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CM  small_loop_hf.nec — TM-ADF-NEC-002 Rev A
CM  Small Magnetic Loop Antenna for HF RDF, 40-meter band
CM  Square loop: 400mm × 400mm (0.4m per side)
CM  Loop in vertical plane, figure-8 pattern in azimuth
CM
CM  Loop dimensions:
CM    Size: 0.4m × 0.4m square, in XZ plane
CM    Wire: 4mm diameter copper (radius=0.002m)
CM    Orientation: loop plane = XZ, broadside = Y axis
CM    Null directions: ±Y (broadside to loop)
CM    Peak directions: ±X (along loop plane)
CM
CM  Analysis goals:
CM    1. Confirm figure-8 azimuth pattern
CM    2. Measure input impedance (small loop: R + jωL)
CM    3. Verify null depth (expect >30 dB if loop is symmetric)
CM    4. Measure Q factor (L/R at resonance with tuning cap)
CM
CM  Note: small loop requires tuning capacitor for resonance.
CM    L ≈ 2.2 µH (for 400mm square, 4mm tube)
CM    C_resonate = 1/(ω²L) = 1/((2π×7.15e6)² × 2.2e-6) ≈ 225 pF
CM    NEC models bare loop; tuning cap is external
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CE
CM
CM Square loop in XZ plane, 4 wire segments (one per side)
CM Loop center at origin, starts at lower-left corner
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CM Bottom side (z=0, from x=-0.2 to x=+0.2)
GW 1 10 -0.2 0.0 0.0 0.2 0.0 0.0 0.002
CM Right side (x=+0.2, from z=0 to z=0.4)
GW 2 10 0.2 0.0 0.0 0.2 0.0 0.4 0.002
CM Top side (z=0.4, from x=+0.2 to x=-0.2)
GW 3 10 0.2 0.0 0.4 -0.2 0.0 0.4 0.002
CM Left side (x=-0.2, from z=0.4 to z=0) — feed gap at bottom of this segment
GW 4 10 -0.2 0.0 0.4 -0.2 0.0 0.0 0.002
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CM Free space (GE 0) — loop is elevated, not ground-mounted
GE 0
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CM No ground (free space analysis for loop pattern)
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CM Frequency: 7.15 MHz (40m band center)
FR 0 1 0 0 7.15 0
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CM Source at lower segment of tag 4 (feed gap at bottom of left side)
EX 0 4 10 0 1.0 0.0
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CM Azimuth radiation pattern (elevation=0°, azimuth 0–360°, 5° steps)
RP 0 1 73 1000 0.0 0.0 0.0 5.0
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CM Elevation pattern (azimuth=0°, elevation 0–90°)
RP 0 46 1 1000 0.0 0.0 2.0 0.0
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CM Also compute at 14.175 MHz (20m) for comparison:
CM FR 0 1 0 0 14.175 0
CM
EN

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CM  EXPECTED RESULTS:
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CM  Impedance: R_rad ≈ 0.008Ω (extremely low — this is correct for small loop)
CM    Series inductance: jωL ≈ j99Ω at 7.15 MHz (for 2.2 µH)
CM    Q factor ≈ X_L / R_loss = 99 / (R_rad + R_wire)
CM    R_wire = ρ_ac × (wire length / area) at 7.15 MHz
CM    For 4mm copper tube, l=1.6m: R_ac ≈ 0.01Ω → Q ≈ 5000 (theoretical)
CM    Practical Q: 200–500 (contact resistance, shield losses)
CM
CM  Pattern: Bidirectional (figure-8) in azimuth
CM    F/B ratio: ~∞ theoretically, > 30 dB in practice
CM    Null direction: perpendicular to loop plane (±Y direction)
CM    Peak direction: along loop plane (±X direction)
CM
CM  FOR RDF USE:
CM    Rotate loop until minimum signal (null) is heard
CM    Null direction = 90° from transmitter bearing (or ±90° ambiguous)
CM    Add sense antenna to resolve 180° ambiguity
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