LDG Z-11ProII Operation Manual - page 22
PAGE 22
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-11ProII 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 DC input or 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 32 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 reduce the SWR as low as possible
(not just to 1.5); it takes about half a second
to run.