QRP Antenna Tuner

DIY kit from Aliexpress or Ebay

Thse kits are sold on Aliexpress and Ebay and at the time of wiring they cost about $13 USD shipped.

Roots of this tuner can be traced to a QRP console designed by Doug DeMaw featured in QST article: “A QRP Console” Doug DeMaw, QST, Sept. 1970, p. 23-25.

SWR indicator used in this tuner is based on Dan Tayloe design:

• SWR Indicator Bridge Kit
• N7VE LED SWR Bridge Manual
• LED SWR Schematic

Tuner is using a high-pass T-network to match low input impedance (Z) to a high output Z.

Detailed explanation of how T-network operates and the best practices of using such tuners covered in QST Jan 1995, Getting the Most Out of Your T-Network Antenna Tuner

Schematic

Build notes

Solder all components of the SWR bridge with the exception of LED. It’ll be easy to estimate require length of the terminals and how they should be bent right before installing the bridge in the enclosure.

FT37-43 - pay attention to the sides: IN 2 turns, OUT 5 turns. Some kits have this mislabeled on the PCB.

Wind the toroid following M1KTA build notes and solder it on rotary switch following the schematic.

Using supplied templates drill the holes in the enclosure. It might be hard to tell what template to use on what side of the enclosure. Pay attention to the position of BNC connectors. Their central connector should be above variable capacitors.

If you decide to remove the templates, they may leave a sticky residue on the enclosure that is hard to clean. In this case you may use WD-40, baby oil or cooking oil and a piece of cloth to wipe it out.

Install the SWR bridge and wire everything following the schematic diagram.

M1KTA Build Notes

M1KTA comments on the Chinese ATU

• Build instructions, English
• Build instructions, Chinese

VK3YE Review and Build Tips

Review

W5CYF / TinkerJohn Build Videos

• Part1

• Part2

Operation

• Connect Transceiver output to the left side BNC connector of the Tuner.
• Connect Antenna to the right side BNC connector of the Tuner.
• Set the inductance switch to the first (A) position and both capacitors to the center of their range.
• SWR bridge should be off (switch in a right position).
• Turn the inductance switch to the position where the noise is heard at its maximum.
• Turn on the SWR bridge (switch in a left position).
• Send a series of dits and observe the LED. Bright LED - high SWR.
• Slowly tune right capacitor (antenna side) to get the minimum LED brightness.
• Slowly tune left capacitor (transceiver side) to get the minimum LED brightness.
• Dim LED is a very good SWR, even better is to tune until the LED goes out. You’ve achieved the lowest SWR for this frequency.
• Turn off the SWR bridge (switch in a right position). Keeping the bridge in the circuit will reduce the power by 6dB.