Revisiting a Budget QRP ATU Project
I first built this project in March 2016. Since then ‘hundreds’ must have been sold! However, I recently came across a reference to this particular ATU on a YouTube video, put together by Carol Milazzo KP4MD in July 2021. This video showed that the inductance had been connected incorrectly on the original schematic. The number of windings at certain points on the toroid were wrong. There was also a mistake on the PCB silkscreen (see references below).
So, I used my original PCB, but re-wound the inductance in accordance with the recommended numbers in the YouTube video.
I checked the antenna’s SWR values with an MFJ Analyser (without the ATU) and then tested the ATU and antenna with a miniVNA (The original antenna was a Windom, the present one, as tested, is a vertical). With the switch in the 'Tune' position, the LED on top comes on if the unit does not 'see' 50 Ohms (this is done by a Wheatstone Bridge).
Test Results
| Band (m) | Mid-Frequency (MHz) | SWR(1) | SWR(2) |
| 160 | 1.92 | >3.00 | >3.00 |
| 80 | 3.70 | >3.00 | 1.38/10 |
| 60 | 5.32 | >3.00 | 1.18/8 |
| 40 | 7.10 | >3.00 | 1.94/5 |
| 30 | 10.12 | 1.20 | 1.35/3 |
| 20 | 14.18 | 1.65 | 1.27/2 |
| 17 | 18.10 | 2.40 | 1.27/1 |
| 15 | 21.23 | 3.00 | 2.22/1 |
| 12 | 24.94 | 3.00 | 1.39/1 |
| 10 | 28.95 | 3.00 | 2.39/1 |
| 6 | 51.00 | 1.60 | 1.25/1 |
Notes:
SWR(1) = Results using MFJ Analyser, Model No 249, on antenna only.
SWR(2) = Results using miniVNA and QRP A.T.U. on antenna.
The secondary figure in SWR(2) column is the band switch position.
The antenna on test alongside this ATU was the Thunderpole SE-HF 360 Fibreglass Vertical Antenna.
I have a few spare PCBs if anybody is interested in taking on this small project.
Kind regards
Alex Henderson M0TOT
