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Home Radiofrequency Transmission Lines Impedancematching Techniques Coupling Networks of Lumped Reactances  
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Coupling Networks of Lumped ReactancesAuthor: Edmund A. Laport This method of coupling is customary in the range of low and medium frequencies where the use of selfmatching line techniques is impractical because of cost. Such networks may be designed for almost any desired impedance match and phase difference. The synthesis of electrical networks 5253 can be quickly solved graphically by the methods described in Chap, 5.
The elimination of advanced mathematics places this method within reach of any radio technician.
Figure 4.31 shows an arrangement for coupling a lowfrequency antenna to a feeder, where the feeder is electrically short and merely acts as additional shunt capacitance across the antenna terminals. Tuning is performed at the input end of the feeder where it is coupled to the transmitter. With this connection the input impedance will vary as the same antenna is used for different operating frequencies.
Figure 4.32 shows a feederterminating and antennacoupling network where the antenna series inductor does not completely neutralize the antenna reactance but leaves a value of capacitive reactance which, when tuned to parallel resonance with a parallel inductor, gives a resistive impedance of a value that will match the impedance of an unbalanced feeder of any prechosen value.
The advantage of this circuit is its easy adjustability to a desired resistance value over a range of operating frequencies with the same antenna. This permits the transmitter to work into a fixed resistive load at all frequencies. Furthermore, the selectivity of the terminal network is nearly the same as that of the antenna itself, assuming the use of highQ inductors in the circuit. The vector diagram of Fig. 4.32B exhibits these conditions. From this vector diagram one can see immediately how the transformation is made. The antenna current I_{0} flowing through the antenna impedance R_{a}jX_{a} (Fig. 4.32A) produces the potential drop V_{0R} = I_{0}R_{a}, in phase with I_{0} , and V_{0X} = I_{0}X_{a} lagging 90 degrees. Their vector sum is V_{0} in a direction that lags I_{0}. Voltage V_{2} is across the inductor L_{p} and sustains the current I_{1} through it. The direction of I_{1} must lag that of V_{2} by 90 degrees, and the reactance of L_{p} is varied until the vector sum of I_{0} and I_{1} is in phase with V_{2}. This makes V_{2}/I_{2} a resistance. By adjusting L_{s} and L_{p} the input impedance to the antenna coupling network can always be made to be a resistance that will match a transmission line of characteristic impedance Z_{0} provided that Z_{0} > R_{a} It is interesting to indicate at this point that if V_{1} > V_{0}x, V_{2} then leads I_{0}. To attain a resistive input impedance to the network, it is then necessary that L_{p} be changed to a capacitance. The vector conditions for this case are shown in Fig. 4.32B by the dotted lines in the upper part of the diagram. The use of an inductor as shown in Fig. 4.32A is preferable to using a capacitor in place of L_{p} for the following reasons: The energy storage in the coupling network is less and therefore has less selectivity (desirable in cases where bandwidth is important). It is sometimes more convenient and economical to use a variable inductor than a variable capacitor (at high power and at low frequencies). The use of a parallel inductor provides a conductive path to ground which serves as a static drain. To make this adjustment in practice, one can preset an impedance bridge for balance at a resistance of Z_{0} ohms, the feeder characteristic impedance. Then the bridge is connected to the input terminals of the coupling network, and L_{p} and L_{s} are manipulated until the bridge is again balanced. The settings of taps and variometers may then be logged and the operation repeated for another frequency. For fixedfrequency operation, permanent connections are made and checked again with the preset impedance bridge. Exact balance is obtained by adjusting the position of the leads themselves by flexing slightly. At exact balance, the movement of the antenna in the wind and the effect of fog and moisture on the system are easily detectable with the bridge. For still more perfect adjustment, the bridge can be connected to the input to the feeder and adjustments made for a specific resistance value at that point. The coupling adjustments between feeder and poweramplifier anodes can also be made precisely by presetting the bridge to the correct operating value of anode resistance at the sockets and adjusting the poweramplifier circuits until balance is obtained at the anodes. The use of this coupling circuit permits the poweramplifier output circuits to be designed for working into a fixed value of resistance at all working frequencies. This enables the transmitter designer to use the coupling circuit of Fig. 4.33 between feeder and tank circuit with excellent harmonicsuppression properties, a minimum number of components, and satisfactory tuning flexibility. Referring now to the more general ranges of antenna impedances such as those encountered with mediumfrequencybroadcast nondirective and directive antennas, one may be required to match almost any antenna impedance whose resistance may be larger or smaller than Z_{0} and whose reactance is positive or negative in any degree.


Home Radiofrequency Transmission Lines Impedancematching Techniques Coupling Networks of Lumped Reactances 