The K3NG ATU (Automatic Antenna Tuning Unit)

under construction, come back soon.

K3NG is well known in Amateur Radio circles for his projects, using the Arduino platform to develop a number of very useful Amateur radio projects.

One of them, which interested me, was the ATU. K3NG has a web page for the project although it is a little limited in content as he never progressed past his working prototype, built 'ugly' style. (Pictures below) It's fairly unique though, in being a balanced tuner, able to feed for example, a doublet antenna, fed with open wire line. It can also be configured for unbalanced output.

I (VK3PE) have long wanted to build such an ATU but being limited in my Arduino programming knowledge decided I would tackle this project as a good starting point. I wasn't that keen on duplicating it K3NG ugly style but preferred to build with PCB's. The ATU uses a standard configuration of variable L & C values in the usual binary fashion with the Inductors duplicated to form the other arm of the balanced format.

This page then is how I built the K3NG tuner and also a 'QRP' version. I made use of some low cost eBay PCB vendors to make the PCB's for the project.

I found that the most economical size for low numbers of PCB's from vendors is 50 x 100mm in size so I used this size for all of the PCB's. (Except QRP version below) This created a problem though as there are many (20) large relays used in this project which simply will not fit on a single board! So, I decided to split the boards into two types for the capacitors and inductor mounting along with their associated relays. Even that was a problem as I found I could fit no more than four relays on a 50x100mm board for the inductors , and six for the capacitor bank. Some 'smart' thinking was needed. I decided to make the two main boards (caps and inductors) so they could be cascaded together. Sounds simple, but actually not so. More on this later.

Two more boards were required to complete the project, a Control board containing the Arduino "NANO" plus an SWR PCB. The control board has a 2 line x 16 character LCD module connected to it as per the original, although I used a standard 2x16 LCD from my junk pile plus an I2C adapter board from Adafruit™, rather than the K3NG version which used an I2C RGB LCD. The RGB version allows the backlight colour to be changed according to the state of the tune process. My version has optional LED's to show the same thing. The RGB board can still be used. Slight changes are required to the firmware or Sketch as it is known in Arduino programming, to use the LCD that I used. The SWR board uses the Bruene single Toroid type detector, rather than the two Toroid type that K3NG used. I don't anticipate any problems doing this, having done some quick tests at 5w and 120W.

Being a modifier at heart, I also decided to fit an analogue meter on the front panel to indicate SWR, even though the LCD shows it numerically. The meter is driven from an MCP4527 DAC, which is connected to the IIC port of the Arduino, along with the LCD.

Draft schematics: These closely follow K3NG's originals, apart from the SWR board as mentioned above. Pictures are of the prototype boards made.

Capacitor PCB (2 used)

Inductor PCB (4 used) Only two required if built as unbalanced only.

Control PCB Schematic & Overlay

SWR PCB Schematic & Overlay

Interconnection details


Once the PCB's arrived I had to find a case to fit them into. Searching through my pile of old instrument cases, a couple looked promising but soon proved to be a little small, so, it was decided make a case using some "U" channel extrusion I happened to have. The channel is 100mm wide with 25mm sides and 3mm thick material. No doubt there are many other ways to make a case. Front and rear are made from the channel material and joined on top by 8mm round rod and a flat aluminium plate on the bottom.

case pics soon



The inductor and capacitor boards are both similar to the one shown above. They are 50x100mm PCB's. In order to get the required eight inductors (or capacitors) identical boards (apart from values) are connected together using ribbon cable for the control lines and thick wire or copper strap, for the RF connections. Larger toroids and relays than the QRP version are used.



QRP Version?

Even though the project is not finished, my busy mind thought that a QRP version might be a handy thing to build also for my WSPR station. But only unbalanced. As it happened, I found a "special" on eBay for a 100x100mm PCB supplier. This spurred me to the QRP version which was duly drawn up, Gerbers made and sent out to the PCB vendor. It's yet to arrive so this information is general only.

'QRP' L & C BOARD Vers 1.0 VK3PE


The previously mentioned Control and SWR boards are unchanged from the 100W version and used in the QRP version also.

This time though I found a nice aluminium case 105 x 125 x 75mm in size which looked perfect to house the project. Of course, being QRP some components can be much smaller, eg relays and Toroids are sized accordingly and my gut feel is that it should be good for about 20W. (The full K3NG version is rated around 100W) Like all ATU's though, tune up should initially done at lower power settings from the Tx. eg 5W.


SWR head for the higher power version only. (see below)


Below, is the QRP version now completed. In testing I found that the Bruene SWR head was not sensitive enough for QRP work so I made a PCB at home using the Stockton Bridge, on a BALUN core, with the outputs fed into two AD8307 Log Amp/detector chips. This works very well and I can tune with about 200mW of RF power now. It did entail some changes to the Arduino Sketch, as the calculations are different. Fairly minor though. The front panel uses an LCD module per original K3NG although in this QRP version, the display is in 4 bit parallel mode rather than using an I2C adapter board. To the right, you can see most of the internals with the SWR bridge and AD8307's on the DIY board to the left, the main L&C board at the bottom of the case and the Arduino control board behind the front panel. If you look carefully, you can see the Arduino NANO board. {the picture is shown during testing, and a 50R load is connected on the output of the SWR board. } The switches on the front panel are Reset, Lock and Manual.

The display shows current SWR, frequency in KHz on top line, L & C values used and Hi or Lo Z antenna on bottom line.


As above the QRP version uses a 100x100mm PCB on which are the required 8 inductors (in a binary sequence from 0.08uH to 10.4uH) giving a total of about 22uH. The capacitors run from 12pF to 1200pF also in a binary sequence as is usual for an ATU controlled by a Micro. Each Inductor and capacitor needs to be switched in our out, using a relay for each. In addition extra relays are required to connect the capacitor bank to either the input or output side of the Inductors. The input and output correspond of course to the Transmitter input and the antenna port. The tuner takes the form of the usual "L" type match. The balanced version described above uses an extra set of identical Inductors, to make the ATU balanced.

The relays used in the QRP version can be smaller due to the lower currents and voltages encountered with low power. A common size of relay used known as a DIP relay. There are many brands available for these but the prototype here uses cheap relays from eBay known as "HK-19F-DC" 12volt relays are used in the QRP version of the ATU. {not in the pictures though}

For the capacitors in both versions, I used SMD parts of 3kV rating, available from Farnell/Element14™, RS-Components™ and no doubt, other suppliers like Mouser™ or Digikey™.

The control PCB is identical to the higher power version though, so a full display and even analogue meter could be used. For the case I have in mind to use though, space is very limited so no 'SWR' meter is fitted.

Schematic QRP board

Board overlay

Interconnection diagram

Control PCB (see link above)

SWR PCB (see link above)

<<< above links coming soon >>

These are K3NG's prototype and his pictures, borrowed from the WWW. I hope he doesn't mind. There is a Yahoo group "Radioartisan" that covers his various projects. I am a member of that group. The display shows final SWR (top left) the inductor and capacitor values and the approx. frequency. (The ATU remembers past tune results)


Firmware K3NG

Original K3NG Schematics

K3NG Youtube video demo

Yahoo Radioartisan group.

Adafruit I2C adapter board for standard 2x16 LCD modules

Arduino. There are a number of Arduino boards, but I used the NANO version, available either as an original part or copies from many ebay vendors.



This page was created on 4th Sept 2016 by VK3PE

Last updated on November 4, 2016