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CM  dipole_cm_current.nec — TM-RFI-NEC-001 Rev A
CM  40-Meter Dipole: Effect of Common-Mode Current on Pattern
CM  Frequency: 7.15 MHz
CM
CM  Two models in one file (run separately by commenting/uncommenting EX cards):
CM    Model A: Ideal balanced dipole (no common-mode current)
CM    Model B: Dipole with 3m feed line acting as unintended radiator
CM             (common-mode current simulated by adding vertical wire at feed)
CM
CM  Purpose:
CM    Demonstrate how common-mode current on feed line:
CM    1. Distorts the azimuth pattern of the dipole
CM    2. Introduces asymmetry and elevation angle changes
CM    3. Creates spurious lobe toward nadir (ground noise pickup)
CM
CM  Geometry (Model B — with feedline radiator):
CM    Dipole: horizontal, +/- 10.05m at height 10m
CM    Feed line stub: vertical wire from feed point down 3m (representing
CM                    unshielded portion of coax braid with CM current)
CM    Feed point: center of dipole at (0, 0, 10m)
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CE

CM ── DIPOLE ELEMENTS ──────────────────────────────────────────
CM West half of dipole: tag 1
GW 1 20 -10.05 0.0 10.0 0.0 0.0 10.0 0.001
CM East half of dipole: tag 2
GW 2 20 0.0 0.0 10.0 10.05 0.0 10.0 0.001

CM ── FEEDLINE STUB (uncomment for Model B — with common-mode current) ──
CM This simulates 3m of exposed coax braid with CM current flowing on it.
CM Tag 3 — comment out for ideal balanced Model A:
GW 3 15 0.0 0.0 10.0 0.0 0.0 7.0 0.001

CM ── GROUND ─────────────────────────────────────────────────────
CM Sommerfeld-Norton real ground (average soil)
GE 1
GN 2 0 0 0 13.0 0.005

CM ── FREQUENCY ──────────────────────────────────────────────────
FR 0 1 0 0 7.15 0

CM ── EXCITATION ─────────────────────────────────────────────────
CM MODEL A (balanced, no CM current):
CM Excite at center of tag 1 (segment 20, the end closest to feed) — or use
CM split feed: one EX card on each half with equal and opposite phases.
CM Simple method: excite center segment of tag 1 (segment 20 = feed end)
EX 0 1 20 0 1.0 0.0

CM MODEL B (with CM current on feed line stub):
CM Comment out MODEL A excitation; use MODEL B excitation:
CM Excite dipole center AND add partial CM current to stub
CM (EX on stub tag 3 segment 1, with fraction of main feed):
CM EX 0 1 20 0 1.0 0.0
CM EX 0 3 1 0 0.3 0.0   ← 30% of feed current = common-mode current on stub

CM ── RADIATION PATTERNS ─────────────────────────────────────────
CM Azimuth pattern at 10° elevation (horizontal component dominates):
RP 0 1 73 1000 10.0 0.0 0.0 5.0
CM Elevation pattern broadside (azimuth = 90°):
RP 0 46 1 1000 0.0 90.0 2.0 0.0
CM Elevation pattern endfire (azimuth = 0°):
RP 0 46 1 1000 0.0 0.0 2.0 0.0

EN

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CM  EXPECTED RESULTS:
CM
CM  MODEL A (ideal, no CM current):
CM    Azimuth: figure-8, nulls at 0° and 180° (along wire axis)
CM    Maximum gain: ~8 dBi broadside (90°, 270°)
CM    Elevation angle of maximum: ~28° at 10m height on 40m
CM    Symmetry: perfect left-right, E/W symmetry
CM
CM  MODEL B (with 3m CM current on stub):
CM    Azimuth: distorted figure-8, nulls filled in
CM    Asymmetry: N-S lobes become unequal (stub breaks symmetry)
CM    Additional low-angle lobe toward nadir: ground noise pickup increases
CM    Pattern tilt: maximum elevation angle shifts slightly
CM    Net effect: 3–10 dB increase in received noise from ground clutter
CM
CM  COMPARING MODELS:
CM    Run both, compare azimuth plots.
CM    Difference = effect of common-mode current.
CM    Installing choke at feed point: returns to Model A pattern.
CM
CM  RFI IMPLICATION:
CM    With CM current, the feed line ADDS to received noise from:
CM    - Ground currents (SMPS return currents in soil)
CM    - Nearby wiring in house (the braid is an antenna for house wiring noise)
CM    - Computer equipment (CM current couples from USB, Ethernet, PSU ground)
CM    Choke at antenna feed point breaks CM path → noise reduction of 10–30 dB
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