I loved that remote controlled ATU for my Dipole 2 x 27 m with
ladderline. But I had the day when it would be in need of repair
always in my mind. The only item I have been missing was a real
With my experiences with the remote controlled Z-Match and all the
good words about the S-Match by PA0FRI it was time to try!
I used a variometer inductor and a variable capacitor.
To turn the axes I borrowed the two servo motors from the Z-Match.
Some new code for the controller and the whole S-Match Tuner was
ready to work for me.
Meanwhile I have made changes to the controller. A Teensy ++2.0 is
in the box now. With the new code the controller can get the frequency
from K3S. After checking a memory with presets the inductor and the
capacitor is moving to the positions for best tuning. That’s comfortable!
But I can’t tune my double zepp antenna on 30m and on 160m either.
Due to that reason I decided to build a new S-Match ATU design.
The controller unit in the shack has a Teensy ++2.0 microcontroller
board a LCD display and 2 rotary encoder. It is connected with the
interface in the ATU through a patch cable via RS 485.
There is an ARDUINO Nano board sitting on the interface board.
The Nano is listening to the Teensy and switches the relays (8)
on the capacitor board and the relays (8) on the inductor board.
C min = 25 pF C max = 4350 pF
L min = 90 nH L max = 24 µH
see the circuit diagrams coming up here a little later
I have designed four PCBs and ordered these boards at JLCPCB
manufacturer in China. The Capacitor Board is ready so far.
Next will be the controller PCB and the interface board.
The second PCB (controller) and the third PCB (interface)
are ready now and the code for the Teensy as well as for the
ARDUINO NANO is tested. At least I have finished the last PCB
(INDUCTOR BOARD) and could start “On The Air – Tests”
Well, what can I say? Whenever HF comes close to low level signals
some interference is usual. But as an active HAM you have to accept
the challenge or search for another hobby.
With some blocking capacitors and some chokes I could eliminate
the interference so far. The tuner works to my needs with 100 Watts
What is coming next? I will test the tuner with more power.
After some QRO and a few contests the coupler is still doing a good
job and it is time to put all boards and the transformer into a box.
See some slides of the status quo …
click here …