================================================================================ SCHEMATIC: Manual Rotary Coaxial Antenna Switches TM-ANT-SW-001 Rev A SP2T, SP4T, SP6T Designs — SO-239 and N-Type Versions HF through 1296 MHz | Power: 1.5 kW CW / 2 kW PEP | IL < 0.2 dB to 500 MHz ================================================================================ DESIGN OVERVIEW ──────────────── Manual rotary coaxial switches route one signal path (common port) to one of N selected ports. In ham use, the common port is usually the transceiver and the switched ports are the antennas; however the topology is bidirectional. TOPOLOGIES COVERED: SP2T — 1 Common + 2 Antenna ports (2-position) SP4T — 1 Common + 4 Antenna ports (4-position) SP6T — 1 Common + 6 Antenna ports (6-position) KEY DESIGN REQUIREMENTS: 1. All-metal construction for RF shielding 2. Low-loss center conductor path (silver or gold plated contacts) 3. Positive detent action — switch cannot rest between positions 4. All unused antenna ports GROUNDED (reduces static, improves isolation) 5. Weatherproof version: IP54 (lid seal), IP65 for outdoor/tower mount ================================================================================ SWITCH THEORY — ISOLATION AND INSERTION LOSS ================================================================================ INSERTION LOSS SOURCES: 1. Contact resistance R_c: IL = 20×log₁₀(1 + R_c/Z₀) [for series element] For R_c = 5 mΩ (silver-plated wiping contact): IL = 0.0004 dB → negligible For R_c = 50 mΩ (corroded contact): IL = 0.004 dB → still acceptable For R_c = 500 mΩ (oxidized brass): IL = 0.04 dB → noticeable 2. Shunt capacitance at junction: At a center conductor junction in a switch housing, stray C adds a shunt load: f₋₃dB = 1/(2π × Z₀/2 × C_stray) For C_stray = 3 pF: f₋₃dB = 2.12 GHz → IL at 500 MHz = 0.055 dB (OK) For C_stray = 10 pF: f₋₃dB = 636 MHz → IL at 500 MHz = 0.29 dB (fail) Keep junction compact; do not run center conductors past unused ports. 3. Discontinuity VSWR from geometry: At HF: negligible (dimension << λ) At VHF/UHF: maintain 50Ω geometry throughout switch body ISOLATION SOURCES (what limits port-to-port isolation): 1. Open contact capacitance C_open: Typical wiping contact open: C_open ≈ 0.1–0.5 pF Isolation from C_open: I ≈ 20×log₁₀(1/(ω × C_open × Z₀/2)) At 14 MHz, C_open=0.5pF: I = 88 dB (excellent) At 144 MHz, C_open=0.5pF: I = 68 dB (good) At 432 MHz, C_open=0.5pF: I = 55 dB (marginal) At 1296 MHz, C_open=0.5pF: I = 45 dB (poor) 2. Stray coupling through enclosure: Limits isolation to ~80–100 dB at HF; 40–60 dB at VHF/UHF Solution: RF absorbing foam or metal dividers between antenna port cavities 3. Common ground path (all shields share chassis): In a properly shielded enclosure this is negligible; in a metal box with poor layout it can reduce isolation to 30–40 dB SPECIFICATION SUMMARY: Frequency | Typical IL | Min Isolation | Notes -------------|---------------|---------------|------------------------------ 1.8–30 MHz | < 0.05 dB | > 80 dB | Contact R dominant 50 MHz | < 0.05 dB | > 70 dB | Stray C beginning 144 MHz | < 0.10 dB | > 60 dB | Good contact geometry 432 MHz | < 0.15 dB | > 55 dB | Use N-type, compact design 1296 MHz | < 0.20 dB | > 40 dB | Marginal; use purpose-built ================================================================================ DESIGN 1: SP2T (2-POSITION) SWITCH — HF VERSION ================================================================================ Two antenna ports, one common. Simplest topology; used for: - A/B antenna comparison testing - Dual-band antenna (switch HF dipole ↔ VHF yagi on one coax run) - Two-radio share one antenna (with TX lockout) SP2T SCHEMATIC: ┌─────────────────────────────────────────────────────┐ │ SP2T SWITCH BODY │ │ │ │ ANT 1 ──[SO-239]────────┬── (detent position 1) │ │ │ │ │ [WIPER]────── [SO-239] ── COMMON (Radio) │ │ │ ANT 2 ──[SO-239]────────┘── (detent position 2) │ │ │ │ ALL CONNECTOR OUTERS ──────── CHASSIS GROUND │ └─────────────────────────────────────────────────────┘ CENTER CONDUCTOR PATH (position 1): ANT1 center → rotary contact → common center CENTER CONDUCTOR PATH (position 2): ANT2 center → rotary contact → common center ISOLATED PORT (not selected): The unselected antenna center conductor floats unless grounded. MUST ADD: 47–100 kΩ resistor from unselected port center to chassis. This resistor: draws negligible RF power, drains static charge, sets DC level without disturbing RF performance. Alternative: use a shorting contact that grounds the unselected port whenever it is not in the selected position (harder to implement mechanically; excellent protection). SP2T WIRING DIAGRAM: [ANT1]─────────────────────────────────────────────────[ANT2] outer → chassis outer → chassis center─────┐ center─────┐ │ │ CONTACT 1 CONTACT 2 ╲ WIPER (rotary) └──────────────────────── center─────[COMMON] outer → chassis STATIC GROUND RESISTORS: From each unselected ANT port center conductor to chassis: 47kΩ to 100kΩ, 1/4W resistor. This has negligible effect on RF (at 1.8 MHz: Z = 100kΩ >> 50Ω; at 14 MHz: Z = 100kΩ >> 50Ω) but bleeds static reliably. ================================================================================ DESIGN 2: SP4T (4-POSITION) SWITCH — HF/VHF VERSION ================================================================================ Four antenna ports, one common. The standard ham "antenna switch." SP4T SCHEMATIC (top view, radial layout): ANT 1 │ [SO-239] │ center ○ ────────┼──────── ○ │ ○ ANT 4 │ ANT 2 [SO-239] ═══[WIPER]═══[SO-239] │ ○ [SO-239] │ ANT 3 COMMON [SO-239] ───── WIPER spindle (center) MECHANICAL DESIGN: Center spindle: brass or silver-plated copper, connects to COMMON SO-239 Rotating wiper: silver-plated beryllium copper or phosphor bronze spring contact Fixed stator contacts: four positions at 0°, 90°, 180°, 270° Detent: 4-position spring-ball or leaf-spring detent, positive click CONTACT RATING: For 1.5kW CW into 50Ω: V_peak = 387V, I_peak = 7.75A Contact must be rated: > 400V isolation when open, > 10A continuous Silver-plated brass contacts: adequate for 1.5kW Gold-plated copper contacts: superior corrosion resistance for outdoor use STATIC GROUNDING OF UNUSED PORTS: When wiper is at Position 1 (ANT 1 selected): ANT 2, ANT 3, ANT 4 centers are isolated. Each shall have a 47kΩ resistor to chassis, OR a shorting relay activated when that position is not selected. SIMPLE IMPLEMENTATION (resistor method): Install 4× 47kΩ, 1/4W resistors from each ANT port center to chassis ground INSIDE the switch housing. This is always connected regardless of switch position. The selected antenna's resistor has negligible effect on RF (100kΩ vs. 50Ω system = 66 dB attenuation of static path). SP4T PORT LAYOUT (SO-239, front panel view): ┌─────────────────────────────────────────┐ │ ANT 1 │ │ [○] │ │ │ │ ANT 4 [KNOB] ANT 2 │ │ [○] (rotate) [○] │ │ [○] │ │ ANT 3 │ │ COMMON [○] │ └─────────────────────────────────────────┘ POSITION INDICATOR: Mechanical: embossed or engraved position markers on panel Visual with LED (optional): see relay-based version for LED circuit Simple: printed arrow on rotary knob + position labels on panel ================================================================================ DESIGN 3: SP6T (6-POSITION) SWITCH — HF/VHF VERSION ================================================================================ Six antenna ports, one common. Used in multi-antenna stations covering all ham bands (160M through 10M requires at least 3 separate antennas in practice; 6 ports provides significant flexibility). SP6T STATOR CONTACT LAYOUT (60° spacing): ANT 1 (0°) ○ ○ ○ ANT 6 ANT 2 (300°) (60°) ○ ○ ANT 5 ANT 3 (240°) (120°) ○ ANT 4 (180°) ↕ WIPER ──── COMMON MECHANICAL SPECIFICATIONS: Shaft diameter: 6mm (standard pot/switch shaft, fits standard knobs) Panel mount: 3/8"-32 UNS (standard rotary switch bushing) Stop: mechanical travel stop at position 1 and position 6 (270° total travel) Detent: 6-position, 45° or 60° click spacing Enclosure clearance: 30mm minimum between adjacent SO-239 connector flanges SP6T PANEL LAYOUT (rectangular enclosure, SO-239 connectors): ┌──────────────────────────────────────────────────────┐ │ [ANT1] [ANT2] [ANT3] [ANT4] [ANT5] [ANT6] │ │ │ │ [SELECTOR KNOB] │ │ 1 2 3 4 5 6 │ │ │ │ [COMMON] │ └──────────────────────────────────────────────────────┘ GROUNDING CONNECTIONS: 5× 47kΩ resistors, one per unselected port. All antenna port outers bonded to chassis. Position 0 (between detents) should never occur with proper detent mechanism; if rotor is between positions both connected ports are effectively shorted to common — this is acceptable (VSWR rises, but no damage; cannot transmit while rotating switch). ================================================================================ VHF/UHF ADAPTATION — N-TYPE CONNECTORS ================================================================================ For applications above 50 MHz (6M, 2M, 70cm): Replace SO-239 with N-type female panel-mount connectors. Maintain 50Ω center conductor diameter throughout. N-type panel spacing: 30mm minimum (outer diameter 20mm + clearance). CONTACT GEOMETRY AT VHF: At 144 MHz, λ = 2.08 m → quarter-wave = 520 mm. Switch body dimensions typically < 50mm — far below λ/4. Lumped-element behavior applies; stray capacitance is the limiting factor. Minimize center conductor trace length between wiper and contacts. VHF SWITCH ISOLATION (additional measures): 1. Silver-plate all internal center conductor surfaces 2. Use Teflon/PTFE bushings for center conductor support (low ε_r) 3. Individual shielded cavities per port (metal dividers between ports) 4. Spring-loaded wiping contact, not sliding flat contact ================================================================================ POWER HANDLING ANALYSIS ================================================================================ For 1500W CW into 50Ω (matched load): V_peak = √(2 × 1500 × 50) = 387V I_peak = √(2 × 1500 / 50) = 7.75A For 2000W PEP SSB (peak envelope power): V_peak = √(2 × 2000 × 50) = 447V I_peak = √(2 × 2000 / 50) = 8.94A With SWR = 2:1 (high-SWR antenna, common in practice): V_max = 447 × √2 = 632V I_max = 8.94 × √2 = 12.6A CONTACT REQUIREMENTS FOR 1.5kW/2kW PEP: Contact material: Silver-plated copper minimum; gold-flashed preferred Contact force: ≥ 200g (prevents micro-arcing during vibration) Contact area: ≥ 10 mm² (limits contact resistance) Voltage standoff: ≥ 700V (2:1 SWR, 2kW PEP margin) HEAT GENERATION: At R_contact = 10 mΩ, I = 7.75A (1.5kW CW): P_contact = I² × R = 7.75² × 0.010 = 0.60W per contact Temperature rise (contact pair in metal enclosure): < 20°C → acceptable ================================================================================ PARTS LIST — MANUAL ROTARY SWITCH ================================================================================ Item | Qty | Description | Source -------|-----|------------------------------------------|------------------- SW-SP4 | 1 | SP4T ceramic rotary switch, 6-pos, 1A | Bourns MPA-series or Alpha, Grayhill 56 SW-SP6 | 1 | SP6T ceramic rotary switch, 1A rated | Grayhill 56D, Alpha SR2912 CONN | 5 | SO-239 silver-plated panel mount | Amphenol UG-58A/U or equiv. CONN-N | 5 | N-type female panel mount, silver | Amphenol 82-836 ENCL | 1 | Die-cast aluminum box, 130×130×40mm | Hammond 1590XX or 1590C KNOB | 1 | Phenolic pointer knob, 6mm shaft | Davies Molding 1100 series R_GND | 6 | 47kΩ, 1/4W, 1% metal film | Mouser (per port) WASHER | 10 | Silver/copper RF gasket washers | per connector HW | 20 | M4 SS screws + lockwashers | LABEL | 1 | Aluminum panel label (engrave/etch) | NOTE ON COMMERCIAL ALTERNATIVES: MFJ-1700C: 6-position SO-239 switch (commercial; ~$65) DX Engineering DXE-AS-4: 4-position SO-239 (~$75) These are adequate references; the designs above are for custom/homebrew stations needing specific power, frequency, or connector type. ================================================================================