The simplest LC impedance matching network is the L-network. Figure 6-40 shows its four variations that have both an inductor and capacitor. (There are four additional variations that either have two inductors or two capacitors, but they are much less common.) The choice of circuit to be used is determined by the ratio of the two impedances to be matched and the practicality of the component values that are required.
The L-network in Figure 6-41 will transform to 50 Ohm any higher impedance presented at the input to the feed line. (At least it will if you have an unlimited choice of values for Land C.) Most antennas and feed lines will present an im¬pedance that can be matched with an L-network.
To adjust this L-network for a proper match, the coil tap is moved one turn at a time, adjusting C for lowest SWR at each step. Eventually a combina¬tion should be found that will give an acceptable SWR value. If the impedance at the input to the feed line is lower than 50 Ohm, the circuit can be "turned around" to reverse the transformation ratio. Matching networks made entirely of inductors and capacitors work equally well in either direction!
The major limitation of an L-network is that a combination of inductor and capacitor is normally chosen to operate on only one frequency band because a given LC combination has a relatively small impedance-matching range. If the operating frequency varies too greatly, a different set of components will be needed.
Pi and Pi-L Networks
Most tube-type amplifiers use pi-network output-coupling circuits as shown in Figure 6-42. The most common form of this network consists of one capacitor in parallel with the input and another capacitor in parallel with the output. An inductor is in series between the two capacitors. The circuit is called a pi-network because it resembles the Greek letter pi - if you use your imagination a bit¬with the two capacitors drawn down from the ends of the horizontally drawn inductor.
To adjust the pi network in a power amplifier for proper operation. the tuning capacitor (CL) is adjusted for minimum plate current. and the loading capacitor (C2) is adjusted for maximum permissible plate current. The adjustments are interactive. so this procedure is usually performed several times to reach the optimum settings.
You can convert the L-network of Figure 6-41 into a pi-network by adding a variable capacitor to the transmitter side of the inductor. This effectively creates two L-networks back-to-back with each L-network sharing half of the series inductance. Using this circuit, any value of load impedance (greater or less than 50 Ohm) can be matched using some values of inductance and capacitance, so it provides a greater impedance-transformation range.
Because of the series coil and parallel capacitors, this circuit acts as a low-pass filter to reduce harmonics, as well as acting as an impedance-matching device. (A pi-network with two coils shunted to ground and a series capacitor would make a high¬pass filter, and is virtually never used as an amateur output-coupling circuit.) Harmonic suppression with a pi-network depends on the impedance-transformation ratio and the circuit Q. While the L-network's Q is fixed as a consequence of the frequency and impedance transformation ratio, the pi-network's Q can be adjusted by selecting different combinations of component values. Circuit design information for pi-networks appears in The ARRL Handbook.
If you need more attenuation of the harmonics from your transmitter, you can add an L-network in series with a pi-network, to build a pi-L-network. Figure 6-43 shows a pi-network and an L-network connected in series. It is common to combine the value of C2 and C3 in single variable capacitor, shown in Figure 6-43B as C4. The pi-L-network thus consists of two series inductors and two shunt capacitors. The pi-L-network provides the greatest harmonic attenuation of the three most-used matching networks - the L, pi and pi-L-networks.
What is a Pi-L network, as used when matching a vacuum-tube final amplifier to a 50-ohm unbalanced output?
A. A Phase Inverter Load network
B. A network consisting of two series inductors and two shunt capacitors
C. A network with only three discrete parts
D. A matching network in which all components are isolated from ground
What is one advantage of a Pi matching network over an L matching network?
A. Q of Pi networks can be varied depending on the component values chosen
B. L networks can not perform impedance transformation
C. Pi networks have fewer components
D. Pi networks are designed for balanced input and output
What advantage does a Pi-L-network have over a Pi-network for impedance matching between the final amplifier of a vacuum-tube type transmitter and an antenna?
A. Greater harmonic suppression
B. Higher efficiency
C. Lower losses
D. Greater transformation range
Which of the following describes how the loading and tuning capacitors are to be adjusted when tuning a vacuum tube RF power amplifier that employs a pi-network output circuit?
A. The loading capacitor is set to maximum capacitance and the tuning capacitor is adjusted for minimum allowable plate current
B. The tuning capacitor is set to maximum capacitance and the loading capacitor is adjusted for minimum plate permissible current
C. The loading capacitor is adjusted to minimum plate current while alternately adjusting the tuning capacitor for maximum allowable plate current
D. The tuning capacitor is adjusted for minimum plate current, while the loading capacitor is adjusted for maximum permissible plate current
How are the capacitors and inductors of a low-pass filter Pi-network arranged between the network’s
input and output?
A. Two inductors are in series between the input and output and a capacitor is connected between the two inductors and ground
B. Two capacitors are in series between the input and output and an inductor is connected between the two capacitors and ground
C. An inductor is in parallel with the input, another inductor is in parallel with the output, and a capacitor is in series between the two
D. A capacitor is in parallel with the input, another capacitor is in parallel with the output, and an
inductor is in series between the two
A T-network with series capacitors and a parallel (shunt) inductor has which of the following properties?
A. It transforms impedance and is a low-pass filter
B. It transforms reactance and is a low-pass filter
C. It transforms impedance and is a high-pass filter
D. It transforms reactance and is a narrow bandwidth notch filter
Which of the following is the common name for a filter network which is equivalent to two L networks back-to-back?