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FS-5000 ATU
Automatic Antenna Tuner

As the FS-5000 radio station had to be operated by non-technical operators, a fully automatic antenna tuner was designed as part of the set. It allows a wire of approx. 15 meters long to be used as antenna. The antenna can be a long-wire, a dipole or an inverted-V. The antenna tuner is mounted on top of the transmitter and is located at the rear left of the radio station.

The image on the right shows the antenna tuner. It is a fairly simple device with two banana-type sockets for the antenna at the rear left and a connector at the bottom that mates with a similar socket on top of the transmitter.

Signal and power are obtained from this plug, and some extra signals are present in the connector for communication between the two units. The antenna tuner contains a series of filters that are selected and adjusted under control of a built-in microprocessor, making the tuner autonomous and transparent to the user.
Automatic Antenna Tuner

Apart from the HF output from the transmitter, the main connector also carries a number of test signals that allows a badly matched antenna, and hence a bad Standing Wave Radio (SWR or VSWR), to reduce the power output from the transmitter. This creates a fail-safe situation. During the tuning stage (i.e. when trying to match the antenna) the RF-output power is less than 5mW.

Automatic Antenna Tuner in storage container Automatic Antenna Tuner Bottom view of Automatic Antenna Tuner Connector to transmitter Transmitter and Antenna Tuner stacked together Complete FS-5000 radio station Antenna connection on top of the Antenna Tuner Push for wire-contact

The antenna tuner is a fully autonomous device with its own logic. It can be accessed from the top, by removing the top panel, and contains a single PCB that fills the entire interior. The PCB is roughly divided in two parts: the matching coils at the left and the digital control at the right.

The left half is the actual antenna matcher that consists of a series of coils and capacitors that are selected by no less than 22 relays. The relays are controlled by a series of SG2003 (ULN2003) transistor arrays on the right half. Feedback signals are provided to the transmitter, to allow output power reduction in case of a mis-match.

The right half of the PCB contains the control logic. At the heart is a National Semiconductor NCS-800 microprocessor. This mil-spec Z-80 compatible CPU [1] is also used in the DSU. The firmware is contained in an EPROM on its left.
Antenna Tuner interior

As a lot of control lines are needed, two NSC-810 I/O expanders are used. These peripherals each have three 8-bit I/O ports, some timers and 1024 bits of static RAM [2]. The smaller chip next to the EPROM is an MM82PC12 8-bit I/O port [3].

Antenna Tuner interior Antenna Tuner interior Antenna Tuner analogue (left) and digital (right) control Antenna matching circuits Antenna matching circuits Antenna output stage Analogue control Digital control

  1. National Semiconductor, NSC800 High-Performance Low-Power CMOS Microprocessor
    June 1992. Retrieved April 2012.

  2. National Semiconductor, NSC810 RAM - I/O - Timer
    June 1992. Retrieved April 2012.

  3. National Semiconductor, MM82PC12 8-bit Input/Ouput Port
    1995. Rev. July 1987. Retrieved April 2012.
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