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Automatic antenna tuner PCB
Introduction
The Automatic antenna tuner is designed to interface the 2050 transceiver to either a vertical whip or a
long wire antenna. The tuner is connected to the base PCB via P2.
Tuning Network
The whip antenna is resonant at approximately 23MHz. Below resonance the whip presents a capacitive
reactance which needs to be tuned out with inductance. A binary network of inductors from 0.33µH to
84µH is used to give a total of 168µH. This is required for tuning at the lowest frequency. When the whip is
operated above resonance or there is a long wire connected a series capacitor (C15) and two shunt
capacitors (C21 & C22) are used to present a capacitive load. Any residual capacitance is then removed
by the inductive network.
To minimise the current draw of the unit all relays are of the latching type. Low voltage relays are used to
switch the smaller inductor coils whilst the larger inductance coils which produce the larger voltages
(>500V) have the higher voltage rated DK1A relays to switch them out of circuit. To minimise the contact
voltages the largest inductor is split into two smaller inductances of 42µH (L9 and L10). After a tune is
performed the power supply to the latching relays is removed. This increases the RF impedance to earth
which minimises the chances of a flash over from contact to coil.
Pad and Phase Detector
A -10dB pad is switched in during the tuning procedure by RL16. The pad has the function of reducing the
RF voltage to minimise arcing and to present a resistive load to the transmitter so that it can provide a full
20W during tuning. Whenever the pad is used a phase detector is also switched in. This comprises of a
current sense transformer (T3) and a voltage sense capacitor (C26). These two signals are mixed and
rectified via a diode network and generate a DC voltage whose polarity directly relates to either leading or
lagging load. The signal is converted to TTL by comparator U7A and is sent to the microprocessor.
Current Sense
The usefulness of the phase detector is limited to below quarter wavelength resonance so for tuning above
this a 10:1 current transformer (T1) is employed. The current sensor measures the RF current flowing into
the tuning network. The maximum current into this network should directly correspond to the minimum
transmitter VSWR. The output of the current transformer is rectified and is sampled by the microprocessor.
Impedance transformer
Once any whip or long wire reactance has been cancelled the impedance transformer is used find the best
VSWR.
Two relays (RL14 and RL15) and a tapped inductor (T2) are used to give three different
impedance transforms (1:2, 1:1 and 2:1). This covers an impedance range of 12.5 to 200. The appropriate
transform ratio is chosen depending on which delivers the most current through the current sense network.
Microprocessor
A PIC18F452 microprocessor is responsible for controlling all operations of the tuner. Each relay is driven
by its own output pin from the micro. To interface the 12V relay coils to the 5V logic gates of the micro,
ULN2003 Darlington transistor array ICs are used.
The microprocessor runs from an external 10MHz crystal oscillator which is multiplied within the
PIC18F452 to create a clock signal of 40MHz.
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