Current in Motion – The Secret Behind Radiation
What really makes an antenna radiate? It’s not black magic—it’s moving current, shaped by wavelength and geometry.
FIGURE 1: Cartoon illustration – “Current in Motion, the Secret Behind Radiation.”
It All Starts with Electricity and Magnetism
Every antenna story begins with the basics of physics. When an electric current flows through a wire, it generates a magnetic field around it. If that current changes rapidly, it also produces a changing electric field. These two fields—the electric (E-field) and magnetic (H-field)—lock together at right angles and sustain each other as they move outward. That’s an electromagnetic wave (see Figure 2).
FIGURE 2: The E-field and H-field are always at right angles, propagating together as an EM wave.
In this diagram, the blue wave is drawn larger and the red wave smaller, but which field is which depends on the antenna type.
For a vertical dipole, the E-field is vertical (aligned with the antenna), while the H-field circles around it. For a small loop, the opposite orientation is emphasized: the H-field dominates in the loop plane and the E-field is orthogonal. The important point is that they are always at right angles, moving together through space.
Ampère’s Law — Current Makes Magnetic Field
A steady current in a wire creates concentric magnetic field lines around that wire. The stronger the current, the stronger the magnetic field; the farther you are, the weaker it gets (~1/r).
- Right-hand rule: Thumb points in current direction; curled fingers show the magnetic field direction.
- Straight wire: Field lines are circles around the conductor.
- At RF: Time-varying current ⇒ time-varying magnetic field — this is half of how antennas radiate.
Later, Maxwell adds the “displacement current” term: a changing electric field also creates a magnetic field. Together with Faraday’s law, this closes the loop and lets E and H fields propagate as a wave.
Movement is Essential
A battery powering a wire produces a steady field but no radiation. To create a wave, the current must oscillate. That’s why radio transmitters use alternating current at high frequencies: the back-and-forth motion shakes the surrounding fields loose, sending energy into space.
Size Matters – Enter Lambda (λ)
The wavelength of a signal, written as λ (lambda), is the ruler by which antennas are measured. A half-wave dipole is about λ/2 long—making it a natural resonator and an efficient radiator. If an antenna is much shorter than the wavelength, it struggles to radiate efficiently. Instead of creating strong fields, it wastes power as heat or reactive energy.
Antennas are physical structures we electrically load. Their length, geometry, and termination define how currents and voltages distribute along them. By shaping those standing waves correctly, we maximize how much of the applied energy becomes radiation instead of heat.
Why Open Ends Radiate
At the ends of an antenna, current doesn’t just vanish—it reflects back, creating standing waves of voltage and current. These oscillations amplify the changing fields, which is why most antennas are open at the tips or tuned to encourage strong oscillations.
But What About Closed Loops?
It’s natural to wonder: if open ends help an antenna radiate, how can a loop antenna—which has no open ends—work so well?
Loops don’t rely on reflections at the tips. Instead, the current flows in a continuous circle. As it oscillates, the loop sets up a changing magnetic field that drives an accompanying electric field. Together, these radiate just as effectively.
Where a dipole launches radiation from the ends, a loop launches radiation from the entire circumference of the wire. The shape and size of the loop, relative to λ, determine how efficient it is and what kind of pattern it produces.
Direction and Pattern
Antennas are not light bulbs. A dipole, for instance, shines broadside to the wire but produces very little off the ends. Figure 3 shows the two classic views of a half-wave dipole:
- Red curve — the vertical-plane (elevation) pattern: a figure-eight that is strongest broadside to the dipole and has deep nulls off the wire ends.
- Green circle — the horizontal-plane (azimuth) pattern: essentially a circle, showing equal radiation in all compass directions around the antenna.
- Blue line — the dipole element itself, drawn at the center for orientation.
FIGURE 3: Half-wave dipole radiation.
Blue = physical dipole element (orientation reference).
Red = vertical-plane figure-eight (strong broadside, nulls off the ends).
Green = horizontal-plane circle (equal azimuthal radiation).
Ground Planes and Counterpoises
Some antennas need a mirror. Vertical antennas, for example, rely on the ground (or an artificial ground plane made of radials) to act as a reflecting surface. Without a return path for current—whether soil, radials, or a counterpoise—radiation efficiency drops dramatically.
Reactance Explained
In electrical terms, an antenna is not just a piece of wire — it behaves like a mix of resistance (R) and reactance (X). Resistance turns power into either heat or radiation, while reactance stores energy temporarily in electric or magnetic fields.
If an antenna is capacitive, it stores energy in electric fields (like a capacitor). If it is inductive, it stores energy in magnetic fields (like a coil). In both cases, that stored energy bounces back and forth instead of being radiated efficiently.
When X = 0, the reactance is canceled out, leaving only resistance. But not all resistance is useful — some is loss resistance (heat), and some is radiation resistance (RF energy leaving as a wave). A dummy load, for example, is purely resistive (X = 0) but all of its resistance is heat, so it radiates nothing.
It’s also important to note that X = 0 does not automatically mean a good match. Impedance is written as Z = R + jX. When X = 0, Z = R. For a 50 Ω system, only when R ≈ 50 Ω will the SWR be 1:1. If R is 10 Ω or 200 Ω (but still with X = 0), the SWR will be high, even though the load is purely resistive.
Resonance and Matching
An antenna works best when it resonates at or near the operating frequency. Resonance means its reactance (X) is close to zero, so voltage and current are in phase. This condition makes the antenna easier to feed, but it does not guarantee efficiency by itself. A dummy load is also purely resistive and well matched, yet it radiates almost nothing. The difference is that a good antenna’s resistance is mostly radiation resistance, while a dummy load’s resistance is heat — and how much is heat depends on the antenna’s physical structure and efficiency.
Matching networks and proper feedline adjustments reduce the bouncing of energy back and forth between the transmitter and antenna. This doesn’t create more radiation—it just ensures the power has the best chance to reach the antenna. Whether that power becomes a strong signal or wasted heat depends on the antenna’s structure and radiating efficiency.
Radiation Resistance – Why Some Antennas Seem “Lossy”
Not all the current in an antenna turns into radio waves. Some is lost as heat in the wire, ground, or surrounding objects. The portion that actually becomes radiation is described as radiation resistance. A small antenna (far shorter than λ/4) often has a very low radiation resistance—meaning most power is lost rather than radiated.
So Why Does an Antenna Radiate?
Boiling it down:
- Changing currents create changing electric and magnetic fields.
- These fields couple and travel outward as electromagnetic waves.
- The antenna’s geometry determines how well those waves are launched.
- Efficient radiation happens when antenna size relates to λ.
- Open ends, loops, and ground planes all provide mechanisms to strengthen the effect.
- Resonance and matching reduce reflections but do not guarantee efficiency.
- Radiation resistance tells you how much becomes signal vs. heat.
- Good SWR does not equal good efficiency — efficiency depends on radiation resistance.
Final Thought
Grasping why antennas radiate is the foundation of all RF work. Whether you’re testing QRP in the backyard or assembling a contest-grade station, remember: it all comes down to alternating current, wavelength, and geometry.
Master these basics, and you’ll already be ahead of the curve.
Mini-FAQ
- Does an antenna need to be resonant to work? — No. Non-resonant antennas can radiate if matched. Resonance usually improves radiating efficiency, but for receiving it makes little difference, since the receiver only needs enough signal above the noise floor.
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Why does SWR matter? — High SWR means energy bounces back and forth between the transmitter and antenna instead of flowing smoothly into the load. This can stress the transmitter and increase feedline losses. A good transmatch (antenna tuner) can protect the rig and reduce these line losses by matching impedances. But matching does not make the antenna itself more efficient — efficiency depends on radiation resistance. It’s entirely possible to have an efficient antenna with poor SWR (hard to feed, but a good radiator), and equally possible to have a perfect SWR but very poor efficiency (dummy load).
[Transmitter] ---> [Tuner/ATU] ---> [Feedline] ---> [Antenna] ^ | |<-- Reflections--|Forward power travels toward the antenna. If the impedance is mismatched, some reflects back. The tuner matches the transmitter to the line, but only the antenna’s radiation resistance determines how much becomes actual RF radiation versus heat.
Case A: Efficient Antenna, Bad SWR [Tx]--->[Feedline]--->[Antenna] mismatch good radiator Case B: Dummy Load, Perfect SWR [Tx]--->[Feedline]--->[50Ω Load] perfect heats, no RF outSWR alone doesn’t tell you efficiency. An antenna can radiate well even with poor SWR if matched properly, while a dummy load gives a perfect SWR but radiates nothing at all.
- Is matching the same as efficiency? — No. Matching just ensures maximum power transfer to the antenna system. Efficiency is determined by how much of that power becomes radiation rather than heat.
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