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Antenna Tuning Unit

I discovered this ATU recently and it looked interesting.

ATU (Automatic Antenna Tuner) developed by N7DDC.

  • Power supply range: 10 - 15 VDC
  • Max current : 450mA
  • Max working power: 100 watts
  • Max measured power: 150 watts
  • Minimum power for tuning start: 1 watt
  • Recommended maximum power while tuning not above 30 watts. (after tuning you can set 100 watts and work on this power)
  • Minimum measured power: 0.1 watt
  • Step for measurement on powers under 10 watts: 0.1 watt
  • Step for measurement on powers above 10 watts : 1 watt
  • Power measurement accuracy : 10%
  • Maximum inductance set: 8.5 uH
  • Minimal step for setting inductance: 0.1 uH
  • Maximum installed capacity: 1870 pF
  • Minimal step for setting capacity: 10 pF

100 Watts tuner, 7x7 (7 capacitors x 7 coils) based on PIC 16F1938.

Last updated on September 5, 2020



The ATU design is by N7DDC and so far, I have not found a lot of information about it, but there appears to be a number of them available on eBay. Also Aliexpress.

Search uTube for "ATU-100" for many videos on this ATU. Make sure you look at the 7x7 versions which is what I have here. ie 7 capacitors and 7 inductors.

A fully built unit here for example.

N7DDC' Github https://github.com/Dfinitski/N7DDC-ATU-100-mini-and-extended-boards

I purchased this kit though for about US$30 and it arrived in a few weeks.


Sept 2020

It's the middle of Covid-19 lockdown here so this seemed to be a project that could be built and be handy.

From what I can tell there are two basic versions of his design. eBay though seems a bit hit and miss as to what you actually get. So choose carefully.

Mine arrived with all of the smd parts already soldered and really about all that is required is to wind the inductors and the SWR pickup.

You have to 'think' a bit when winding the supplied toroids to ensure the start and finish are aligned with the holes in the PCB. The supplied wire is 0.8mm but it's a bit difficult to wind neatly and since i had some 0.71mm wire I decided to us that instead.

The SWR pickup is a bit on the fiddly side to wind, and it appears that the windings need to be wound in opposite directions?

There are two schematics about on the WWW and for my kit it looks like I have the ATU-100 (EXTended board) version. It's not obvious initially how to wire up the supplied OLED display but it connects to the PROG port. (MCLR is not used.) The schematic shows 2 buttons but connection points seem to be undr the PCB. Marked "B". On top there is a connector marked a "B" also, so not sure yet what the functions are.

For some reason when I tried to download some of the files from N7DDC's Github, they were corrupted. HA3HZ has some information on his pages which are interesting, especially as the version he has is totally different.

I connected it all up and this is the display. Initially it shows the version number, 3.0 which is the latest and note he no longer supports this project. Looking at HA3HZ's pages, it appears that other screen displays can be cycled through, eg PWR, SWR, Ant PWR and EFF%.

This is a picture of my completed board with display connected.

And the PCB itself.

Note that all the tune caps supplied are smd parts. N7DDC's web page actually shows Mic caps used for the upper (higher) values so keep this in mind if trying it at full 100watts ? In any case, tune up should be carried out with less than 30Watts.

The board is quite small, just 62 x 130mm in size.

So far (Sept 5th 2020) this is all I have done.

to be continued............




Page created by vk3pe on Sept 4th 2020