Generally Automatic Atu are very expensive.
However the atu is composed essentially of relays commanding inductors and capacitors. Most of the time the L match circuit is used. For several months I have been thinking about a way to make one around an Arduino and using economic cards. It will not be necessary to spend more than 70$ for an ATU of 100W.
In addition it can be installed outdoors and controlled remotely by an rs485 link
- Modular design.
- Up to 100W or more (not tested)
- Tandem match detector
- Arduino nano CPU.
- Remote controller by RS485
- Balun integrated symmetric and asymmetric output
- Can be installed outdoors
- SWR and POWER meter.
- Ajustable trigger power (lower than 1W)
- Simple protocol between ATU and remote control
- Cost 70$ max without box
The ATU is based on modular cards:
- 12V Relais x8 (two cards)
- 12V Relais x2
- 4×20 LCD display.
- PCF8574 expander.
- Specials PCB for detector
Pcb available mail me : email@example.com
Antenna tuning unit:
The L matching network is a classical design in amateur radio.
Prototyping an ATU is not easy nor cost efficient with spare parts.
The trick was to use eight 12V relays manufactured modules for Arduino. They are optocoupled insulated and have screw terminal blocks, reducing cost and assembly time.
They can be found on online merchant sites. (atu-ebay-links)
Those relays are enabled at 0 logic Level.
12V input is used for relays power
5V input is used for the optocoupler and led
One is used to switch eight inductors and another to switch eight capacitors (another relay module is high/low impedance switcher and symmetric / asymmetric inverter)
To prevent HF return, you need to weld capacitor on each transistor
Values of inductors and turns depending of toroid for QRO:
|µH||Tore and turns QRO||Capacitors|
|L1||0,07||4 turns 10mm diameter on air||10pF|
|L2||0,18||6 turns 10mm diameter on air||20pF|
|L3||0,3||5 turns T94-2||40pF|
|L4||0,6||8 turns T94-2||80pF|
|L5||1.2||12 turns T94-2||160pF|
|L6||2.2||13 turns T106-2||320pF|
|L7||4.4||19 turns T106-2||640pF|
|L8||10||29 turns T106-2||1280pF|
Values of inductors and turns depending of toroid for QRP:
You can use other toroid models and calculate them with Mini Tore Calculator from DL5SWB for example.
If ATU being used at low power, capacitors voltage is not critical.
Directly soldering inductors and capacitors on relay cards, placing the two cards in front of each other, making connections with wires.
You can use two types of card
An OM made with MCP23008 or manufactured modules like PCF8574 expander
MCP23008 expander :
Two I²C I/O expanders drive all relays.
12V power supply is plugged to both J21 terminal blocks, 5V power supply comes with the I²C bus.
Take care of J19 jumper position setting the I²C address.
Inductors relays card: J19 2-3 position, address 0x20
Capacitors relays card: J19 1-2 position, address 0x21
If you want to use another I²C address, change line in source code here
#define mcpLRAdr 0x20 //adresse i2c mcp23008
#define mcpCRAdr 0x21
PCF8574 expander :
This expander is easy to use, cheaper, but be careful to the I²C address
There was 2 PCF8574 package
PCF8574 Address range is 0x20-0x27
PCF8574A Address range is 0x38-0x3F
In my case I use 2 types
#define pcfLRAdr 0x38 //address i2c PCF8574AT
#define pcfCRAdr 0x20 //address i2c PCF8574T
You must add 2 wires to powering 5V to the relays card. Interrupt input isn’t used.
If is necessary add some decoupling capacitor on 5V power like the scheme of the MCP23008
A dedicated printed circuit has been specially designed to facilitate assembly of the coupler
RF transformer is made with 3 T50-2 side by side and 25 turns (0.6mm wire)
Two AD8307 is added on this board. This avoids the use of diodes and one can measure small powers. So we can use the ATU in QRP.
You need to insert -14 dB attenuator between coupler and AD8307. Indeed AD8307 can be measure power lower than 17dbm (50mW). Here is the equivalent circuit to calculate the attenuator if you want to change the gain.
Arduino PCB :
Another dedicated printed circuit has been specially designed to facilitate assembly of the Arduino nano.
Two PCB is need. One for the ATU and another for the remote control.
Schematic is classical. An RS485 module is inserted on board. The LM385-2.5V is soldering for VREF ADC converter. Q1 and Q3 is used to control Green and Red external LED. That inform user can be transmit (LED green ON) or not (LED red ON).
You don’t need to sold LM385, Q1, Q3, C1 and C3 for the remote control. Just add lcd 4×40 char with his I2C expander. Don’t forget a 5V regulator.
To configure parameters of ATU from the remote control, connect a little keypad. The output delivers a voltage corresponding to the pressed key.
If you want to use ATU without remote control, you need to learn how to configure it by serial terminal using a specific protocol.
Arduino firmware programming :
Use ATUV0.8 for ATU or remoteV02 for the remote control from the following link:
Specific protocol and Testing ATU:
Tuning search is made in two passes:
A coarse search using each capacitor for each inductor in high and low impedance.
A fine search using combinations of inductors and capacitors around the coarse search.
Use HTerm on ATU (usb on NANO directly or USB dongle on RS485 line)
If you use usb on NANO directy,RS485 adapter (on ATU) need to be disconnected to prevent serial bus error.
ATU Commands list : (from serial terminal to ATU)
b00/01 : symetric or asymetric
w22 : swr trigger x10 (22 = 2,2)
u00/01 L/C UP
d00/01 L/C Down
c00 record config
m47 tandem coupler correction (in dBm)
n00 force search
v00/01 auto save (auto save LC config)
i00 init eeprom (set config as default)
t30 trig level Power
r00 read config
ATU Commands list : (from ATU to serial terminal)
S45 : swr x10 (45= 4,5) 99 max
R00 : Start coarse search
F00 : Start fine search
L(byte) : inductors values
C(byte) : capacitors values
H(byte) : hzlz state
P 000/999 : x10 power level (ex : 455 = 45,5W)
B 00/01 balun state
W 22 swr trigger
M 47 m47 tandem coupler correction (dbm)
V 01 save auto state
T 30 power level trigger
With hterm, send ATU Commands list in lowercase
You could check all relays and force searching tuning
Making this ATU was a good learning path either on hardware and software aspects.
Antenna tuning unit conception was challenging to reduce cost and footprint and also to integrate it in the little programming space available in the Arduino.
I am deeply involved in programming search algorithm and RS485 protocol almost from scratch.
It was a good opportunity using AD8307 logarithmic amplifiers for SWR measures and calculation in the QRP/QRO environment.
This ATU will be improved in the future with the prevision of Java oriented multi-platform configuration software
73 from Anthony F4GOH