This simulator explores a center-fed half-wave dipole at slowed RF time. Watch current reverse along the arms, opposite-arm charges accumulate and decay through the continuity equation, and a distant field probe respond only after a realistic propagation delay.
• A real-time 3D scene of two conductive dipole arms with an insulating feed gap, a balanced feed/coax/ferrite balun, a standing current distribution shown as teal arrows, an alternating orange/blue arm-charge display, outgoing energy markers, and a distant field probe with oscilloscope, with home view, focus-selected-part, auto-rotate, expand and show/hide instrument-cover controls. • Seven experiment controls: drive frequency (150–600 MHz), peak feed current (0.1–2 A), observation angle from the antenna axis (0–180°), far-field probe distance (5–50 m), and show/hide toggles for current arrows, charge distribution and radiation markers. • Live metrics: ideal half-wave tip-to-tip length, free-space wavelength, RF period, instantaneous feed current, charge on the positive-coordinate arm, average radiated power, radiation resistance, probe propagation delay, instantaneous and RMS far-field Eθ, and average far-field power density. • Playback controls: restart animation and advance T/4 (quarter-cycle), plus the standard pause/resume and speed controls. • A Curves & measurements tab with two live charts (feed current/arm charge, and the distant probe's retarded field), the full driven half-wave dipole equations, and snapshot measurement readouts. • An Experiments tab with four guided fixtures (quarter-cycle charge buildup, find the axial null, double the feed current, increase observation distance) and a Model verification bench, plus a timestamped event log and a copyable trial report. • A Learn & assess tab with four guided lessons, a knowledge-check quiz with reset, and a written model-scope statement with a technical-background reference link.
The prescribed standing-current amplitude follows Ipk cos(kx), measured from the center feed, with the maximum at the feed and current vanishing at both open tips — conduction current cannot flow off the end of an open conductor. The continuity equation ∂ρ/∂t = −∂I/∂x links current and charge: opposite arms always carry equal and opposite excess charge, and when feed current is at its maximum, arm charge passes through zero — a quarter cycle later, charge is maximal while current is zero.
The distant field depends on source time t − r/c, so the field probe registers what the antenna was doing at an earlier moment, delayed by the signal's travel time. A half-wave dipole has an exact axial null and a broadside maximum; changing the drive frequency in this lab also changes the ideal antenna length to keep the half-wave condition, rather than detuning a fixed-length antenna.
This is an ideal isolated thin half-wave dipole in free space with a prescribed sinusoidal current. It excludes finite-radius correction, input reactance, matching networks, nearby objects, ground effects, conductor loss and a full near-field solution. One playback second represents 1 ns of physical time.
Conduction current cannot flow off the end of an open conductor into free space, so the prescribed sinusoidal current distribution is forced to zero at both tips, with its maximum at the center feed.
It is zero. Current and charge are a quarter cycle out of phase in this prescribed standing-wave model, governed by the continuity equation relating charge density and current along the antenna.
No. This lab keeps the half-wave condition by adjusting the ideal antenna length whenever frequency changes, so it always models a properly sized half-wave dipole rather than a fixed-length antenna going off-resonance.
The far-field Eθ and power density both go to exactly zero along the antenna axis (the axial null) even though the antenna current and charge keep oscillating — a half-wave dipole simply does not radiate in that direction.