================================================================================ VHF/UHF REMOTE ELEMENT SWITCHING SYSTEM RELAY-BASED ELEMENT SWITCHING FOR HF/VHF (6M, 2M) DPDT Relay Switching for Reflector/Director Mode Selection ================================================================================ OVERVIEW ──────── At 6M (50 MHz) and 2M (144 MHz), SPDT relay contacts can be used for element switching with acceptable RF loss (< 0.3 dB). The relay shorts the parasitic element to the boom (making it act as a reflector) via its NC (Normally Closed) contact, or leaves it free (acting as director) via NO (Normally Open) contact. CRITICAL: Above 432 MHz, relay contact inductance (~1-2 nH) causes significant losses and must be replaced with PIN diodes. ================================================================================ SINGLE ELEMENT RELAY SWITCHING CIRCUIT - 6M/2M (50-144 MHz) ================================================================================ Parasitic Element (Reflector or Director): Element ────[C_iso]────●────┬──────── Driven Element / Feed Point │ │ (R_1NC) │ (Relay │ Contact) │ │ │ Boom ───┴────┘ Return (GND) RELAY DPDT G5V-2 (Omron, 5V coil, 200 Ω coil resistance): Logic Level Relay Coil Element (from ESP32) (via ULN2803) State ────────────────── ────────────────────────────── 3V3 (HIGH) → 5V → Coil energized (ON) → NO path: Element FREE (Director) GND (LOW) → OFF → Coil de-energized → NC path: Element SHORTED (Reflector) ================================================================================ RELAY DRIVER CIRCUIT (For Single Element Relay) ================================================================================ From ESP32 GPIO (3V3 Output, max 40 mA) │ o──────────[R_base]────┬──── Base of ULN2803 Array 10 kΩ │ (or single 2N2222) │ GND ───────────────────────────┴──── Emitter ULN2803 OUTPUT (Darlington Array - 8 channels): ┌──────────────────────────────────────────────────┐ │ INPUT VOUT COM OUTPUT (to relay) │ ├──────────────────────────────────────────────────┤ │ 1 ─────→ 1 (50mA) ────┬────────────┬─ RELAY1 │ │ 2 ─────→ 2 (50mA) ────│ +12V Rail │ │ │ 3 ─────→ 3 (50mA) ────│ GND Com ├─ RELAY2 │ │ 4 ─────→ 4 (50mA) ────┤ (COM pins)│ │ │ 5 ─────→ 5 (50mA) ────│ ├─ RELAY3 │ │ 6 ─────→ 6 (50mA) ────│ │ │ │ 7 ─────→ 7 (50mA) ────├────────────┴─ RELAY4 │ │ 8 ─────→ 8 (50mA) ────┘ │ │ │ │ V- pin ────────────── GND │ │ COM pin ────────────── GND │ │ Vcc pin ────────────── +12V │ └──────────────────────────────────────────────────┘ ================================================================================ FULL 5-ELEMENT ARRAY RELAY MATRIX (6M Example: 50.1 MHz) ================================================================================ System requires control of: - Reflector: ON (shorted to boom) / OFF (free) = 1 relay (G5V-2 DPDT) - Director 1: ON (free) / OFF (shorted) = 1 relay - Director 2: ON / OFF = 1 relay - Director 3: ON / OFF = 1 relay Total: 4 relays minimum for independent element control ELEMENT STATE TABLE (5-Element Yagi, All Patterns): Pattern | Reflector | Dir1 | Dir2 | Dir3 | Relay State ──────────────────────────────────────────────────────────────── BASELINE 5-EL | ON | ON | ON | ON | 0101 (all free) COMPACT 3-EL | ON | ON | OFF | OFF | 0100 (D2,D3 shorted) REVERSE BEAM | OFF | ON | ON | ON | 1101 (Refl shorted) CARDIOID +Z | ON | ON | ON | OFF | 0100 (only D3 shorted) (etc. for 12+ patterns) Relay wiring convention: - Relay NC (Normally Closed) → Element shorted to boom (acts as reflector) - Relay NO (Normally Open) → Element free (acts as director or inactive) - Relay COM → Boom RF ground ================================================================================ ELEMENT SHORTING STUB DETAIL (Critical for Reflector Action) ================================================================================ When relay NC contact connects element to boom: Element │ (0.494λ) = 2.956 m at 50 MHz │ │ ├───────────────────┐ │ │ ├──[Relay NC]──┬────┘ Boom conductor │ │ │ Lambda/4 tuning stub (optional) │ (short circuit stub for impedance xfm) │ GND ──────────── For reflector action, the element needs to be about 5% longer than resonance. Coupled to driven element, this creates the reflector impedance. If relay shorts element directly to boom with no stub: - Effective frequency shift: ~2-3% depending on boom size - Element becomes reflector naturally due to length mismatch - For 50.1 MHz: 0.494λ × 1.05 = 2.956 m (already accounted for in NEC models) ================================================================================ RELAY SPECIFICATIONS & SELECTION ================================================================================ Omron G5V-2-5 (Recommended for 6M/2M): ────────────────────────────────────── Coil Voltage: 5V DC Coil Current: 40 mA @ 5V (Relay ON) Coil Resistance: ~125 Ω Contact Configuration: DPDT (Double Pole Double Throw) Contact Rating: Max 2A @ 30V DC Contact Bounce: ~1 ms typical Operating Frequency: Up to ~10 MHz (contact parasitic inductance) RF Loss @ 50 MHz: ~0.1 dB RF Loss @ 144 MHz: ~0.2 dB RF Loss @ 432 MHz: NOT ACCEPTABLE (> 5 dB loss + reactance) Isolation (OFF): > 40 dB Contact Resistance: ~100 mΩ when closed Plug-in Relay Package: ┌────────┐ │ │ │ K1 │ Coil connections: pins 2,8 (coil) │ │ NO contact: pins 3,4 │ ◆◆◆◆◆◆ │ NC contact: pins 5,6 │ ◆ ◆│ Common: pins 1,7 │ ◆ K1 ◆│ │ ◆ ◆│ │ ◆◆◆◆◆◆ │ │ │ └────────┘ 1 2 3 4 5 6 7 8 │ │ │ │ │ │ │ │ A B N N C C C C ================================================================================ MULTI-ELEMENT DRIVER: ULN2803 DARLINGTON ARRAY ================================================================================ 8 Relays (up to 8 elements) driven from single ULN2803 IC: ESP32 GPIO Pins: 0 1 2 3 4 5 6 7 │ │ │ │ │ │ │ │ │ └──[1kΩ]──┬──┴───┴───┴───┴───┴───┴───┴───┴─→ ULN2803 IN(1-8) │ GND ────────────┴─────────────────────────────→ ULN2803 GND ULN2803 Outputs (collectors, active HIGH): OUT(1-8) ────────→ +12V (relay supply) via relay coils Each output can sink 500 mA, sufficient for ~6 relays in parallel (2 relays × 40 mA coil current = 80 mA per output channel, < 500 mA limit) Example: 5-Element Array with 4 Relays (Reflector + 3 Directors) ESP32 GPIO2 ──[1kΩ]──→ ULN2803 IN(1) ──→ OUT(1) ─┬──[Relay1 Coil]──┬── +12V │ │ COM ────────────────┴─ GND ESP32 GPIO4 ──[1kΩ]──→ ULN2803 IN(2) ──→ OUT(2) ──[Relay2 Coil]── +12V/GND ESP32 GPIO5 ──[1kΩ]──→ ULN2803 IN(3) ──→ OUT(3) ──[Relay3 Coil]──+12V/GND ESP32 GPIO17──[1kΩ]──→ ULN2803 IN(4) ──→ OUT(4) ──[Relay4 Coil]──+12V/GND Relay coils are 12V @ 40 mA each ================================================================================ TIMING & BOUNCE CONSIDERATIONS ================================================================================ Contact Bounce Time (Mechanical): G5V-2 relay: ~1 ms typical bounce Solution: - Add 50 ms settling delay in ESP32 firmware after relay energization - Use debounce in software (read relay state 3× with 5 ms delays) - OR add 100 nF ceramic capacitor across relay coil for RC filtering Element Switching Procedure (Firmware): 1. De-energize ALL relays (set all GPIO to LOW) → all elements shorted 2. Wait 50 ms for settle 3. Energize target relay (set GPIO HIGH) 4. Wait 50 ms for settle 5. Read SWR via RF sensor (NanoVNA or AD8307 power detector) 6. Confirm lock before declaring pattern active ================================================================================ RELAY CONTACT FAILURE MODES (Field Service) ================================================================================ Symptom: Relay clicks but element doesn't switch state Cause: Contact oxidation or corrosion Remedy: Use sealed relay with gold-plated contacts (e.g., G5V-2-5) Periodic exercise (switch contacts 10× per month) Symptom: SWR changes unpredictably when switching Cause: Relay bounce creating transient impedance variations Remedy: Increase settling delay to 100 ms, verify bounce with scope Symptom: RF loss increases over time Cause: Contact surface corrosion; contact resistance degradation Remedy: Check contact resistance with ohmmeter (should be < 150 mΩ) If > 200 mΩ, replace relay ================================================================================ SUPPLY VOLTAGE & DECOUPLING ================================================================================ Relay Coil Supply: +12V DC @ ~200 mA maximum (4 relays × 40 mA + margin) Decoupling Capacitors: - 1000 µF electrolytic across +12V/GND bus (bulk storage) - 100 nF ceramic at each relay coil (local high-frequency bypass) - Place ceramic caps within 10 mm of relay coil pins Flyback Diode (Protection): 1N4148 or 1N4007 across relay coil Cathode → +12V rail (anode to coil terminal during OFF state) Prevents voltage spike when coil is de-energized ================================================================================ EQUIVALENT CIRCUITS FOR NPEC ================================================================================ At UHF frequencies (432+ MHz), relay contacts exhibit: - Series inductance: ~0.5-2 nH per contact pair - Contact resistance: ~100 mΩ - Effective impedance at 432 MHz: Z ≈ 100 mΩ + j(2π × 432e6 × 1e-9) ≈ 100 mΩ + j2.7 Ω ≈ 2.7 Ω (inductive) This causes ~3-5 dB insertion loss and severely impacts patterns. PIN diodes MUST be used above 432 MHz. ================================================================================ RELATED DOCUMENTS ================================================================================ - pin_diode_switching_ascii.txt: For UHF bands (432+ MHz) - relay_switching_6m_ascii.txt: 6M specific component values - relay_switching_2m_ascii.txt: 2M specific component values - rf_isolation_filter_ascii.txt: Control line shielding & filtering