Radiation resistance
The effective resistance representing power radiated as radio waves from an antenna.
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Radiation resistance is the component of an antenna's feedpoint resistance that arises from the emission of radio waves. When a radio transmitter sends a radio-frequency alternating current into an antenna, the antenna radiates that energy as radio waves.
Radiation resistance and loss resistance
Because the antenna absorbs the energy it radiates from the transmitter, its input terminals present a resistance to the current. This is an effective resistance, representing the power carried away as radio waves, and is conventionally defined as the value of electrical resistance that would dissipate the same amount of power as heat as the antenna radiates. It is not the same as ordinary ohmic resistance, which comes from the imperfect conductivity of the antenna's materials.
The feedpoint impedance and radiation resistance depend on the antenna's geometry, the operating frequency, and its location, especially relative to the ground. The relationship between feedpoint impedance and radiation resistance changes based on where the feedline is attached. When the feedpoint is placed at the antenna's point of minimum voltage and maximum current—the usual arrangement—the total feedpoint impedance equals the sum of the radiation resistance and the loss resistance from ohmic losses in the antenna and nearby soil. If the feedpoint is elsewhere, a correction factor is needed.
Receiving antennas
In a receiving antenna, the radiation resistance acts as the source resistance, and the power consumed by it represents radio waves re-radiated (scattered) by the antenna.
Cause
Electromagnetic waves are radiated when electric charges accelerate. In a transmitting antenna, time-varying electric currents—electrons flowing back and forth—generate radio waves, driven by the oscillating voltage from the transmitter. An electromagnetic wave carries momentum away from the electron that emitted it.
The cause of radiation resistance is the radiation reaction: a recoil force on the electron when it emits a radio wave photon, which reduces its momentum. This is known as the Abraham–Lorentz force. The recoil force opposes the electric field accelerating the electron, reducing the average electron velocity for a given driving voltage, thus acting as a resistance opposing the current.
Radiation resistance is only part of the feedpoint impedance.
Definition and Distinction from Ohmic Resistance
Radiation resistance is the portion of an antenna's feedpoint resistance that arises specifically from the emission of radio waves. When a transmitter drives an alternating current through an antenna, the antenna absorbs energy from the source and converts it into electromagnetic radiation. This energy absorption manifests at the input terminals as an effective resistance. Crucially, this is not the same as the resistive opposition you would encounter in a copper wire or any imperfect conductor.
It is not caused by the material's inherent resistivity. Instead, it is a bookkeeping concept: the value of resistance that would dissipate the identical amount of power as heat as is actually carried away by the radiated waves. Formally, it equals the total radiated power divided by the square of the root-mean-square current flowing into the antenna terminals. This makes it a purely electromagnetic quantity tied to the act of radiation itself, independent of the physical conductor's quality.
The Physical Mechanism: Radiation Reaction
At the microscopic level, radiation resistance traces back to a phenomenon known as the Abraham-Lorentz force, or radiation reaction. When electrons in an antenna are accelerated by the oscillating electric field of the applied transmitter voltage, they emit electromagnetic wave energy. Each emitted photon carries momentum away from the electron, producing a recoil force that opposes the very field accelerating the electron.
This recoil reduces the average velocity of the electron flow for a given driving voltage, effectively acting as a drag on the current. The cumulative effect across all moving charges in the antenna structure is what we observe macroscopically as radiation resistance at the feedpoint. In essence, the antenna feels a back-reaction from its own radiation, and that back-reaction is indistinguishable from a resistive load from the perspective of the driving source.
Feedpoint Impedance and the Role of Geometry
The numerical value of radiation resistance is not a fixed property of a material but is determined entirely by the antenna's physical geometry, the operating frequency, and its spatial relationship to the ground. The relationship between the total feedpoint impedance and the radiation resistance further depends on where along the antenna the feedline is connected. The simplest and most common case occurs when the feedpoint is placed at the antenna's natural voltage-minimum, current-maximum point.
In that configuration, the total feedpoint resistance equals the straightforward sum of the radiation resistance and the loss resistance, which accounts for ohmic heating in the metal elements, ground losses from induced currents in the soil, and dielectric losses in insulating materials. If the feedpoint is relocated to any other position on the antenna, a correction factor must be applied to the basic relationship, making the calculation more complex.
Radiation Resistance in Receiving Antennas
In the receiving context, radiation resistance takes on a different but equally important role. It represents the source resistance of the antenna as seen by the connected receiver circuit. The portion of the received radio power that is consumed by this radiation resistance does not represent useful signal energy delivered to the receiver. Rather, it corresponds to radio waves that are re-radiated, or scattered, back into space by the antenna structure.
This means that even a perfectly lossless antenna with zero ohmic resistance will still present a nonzero feedpoint resistance due to radiation resistance, and some fraction of the captured energy will inevitably be re-emitted rather than transferred to the load. Understanding this distinction is essential for evaluating the true efficiency of a receiving antenna, since the radiation resistance sets a fundamental lower bound on the source impedance that no amount of material improvement can eliminate.
Frequently Asked Questions
Who is Radiation resistance?
Radiation resistance is the portion of an antenna's feedpoint resistance that accounts for the power actually launched into free space as radio waves. It is not a true ohmic loss but an effective resistance representing the energy carried away by the radiated electromagnetic field.
What is Radiation resistance's signature number?
For a zero-thickness half-wave dipole in free space, radiation resistance works out to roughly 73 ohms. This value emerges from the interplay between the dipole's geometry and the intrinsic impedance of free space, which is approximately 120 ohms.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Radiation resistance (CC BY-SA 4.0).
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