AG-430 Antenna Tuner (Zellweger, Switzerland)
Basically the unit is an analog device and needs HF of some 20W and a DC supply of 30V to tune. RF is routed through a balun to obtain 200ohms for a 4-wire dipole which is originally 2x30m long in an inverted-V configuration and was designed to cover 1.6-12MHz. Discriminators then sense the phase and impedance of the antenna and digital circuits control the overall operation. End of tune is signalled by a load pulse on the 30V supply which needs to be detected.
Apart from the need to supply the 30VDC through the 50ohm coax cable with a bias-tee, there is the additional problem of providing the 20W RF through wildly varying SWRs during the tuning cycle. Some radios have tuner output modes for that, but otherwise a power attenuator of 4-6dB has to be temporarily inserted to prevent the protection circuit from reducing the transmitter output power. Such protective action results in errors in the tuner logic and unsatisfactory results. In this context it might be worthwhile to read the notes on the preceding system AGD-415 or the detailed write-up by hb9tka on this matter:
There is room for experiments with other dipole lengths, my installation has a low 2x20m dipole - two wires to 2.5m poles with about 1.5m spacing and interconnected end points - this makes worthwhile difference - and I am able to tune down to 1990kHz. It's close and perhaps 21m would be safer if you have the space. Based on my work with the AGD-415 tuner operation on 20m should also be possible with suitable dipole lenghts.
Some time ago I produced a .pdf file from original document scans which I would like to present here. Not everything is there - no detailed diagrams of most of the circuit cards - but it is certainly a step in the right direction describing its operation in depth.
In the issue 5/13 periodical of the Swiss Radio Amateurs "HBradio" a technical article by hb9tka describing a controller he built for this tuner (in German) can be found. It is on pages 45/46 and includes a circuit diagram (less power supply).
