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automates this process.
No tuner will fix a bad antenna. If the antenna is far from resonance, the inefficiencies inherent
in such operation are inescapable; it’s simple physics. Much of the transmitted power may be
dissipated in the tuner as heat, never reaching the antenna at all. A tuner simply “fools” the
transmitter into behaving as though the antenna is resonant, avoiding any damage that might
otherwise be caused by high reflected power. For best performance, the antenna used should
always be as close to resonance as is practical.
THE LDG Z-100PLUS
In 1995, LDG Electronics pioneered a new type of automatic antenna tuner. The LDG design
uses banks of fixed capacitors and inductors, switched in and out of the circuit by relays under
microprocessor control. An additional relay switches between high and low impedance ranges. A
built-in SWR sensor provides feedback; the microprocessor searches the capacitor and inductor
banks, seeking the lowest possible SWR. The tuner is a “Switched L” network, consisting of series
inductors and parallel capacitors. LDG chose the L network for its minimum number of parts and
its ability to tune unbalanced loads, such as coax-fed dipoles, verticals, Yagis, and, in fact,
virtually any coax-fed antenna.
The series inductors are switched in and out of the circuit, and the parallel capacitors are
switched to ground under microprocessor control. The high/low impedance relay switches the
capacitor bank either to the transmitter side of the inductor bank, or to the antenna side. This
allows the Z-100Plus to handle loads that are either greater than or less than 50 ohms. All relays
are sized to carry 125 watts continuously.
The SWR sensor is a variation of the Bruene circuit. This SWR measuring technique is used in
most dual-meter and direct-reading SWR meters. Slight modifications were made to the circuit to
provide voltages instead of currents for the analog-to-digital converters that provide signals
proportional to the forward and reflected power levels. The single-lead primary through the center
of the sensor transformer provides RF current sampling. Diodes rectify the sample and provide a
DC voltage proportional to RF power. These two voltages are read by the ADCs in the
microprocessor, and are used to compute SWR in real time.
The relays are powered by the 12VDC input provided by the DC coax jack or the optional
internal batteries. The relays are a latching type, and so they consume no current when not
actively switching. Although the microprocessor’s oscillator runs at 8 MHz, which allows the main
tuning routine to execute in only a few milliseconds, the relays require several milliseconds of
settling time for every combination of inductors and capacitors. Thus, it may take several seconds
before all relay combinations are exhausted, in the case of a difficult tune.
The tuning routine uses an algorithm to minimize the number of tuner adjustments. The routine
first de-energizes the high/low impedance relay if necessary, and then individually steps through
the inductors to find a coarse match. With the best inductor selected, the tuner then steps through
the individual capacitors to find the best coarse match. If no match is found, the routine repeats the
coarse tuning with the high/low impedance relay energized. The routine then fine tunes the
inductors and capacitors.
The microprocessor runs a fine tune routine just after the tuner finds a match of 1.5:1 or less.
This fine tune routine now tries to adjust the SWR as low as possible (not just to 1.5); it takes
