LDG YT-100 Operation Manual - page 20
PAGE 20
antenna at all. A tuner simply “fools” the transmitter into behaving as though the antenna were 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 YT-100
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 YT-100 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 CAT interface cable. 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 program checks LC combinations to see if a 1.5:1 or lower SWR can be obtained and
stops when it finds a good match.
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 the SWR as low as possible (not just to 1.5); it takes about half a second to
run.