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