Automatic Antenna Tuner ATU-100 by N7DDC

by Leon Uys Personal

R1 300.00

Automatic Antenna Tuner (ATU-100) with power and SWR Indication. Design by N7DDC. Very lightweight and useful for field work. Quick tuning!


5 in stock (can be backordered)

SKU: ATU-100 Categories: , , , , Tags: , , , , , ,


Automatic Antenna Tuner (ATU-100) from original N7DDC design with power and SWR Indication. Very lightweight and useful for field work. Requires external 12v power like most other ham devices but can be 9v battery operated. Quick tuning!

After recovering from the shock of the price of the Z-100+/MFJ-939 auto tuners (the US$ is killing us) I started looking for a tuner that isn’t going to put me back in hospital. And lo and behold I found the design from N7DDC on the internet at N7DDC-ATU-100-mini-and-extended-boards . This is the perfect tuner to match your standard Yaesu or ICOM 100 watt radio at a third of the cost of the brand names! I’m going to lovingly call this the poorman’s Z-100 plus.

There is only one small problem. Yes, it is small … as in surface mount! There are many kits on eBay and AliExpress in various stages of completeness, but you are on your own if it doesn’t work. So I have put together a team in the workshop that can do the assembly and test locally. What you get is a working unit.

The unit can be programmed for QRP work (20W) or normal (100W) – please specify your choice. The type of antenna connector is also important: BNC is far superior to anything and comes standard but SO239 connectors can optionally be fitted.

Some notes from Dave Plummer 2M0CX

I built an ATU100 then after exhaustive testing cased same. Its specification claims:-

*Supply voltage of 10-15V dc,
PCB has a 5v regulator for both the switching transistors relays and PIC processor. My unit works from 8-14Vdc, my PSU max output is 14V

*Maximum current consumption 400mA.
During Tune mode all relays can be used, but not all are simulataneously used,
In test mode all relay driver devices (2n2002) are turned on, respective relays are energised. 15 Off relays plus 2n2002 devices and respective PIC outputs which is 400mA

  • Maximum working throughput of the standard 7×7 variant is 100W
    the relay contacts are only rated at 1.3A, therefore before entering tune mode the recommended power source is less than 10W. Once a tuned solution is found, the power can then be icreased to 100W.
    There are pads on the underside of PCB to wire connection to transceiver for tune initiate and from ATU100 to transceiver to initiate power reduction, provided the transceiver has these options.
  • Maximum possible measured power 150W
    I had an input of 118W through the unit, with no obvious signs of stress or damage, though with respect to relay contacts, 100W would be the limit.

*minimum power requoired to start tuning 5W
5w is default. This can be changed using a software / hardware option
it can go less than 5W, however the sense coil will require rewinding, there are details on internet showing how this is done.

  • Minimum possible measured power 0.1W. True

*Accuracy of power measurement 10%
My unit tracks a Bird 4413 with both 5 / 50 / 100W elements used

  • Typical consumtion 150 – 200mA
    Once tuned solution is found the current draw is low
  • Frequency range 3 – 54Mz
    My unit has been proven to work between 3.5 to 29MHz, not tried on 6m.
    Only tried with dummy load and off resonance dipoles (purposely lengthened and shortened dipole elements to give high SWR of 4:1)
    Unit found a tuned solution all bands 80-10m with exception of 30m had to reset and retry Tuned mode. Equipment Elecraft K2/100

Unit is not easy to build if you are not used to SMD components ranging in size from 1206 to 0603. Also uses air coils, single and double toroids.

Relays are made in Hong Kong, no equivalent in size or pinout, so expect usual delays for these. Relays are HK1400’s pinout does not conform to usual standards.
Most SMD’s are on rear of PCB, I was encasing mine, so soldered M2 nuts on bottom 4 pads for same on PCB, this protects components on underside from damage.
Relays are last to be installed. They are very susceptable to heat damage.

Also take care winding toroids they will only mount in PCB through holes if wound the correct way. I had to rewind all mine.

If your PIC is not programmed, then you will need a PICKIT 2 or 3 to program same, also need the PICKIT to change any parameters that includes display type and OLED size.

If you choose Autotune mode, then the power throughput must be less than 10W

In power down, the unit defaults to Bypass mode

Additional information

Weight 0.5 kg
Dimensions 15 × 15 × 8 cm


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