================================================================================
UNUN 49:1 — END-FED HALF-WAVE (EFHW) ANTENNA MATCHING
================================================================================

Purpose: Impedance transformation 2450Ω unbalanced → 50Ω unbalanced
Type: Voltage transformer (7:1 turns ratio unun)
Insertion Loss: 0.35–0.50 dB (expected for extreme 49:1 ratio)
Primary Use: EFHW multiband antennas (classic 3:21 turns design)

================================================================================
CIRCUIT DIAGRAM — 49:1 EFHW UNUN
================================================================================

End-Fed Half-Wave Antenna (135–270 feet depending on lowest band)
        │
        │ 2450Ω feedpoint impedance
        │
   ┌────┴────┐
   │          │
(antenna)   (counterpoise: 10–30 feet at ~45° angle)
   │          │
   ├─[UNUN]──┤
   │  49:1    │
   │ 3:21     │
   │ FT-240-43│
   └────┬────┘
        │
      50Ω Coax
        │
    ┌───┴───┐
    │ Radio │
    │ 50Ω   │
    └───────┘

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

EFHW Configuration (Multiband 80M–10M):
  Core: Single FT-240-43 (exact 3:21 turns, never vary)
  Material: Mix 43 (µ=850, AL=1100 µH/100T)
  Wire: #14 enameled copper
  Primary: 3 turns (antenna side, 2450Ω)
  Secondary: 21 turns (coax side, 50Ω)
  Turns Ratio: EXACT 7:1 (critical for EFHW performance)
  Impedance Ratio: 7² = 49:1 (exact 2450→50Ω)
  Max Power: 400 W (QRP to QRO)
  Insertion Loss: 0.40–0.50 dB @ 80M, decreasing to 0.25–0.30 @ 20M

================================================================================
IMPEDANCE TRANSFORMATION
================================================================================

CRITICAL: Exact 3:21 turns ratio required for EFHW operation.

TURNS RATIO:
  N_ratio = 21 / 3 = 7:1 (EXACT ratio)
  Z_out = Z_in / N² = 2450 / 49 = 50Ω ✓

VOLTAGE STEP-DOWN:
  V_out = V_in × (1/7) ≈ 0.143 × V_in

EFHW FEEDPOINT IMPEDANCE:
  A half-wave antenna fed at its end (non-resonant point) exhibits
  very high impedance: ~2450Ω at center frequency (approximately).

  This impedance varies across bands:
  - 80M: ~2700Ω (above 2450Ω nominal)
  - 40M: ~2450Ω (nominal)
  - 20M: ~2100Ω (below nominal)
  - 10M: ~1800Ω (lowest on HF)

  The EFHW's impedance variation is INTENTIONAL (antenna tuner expected)
  or managed by careful antenna design (loading coils, counterpoise tuning).

================================================================================
WHY 49:1 = 7² from 7:1 TURNS RATIO
================================================================================

Derivation (Voltage Transformer):

For a voltage transformer (ideal, lossless):
  V₁ / V₂ = N₁ / N₂   (voltage ratio)
  I₁ / I₂ = N₂ / N₁   (current inverse ratio)
  Z₁ / Z₂ = (N₁ / N₂)²   (impedance ratio)

For 3:21 turns ratio:
  N₁ / N₂ = 3/21 = 1/7
  Z₁ / Z₂ = (1/7)² = 1/49

Therefore:
  Z₂ / Z₁ = 49   (inverted for output → input perspective)
  If Z₁ = 2450Ω, then Z₂ = 2450 / 49 = 50Ω ✓

This is NOT a coincidence. The EFHW antenna feedpoint impedance
was DESIGNED historically to be 2450Ω, specifically to use a 49:1
transformer (standard transformer core available at the time).

================================================================================
FREQUENCY RESPONSE
================================================================================

FT-240-43, 3:21 turns (EXACT ratio, all bands):

Frequency    Z_in (measured)    Z_in (design)    Insertion Loss
160M         3000Ω              (not used)        0.50 dB
80M          2700Ω              2450Ω            0.45 dB
40M          2450Ω              2450Ω            0.35 dB
30M          2300Ω              (not classic)     0.32 dB
20M          2100Ω              2450Ω            0.28 dB
17M          1900Ω              (not classic)     0.25 dB
15M          1850Ω              (not classic)     0.24 dB
12M          1750Ω              (not classic)     0.22 dB
10M          1700Ω              2450Ω            0.20 dB

Notes:
  - Classic EFHW designed for 80M–10M coverage
  - Feedpoint impedance varies ~1700–2700Ω across band
  - Antenna tuner compensates for variation
  - OR: Careful antenna design minimizes impedance change

================================================================================
WINDING TECHNIQUE — EFHW EXACT 3:21
================================================================================

CRITICAL: Count turns EXACTLY. No tolerance acceptable.

Materials:
  - 1× FT-240-43 core (MUST use correct core size)
  - Spool of #14 enameled copper wire
  - Small vice and wire counter

Step 1: Primary winding (3 turns only)
  - Wind exactly 3 turns of #14 wire
  - Keep spacing even and tight
  - Count on fingers: 1... 2... 3... (verify twice)

Step 2: Secondary winding (21 turns)
  - Without removing primary, wind 21 turns directly on top
  - Count carefully: 1, 2, 3... 19, 20, 21 (verify count twice)
  - If you lose count, start over (critical application)

Step 3: Final verification
  - Recount primary: 3 turns ✓
  - Recount secondary: 21 turns ✓
  - Measure turns ratio: 21/3 = 7:1 ✓

Step 4: Connect antenna
  - Primary (3 turns) to antenna feedpoint
  - Counterpoise (10–30 feet) to primary return

Step 5: Connect coax
  - Secondary (21 turns) center to coax center conductor
  - Secondary return to coax braid

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

1. END-FED HALF-WAVE (EFHW) ANTENNAS:
   - Classic multiband design (80M–10M, some 160M with tuner)
   - 135-foot length typical (resonant @ 40M)
   - 49:1 unun DESIGNED specifically for this antenna
   - Best overall performance: EFHW + 49:1 unun + small tuner

2. PORTABLE EFHW (Field Operation):
   - Lightweight, single wire design
   - 10-foot counterpoise (short, portable)
   - 49:1 unun as key component of portable station

3. QRP OPERATIONS:
   - End-fed antennas ideal for low-power (inherently high-Z fed)
   - 49:1 unun minimizes insertion loss impact on QRP (dB loss less critical)

4. MULTIBAND MATCHING:
   - Unlike balanced antennas (require per-band tuning), EFHW + tuner
     works across entire 80M–10M band with single tuner

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

Setup:
  Port 1: Coax 50Ω output
  Port 2: Primary antenna side (2450Ω load)
  Use 2450Ω resistive load for testing

Measurement 1 — Impedance Ratio (2450Ω load):
  Sweep 1–30 MHz
  Expected: Z_out ≈ 50Ω (with 2450Ω load)
  S₁₁ <-20 dB (excellent match)

Measurement 2 — Insertion Loss:
  Sweep 1–30 MHz
  Expected: S₂₁ ≈ -0.30 to -0.50 dB (accept high loss)
  Note: Loss decreases at higher frequencies

Measurement 3 — Turns Count Verification:
  Use NanoVNA to measure inductance ratio
  L_secondary / L_primary should equal (N_sec / N_prim)² = 49:1
  Verification: L₂₁ / L₃ should be 49±2 (tolerance)

================================================================================
DESIGN EQUATIONS
================================================================================

Turns ratio for 2450→50Ω:
  N_ratio = sqrt(2450/50) = sqrt(49) = 7:1
  Use 3 primary, 21 secondary (EXACT 7:1 ratio)
  Note: 3/21 = 1/7, so 21/3 = 7 ✓

Why 2450Ω?
  Historical standard from 1950s–1960s EFHW antenna designs
  Chosen to use available transformer cores of the era
  Matches half-wave end feedpoint impedance (approximately)

Insertion Loss:
  L_dB ≈ 0.25 + 0.003 × f_MHz (FT-240-43, 49:1)
  Example @ 14.2 MHz: L ≈ 0.25 + 0.043 = 0.29 dB ✓

================================================================================
FAILURE MODES
================================================================================

SYMPTOM: High SWR on all bands
ROOT CAUSE 1: Turns ratio incorrect (most likely: miscounted)
SOLUTION: Unwind entire balun, recount 3:21 exactly, rewind

ROOT CAUSE 2: EFHW antenna not properly tuned
SOLUTION: Use antenna tuner to match impedance variation across band

ROOT CAUSE 3: Counterpoise inadequate (10 feet minimum)
SOLUTION: Install counterpoise at 45° angle (10–30 feet length)

SYMPTOM: Good match on 40M, poor on 80M and 20M
ROOT CAUSE: Normal EFHW behavior (impedance varies with frequency)
SOLUTION: Use antenna tuner (mandatory for EFHW operation)

SYMPTOM: Excessive heating
ROOT CAUSE: High SWR causing high current on primary
SOLUTION: Tune antenna with tuner, or verify counterpoise installed

================================================================================
POWER HANDLING
================================================================================

FT-240-43 single core, 49:1 unun:
  Continuous (CW): 400 W (adequate for most EFHW)
  Peak (SSB): 600 W
  Thermal rise: ~20°C @ 400W
  Note: High impedance primary = low current, efficient

EFHW Power Recommendations:
  QRP: 5–10 W (excellent DX capability)
  QRO: 100–500 W (400 W typical)
  Contest: 1500 W (industrial design, special core)

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

Mechanical Assembly:
  - Mount unun at antenna feedpoint (minimize feedline losses)
  - Use weather-resistant enclosure (aluminum or PETG)
  - Counterpoise support: 45° angle, insulated from structure

Weatherproofing:
  - Enamel insulation sufficient
  - Potting recommended (silicone, not epoxy)
  - O-ring seal on enclosure (prevents moisture ingress)

Field Deployment:
  - Tree mount: Secure unun enclosure to tree (clear of feedline)
  - Portable: Small enclosure fits in backpack (weight ~300 grams)
  - Field repair: Turns count critical; rewind if any doubt

Spare Parts Kit:
  - Extra FT-240-43 core
  - Spool of #14 enameled wire (for rewinding)
  - Connectors (SO-239, BNC)
  - Potting compound (silicone)

================================================================================
HISTORICAL BACKGROUND
================================================================================

The EFHW antenna was popularized by George Dobbs (G3RJV) in 1936.
The 49:1 transformer was the standard matching device.

Modern variations include:
  - Loading coils (flatten impedance across band)
  - Multiple counterpoise elements
  - Tuning stub (series resonance cancellation)

But the classic design + 49:1 unun remains the most popular
multiband antenna for portable and field operation.

================================================================================
RECOMMENDED SYSTEM: EFHW + 49:1 UNUN + SMALL TUNER
================================================================================

For multiband portable HF operation:

1. EFHW antenna: 135 feet total length
   - Feedpoint end at 35–60 feet height
   - Counterpoise: 10–20 feet at 45° angle, 5–10 feet high

2. 49:1 unun: FT-240-43, 3:21 turns
   - Mounted at antenna feedpoint in weatherproof enclosure
   - Total weight: ~300 grams (portable)
   - Cost: ~$25

3. Small antenna tuner: L-match or similar
   - Adjustable to match 50Ω impedance variation across band
   - 80M–10M coverage (automatic or manual tuning)

4. Coax feedline:
   - 50–100 feet RG-58 or RG-8 (low loss preferred)

Total system cost: ~$120
Total system weight: ~2–3 kg (portable backpack operation)
Performance: Excellent DX capability, 100+ countries reportable with QRP

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