EM Principles

Part of Radio

Electromagnetic principles explain how changing electric and magnetic fields propagate through space as waves, carrying energy and information.

Why This Matters

Radio works because of a relationship that Maxwell formalized in 1865: a changing electric field creates a magnetic field, and a changing magnetic field creates an electric field. Each generates the other, and together they propagate outward from their source as a self-sustaining electromagnetic wave moving at the speed of light. This is not intuition — it is mathematics that unifies electricity, magnetism, and optics.

For a builder of radio equipment, understanding electromagnetic principles is the difference between following recipes and understanding why those recipes work. When you know that wavelength equals the speed of light divided by frequency, you can calculate antenna lengths without memorizing tables. When you understand why an inductor opposes current changes while a capacitor opposes voltage changes, you can design filters and matching networks. When you grasp that near-field and far-field behave differently, you understand why your antenna needs to be distant from metal objects.

These principles also reveal the limits and possibilities of radio in a rebuilding scenario: why higher frequencies travel in straight lines while lower frequencies follow the Earth’s curve; why the ionosphere reflects certain frequencies back to Earth enabling long-distance communication; why your spark-gap transmitter occupies a wide swath of spectrum and why a tuned oscillator is far more efficient.

Electric and Magnetic Fields

An electric field exists around any charged object or between voltage differences. Its strength (field intensity, E, in volts per meter) decreases with distance and is directed from positive toward negative charge. Accelerating charges — charges that are speeding up, slowing down, or changing direction — radiate electromagnetic energy. This is the physical basis of antenna operation: when you drive alternating current through a wire, the charges in that wire are continuously accelerating back and forth, radiating electromagnetic waves at the frequency of oscillation.

A magnetic field exists around any moving charge (current). A straight wire carrying current is surrounded by circular magnetic field lines. The field strength H (amperes per meter) is proportional to current and inversely proportional to distance. In an inductor (coil of wire), the magnetic field is concentrated through the coil’s center and forms the basis for energy storage in the magnetic field.

Maxwell’s equations express four relationships: Gauss’s law for electric fields (charges create electric fields); Gauss’s law for magnetic fields (no magnetic monopoles exist); Faraday’s law (changing magnetic flux creates electric field — this is how transformers and generators work); and the Ampere-Maxwell law (current and changing electric field create magnetic field).

From these four equations, Maxwell derived that electric and magnetic fields can propagate together as waves, with the wave speed equal to 1/√(ε₀μ₀), where ε₀ is the electric permittivity of free space and μ₀ is the magnetic permeability. The calculated value is exactly the speed of light: 2.998 × 10⁸ meters per second. This is how Maxwell predicted that light itself is an electromagnetic wave decades before it was confirmed experimentally.

Frequency, Wavelength, and the Spectrum

The fundamental relationship: c = fλ, where c is the speed of light (3 × 10⁸ m/s), f is frequency in hertz, and λ (lambda) is wavelength in meters. Rearranged: λ = c/f = 300/f(MHz) meters.

The electromagnetic spectrum spans from DC (0 Hz) through radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Radio waves conventionally span from about 3 Hz (extremely low frequency, ELF, used for submarine communication) to 300 GHz (millimeter wave). Human radio communication uses primarily:

• Very Low Frequency (VLF): 3–30 kHz. Penetrates seawater, used for naval communication. Wavelengths 10–100 km. Earth-following propagation.
• Low Frequency (LF): 30–300 kHz. AM broadcast band low end, navigation beacons. Wavelengths 1–10 km.
• Medium Frequency (MF): 300 kHz–3 MHz. AM broadcast (535–1705 kHz). Wavelengths 100 m–1 km.
• High Frequency (HF): 3–30 MHz. Amateur radio, shortwave broadcast, international communication. Wavelengths 10–100 m. Ionospheric propagation enables global reach.
• Very High Frequency (VHF): 30–300 MHz. FM broadcast, amateur radio, aviation. Mostly line-of-sight.
• Ultra High Frequency (UHF): 300 MHz–3 GHz. Television, cellular, Wi-Fi. Strictly line-of-sight.

For post-collapse communication planning, HF (3–30 MHz) is the critical range. It offers the combination of long-distance propagation (via ionospheric reflection) with antenna and equipment complexity that is achievable with first-generation fabricated equipment.

Reactance, Impedance, and Resonance

In AC circuits — including all radio circuits — resistors dissipate energy (they convert electrical energy to heat, following Ohm’s law at all frequencies). But inductors and capacitors store energy temporarily, and their opposition to current flow is frequency-dependent. This frequency-dependent opposition is called reactance.

Inductive reactance: XL = 2πfL (ohms), where f is frequency in Hz and L is inductance in henries. At higher frequencies, an inductor opposes current more strongly — it “resists” the rapid current changes that high frequencies demand. At DC (f = 0), inductors are just wire.

Capacitive reactance: XC = 1/(2πfC) (ohms), where C is capacitance in farads. Capacitors oppose current flow more strongly at lower frequencies; at high frequencies they pass current easily. At DC, capacitors are open circuits (no DC passes).

Impedance Z combines resistance R and reactance X: Z = √(R² + X²). For a series circuit, Z = R + j(XL - XC) where j indicates the reactive component. When XL = XC (inductive reactance equals capacitive reactance), the imaginary part cancels: Z = R. This is resonance, and it occurs at: f_resonant = 1/(2π√(LC)).

At resonance, a series LC circuit presents minimum impedance (essentially just the small resistance of the wire) and maximum current flows. A parallel LC circuit presents maximum impedance at resonance — it is “transparent” to the resonant frequency, presenting high impedance that prevents the signal from being shunted away. This is the principle of the tank circuit in every radio receiver: it resonates at the desired frequency, passing it on to the detector while presenting high impedance to all other frequencies.

Near Field and Far Field

Close to an antenna, the electromagnetic field is complex — it has reactive components (stored energy that sloshes back and forth near the antenna like the magnetic field of an inductor) that do not radiate and a radiating component that propagates outward. The transition between these regions happens at roughly λ/(2π) from the antenna — for a 7 MHz antenna (λ = 42 m), this is about 7 meters.

In the near field, coupling between antennas is inductive or capacitive — similar to transformer coupling. Metal objects in the near field “load” the antenna, shifting its resonant frequency and dissipating energy. Keep metal objects, feedlines, and structures at least λ/4 from the antenna for best performance.

In the far field (beyond about 2λ from the antenna), the fields are purely radiating: E and B fields are perpendicular to each other and perpendicular to the direction of propagation. The ratio E/H = 377 ohms (the impedance of free space) everywhere in the far field. An antenna can be meaningfully characterized by its radiation pattern — the angular distribution of radiated power — only in the far field.

Understanding near-field effects explains practical antenna placement: why your dipole works poorly when run next to a metal roof; why the SWR on your feedline changes when someone walks near the antenna; why the tuning of your tank circuit shifts when you bring your hand close to the coil. The reactive near field extends farther than people expect, and good antenna practice keeps structures and conductors well clear.