ATU-100 Automatic Antenna Tuner

Core design based on the open-source project by N7DDC.

Technical Characteristics (Basic Model)

Permissible power supply voltage range: 10-15VDC
Maximum current consumption: 400 mA
Maximum working power: 100W
Maximum possible measured power: 150W
Minimum power required to start tuning: 5W
Minimum possible measured power: 0.1W
Measurement resolution (up to 10 watt hours): 0.1W
Measurement resolution (greater than 10 watts): 1W
Power measurement accuracy error: ± 10%
Maximum installed inductance: 8.53 μH
Minimum installation step of inductance: 0.05 μH
Maximum installed capacity: 1869 pF
Minimum capacitor installation steps: 10 pF
Typical consumption: 100-200 mA

Product Overview

The ATU-100 is an automatic antenna tuner constructed with manual welding. It features a 3.1 program version, an SSD1306 controller, and a 0.91-inch monochrome OLED display with a resolution of 128x32 pixels. The internal circuit comprises 7 inductors and 7 capacitors, enabling operation across a wide frequency range, specifically covering the amateur radio spectrum from 1.8MHz to 50MHz.

Front view of the ATU-100 HF Auto Tuner.

The front panel of the ATU-100 HF Auto Tuner displays a power switch labeled "Power", a button labeled "Reset Tune", and a 0.91-inch monochrome OLED screen. The screen displays "ATU-100 HF Auto Tuner 1.8MHz-30MHz".

Rear view of the ATU-100 HF Auto Tuner.

The rear panel features two SO-239 connectors labeled "ANT" and "TX", and a DC power input socket with a label indicating "2VDC 0.5A".

Internal Structure and Operation

Internal view of the ATU-100.

An internal view reveals the printed circuit board (PCB) populated with multiple relays, the SSD1306 display, and other electronic components. A notable component is described as a fixed hollow inductor and magnetic ring inductor.

The ATU-100 is marked with "Salute To N7DDC", "Ver:1.21 20.12.14", and "Made By BH3SAP".

Operating Instructions

Connect the DC socket to a 10-15V DC power supply, ensuring correct polarity.
Ensure the POWER switch is in the 'open' position.
Set the connected radio's output power to 5W-10W.
To initiate automatic tuning: Long press and hold the RESET/TUNE button. The word "TUNE" will appear on the antenna tuner's screen, indicating that the transmitter's continuous power is being automatically adjusted.
To release previous settings: Short press the RESET/TUNE button. The antenna tuner will disengage the LC relay, returning to a straight-through position.

Modes of Operation

For environmental protection and energy conservation, the antenna tuner enters a screen sleep mode after 10 seconds of inactivity.

Manual Mode: The relay adjusts only after the RESET/TUNE button is long-pressed.
Automatic Mode: The system automatically begins tuning when the standing wave ratio (SWR) is detected to be greater than 1.3:1. This SWR threshold is adjustable.

The black and red terminals on the unit are experimentally provided for adjusting long line antennas.

A microswitch is located below the DC socket. Pressing the A/M button activates AUTO mode, indicated by a "." appearing next to the PWR value on the screen. In this mode, tuning commences automatically when the SWR is detected to be greater than 1:1.3. Pressing the A/M button again deactivates AUTO mode, and the "." will disappear.

Version 1.21 Enhancements and DIY Modifications

Version 1.21 introduces an SX1308 boost circuit, which can be installed in a spare motherboard position. These components must be purchased separately by the customer. The system allows switching between power supply modes via reserved SMT pads and includes an interface for fully automatic mode to facilitate DIY modifications.

Note: Any noise interference resulting from the addition of this DC-DC boost circuit is not considered a product quality issue.

PCB layout and connection points for the SX1308 boost circuit.

The PCB shows connection points for the SX1308 boost circuit. Text indicates that shorting the SMT pads within the red box enables external power supply, with the yellow-bordered pads serving as the switch interface. When the SMT pads within the red box are disconnected, the blue-bordered pads act as the switch interface, and the boost circuit is integrated into the power supply system.

The SX1308 is a fixed-frequency, SOT23-6 enclosed current mode boost converter operating at up to 1.2MHz. This allows for the use of smaller peripheral inductors and capacitors. It features a built-in soft-start function to reduce initial current surges and integrates an 80 mΩ power MOSFET, achieving a maximum boost efficiency of 97%.

Schematic diagram of the SX1308 boost circuit.

The schematic illustrates the SX1308 boost converter circuit. Key components include: L1 (4.7-22μH inductor), D12 (1N5822 diode), R35 (10KΩ resistor), R36 (191KΩ resistor), C16 (47μF capacitor), C17 (47μF capacitor), C18 (104 capacitor), and the SX1308 integrated circuit (U9). The circuit connects to VCC and GND, with an output labeled "12V".