================================================================================
BALUN 1:1 CURRENT CHOKE — FERRITE TOROID
================================================================================

Purpose: Common-mode suppression on dipole and balanced antenna feeds
Common-Mode Impedance: 250+ Ω @ HF, 500+ Ω @ 10M, 5000+ Ω @ VHF
Insertion Loss: 0.1–0.3 dB (ferrite) to <0.05 dB (air-core VHF)

================================================================================
CIRCUIT DIAGRAM (RG-58 coax threaded through toroid core)
================================================================================

          ┌────────────────────┐
          │  ANTENNA (Balanced)│
          │    (dipole feed)   │
          └──────┬─────────────┘
                 │
         ┌───────┴──────┐
         │              │
      Conductor A    Conductor B
         │              │
      ┌──┴──────┐   ┌──┴──────┐
      │         │   │         │
      │  RG-58  │   │  RG-58  │
      │ through │   │ through │
      │ FT-240  │   │ FT-240  │
      │  core   │   │  core   │
      │         │   │         │
      └──┬──────┘   └──┬──────┘
         │              │
         ├──────┬───────┤
                │
          [Coax Cable]
                │
          ┌─────┴─────┐
          │  Receiver  │
          │ or TX      │
          │  50Ω IN    │
          └───────────┘

================================================================================
CORE SPECIFICATIONS
================================================================================

160M–10M (HF):
  Primary Core: FT-240-43 (2× stacked for low frequencies)
  Material: Mix 43 (µ=850, AL=1100 µH/100T)
  Winding: 12–14 turns RG-58 or RG-174 coax threaded through core
  Wire Gauge: AWG 8 (RG-58) or AWG 10 (RG-174)
  Max Power: 700 W (single core), 500 W (stacked)

6M–2M (VHF):
  Primary Core: FT-240-61 (Mix 61, µ=125)
  Winding: 5–7 turns RG-174 coax
  Max Power: 600 W
  CM Impedance: 5000+ Ω @ 145 MHz

70cm–20cm (UHF):
  Core: Air-core RG-58 on 3.5"–2.5" diameter PVC form
  Winding: 8–5 turns depending on frequency
  Insertion Loss: <0.05 dB (minimal ferrite loss at microwave)
  CM Impedance: 2000+ Ω @ 432 MHz

================================================================================
WINDING TECHNIQUE — FERRITE TOROID
================================================================================

1. Prepare coax cable:
   - Remove ~6 inches of jacket from end
   - Do NOT separate inner/outer conductors — keep intact
   - Tin inner conductor and braid separately for field connection

2. Thread coax through core:
   - Pass entire coax through center hole, multiple passes
   - 12 passes = 12 turns through core (counts bifilar: goes in, comes out)
   - Evenly space turns around circumference (avoid bunching)
   - Keep coax straight — do not crimp or fold back sharply

3. Field connections:
   - Balanced feedpoint: Connect center conductor to one antenna leg,
     braid to other leg, with chassis/counterpoise at choke input
   - Coax feed: Simply insert choke in feedline (no signal path disruption)

================================================================================
COMMON-MODE IMPEDANCE CALCULATION
================================================================================

Z_CM = 2π × f × L   (inductive reactance, ohms)
L = AL × N² / 10000 (inductance in µH)

Example @ 80M (3.75 MHz) with FT-240-43, N=12 turns:
  L = 1100 × 144 / 10000 = 15.84 µH
  Z_CM = 2π × 3.75×10⁶ × 15.84×10⁻⁶ ≈ 375 Ω

Example @ 10M (28.4 MHz) with FT-240-43, N=6 turns:
  L = 1100 × 36 / 10000 = 3.96 µH
  Z_CM = 2π × 28.4×10⁶ × 3.96×10⁻⁶ ≈ 708 Ω

DESIGN RULE: Z_CM ≥ 5 × Z₀ (250 Ω minimum for 50 Ω system)

================================================================================
IMPEDANCE TRANSFORMATION (1:1 CURRENT BALUN)
================================================================================

THEORY:
  A 1:1 current balun provides NO impedance transformation. Its sole purpose
  is to suppress common-mode (CM) currents on the feedline.

OPERATION:
  - Differential (balanced) signal passes through coax with minimal insertion loss
  - Common-mode currents (equal magnitude, same phase on braid + center)
    encounter high inductance Z_CM and are attenuated
  - Attenuation ratio: typically 20–40 dB @ fundamental frequency

BALANCING MECHANISM:
  Pure inductive choke on the feedline. Ferrite acts to increase inductance
  at the target frequency range.

================================================================================
FREQUENCY RESPONSE & BANDWIDTH
================================================================================

FT-240-43 (12 turns) typical response:
  1.8 MHz (160M):   Z_CM ≈ 280 Ω (broad, flat response)
  3.75 MHz (80M):   Z_CM ≈ 375 Ω (peak effectiveness)
  7.15 MHz (40M):   Z_CM ≈ 530 Ω (still excellent)
  14.2 MHz (20M):   Z_CM ≈ 950 Ω (very good)
  28.4 MHz (10M):   Z_CM ≈ 1700 Ω (excellent but approaching SRF)

Insertion Loss (through-line):
  <0.1 dB @ 80M
  <0.15 dB @ 40M
  <0.2 dB @ 20M
  <0.3 dB @ 10M

SRF (Self-Resonant Frequency) considerations:
  FT-240-43, 12 turns: SRF ≈ 40–50 MHz (stray capacitance limits upper reach)
  Beyond SRF, impedance drops and core losses increase

================================================================================
PRIMARY APPLICATIONS
================================================================================

1. DIPOLE FEEDS (symmetric antennas):
   - 80M–10M dipoles fed with balanced 200Ω or 300Ω window line
   - Choke suppresses common-mode radiation and reduces feedline loss

2. END-FED HALF-WAVE ANTENNAS (EFHW):
   - Acts as RF ground isolation between antenna and equipment ground
   - Prevents RF from coupling into power supplies and audio

3. DELTA LOOP ANTENNAS:
   - Symmetric feeding reduces asymmetric radiation patterns
   - Improves pattern directivity

4. LADDER LINE TRANSITIONS:
   - 1:1 choke at junction between open-wire line and coax
   - Maintains CM suppression without impedance transformation

5. VERTICAL ANTENNA (SINGLE-ENDED) FEEDS:
   - Acts as a RF-choke for counterpoise currents
   - Reduces vertical radiation pattern distortion

================================================================================
TESTING PROCEDURE — NANOVNA MEASUREMENT
================================================================================

Setup:
  1. Connect NanoVNA Port 1 → choke input (center conductor of coax)
  2. Connect NanoVNA Port 2 → choke output (center conductor of coax)
  3. Leave braids open-circuit (capacitive coupling minimal)
  4. Perform SOLT calibration at both ports

Measurement 1 — Insertion Loss S₂₁ (magnitude):
  Sweep 1–30 MHz
  Expected result: S₂₁ ≈ -0.1 to -0.3 dB (roughly flat across band)

Measurement 2 — Common-Mode Impedance (Port 1 input, Port 2 open):
  Measure S₁₁ (reflection), convert to impedance: Z_in = 50 × (1+S₁₁)/(1-S₁₁)
  At each frequency, Z_in ≈ Z_CM (inductive)
  Expected: Rising impedance with frequency (inductive slope)

Measurement 3 — Phase (Port 1 to Port 2):
  Phase should track approximately 0° ± small ripple (feedthrough)
  Large phase deviation indicates ferrite saturation or core resonance

================================================================================
FAILURE MODES & TROUBLESHOOTING
================================================================================

SYMPTOM: High SWR on balanced antenna
ROOT CAUSE 1: CM choke missing or installed backwards
SOLUTION: Verify choke is in primary feedline, not a secondary path

ROOT CAUSE 2: Ferrite core saturated (heating observed)
SOLUTION:
  - Reduce transmit power
  - Add second core in series (stacked configuration)
  - Use larger core (FT-240 instead of FT-140)

ROOT CAUSE 3: Coax damaged or shorted in core winding
SOLUTION: Test choke with multimeter (ohms, between center and braid)
         Expected: Open circuit; replace if shorted

SYMPTOM: Excessive heating during transmission
ROOT CAUSE: Core loss at high power (ferrite dissipates RF as heat)
SOLUTION:
  - Reduce power, or
  - Switch to air-core design for VHF/UHF
  - Use multiple cores (parallelized)

SYMPTOM: Pattern null in wrong direction
ROOT CAUSE: CM currents still flowing on feedline (choke not working)
SOLUTION:
  - Check choke impedance with NanoVNA
  - If Z_CM << 250 Ω, remove core and re-measure (verify core is functional)
  - Consider using ferrite tiles (broader bandwidth) instead of toroid

================================================================================
POWER HANDLING & THERMAL CONSIDERATIONS
================================================================================

Ferrite loss: P_loss = I² × R_loss
  where R_loss ≈ tan(δ) × X_L (frequency dependent)

For FT-240-43 @ 80M, 12 turns, 100W input:
  I_CM ≈ 0.5 A (typical CM current, worst case ≈ 2 A)
  Core loss ≈ 0.2–0.5 W (tolerable)

For FT-240-43 @ 10M, 6 turns, 500W input:
  Core loss ≈ 2–5 W (watch for heating)
  Temperature rise ≈ 20–30°C above ambient

DESIGN RULE: Monitor core temperature during high-power transmission.
  If core too hot to touch (>60°C), reduce power or use larger core.

================================================================================
CONSTRUCTION NOTES
================================================================================

WEATHERPROOFING:
  - Potting epoxy (two-part) optional for field durability
  - Silicone rubber gasket between chassis and coax strain relief
  - O-ring seal recommended for outdoor installations

FIELD DEPLOYMENT:
  - Can be mounted on antenna feedpoint (with waterproof enclosure)
  - Recommended: Feed into portable antenna coupler for impedance adjustment
  - Typical installation: Base of mast or feed tower

SPARE PARTS:
  - Keep extra FT-240-43 and FT-140-43 cores (ferrite is generic)
  - Stock RG-58 and RG-174 coax for field rewinding
  - Small alligator clips useful for test connections

================================================================================