Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers remote switching of antenna parasitic elements (directors and reflectors) to change the radiation pattern of Yagi-Uda and phased arrays. Four switching technologies are covered: PIN diode RF switching (70 cm through 23 cm), bias-T DC injection (for remote power), RF isolation filters, and relay sequencer (HF and VHF use). The ESP32 controller manages switching sequences, CYD display, and BLE remote control.

Chapter 2 — Theory of Operation

2-1 PIN Diode RF Switch

A PIN (p-intrinsic-n) diode has a wide intrinsic region that stores minority carriers under forward bias. Under forward bias (15–50 mA): the diode becomes a small resistance (<0.5Ω), passing RF. Under reverse bias (−5–−20V): the carriers sweep out and the diode becomes a small capacitor (<0.2 pF), blocking RF.

For parasitic element switching: the element is connected to the boom through the PIN diode. Forward bias connects the element (director mode); reverse bias opens the circuit (the element is now a passive wire, which shortens effective reflector length).

2-2 Bias-T for Remote Control

The PIN diode bias current is injected onto the coaxial feedline through a bias-T at the feed point. A high-impedance choke (RFC, >1 kΩ at operating frequency) in series with the bias supply passes DC but blocks RF. At the element, the reverse bias circuit is isolated by capacitors in the RF path.

2-3 Relay Sequencer for TX/RX Switching

During transmit, the sequencer ensures: (1) preamp is bypassed BEFORE (2) transmit relay closes BEFORE (3) PA is keyed. Reverse on receive. This prevents preamp burnout from TX power leakage.

Chapter 3 — Equipment and Materials

Chapter 4 — Construction

4-1 PIN Diode Element Switch (70 cm)

1. Mount MA4P504 diode on a small PCB (10×10 mm) at the element attachment point on the boom. Orient cathode toward the bias supply, anode toward the element.
2. Wire the RF choke (47 nH SMD) in series between the bias supply conductor and the cathode. This prevents RF from entering the bias circuit.
3. Connect 100 pF NP0 capacitor in series with the RF path (between boom and element) to block DC while passing RF.
4. Run a twisted pair (bias supply and return) along the boom to the controller box. Use shielded twisted pair for runs >300 mm.

4-2 Relay Sequencer

1. Connect Relay 1 (preamp bypass) to ESP32 GPIO with a 2N2222 driver. Relay 2 (TX/RX antenna relay) to a second GPIO + driver.
2. On PTT input (low = key): fire Relay 1 (bypass preamp) after 10 ms delay; fire Relay 2 (TX relay) after another 20 ms delay. Then assert PA key output.
3. On PTT release: de-assert PA key; wait 20 ms; release Relay 2; wait 10 ms; release Relay 1 (reconnect preamp).

Chapter 5 — Operating Procedures

1. Select antenna pattern via CYD touchscreen or BLE remote. The ESP32 sets the correct PIN diode bias states for the chosen pattern (cardioid, broadside, endfire, or omnidirectional).
2. Pattern switching may occur on receive only; switching during transmit is not recommended (momentary impedance mismatch during transition).
3. Inspect all bias-T connections at antenna installation and after any storm. Water ingress at a bias-T can short the bias supply, permanently forward-biasing one PIN diode and locking the pattern.

Chapter 6 — Calibration

1. Verify PIN diode states: with a NanoVNA and TinySA, measure S21 through each switched element. Forward-biased: S21 must be <−0.5 dB (low loss connected state). Reverse-biased: S21 must be <−35 dB (high isolation open state).
2. Sequencer timing: measure delay between PTT closure and TX relay closure with an oscilloscope. Must be ≥20 ms to protect the preamp.

Chapter 7 — Verification and Acceptance

1. PIN diode insertion loss (forward bias) <0.5 dB at operating frequency.
2. PIN diode isolation (reverse bias) ≥35 dB at operating frequency.
3. Pattern repeatability: switching to the same pattern twice should give the same S21 phase through each element within ±5°.
4. Sequencer: TX relay must not close before preamp bypass relay; verified with dual-trace oscilloscope measuring each relay coil drive.
5. Log: date, diode types, forward loss, reverse isolation, sequencer timing, pattern repeat error, operator.

Appendix A — PIN Diode Bias Current vs. Loss

Appendix B — TX/RX Sequencer State Table