Practical_Antenna_Handbook_0071639586

CHAPTER 24
Antenna Tuners (ATUs)
The primary task of an impedance-Âmatching or antenna tuning unit (ATU) located
at the transmitter end of the transmission line is to present the transmitter output
stage with an apparent antenna feedpoint impedance (after transformation
through an arbitrary-Âlength transmission line) equal to the output impedance of the
transmitter. This results in maximum power transfer from the power amplifier stage of
the transmitter to the feedline and antenna. So located, a properly designed ATU provides
the station owner with multiple benefits:
• It minimizes the standing wave ratio (SWR) that the transmitter or power
amplifier—whether vacuum tube or solid-Âstate—sees. RF power amplifiers are
not tolerant of high SWR; when operating into a mismatched load, expensive
tubes, transistor(s), and other components can be destroyed instantly.
• It minimizes the occurrence of reduced output power triggered by power
amplifier “fold-Âback” protection circuitry when a high VSWR is sensed.
• It facilitates operation of the transmitter final amplifier stage at maximum
efficiency.
• It provides additional attenuation of out-Âof-Âband emissions, including
harmonics generated in, or amplified by, the amplifier.
• If left in the line to the receiver during “key up” periods, it helps protect the
receiver from overload caused by strong out-Âof-Âband signals.
If, on the other hand, the ATU is located at the antenna end of the transmission line,
it not only accomplishes all those benefits listed here but it also transforms the feedpoint
impedance of the antenna to the system impedance or principal station transmission
line impedance—totally eliminating the reactive part of the feedpoint impedance
and presenting the transmission line with an R RAD that is identical to Z 0 . Thus, an ATU
located at the antenna provides the user with the following additional benefits:
• It minimizes the occurrence and amplitude of high-Âvoltage standing waves
along the line that can destroy the transmission line and any associated switches,
relays, or other components when high power is employed.
• It minimizes additional, or mismatch, signal loss on long transmission lines
relative to the loss that is present with line SWRs greater than 1.0:1.
• It can include additional protection for station electronics (receivers, transmitters,
accessories, etc.) from lightning-Âinduced events, serving as a first line of
defense in preventing voltage or current surges from entering the radio room.
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