Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers a compact portable transmatch (antenna tuner) for all fifteen amateur radio bands from 160 meters through 20 centimeters. Two sections are provided: a balanced T-network for HF (1.8–30 MHz, 100W continuous) and an L-network or stub tuner for VHF/UHF (50–1300 MHz, 50W). The transmatch transforms antenna impedances of approximately 5–2000Ω to the 50Ω transceiver impedance, enabling operation with non-resonant or slightly mistuned antennas.

Chapter 2 — Theory of Operation

2-1 T-Network (HF)

The T-network (input capacitor C1, series inductor L, output capacitor C2) is the most common antenna tuner topology. Both capacitors in shunt (to ground) with an inductor in series create a low-pass filter whose cutoff and impedance transformation ratio are controlled by the component values. The T-network can match any impedance in its range, though it introduces some loss at high transformation ratios. Loss increases at extremes: matching 2000Ω to 50Ω introduces 1–2 dB loss; matching near 50Ω introduces <0.1 dB.

2-2 L-Network (VHF/UHF)

An L-network (one shunt reactive element + one series reactive element) provides a two-element matching solution. It can match either upward or downward impedances depending on component arrangement. For VHF/UHF, lumped LC components are replaced by transmission-line stub sections (open or short-circuit stubs) for lower loss and higher power handling.

2-3 SWR Indicator

The built-in SWR indicator uses a directional coupler (see TM-GEAR-015) with LED or analog meter to indicate SWR. Tuning goal: minimum SWR (minimum reflected indicator deflection).

Chapter 3 — Equipment and Materials

Chapter 4 — Construction

1. Mount variable capacitors C1 and C2 on the front panel with lockwashers. Keep capacitor leads to the inductor and SWR bridge as short as possible (<50 mm) to minimize stray inductance and radiation.
2. Mount the roller inductor vertically on the center of the main board. The inductor lead to C1 and C2 should be heavy-gauge solid wire (#12 AWG or larger) capable of 100W continuous RF current.
3. Wire the balun output option: a 4:1 Guanella balun (see TM-GEAR-003) mounted on the rear panel provides a balanced output for dipoles and open-wire feedlines.
4. Install the SWR bridge (from TM-GEAR-015) inline between the radio input SO-239 and the T-network C1. Connect VFWD to forward LED driver; VREF to reflected LED driver.

Chapter 5 — Operating Procedures

5-1 Initial Tuning on a New Band

1. Set C1 and C2 to midscale. Set L to minimum (roller at bottom).
2. Key the transmitter at 5 W (reduced power protects transistors during tuning on a badly mismatched antenna).
3. Watch the SWR indicator. Adjust L for minimum reflected LED; then adjust C1 and C2 alternately for minimum reflected, returning to L adjustment if a new minimum appears.
4. Iterate until reflected indicator is at minimum and forward indicator shows full power. Typical tuning time: 30–60 seconds.
5. Increase to full operating power. Verify SWR indicator does not change.

5-2 Band-to-Band Switching

1. Record successful L, C1, C2 positions for each band on the band card (laminated chart in the lid of the tuner).
2. On band change: set L, C1, C2 to the recorded positions; tune briefly at 5W to confirm the antenna has not changed.

Chapter 6 — Calibration

1. Verify SWR indicator zero: connect 50Ω load to the antenna port. Transmit 10W. Reflected LED must not illuminate. Adjust SWR bridge null trimmer if reflected shows any indication.
2. Verify SWR indicator response: connect a 100Ω load (SWR 2:1). Reflected LED must illuminate at approximately 30% of the forward LED intensity (indicating ∼2:1 SWR).

Chapter 7 — Verification and Acceptance

1. Matches a 200Ω non-reactive load to SWR <1.5:1 on all HF bands.
2. Matches a 450Ω random wire (via 9:1 un-un) to SWR <2:1 on all HF bands.
3. Insertion loss with 50Ω load on both ports: <0.2 dB at 14 MHz.
4. VHF section tunes 2M vertical to SWR <1.5:1 from a 75Ω feedpoint.
5. All controls rotate smoothly over full range with no mechanical binding.
6. Log: date, match test (200Ω at 7 and 14 MHz, SWR achieved), insertion loss check, VHF section test, operator.

Appendix A — T-Network Component Value Guidelines

Appendix B — Matching Range and Loss Chart