Practical_Antenna_Handbook_0071639586
534 p a r t V I I : t u n i n g , T r o u b l e s h o o t i n g , a n d D e s i g n A i d s As the second set of bullets makes clear, it is generally preferable to locate the ATU at the antenna. However, at low frequencies, low power levels, and/or for relatively short distances between the transmitter output and the antenna, an ATU at the transmitter can provide acceptable results (except with respect to keeping lightning out of the radio shack). Today most commercially available transceivers sport an internal ATU—either standard or as an option. When operated “barefoot” (i.e., with no amplifier) and with short feedlines (such as mobile or portable installations) on HF, this configuration is compact and provides nearly as efficient power transfer to the antenna as a separate box located at the antenna would. In many stations, however, the approach is to incorporate matching devices at both ends of the system transmission line—that is, to locate fixed-Âtuned impedance-Âmatching networks at the antenna feedpoint to get the SWR on the system transmission line down to a manageable level and a tunable ATU back at the transmitter and receiver end of the main transmission line to complete the task of presenting the transmitter with a matched load, especially when the operating frequency is apt to be varied by a modest amount— typically no more than a few percent. Often, the impedance-Âmatching network at the antenna is a simple balun or a distributed network, such as the transmission line transformers described in the second half of Chap. 4. Occasionally fully tunable ATUs are located at or near the antenna end of the system transmission line. In that case, they are often capable of being tuned remotely, or the intended operating range is over such a small percentage bandwidth that adjustment of the ATU is required infrequently or never. An example of such a configuration is found in Fig. 8.2; open-Âwire line (which is the best choice when the possibility of high SWR on the line exists) connects the center of a multiband dipole to a tunable ATU directly beneath it. The ATU is adjusted to provide minimum SWR on the coaxial transmission line that comes from the station equipment. It may also incorporate RF chokes to drain static buildup from the antenna and OWL, or remotely actuated mechanical contactors to directly ground the OWL and dipole when they’re not in use. An even more compelling case for a remote ATU is the bobtail curtain of Chap. 10. Since the feedpoint for this array is at a high-Âimpedance point, the SWR on any common coaxial cable back to the transmitter would put the cable at risk of destruction when substantial power levels are used. Any attempt to run open-Âwire line over that distance could materially degrade the antenna pattern. Locating a tunable ATU at the base of the antenna, as shown in Fig. 10.11, is by far the best possible approach. ATU Circuit Configurations While it is tempting to treat the ATU as a black box, in truth there are many different kinds of impedance-Âmatching circuit configurations, each with its own list of advantages and disadvantages. At any given operating frequency, the antenna feedpoint impedance, Z ANT , can be represented as a resistive part, R ANT , in series with a reactive part, X ANT : ZANT = RANT + jX (24.1) ANT where X ANT can be either positive or negative. If we could count on R ANT always equaling the system transmission line impedance, Z 0 , we could use a single variable capacitor in series to tune out a positive X ANT and a single variable inductor in series to tune
C h a p t e r 2 4 : a n t e n n a T u n e r s ( A T U s ) 535 out a negative X ANT . Unfortunately, as the operating frequency changes, not only does X ANT vary—often between positive and negative values—but R ANT does so, as well. As explained in Chap. 3, neither R ANT nor X ANT corresponds to a specific resistor, capacitor, or inductor except in the very simplest electronic circuits. The magnitude and frequency dependence of either R ANT or X ANT —and most likely both—are, in general, complicated functions of many components in a lumped-Âelement circuit and of many geometrical interrelationships in a distributed circuit such as an antenna. Thus, the task of the successful antenna matching unit or ATU is to be flexible enough to present the system transmission line with an apparent antenna feedpoint impedance of Z ANT = Z 0 = R 0 + j0 over a reasonable range of operating frequencies and a practical range of complex antenna impedances. In the process, the ATU should dissipate as little of the transmitter RF power output in internal losses as possible. As you might expect, various network topologies can do this—but with varying degrees of success. L-ÂSection Network Judging by the number of times it has appeared in print, the L-Âsection network is one of the most used antenna matching networks in existence, rivaling even the pi network. A typical circuit for one form of L-Âsection network is shown in Fig. 24.1A. Values for L and C can be found that will allow this circuit to match all possible load impedances having R 1 , the resistive part of the source impedance, less than R 2 , the resistive part of the load impedance. The circuit can also match some load impedances for the range R 1 > R 2 , depending on the magnitude of X 2 , the reactive part of the series-Ârepresentation load im- L C R 1 C R 2 R L 1 R 2 R 1 R 2 A R 1 R 2 B L R 1 R C 2 R 1 R 2 C Figure 24.1 (A) L-Âsection network. (B) Reverse L section. (C) Inverted L-Âsection network.
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C h a p t e r 2 4 : a n t e n n a T u n e r s ( A T U s ) 535<br />
out a negative X ANT . Unfortunately, as the operating frequency changes, not only does<br />
X ANT vary—often between positive and negative values—but R ANT does so, as well. As<br />
explained in Chap. 3, neither R ANT nor X ANT corresponds to a specific resistor, capacitor,<br />
or inductor except in the very simplest electronic circuits. The magnitude and frequency<br />
dependence of either R ANT or X ANT —and most likely both—are, in general, complicated<br />
functions of many components in a lumped-Âelement circuit and of many geometrical<br />
interrelationships in a distributed circuit such as an antenna. Thus, the task of the successful<br />
antenna matching unit or ATU is to be flexible enough to present the system<br />
transmission line with an apparent antenna feedpoint impedance of Z ANT = Z 0 = R 0 + j0<br />
over a reasonable range of operating frequencies and a practical range of complex antenna<br />
impedances. In the process, the ATU should dissipate as little of the transmitter<br />
RF power output in internal losses as possible. As you might expect, various network<br />
topologies can do this—but with varying degrees of success.<br />
L-ÂSection Network<br />
Judging by the number of times it has appeared in print, the L-Âsection network is one of<br />
the most used antenna matching networks in existence, rivaling even the pi network. A<br />
typical circuit for one form of L-Âsection network is shown in Fig. 24.1A. Values for L and<br />
C can be found that will allow this circuit to match all possible load impedances having<br />
R 1 , the resistive part of the source impedance, less than R 2 , the resistive part of the load<br />
impedance. The circuit can also match some load impedances for the range R 1 > R 2 , depending<br />
on the magnitude of X 2 , the reactive part of the series-Ârepresentation load im-<br />
L<br />
C<br />
R 1<br />
C R 2<br />
R<br />
L<br />
1<br />
R 2<br />
R 1 R 2<br />
A<br />
R 1 R 2<br />
B<br />
L<br />
R 1 R<br />
C<br />
2<br />
R 1 R 2<br />
C<br />
Figure 24.1 (A) L-Âsection network. (B) Reverse L section. (C) Inverted L-Âsection network.