Kanga Products Phoenix QRP Pocket Transmatch ATU (PTM-ATU)

Introduction

This document provides instructions for building the Kanga Products Phoenix QRP Pocket Transmatch ATU (PTM-ATU). It is described as a challenging but rewarding project for builders with attention to detail. The ATU is designed to be compact, fitting into a pocket, and offers advantages over simpler L-match circuits by including a band-pass filter to reduce out-of-band interference and harmonics.

The design is based on the work of Charlie Lofgren W6JJZ, combined with a resistive SWR bridge by Kanga Products to create a compact unit. The pocket transmatch is designed to work from 80 to 10 meters, with a maximum power of 10 Watts. It features BNC connectors and has dimensions of 85 x 55 x 27mm, weighing less than 110 grams.

Parts List

Before beginning assembly, check that all parts are present:

• Pocket Transmatch case
• Pocket Transmatch Front Panel
• Pocket Transmatch PCB (Version 2)
• DPST Switch
• SPST switch
• Resistors: R2, R3, R4 (51 Ohm 2 Watt), R1 (1K 1/4w), R5 (Sensitivity Adjust Trimmer)
• Capacitors: C1, C2 (10nF Disk Capacitor, Yellow 5mm spacing), C3 (10nF Disk Capacitor, 2.5mm spacing)
• 1 x IN5711 Diode
• T68-2 Core
• Enamelled Copper Wire: 1m (0.5mm, Colour 1), 250mm (0.5mm, Colour 2)
• 2 x 270pf Dual Poly Capacitors
• 1 x 5mm RED LED
• 2 x BNC Sockets
• Socket mounting Hardware: 4 x M2.5 4mm var-cap mounting screws
• 2 x M2.5x12mm Shaft extender Screws
• 2 x 10mm shaft extenders
• Two control knobs
• 4 x Stick on feet
• Connecting Cable 150mm

Circuit Description

The Pocket Transmatch is divided into two main sections: a resistive SWR bridge and the ATU section.

SWR Bridge

The SWR bridge circuit is similar to that used in Kanga Products' SWR Indicator bridge kit. It features a switchable LED that illuminates if the antenna presents an impedance significantly different from 50 ohms. The brightness of the LED can be adjusted via a sensitive trimmer to suit the user's typical power level.

ATU Section

The ATU section utilizes a T68-2 core with windings configured as 13 turns on the primary and 7 turns on the secondary. An additional switch allows the output to be grounded, enabling the use of either balanced or unbalanced antennas. The number of turns on the output stage can be adjusted for lower impedance antennas, though this may affect efficiency.

Winding the Toroidal Coil

The toroidal coil winding is a critical step. The kit provides two wires of different colors: a longer wire for the main winding and a shorter wire for the coupling winding.

1. Take the longer wire (approx. 1m) and pass it through the toroidal core, leaving about 50mm at one end. Each pass through the core counts as one turn.
2. Wind a total of 13 turns.
3. Form a loop about 30-50mm long, feed the wire back into the core, and wind another 6 turns.
4. Form another loop and wind a final 13 turns.
5. Double-check the winding count (13 turns, loop, 6 turns, loop, 13 turns). Using a phone camera to zoom in on a picture can help verify the turn count.
6. For the second winding, use the shorter wire (approx. 25cm). Wind 7 turns BETWEEN the existing windings on the core, starting about 3 turns from the end. Ensure this winding is in the same direction as the first.

The enamel on the wire needs to be burned off using a soldering iron. Lightly sanding the wire can help. Apply the soldering iron and solder for about 10 seconds until the enamel burns off and the solder flows. Ensure good ventilation and avoid inhaling fumes.

The main winding has four connection points. The earth side connects to the pad marked CL4, and the 'hot' end goes to CL1. The coupling coil connects to LNK2 and LNK1.

Component Assembly

1. Prepare the toroidal coil by tinning its leads. This is described as the hardest part of the kit.
2. Solder the four wires of the toroidal to the PCB, ensuring the toroidal sits flush and the leads have no slack.
3. Prepare the BNC sockets by soldering the supplied wires to them and their earth tags. Ensure the wires exit at a 90-degree angle to the brass pin for proper case fitting.
4. Prepare the case by snipping off two plastic PCB guides near the BNC mounting holes to ensure the BNC sockets mount correctly.
5. Fit the BNC sockets into the case, ensuring the earth tags are nearest to the sides and pointing downwards. Tighten them securely.
6. Trim and attach the wires from the BNC sockets to the main PCB, keeping them as short as possible.
7. Fit the main PCB into the case. A green spacer is provided to ensure the front panel sits flush with the case edge. Place this spacer before tightening the front panel.
8. Attach the front panel to the case using four 8mm self-tapping screws.
9. Attach the nuts to the switches for cosmetic purposes. Fit the control knobs and stick-on feet if desired.
10. Solder the remaining components to the PCB, including the two 10nF capacitors (C1 & C2), C3 (10nF), the 1K resistor (R1), the 1N5711 diode (D1), and the sensitivity trimmer (R5).
11. Fit the switches carefully, ensuring their legs are flush with the back of the PCB and the switches are straight. Do not push the switches fully down onto the PCB.
12. Prepare the two variable capacitors: snip off excess leads on the 4-pin side. On the 5-pin side, identify and keep the two outer leads; snip off the other three leads close to the body.
13. Attach the shaft extenders to the variable capacitors using the 2.5mm x 12mm screws, being careful not to damage the capacitor internals. Use pliers to hold the brass shaft.
14. Set the capacitor trimmers fully open (un-meshed), although this is not critical.
15. Attach the prepared variable capacitors to the front plate using the 2.5mm short screws.
16. Fold back the remaining terminals from each capacitor.
17. Apply thin foam tape to the back of the capacitors to prevent short circuits when the assembly is bolted together.
18. Place the LED into the PCB from the switch side, ensuring the longer leg goes into the round hole.
19. Position the main PCB over the back of the capacitors, aligning the capacitor pins with the PCB fixing slots. Ensure the LED protrudes through the front plate.
20. Add capacitor C3 (10nF) into the same holes as the LED leads before soldering. Solder the capacitor and LED leads from the top of the board.
21. Solder the capacitor legs and trim them.

Testing and Operation

To test the transmatch:

1. Connect your antenna to the ANT side of the ATU and your transmitter to the TX side.
2. Tune your radio to the desired frequency.
3. Set the switch to 'OPER'. If using a coax-fed antenna, set the other switch to 'Un-Bal'; for a balanced feeder, select 'BAL'.
4. Adjust the two ATU controls to find the spot with maximum band noise. This is often close to the correct match.
5. Reduce transmitter power to 5 watts or less and switch the ATU to 'Tune'.
6. Supply carrier and adjust the two controls until the LED goes out or becomes as dim as possible. This indicates a match.
7. Once matched, switch from 'Tune' to 'OPER'.

If your radio has a built-in SWR meter, you can fine-tune the match. However, a 1:1 match does not guarantee a good antenna; a dummy load will also provide a perfect match. Power can be increased up to 10 watts after a match is achieved.

The LED serves as a reliable SWR indicator, often making a separate SWR meter unnecessary.

Troubleshooting

• LED not working: Check if the Sensitivity control is set too low or if the LED is inserted backwards.
• No Match on certain bands: Some antennas may have impedances that are difficult for the ATU to match on specific bands (e.g., a quarter-wave antenna on 40m might be a half-wave on 20m). Adjusting antenna length slightly can help.
• LED lights dimly during OPER mode: This can be due to stray RF pickup. Newer PCB versions with an extra capacitor (C3) help mitigate this.
• SWR LED vs. Radio SWR Meter: Minor differences in indicated SWR may occur due to the patch lead length. If your radio has an SWR meter, use it to adjust the tuner for the lowest reading. The SWR LED is primarily for situations without additional indication.