
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supply A 22- F bypass capacitor is used to put the drain terminals of Q1 and Q2 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 center-tapped transformer, the center tap of which is grounded To keep the dc on the source terminals from being shorted through the transformer winding, a pair of blocking capacitors (C4, C5) is used The input signals are applied to the gate terminals of Q1 and Q2 through dc blocking capacitors C2 and C3 A pair of diodes (D1, D2) is used to keep high-amplitude noise transients from affecting the operation of the amplifier These diodes are connected back to back in order to snub out both polarities of signal Tuning capacitor C1 is used in lieu of the capacitor in the loop and is used to resonate the loop to a specific frequency Its value can be found from the equation given earlier The transistors used for the push-pull amplifier (Q1, Q2) can be nearly any general-purpose 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
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15-12 A practical loop amplifier
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Special problem for VLF/LF loops
A capacitance is formed whenever two conductors are side by side A coil produces 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 causes a self-resonance with the inductance The loop does not work well at frequencies above the self-resonant point, so it is sometimes important to raise the self-resonance to a point where it does not affect operation at the desired frequencies Figure 15-13 shows a solution that raises the self-resonant point The turns are broken into two or more groups and separated by a space This method reduces the effective capacitance by placing the capacitances of each group of wires in series with the others
15-13 Raising the self-resonant point
318 Small loop receiving antennas
Coaxial-cable loop antennas
One of the more effective ways to make a shielded loop is to use coaxial cable Figure 15-14 shows the circuit of such a loop Although only a single-turn loop is shown, there can be any number of turns One reader made a 100-kHz LORAN (a navigation system) loop using eight turns of RG-59/U coaxial cable on an 8-ft diameter Note the special way that the coaxial cable is connected This method is called the Faraday connection after the fact that the shield of the coax forms a Faraday shield At the output end, the center conductor of the coaxial cable is connected to the center conductor of the coaxial connector The coax shield is connected to the connector ground/shield terminal At the other end of the loop, the shield is left floating, but the center conductor is connected to the shield Note very carefully that the center conductor at the far end is connected to the shield at the connector, not at just any convenient point
15-14 A coaxial-cable shielded loop
Small transmitting loop antennas
Small transmitting loops are those which are less than one-third wavelength ( /3) according to Hart (1986) or between 004 and 010 according to Belrose (1993) Other sources claim that loops up to about 020 are small The key characteristic is that the current is equal in all points of the loop rather than varying with loop length the way the current in a large loop (or other antenna) does One of the parameters of the small loop antenna is a very low radiation resistance (it is a function of the area of the loop) The radiation resistance of the small loop antenna is less than 1 and often considerably less than this figure (eg, 005 ) This means that the copper or aluminum losses of the antenna must be kept very, very low For such reasons, it is common to assemble the small transmitting loop antenna from 1- to 4-in copper or aluminum pipe For this reason, the square (Fig 16-1A) or octagon (Fig 16-1B) shapes are preferred The reason this is true is purely mechanical: The square loop can be built with 90 elbow joints, and the octagon can be built with 45 elbow joints For antennas whose circumference is greater than /8, standard 3/4-in copper pipe can be used in construction, but for smallersized loops, larger-sized copper piping is used The small transmitting loop tends to be highly inductive in its reactance Values of inductive reactance to 1000 are not unusual, although closer to 100 is the norm Because of this inductive reactance, the small transmitting loop antenna is tuned by one or more series capacitors A sample loop was modeled by Belrose (1993) It was 1 m in diameter and was made of 254-cm (1-in) copper pipe For this loop, the gains were 10 MHz 14 MHz 21 MHz 30 MHz 288 dBi 022 dBi 114 dBi 142 dBi
Note that the gains were all less than that of a dipole (1215 dBi) but in the upper regions of the frequency spectrum were quite decent Even at 10 MHz, the 1-m
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