This simulator follows an RF signal across a free-space link from an ideal isotropic transmitter to a receiver. Compare spherical power-density spreading, receiver effective aperture and the resulting free-space path loss, while a travel marker reveals the actual propagation delay across the link distance.
• A real-time 3D scene of a transmitter cabinet with an isotropic reference radiator, an expanding spherical wavefront sector, a travel marker and distance rail, a receiver aperture on a movable carriage, and a power detector/oscilloscope, with show/hide toggles for the wavefront sector and travel markers. • Five experiment controls: carrier frequency (300–6,000 MHz), link distance (10–1,000 m), transmitter power (−30 to 30 dBm, isotropic reference), receiver assumption (fixed unity gain or fixed effective aperture), and effective receive aperture (0.002–0.05 m², used only in fixed-aperture mode). • Live metrics: wavelength, isotropic free-space path loss in dB, steady power density at the receiver, effective receive aperture, receive gain in dBi, steady received power (nW and dBm), one-way flight time, ideal detector power after arrival, and the area of the sphere at the receiver distance. • A Curves & measurements tab with two live charts (wavelength/path loss), the full Friis link-budget equations, and snapshot measurement readouts. • An Experiments tab with four guided fixtures (double the range, double frequency at fixed gain, hold collecting area instead, watch first arrival) 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.
An isotropic source distributes its power evenly over a sphere of area 4πr². Doubling range quadruples this area, so power density falls to one quarter — even in perfectly lossless free space with no atmosphere to absorb anything. Free-space path loss, FSPL = (4πr/λ)², is defined between unity-gain antennas; it isn't a physical energy loss so much as a bookkeeping term that accounts for both geometric spreading and the frequency-dependent size of a unity-gain receive aperture.
With a fixed effective aperture instead of fixed unity gain, collected power S·Ae becomes independent of frequency in this ideal model — because receiver gain rises as 1/λ² exactly as fast as isotropic path loss rises with frequency, the two effects cancel for a physically fixed collecting area. Signal energy itself always propagates at c; the travel marker and detector-arrival readouts show this real latency, while the steady-state readouts show the power expected once the signal has arrived.
This is an ideal far-field free-space link with an isotropic transmitting reference, matched polarization and no atmospheric, cable, mismatch, obstacle, multipath or noise losses. One playback second represents 100 ns, and runs stop at 20,000 ns.
It drops to one quarter. The same radiated power now crosses a spherical surface with four times the area, so power density falls with the square of distance.
No. Power simply spreads across a larger sphere; most of it never enters the small receiver aperture, but none of it is absorbed by free space itself in this lossless model.
In this ideal model, it does not change collected power at all — receiver gain and isotropic path loss both change with frequency by exactly offsetting amounts when the physical aperture area is held fixed.
Flight time equals distance divided by the speed of light. At 1,000 m this is about 3,335.64 nanoseconds — the simulator's ideal detector only becomes active after this delay, illustrating that steady-state power readouts describe the link after signal arrival, not instantaneously.