ATU 7×7

I have been searching already for a long time for a design of an automatic antenna tuner that can handle 100 Watts. It was a surprise for me that I only recently came across the N7DDC 7×7 ATU that was sold in all kind of combinations (PCB only, PCB with relays and torroids, completely assembled). It is a very compact design with different options for the use of Oled displays and adding buttons for Bypass and Auto tuning.

I ordered the PCB with torroids and some components via ebay. However, the there are many sources selling some or all components or even fully built versions.

The ATU is build around 7 coils and 7 capacitors that can be selected in different configurations depending on the antenna adjustment needed. Maximum inductance is 8.5 uH with 0.1 uH steps and maximum installed capacity is 1870 pF with minimal steps of 10 pF.

The building of the ATU was fun but soldering the SMD components may be a challenge. Still with normal tools it was possible to solder the tiny components. After the soldering was completed, I tested the ATU and worked correctly. I have chosen a nice aluminum case (matching with my SunSDR2 radios) to cover the PCB and display.

The documentation can be found on different web sites and the prime source is Some further information can be found on the web site of R0AEK. What is also not clearly documented is how to connect the two push buttons for “Bypass” and “Auto tuning”. See the picture below where to solder the wires on the bottom of the PCB for these buttons. Connect Auto to RB1 and Bypass to RB2. Also apply a 1 nF capacitor across each button to avoid RFI trouble.

Recently I found the same 7×7 N7DDC tuner (called ATU-100 EXT) on Aliexpress, complete with housing and different OLED display than in my other tuner. Just for fun I purchased this building project and assembled the tuner in just a couple of hours. In the “line up” picture you see the following: 7×7 tuner with home made enclosure, on top of the home built 12V-30A power supply, SunSDR2DX, SunSDR2PRO and ATU-100 EXT.