TRANSMATCH ANTENNA TUNING UNIT

TECHNICAL MANUAL

Document Number: TM-ATU-001 Rev A
Equipment: Transmatch Antenna Tuning Unit, Portable, HF/VHF/UHF
Frequency Coverage: 1.8 MHz – 1300 MHz (160M through 23cm)
Power Rating: 100 W continuous HF; 50 W continuous VHF/UHF
Classification: UNCLASSIFIED — Amateur Radio / Field Use
Date: 2026-05-24
Supersedes: None (initial issue)

CHAPTER/SECTION                                                    PAGE
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CHAPTER 1   GENERAL INFORMATION . . . . . . . . . . . . . . . . .  1-1
CHAPTER 2   THEORY OF OPERATION . . . . . . . . . . . . . . . . .  2-1
CHAPTER 3   EQUIPMENT DESCRIPTION  . . . . . . . . . . . . . . .  3-1
CHAPTER 4   COMPONENT SELECTION AND PROCUREMENT . . . . . . . . .  4-1
CHAPTER 5   CONSTRUCTION PROCEDURES  . . . . . . . . . . . . . .  5-1
CHAPTER 6   INSTALLATION  . . . . . . . . . . . . . . . . . . . .  6-1
CHAPTER 7   OPERATING PROCEDURES  . . . . . . . . . . . . . . . .  7-1
CHAPTER 8   CALIBRATION . . . . . . . . . . . . . . . . . . . . .  8-1
CHAPTER 9   MAINTENANCE . . . . . . . . . . . . . . . . . . . . .  9-1
CHAPTER 10  TROUBLESHOOTING  . . . . . . . . . . . . . . . . . . 10-1
APPENDIX A  SPECIFICATIONS  . . . . . . . . . . . . . . . . . . .  A-1
APPENDIX B  BILL OF MATERIALS  . . . . . . . . . . . . . . . . .  B-1
APPENDIX C  CIRCUIT SCHEMATICS  . . . . . . . . . . . . . . . . .  C-1
APPENDIX D  GLOSSARY  . . . . . . . . . . . . . . . . . . . . . .  D-1
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RECORD OF CHANGES

CHAPTER 1 — GENERAL INFORMATION

1.1 Scope

This manual provides design, construction, operating, and maintenance instructions for the TM-ATU-001 Portable Transmatch Antenna Tuning Unit. The ATU covers all amateur radio bands from 160 meters through 20 centimeters using a hybrid topology: T-network for HF and L-network for VHF/UHF.

1.2 Purpose

A transmatch transforms antenna feedpoint impedance to the 50 Ω impedance required by modern transceivers. This allows full-power operation with non-resonant antennas, random wire antennas, or antennas that are slightly off resonance due to environmental conditions.

NOTE: A transmatch does not improve antenna radiation efficiency.  It
presents a 50 Ω match to the transmitter, eliminating SWR-induced power
reduction, but any loss within the antenna system remains unchanged.

1.3 Applicable Documents

1.4 Equipment Identification

CHAPTER 2 — THEORY OF OPERATION

2.1 Why Impedance Matching Is Required

Modern transceivers are designed for 50 Ω loads. When antenna impedance differs from 50 Ω: - Reflected power travels back toward the transmitter - SWR protection circuitry reduces output power - Transmission efficiency decreases - In extreme cases (SWR > 10:1), transmitter finals may be damaged

The transmatch absorbs the impedance mismatch by presenting 50 Ω to the transmitter while driving the antenna at its actual impedance.

2.2 T-Network Topology (HF Bands)

The T-network uses three reactive elements:

  Transceiver      C1         L1         C2       Antenna
    (50 Ω)      (Input)    (Series)   (Output)
       │            │          │          │           │
       ├────────────┴──────────┴──────────┴───────────┤
       │                                               │
      GND                                            GND

C1: Input variable capacitor — matches transceiver to inductor
L1: Roller inductor or switched coils — provides reactance range
C2: Output variable capacitor — matches inductor to antenna

All three elements are adjusted simultaneously for minimum SWR. The T-network can match impedances from 10 Ω to 1000 Ω across HF.

T-network advantages: wide impedance matching range; works with both high and low impedances; adjustable Q factor (affects bandwidth and efficiency).

2.3 L-Network Topology (VHF/UHF Bands)

The L-network uses two reactive elements:

  Transceiver       L1      Antenna
    (50 Ω)       (Series)
       │             │           │
       ├─────────────┴───────────┤
       │        ┌─ C1 ─┐        │
       │        │       │        │
      GND      GND     GND      GND

Fewer components reduce insertion loss at VHF/UHF. Switched fixed values are used for each band; the L-network matches SWR up to approximately 3:1 input.

2.4 Q Factor and Efficiency

The T-network Q is set by the ratio of inductor reactance to the matched load impedance. High Q: narrow bandwidth, higher losses in reactive components. Low Q: wider bandwidth, lower losses.

For portable QRP field use, set Q ≤ 10 for efficiency.

CHAPTER 3 — EQUIPMENT DESCRIPTION

3.1 HF Section (160M–10M)

Component               Specification
──────────────────────────────────────────────────────────────────
C1 (input capacitor)    15–500 pF variable, 500 V minimum rating
L1 (inductor)           5–30 µH, roller or switched-coil
C2 (output capacitor)   15–500 pF variable, 500 V minimum rating
Connectors              SO-239 input and output
Power rating            100 W continuous, 200 W PEP
Insertion loss          < 0.5 dB typical

3.2 VHF/UHF Section (6M–23cm)

Component               Specification
──────────────────────────────────────────────────────────────────
L1 (series inductor)    Switched; band-specific values
C1 (shunt capacitor)    Switched; band-specific values
Connectors              SO-239 (6M, 2M); SMA (70cm and above)
Power rating            50 W continuous, 100 W PEP
Insertion loss          < 0.3 dB typical

3.3 Enclosure

3D-printed PETG; 200 × 150 × 80 mm. OpenSCAD source files in mechanical/ subdirectory. Minimum wall thickness: 2.5 mm.

CHAPTER 4 — COMPONENT SELECTION AND PROCUREMENT

4.1 Variable Capacitors

Critical component — quality directly affects insertion loss and maximum SWR matching range. Do not substitute ceramic or mica dielectric capacitors; use air-dielectric only.

WARNING
-------
For 100 W CW operation, variable capacitor voltage rating must be
≥ 500 V DC (1000 V preferred).  At 100 W into 50 Ω, peak voltage is
100 V RMS × √2 = 141 V peak.  High-impedance antenna ports may see
several times this voltage.  Insufficient voltage rating causes arc-over,
capacitor damage, and possible fire.

Specifications required: - Capacitance range: 15–500 pF (or dual-gang 2 × 250 pF) - Voltage rating: 500 V minimum; 1000 V recommended; 2500 V for high-SWR loads - Dielectric: air (not ceramic or mica for transmitting use) - Plate material: brass or silver-plated brass

Sources: Fair Radio Sales, Ocean State Electronics (surplus, $15–40); Surplus Sales of Nebraska, Antique Electronic Supply (new, $40–100); Cardwell, Hammarlund, Johnson vintage (eBay, $50–150).

4.2 Roller Inductor

• Inductance range: 5–30 µH (covers all HF bands)
• Current rating: ≥ 10 A continuous
• Wire: AWG #14 silver-plated copper minimum

Pre-made: MFJ, Palstar, Barker & Williamson ($80–200); vintage Hammarlund, Johnson ($50–150 used). DIY option: wind from 6 mm copper tubing on threaded fiberglass rod (total cost $15–30; detailed procedure in Section 5.4).

Alternative: switched inductors (separate fixed coils selected by rotary switch). Less flexible but easier to build.

4.3 Connectors

• Input/output HF: SO-239 (UHF female); silver-plated preferred
• VHF/UHF above 70cm: SMA female; 50 Ω rated

CHAPTER 5 — CONSTRUCTION PROCEDURES

5.1 Safety Precautions

WARNING
-------
RF voltages during transmission are hazardous.  At 100 W into 50 Ω, RMS
voltage is 70.7 V; peak voltage is 100 V.  At high SWR conditions or
antenna impedances far from 50 Ω, voltages across the variable capacitors
may exceed 1000 V peak.  NEVER touch any internal component while
transmitting.  De-energize the transmitter and wait 10 seconds before
opening the enclosure.

5.2 Printed Enclosure Parts

Print order: 1. enclosure_main.scad — main body (longest print; start first) 2. front_panel.scad — control cutouts and labels 3. inductor_mount.scad — roller inductor mounting bracket 4. capacitor_mount.scad — variable capacitor mounting hardware

Print settings: PETG; 0.2 mm layer height; 25% infill; 3 perimeter walls. Total print time: approximately 15–20 hours.

5.3 HF T-Network Assembly

1. Mount variable capacitors C1 and C2 in enclosure front panel cutouts. Secure with flat washers and locknuts; do not overtighten.
2. Mount roller inductor L1 on inductor bracket.
3. Wire C1 input terminal to SO-239 input center pin.
4. Wire C1 output terminal to L1 input terminal.
5. Wire L1 output terminal to C2 input terminal.
6. Wire C2 output terminal to SO-239 output center pin.
7. Connect all chassis points (capacitor frames, inductor frame, SO-239 shields) to single ground point via AWG #14 braid.
8. Keep all wiring short; maximum lead length 50 mm.

5.4 DIY Roller Inductor Construction

Materials: 13 mm OD copper tubing, fiberglass threaded rod 12 mm dia, AWG #14 copper braid, two brass end plates.

Wind copper tubing in a helix on the threaded rod; secure ends to brass plates. Roller contact: silver-plated roller bearing riding along the outer surface of the coil. Inductance per turn varies by winding pitch; verify with NanoVNA at completion.

5.5 VHF/UHF L-Network Assembly

Install switched inductors and capacitors on auxiliary PCB. Band selection via rotary switch on front panel. Component values are in schematics/ subdirectory.

CHAPTER 6 — INSTALLATION

6.1 Location

Place transmatch between transceiver output connector and antenna feedline. Keep RF path leads as short as possible (< 150 mm each side).

6.2 Grounding

Connect transmatch chassis ground to station ground bus via shortest practical path. Do not rely on the coaxial cable shield as the sole ground path; add a dedicated bond wire if using long feedlines.

6.3 Balun Placement

When used with balanced antennas (dipoles, doublets), install a 4:1 or 1:1 balun at the transmatch output, between the ATU and the antenna feedpoint. See TM-BAL-001 for balun design and selection.

CHAPTER 7 — OPERATING PROCEDURES

7.1 Initial Setup

1. Set transceiver power to minimum test level (5 W).
2. Connect transmatch between transceiver output and antenna.
3. Set C1 and C2 to midrange; set L1 roller to approximate band position (see band preset reference chart posted inside enclosure lid).
4. Do not transmit until SWR meter is installed in line.

7.2 Tuning Procedure

CAUTION
-------
Tune at reduced power (5–10 W) until minimum SWR is achieved.  Transmitting
at full power into a mistuned transmatch may cause voltage arc-over in the
capacitors or overheating in the inductor.  Only increase to operating power
after SWR < 2:1 is confirmed.

1. Key transmitter (CW or tune mode) at 5–10 W.
2. Adjust C1 and C2 alternately while observing SWR meter.
3. Adjust L1 (roller position or switched tap) when C adjustment alone cannot reduce SWR below 2:1.
4. Iterate: after each L1 change, re-optimize C1 and C2.
5. When SWR < 1.5:1, increase power to operating level and verify SWR does not change.
6. Record L1 tap/position and C1/C2 scale readings for this antenna and band (for quick recall on future activations).

7.3 Band Preset Chart

Mark a card with C1, L1, and C2 settings for each band-antenna combination. Attach inside enclosure lid. Pre-tuning time in the field is typically 5–30 seconds with a known preset.

7.4 VHF/UHF Tuning

Select the appropriate band switch position. Adjust L1 and C1 for minimum SWR. VHF/UHF tuning range is narrower; the input SWR of the antenna must be < 3:1 for the L-network to achieve a satisfactory match.

CHAPTER 8 — CALIBRATION

8.1 Initial Calibration

After construction, verify with NanoVNA before first RF application: 1. Measure insertion loss (S21) at minimum loss setting: expect < 0.5 dB HF. 2. Verify capacitance range at both extremes with NanoVNA impedance mode. 3. Verify inductance range across roller travel. 4. No calibration adjustments available; replace components out of specification.

CHAPTER 9 — MAINTENANCE

9.1 Periodic Inspection (Annual or After Field Deployment)

[ ] Inspect all solder joints — look for cold joints and corrosion
[ ] Clean variable capacitor plates with dry compressed air
[ ] Check roller inductor contact for arcing marks (pitting)
[ ] Verify all SO-239 connectors are tight; check center pin alignment
[ ] Check enclosure screws for tightness
[ ] Verify all ground connections are secure

9.2 Roller Inductor Maintenance

Clean roller contact surface with 600-grit crocus cloth. Silver-plate the roller with conductive silver paint if plating is worn. Check spring tension of roller mechanism; replace spring if contact pressure is inadequate.

9.3 Variable Capacitor Maintenance

Clean capacitor plates annually with dry compressed air. Do not use liquid cleaners. Inspect for bent plates (touching plates cause shorts during rotation). Apply a small amount of conductive grease to shaft bearings.

9.4 Storage

Store with variable capacitors at midrange position (reduces bearing fatigue). Place silica gel desiccant packet inside enclosure. Keep enclosure closed.

CHAPTER 10 — TROUBLESHOOTING

10.1 Symptom Index

10.2 Cannot Achieve SWR < 2:1

10.3 SWR Increases at High Power

Cause: component nonlinearity or partial arc-over. Check capacitor voltage rating. Inspect inductor roller contact for pitting. Increase SWR check power in 10 W steps; note where SWR begins to change.

10.4 Arcing Sound During Transmit

WARNING
-------
Disconnect from transmitter immediately.  Arc-over indicates capacitor plate
gap exceeded.  Continued operation with arc-over present may cause fire.
Inspect capacitor plates; clean or replace.  Reduce operating power or
change to a higher-voltage-rated capacitor.

10.5 High Insertion Loss (> 1 dB)

Causes: roller inductor contact pitting; capacitor plate oxidation; cold solder joint in RF path. Trace with NanoVNA S21 while manipulating controls to identify location.

10.6 Roller Inductor Intermittent

Clean roller contact and coil surface with crocus cloth. Verify spring pressure. Check for stripped turns on coil (physical inspection).

APPENDIX A — SPECIFICATIONS

APPENDIX B — BILL OF MATERIALS

APPENDIX C — CIRCUIT SCHEMATICS

See schematics/TRANSMATCH_SCHEMATICS.txt for full ASCII circuit diagrams: - HF T-network (160M–10M) - VHF/UHF L-network (6M–23cm) - Integral SWR indicator (optional)

APPENDIX D — GLOSSARY

ATU (Antenna Tuning Unit): Device that transforms antenna feedpoint impedance to 50 Ω for the transceiver.

Insertion loss: Power lost in the matching network, expressed in dB.

Q factor: Ratio of reactive power stored to resistive power dissipated; determines bandwidth and efficiency of matching network.

SWR (Standing Wave Ratio): Ratio of maximum to minimum voltage on a transmission line; 1.0:1 is perfect match.

Transmatch: Synonym for ATU; emphasizes the network’s role in transforming (matching) the transmission-line impedance.

End of TM-ATU-001 Rev A.