This ATU consists of a remotely-mounted balanced matching unit controlled by a PC via a serial data link. The serial data is converted from RS232 to differential RS422 to increase its immunity to RF. Band information is read by monitoring the PC's LPT port. Whenever a logging program changes band and writes to the LPT port, the software reads this and sends the appropriate data to the Tuner. Antenana matching is initially performed by manually tuning up the antenna from the computer and saving the settings to hard disk.
In recent years in SSB Field Day, we've used an SGC Smartuner feeding a double-extended Zepp. This has served us well and we won the Restricted Section using it in 2002. However there are some shortcomings and this ATU was developed to address them.
- Large high-Q inductors to minimise loss on 80m. The double extended Zepp has low radiation resistance on 80m. High-Q ATU inductors give lower loss.
- Direct control from operator's PC, i.e., no auto-tuning. Allows fast band-changing in contests.
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The ATU and choke balun being mounted on the tower for SSB Field Day 2003.
The complete ATU and Controller box.
Antenna tuning unit showing large high-Q inductors in a balanced Pi-tank configuration.
View of the ATU showing power and data connectors. The tuner serial interface is in the small die-cast box in the bottom.
The "controller" box connects to the operator's PC. Inputs are DC power & RS232 serial data on a DB9 connector. Outputs are DC power & RS422 differential data on 5-pin XLR connectors.
The "tuner" serial interface mounts inside the remote ATU. The serial data is fed into a Basic Stamp micro-controller. 21 data bits are fed to three 8-bit shift registers. The outputs are buffered by ULN2003's to drive the relays.
Testing the serial interface with the 50m power & data cables.
The tuner interface box being tested with the LED diagnostic boards.