Explore the electromagnetic spectrum through interactive simulations. Understand how signals travel, why AM fades at night, how GPS knows where you are, and what's actually happening inside a radio receiver.
Scroll to explore each topic — every visualization is interactive.
Radio waves occupy frequencies from about 3 kHz to 300 GHz. Each band has distinct propagation characteristics determined by wavelength — longer waves diffract around obstacles and reflect off the ionosphere; shorter waves travel line-of-sight and can carry more data.
Hover or click on the spectrum to explore frequency bands
30–300 MHz · λ = 1–10 m · FM radio, TV broadcast, air traffic control, marine VHF. Line-of-sight propagation with some diffraction. The sweet spot for many services.
A 1 MHz AM signal has a 300m wavelength — it literally wraps around buildings. A 2.4 GHz WiFi signal at 12.5 cm barely passes through walls.
Shannon's theorem: C = B·log₂(1+SNR). Higher carrier frequencies allow wider channels. This is why 5G uses mmWave — bandwidth is plentiful above 24 GHz.
FSPL = 20·log(d) + 20·log(f) + 32.44 dB. Signal strength drops with square of distance AND frequency. Double the frequency = 6 dB more loss at same distance.
Adjust frequency, transmit power, terrain type, and atmospheric conditions to see how they affect signal coverage. The simulation models free-space path loss, terrain attenuation, atmospheric absorption, and basic diffraction.
Amplitude Modulation varies signal strength to encode audio. Frequency Modulation varies the carrier's frequency instead. This fundamental difference explains everything — noise immunity, bandwidth, fidelity, and propagation behavior.
↑ Increase noise to see why FM is resistant — noise affects amplitude, which AM encodes data in, but FM encodes in frequency shifts unaffected by amplitude noise.
| Property | Detail |
|---|---|
| Bandwidth | ~10 kHz per channel |
| Audio range | ~50 Hz – 5 kHz |
| Noise immunity | Poor — noise is additive to amplitude |
| Propagation | Ground wave + skywave (ionospheric bounce) |
| Range | Hundreds to thousands of km at night |
| Capture effect | None — signals add together |
AM stations skip off the ionosphere at night (F-layer moves higher, reflecting longer wavelengths). This is why you can hear distant AM stations after sunset — and why the FCC reduces power for many stations at night to prevent interference.
| Property | Detail |
|---|---|
| Bandwidth | ~200 kHz per channel |
| Audio range | ~30 Hz – 15 kHz (stereo) |
| Noise immunity | Excellent — amplitude noise rejected by limiter |
| Propagation | Line-of-sight only (VHF doesn't reflect off ionosphere) |
| Range | ~100 km typical (horizon-limited) |
| Capture effect | Strong — strongest signal dominates |
FM's capture effect: if two stations overlap, the stronger one (even by just 1 dB) completely silences the weaker. AM signals just mix together. This is a direct consequence of the limiter circuit stripping amplitude variation before demodulation.
FM broadcasting boosts high frequencies before transmission (pre-emphasis, 75µs time constant in NA) and attenuates them on receive. Since noise energy is proportional to frequency in FM (triangular noise spectrum), this effectively cancels high-frequency noise. It's spectral noise shaping — giving FM another ~10-12 dB of SNR improvement at high audio frequencies.
GPS doesn't use triangulation (angles). It uses trilateration (distances). Each satellite broadcasts its precise position and exact time. Your receiver measures signal travel time, multiplies by c to get distance, then finds the intersection of spheres. In 2D: three circles. In 3D: four spheres (the fourth solves for clock error).
Click anywhere on the map to set your position. Watch how satellite ranges intersect to locate you. Drag the noise slider to see timing errors degrade accuracy.
Click the map to place your receiver
3 unknowns (x, y, z) + 1 unknown (clock bias). Your receiver's quartz crystal is off by milliseconds — at light speed that's ~300 km error. The 4th satellite solves for this, letting a $2 crystal replace a $50,000 atomic clock.
GPS clocks tick 38µs/day faster due to general relativity (gravity is weaker in orbit) minus 7µs/day from special relativity (orbital velocity). Net: +31µs/day. Without correction: ~10 km/day drift.
L1 = 1575.42 MHz, L2 = 1227.60 MHz. Each satellite transmits a unique PRN code at 1.023 Mchip/s. Your receiver cross-correlates to measure time-of-arrival with ~1m precision. Signal arrives at about -130 dBm — 20 dB below thermal noise floor.
Nearly every radio since the 1930s uses superheterodyne architecture. The key insight: instead of building a tunable narrowband filter (mechanically impossible at RF), convert the desired signal to a fixed intermediate frequency (IF) where a single high-quality filter works for all stations.
Hover over each block to learn its function ↓
The mixer produces f_IF = |f_RF - f_LO|. But two different RF frequencies produce the same IF: f_LO + f_IF and f_LO - f_IF. The unwanted one is the image frequency. For FM at 99.1 MHz with IF = 10.7 MHz and LO = 109.8 MHz, the image is 120.5 MHz. The RF front-end must reject it — at 21.4 MHz separation, not hard. AM with its 455 kHz IF has images only 910 kHz away, making rejection harder — explaining why AM radios need better front-end filters or dual-conversion architectures.
An antenna converts between guided electromagnetic waves (in a transmission line) and free-space radiation. Its properties — gain, radiation pattern, impedance, bandwidth — are entirely determined by physical geometry relative to wavelength.
Interactive antenna radiation pattern — select type:
An antenna has identical properties whether transmitting or receiving. Gain, impedance, efficiency — same in both directions. Design once, use for both.
Max power transfer requires matched impedance. Dipole is ~73Ω, coax is 50/75Ω. Mismatch causes reflected power (SWR > 1). At SWR=3, 25% of power reflects back.
Antenna gain is passive — it redistributes radiation directionally. A Yagi focuses energy forward like a flashlight vs. a bare bulb. Total radiated power unchanged.
A_e = G·λ²/(4π). A 1m dish at 10 GHz ≈ 38 dBi. Same dish at 1 GHz ≈ 18 dBi. Higher frequency = more gain for same physical size.
Radio waves don't just fly in straight lines. Depending on frequency and conditions, they can hug the surface, bounce off the ionosphere, scatter off rain, or duct through temperature inversions.
Surface wave follows Earth's curvature via diffraction. Dominant for AM broadcast (below ~2 MHz). Range depends on ground conductivity — seawater is excellent (σ ≈ 5 S/m), dry rocky soil is poor (σ ≈ 0.001 S/m). Reliable day and night. Attenuation increases sharply with frequency.
Solar UV ionizes atmospheric gases at 60–600 km altitude, creating conductive layers (D, E, F1, F2). These refract/reflect radio waves below ~30 MHz. The D-layer (60-90 km) absorbs MF signals during the day and disappears at night — this is why AM radio range dramatically increases after sunset. The F2-layer at ~300 km provides the longest skip distances, up to ~4000 km per hop.
The Maximum Usable Frequency (MUF) varies with solar activity, time of day, and season. During solar max, MUF can reach 50+ MHz, occasionally allowing VHF signals worldwide. During solar min, HF ops may find frequencies above 10 MHz dead. Ham operators obsessively track Solar Flux Index (SFI) and K/A geomagnetic indices to predict conditions.