Building & Improving the ATU-100 (N7DCC) Automatic Antenna Tuner.

Like a lot of amateur radio enthusiasts I looked at many articles and videos on the inexpensive ATU-100 which is based on a design by N7DCC and sold by mostly Chinese vendors either built or as a kit.

Generally speaking there are normal and extended board versions.

Mini = 5 transformers and 5 capacitors for antenna matching permutations.

Extended = 7 transformers and 7 capacitors for antenna matching permutations.

As I write this article The firmware current is v3.2 , this can be found with guide notes at the link below

There is an excellent video guide enclosed on the page above for converting the ATU-100 to QRP operation by

1. Altering firmware settings slightly by using an inexpensive Pickit 3 PIC programmer.

2. Make adjustment to tandem match from 10:1 and convert to 5:1 for each of the 2 transformers in the binocular cored ferrite.

The second item above is critical for the RF power and SWR readings on the display to be correct.

If you do not proceed with step 2, Yes the unit will appear to work and tune at just 1 watt power but the display will show a match but the reading will not be true as the voltage and current reaching the forward and reverse monitoring diodes D1 and D2 will be wrong, if that is wrong the microprocessor will find the wrong match of components to tune the antenna.. More on this later.


And the schematic diagrams for both versions of ATU-100 are here

Mini Version 5 x 5 :

Extended Version 7 x 7 :×7.pdf



Pre-Assembled ATU-100.

I bought one of the units pre built from Bangood in China for around $80 USD equivalent.

Banggood ATU-100


It is an enclosed version with integral “gum stick” lithium battery and usb 2.0 socket on the front panel for charging.

The construction inside was terrible, glue all over the place and the transformers looked like they were put together by a 5 year old.

The torroid rings were poor clones of the original Amidon T68-2 used in the schematic. The give away was their bright scarlet colour unlike the maroon of the Amidon rings.

Speaking of the rings, the wire of the windings was far too thick. The assembler obviously had no clue that RF fields would be passing around the exterior of the rings and just did a fix and forget job of soldering the inductors to the board. They were slanted all over the place with no care to mount them neatly in a straight row.

Updating firmware on this one was a challenge as the 4 leads inside were soldered to the board, cheap and nasty instead of fitting a board header for an extra few cents cost.

Looking back I got what I paid for, it was cheap and boy was it cheaply made !

I converted the unit to QRP by first swapping out the ratio of the tandem match to 5:1



I used the Pickit 3 programmer to edit values in the PIC table and I spent a few hours putting right the mess of a inductor job and removing that glue.

I can understand why the glue is used on inductors as in service they should not move around but the assembler used that glue everywhere, battery, connectors the lot !



Self Assembly ATU-100 Kit

Searching for better I decided to buy a kit for equivalent of $23 USD and build it myself.

The kit came from Chinese vendor on Aliexpress and it came with original T68-2 rings and updated/latest firmware to v3.2.

Delivery was very quick, 10 days China to UK.


A small bag of accessories came along with the enclosure. Inside 4 pieces of FR4 board, These are for fixing to the 4 corners of the main PCB to enable mounting the whole main board into enclosure internal slots.


I was just about to embark on building when I came across this really excellent video from Brazilian Radio Amateur Rafael Pinto PU1OWL on Youtube.

If you are building an ATU-100 , this is a must watch for you BEFORE BUILDING.

Some really fantastic detective work and great tips.

A really good reference page is given on the youtube page by authored by Jeff K6JCA to help users understand SWR measurement and what is going on in the ATU-100 SWR bridge.


This is Rafael’s first attempt at making a youtube video for the Amateur Radio Community and he should be congratulated for an excellent presentation, after all our Amateur Radio hobby is all about knowledge exchange, Top marks, first class all round..


So with the hints that Rafael gave in his video I decided to take a look at my kit with particular emphasis on the D1 and D2 diode pair in the SWR bridge circuit.

Here is what I found

A mis matched pair at the D1 and D2 locations. The bottom diode is correct (1N5711W marking “SA” SOD-123) but the one above is not correct as it has DO-80 package.

So with that I searched around for replacements.

Finding a few 1N5711W SMD or BAT41ZFILM in SOD-123 package is very challenging without paying a penalty for carriage charges.

Believe me I have checked !

Meanwhile I explained through Aliexpress messaging to the Chinese vendor of the ATU kit the issue I was having and good enough they sent another kit to me very fast indeed.

The vendor told me that the replacement would have BAT41 diodes.

The package arrived in 5 days.

Guess what ?

The diodes were a matched pair but in DO-80 package.

Detective work revealed that BAT41 is not available in DO-80 so the question now is

What diodes are they ? They certainly are not BAT41 or 1N5711 as per schematic specifications.

I have to say the vendors customer service was very impressive. All messages were answered in a timely manner and they took the trouble to send over another kit to me here in UK on a very speedy courier service at no cost. I wish all Aliexpress sellers were the same.

So what about good old through hole components THCs ?

I bought some original ST Micro 1n5711 here in the UK from a very reputable seller which are blue in colour. (ST Micro BAT41 are exact same dimensions and colour)

I include a photo below showing the issue and along side an original THC ST Micro 1N5711.

The leads of the diode are 0.7-0.8mm diameter. The PCB holes are around 0.5mm and not much room to work with widening the hole and not damaging the ring solder pads or tracks. What a pain !



A few days later an order arrived for replacement diodes,

As if by fate they are the correct type but wrong size, the vendor sent 1n5711w but in tiny SOD-323 and not SOD-123 which I ordered.

These are too small for my eyes !  You can see the 323 on the left side .


All very taxing on the mind but I await more supplies and I will continue to assemble this ATU and update this page.



Further Scheduled Improvements

I purchased some original Motorola 78L05 regulators and some Panasonic 25v rated low ESR 105C capacitors to replace the cheap ones in the kit.


I have ideas of using a larger enclosure with 1602 green LCD so that the display can be used outdoors


I successfully built the ATU-100 kit.

Here is a short video regarding some useful additional features I found.





For your reference RF Shottky Diode datasheets below :


Surface Mount Device SMD  (Package SOD-123)


BAT41ZFILM Package Marking “Z41”


1N5711W Package Marking “SA”


Through Hole Component THC


BAT41 (ST Micro)



1N5711 (ST Micro)



Thanks again to Rafael PU1OWL for his help on twitter. He freely admitted to me that he only found out that his ATU-100 tuner was essentially SWR/POWER lying to him after he took some readings from the meter on his rig and the two did not correlate.


ATU-100M New Version

I have noticed a new version of ATU-100 , the ATU-100M which has a very clever display.

As far as I can see it is well put together ,The construction is all very neat and tidy unlike some other factory ATU-100  generic models.

Here is a video from “Temporarily Offline Ham Radio”  where the cover comes off to let us see what is inside.