Also known as: field strength, electric field strength, E-field strength
Field strength is the amplitude of the electric field a radio wave produces at a given point in space, expressed in volts per metre (V/m) — or, for the weak fields of received signals, in microvolts per metre (µV/m) or dBµV/m.1 It is a point property of the wave itself, independent of any particular receiving antenna, which makes it the natural language for describing coverage, exposure limits, and how strongly a signal arrives before an antenna converts it into a voltage.
How it works
A propagating radio wave carries coupled electric (E) and magnetic (H) fields. In the far field — many wavelengths from the antenna — these are locked in a fixed ratio, the impedance of free space, η₀ ≈ 377 Ω, and the wave behaves as a simple plane wave. There, field strength and power density S (watts per square metre) are two views of the same thing:
S = E² / η₀ and E = √(S · η₀)
So a measured or predicted power density converts straight to a V/m figure, and vice versa. Power density itself falls as the inverse square of distance from a point source, so field strength falls as 1/d — every doubling of distance halves the volts per metre (−6 dB).
The near field, within roughly a wavelength (or a couple of antenna diameters) of the radiator, behaves differently. There the E and H fields are not in the 377 Ω ratio, energy sloshes reactively in and out of the antenna’s surroundings, and E and H must be specified separately — a single “field strength” number does not capture the situation. This is why exposure and measurement standards treat near-field and far-field regions with different rules.
In practice
Field strength is what regulators actually limit and measure. Broadcast coverage contours are drawn at specific V/m or dBµV/m levels (for example, a protected service contour), and RF-exposure limits are stated as maximum permitted E-fields. Calibrated field-strength meters and antennas with a known antenna factor convert the voltage at a receiver back to the incident field, letting a measured receiver reading be reported as an absolute, antenna-independent field level.
Relevance to SDR
An SDR never reads field strength directly — its front end sees the voltage an antenna develops, which depends on that antenna’s gain, aperture, and matching. Field strength is the upstream, antenna-independent quantity: knowing the E-field at your location plus your antenna’s characteristics tells you the power delivered to the receiver, hence whether the signal clears the noise floor. It is the bridge between propagation predictions (which produce V/m or power density) and the dBm at the SDR input.
GopherTrunk works in receiver-side power terms and does not compute field strength, but the concept explains coverage maps and why a published field-strength contour translates into a predictable — or marginal — decode at a given site.
Sources
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Field strength — Wikipedia, definition in volts per metre and the relationship to power density via free-space impedance. ↩