================================================================================ SCHEMATIC: Sleeve Balun (λ/4 Choke Stub) — VHF and UHF Bands TM-CMC-001 Rev A Covers: 50 MHz (6M), 144 MHz (2M), 222 MHz (1.25M), 432 MHz (70cm), 902 MHz (33cm), 1296 MHz (23cm/20cm) ================================================================================ THEORY OF THE SLEEVE BALUN ─────────────────────────── A sleeve balun consists of a conductive sleeve (tube or foil cylinder) mounted coaxially around the outer conductor of the feed coax, connected to the outer conductor at one end. The sleeve and the outer conductor of the coax form a λ/4 coaxial stub, shorted at the antenna end. A shorted λ/4 stub appears as OPEN CIRCUIT at the free end (the end facing the transmitter/receiver). This open circuit blocks common-mode current from flowing back along the outer shield toward the shack. Impedance at the free end of a λ/4 shorted stub: Z_input = Z₀² / Z_load = Z₀² / 0 → ∞ (ideal: blocked, open circuit) Practical: |Z| = 1000 to 10000 Ω depending on Q of stub Bandwidth of λ/4 sleeve: −3 dB of maximum impedance: BW ≈ f₀ / Q ≈ f₀ × (loss of stub) / π Typical Q for copper sleeve: 50–200 → BW ≈ 0.5–2% of f₀ Works well for single-band use; for multi-band use stack or use ferrite Physical λ/4 length in sleeve gap: l = c / (4 × f₀ × √ε_eff) For air gap (ε_eff = 1.0): l = 75,000,000 / (4 × f₀_Hz) mm [= c/(4f) in mm] Velocity factor of coaxial space between sleeve and outer conductor: ~0.95–0.97 Therefore: l_physical = 0.96 × (75,000,000 / (4 × f₀_Hz)) mm ================================================================================ λ/4 SLEEVE LENGTHS BY BAND ================================================================================ Using copper sleeve, velocity factor = 0.96 (air + connector body correction): Band | f₀ (MHz) | λ/4 air (mm) | λ/4 physical (mm) | Notes -----|---------|-------------|------------------|---------------------------------- 6M | 50.1 | 1497 | 1437 | Use coiled coax or ferrite instead 2M | 144.0 | 520.8 | 500 | Very practical 500mm sleeve 1.25M| 222.0 | 338.0 | 325 | Compact 325mm sleeve 70cm | 432.0 | 173.6 | 167 | 167mm — very portable 33cm | 902.0 | 83.1 | 80 | 80mm microstrip or tube 23cm |1296.0 | 57.9 | 56 | 56mm; precision required PRACTICAL NOTE: 6M (50 MHz) sleeve would be 1.44 METERS — impractical for portable use. Use a toroid choke (FT-240-43, 8 turns) for 6M feedpoint common mode suppression. Sleeve baluns are appropriate for 2M and above. ================================================================================ DESIGN 1: 2M SLEEVE BALUN (144 MHz, 500mm) ================================================================================ MATERIALS: Inner coax: RG-58 or RG-8X feed cable (feeds the antenna) Sleeve: copper tube 19mm ID × 0.5mm wall (fits over RG-8X with 0.5mm gap) or: 2.5cm wide copper foil tape wound over foam spacer Sleeve length: 500mm ± 5mm Connection: solder sleeve to outer conductor at ANTENNA end (feedpoint) CONSTRUCTION — COPPER TUBE METHOD: 1. Cut 500mm length of 19mm copper or brass tube. 2. Slide over RG-8X feed cable; one end should be at the antenna feedpoint. 3. Solder ONE END of tube to the outer conductor of coax at feedpoint. The OTHER END (facing transmitter) is OPEN (floating electrically). 4. The tube-coax combination is the sleeve balun. CONSTRUCTION — COPPER FOIL TAPE METHOD: 1. Wrap 3mm closed-cell foam tape (3mm thick) around outer jacket of RG-8X. This creates the spacing for the λ/4 stub cavity. 2. Wrap 1.5" wide copper foil tape tightly over foam. Length: 500mm; overlap first and last wrap for electrical continuity. 3. Solder one end of copper foil to outer conductor at feedpoint. 4. The other end is the open (free) end toward transmitter. COPPER FOIL SLEEVE BALUN — CROSS-SECTION: ════════════════════════════════════════════ coax center conductor ──────────────────────────────────────────── coax dielectric ════════════════════════════════════════════ coax outer conductor (braid/foil) ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░ coax outer jacket ◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌◌ foam spacer (3mm) ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓ copper sleeve (foil or tube) Side view: Tx/Rx end ◄─────────────────────────────────────► Antenna (open end of sleeve) (shorted end) ══════════════════════════════════════════════════ coax center ┌──────────────────────────────────────────────┐ │ sleeve (shorted at right, open at left) │ └──────────────────────────────────────────────┘ ←────────── l = λ/4 = 500mm at 144 MHz ────────→ │ │ ├─ OPEN (high Z, blocks CM current) ────── SHORT (solder here to coax shield) ================================================================================ DESIGN 2: 70cm SLEEVE BALUN (432 MHz, 167mm) ================================================================================ At 70cm, the 167mm sleeve fits conveniently as part of the antenna mounting hardware. MATERIALS: Inner coax: RG-58 or LMR-195 Sleeve: 12mm copper tube (1/2" plumbing tube) or aluminum tube Sleeve OD needs to fit inside antenna element tube for integrated construction OR attach externally with insulating spacers INTEGRATED MAST-MOUNTED CONSTRUCTION: 1. Cut 167mm × 12mm copper tube. 2. Drill 5mm hole through sleeve wall at bottom (shorted end). 3. Feed RG-58 through sleeve. 4. Solder coax outer conductor to sleeve at bottom (shorted end). 5. Upper (open) end: sleeve simply ends — do not connect to coax. 6. Weatherproof with self-amalgamating tape above and below sleeve. PERFORMANCE: At 432 MHz: |Z_sleeve| ≈ 5000–10000 Ω (Q of copper stub) CMR at 432 MHz: 20 × log₁₀(1 + 5000/50) = 40 dB Bandwidth (−3 dB): ~10 MHz (432 ± 5 MHz) typical for copper For 70cm SSB/CW/FM frequencies (432–433 MHz): easily covered. ================================================================================ DESIGN 3: 33cm/23cm SLEEVE BALUN (902/1296 MHz) ================================================================================ At 902 MHz (33cm): sleeve length = 80mm At 1296 MHz (23cm): sleeve length = 56mm These short sleeves are ideal integrated into antenna construction. CONSTRUCTION (MICROSTRIP VERSION for PCB-mounted antenna): The sleeve can be implemented as a λ/4 section of 50Ω microstrip line on FR-4 PCB, connected to the coax shield at one end, open at the other. For 1296 MHz on Rogers RO4003 (ε_r = 3.55): ε_eff = (ε_r + 1)/2 + (ε_r-1)/2 × (1/√(1 + 12h/W)) where W/h = 2.1 for 50Ω ε_eff ≈ 3.15 → VF = 1/√3.15 = 0.564 λ/4 = 56mm × 0.564 = 31.6mm microstrip line COAXIAL SLEEVE BALUN — MOUNTING OPTIONS: Option A — External tape sleeve (field-expedient): Apply 20mm copper tape in 80mm length over outer jacket, gap-fill with tape. Quick; removable; weatherproof with self-amal. tape over. Option B — Machined aluminum collar (permanent installation): Turn a 30mm OD × 26mm ID aluminum collar (fits 1/2" Heliax or LMR-400). Solder/silver-braze bottom to coax outer conductor. Machine relief slot for connector clearance. Weather seal with O-ring at top (open end). ================================================================================ SLEEVE BALUN COMPARISON TABLE ================================================================================ Band | Type | Length (mm) | CMR (dB) | BW (MHz) | Construction -----|-----------|-------------|----------|----------|------------------ 2M | Copper tube| 500 | 30–40 | ±5–10 | Practical, durable 1.25M| Copper foil| 325 | 30–40 | ±8–15 | Simple tape wrap 70cm | Copper tube| 167 | 35–45 | ±15–20 | Integrated in mount 33cm | Al collar | 80 | 35–45 | ±30–40 | Machined or tape 23cm | Microstrip | 32 | 30–40 | ±50–70 | PCB or tape SLEEVE vs. FERRITE COMPARISON: Method | 2M (144MHz) | 70cm (432MHz) | 1296MHz | Wideband? -----------------|-------------|---------------|---------|---------- Sleeve balun | Excellent | Excellent | Excellent | No (1 band) Mix 61 toroid | Good | Poor | Very poor | No FT-82-43 ×10 | Excellent | Good | Marginal | Yes (30M-2M) Bead-on-coax×20 | Excellent | Excellent | Good | Yes (VHF/UHF) RECOMMENDATION: For single-band VHF/UHF antenna feedpoints: sleeve balun (best performance) For multiband or portable use: bead-on-coax with Mix 43 + Mix 61 ================================================================================ WEATHERPROOFING SLEEVE BALUN FOR OUTDOOR INSTALLATION ================================================================================ 1. After solder connection at antenna end: apply 2 wraps self-amalgamating tape over the solder joint to seal against moisture ingress. 2. Over the full sleeve length: wrap single layer of self-amalgamating tape. 3. Where the open end of sleeve terminates: cap with electrical tape fold-over or heat-shrink cap. 4. UV protection: outer wrap of 3M UV-resistant vinyl tape (black). 5. Water drain: orient so any condensed moisture drains away from solder joint. INSPECTION INTERVAL: Inspect all weatherproofing annually. Soft self- amalgamating tape degrades after 3–5 years outdoors in UV exposure. Replace outer UV tape layer every 2–3 years; inner seal may remain intact longer. ================================================================================ PARTS LIST — SLEEVE BALUN ================================================================================ Item | Qty | Description | Notes -----|-----|------------------------------------------|--------------------------- | 1 | Copper tube 19mm OD, 500mm | 2M version | 1 | Copper tube 12mm OD, 167mm | 70cm version | 1m | Copper foil tape 1.5" (38mm) | Foil tape method | 1m | Closed-cell foam tape 3mm | Spacer under foil | 1 | Self-amalgamating tape roll | Weatherproof seal | 50cm| UV-resistant vinyl tape (black) | UV protection | | Solder, silver-bearing (Ag4) | Sleeve-to-coax joint | 1 | IPA wipes | Pre-solder cleaning ================================================================================