How Antennas Radiate

Every ham radio operator uses antennas, but very few understand the physics of why they work. How does a piece of wire hanging in the air convert a radio-frequency voltage from your transmitter into a wave that travels thousands of miles through empty space? The answer lies in one of the most elegant principles in physics: an accelerating electric charge radiates energy. This lesson explains that principle from the ground up and shows you exactly what happens inside an antenna when you key up.

An alternating current flowing in a conductor causes the surrounding electric and magnetic fields to detach and propagate outward as electromagnetic waves.

View Larger

Accelerating Charges and Radiation

To understand how antennas radiate, you first need to understand what happens when electric charges move. A charge sitting still creates an electric field that extends outward in all directions — like the spokes of a wheel radiating from a hub. A charge moving at constant velocity creates both an electric field and a magnetic field that accompany it as it moves, but those fields travel with the charge and do not detach. Nothing radiates. Nothing propagates away from the charge independently.

Now accelerate the charge — speed it up, slow it down, or reverse its direction. The moment you do that, something dramatic happens. The electric field lines, which were nicely organized around the moving charge, get "kinked." The field configuration near the charge rearranges itself quickly to match the new velocity, but the field far away cannot respond instantaneously because the speed of light is finite. The far field still shows the old configuration. At the boundary between the old field and the new field, there is a kink — a disturbance that propagates outward at the speed of light. That propagating disturbance is electromagnetic radiation.

In a radio antenna, the charges that radiate are the electrons in the wire. Your transmitter drives an alternating current through the antenna conductor. The electrons accelerate forward as current increases, then decelerate as current decreases, then accelerate in reverse as current reverses direction. This is happening at your transmitter frequency — for example, at 14 MHz the current direction reverses 14 million times per second. That constant acceleration and deceleration of billions of electrons produces a continuous stream of electromagnetic radiation.

The power radiated depends strongly on how much the charges are accelerating and on the length of the conductor over which that acceleration occurs. A very short wire with very little current produces very little radiation. A wire that is a significant fraction of a wavelength long, carrying a substantial current, radiates much more efficiently. This is the fundamental reason why antenna size is related to wavelength — the physics of radiation demands it.

Electric and Magnetic Fields in a Wave

The electromagnetic wave that leaves an antenna consists of two inseparable components: an electric field (E-field) and a magnetic field (H-field). These two fields are always present together — you cannot have one without the other in a propagating wave. They are perpendicular to each other and both are perpendicular to the direction the wave is traveling.

Imagine you are looking down a long corridor. An electromagnetic wave traveling down that corridor toward you would have its E-field oriented either up-down (vertical polarization) or left-right (horizontal polarization), and its H-field would be oriented at exactly 90 degrees to the E-field. The wave energy flows along the corridor toward you, and the two fields oscillate back and forth as the wave passes.

The ratio of the electric field strength to the magnetic field strength in a plane wave in free space is always the same: approximately 377 ohms. This is called the impedance of free space (often written as η₀ or Z₀). It is a fundamental constant of the universe. Just as a resistor has a fixed ratio of voltage to current (Ohm's Law), free space has a fixed ratio of E-field to H-field for a propagating wave. This impedance of free space is why antennas must be matched to 50-ohm or 75-ohm transmission lines through specific geometric arrangements — there is a large impedance transformation to be managed between 377 ohms in space and 50 ohms in your coaxial cable.

As a wave travels away from the antenna, the E-field and H-field amplitudes decrease with distance. In the far field — more than a few wavelengths away from the antenna — the field strengths decrease in proportion to 1/distance (inverse of distance). This means doubling your distance from a transmitting antenna reduces the field strength by half (a 6 dB reduction in field strength, or a 6 dB reduction per octave of distance). This is why distance has such a dramatic effect on signal levels and why more transmit power is always at a disadvantage compared to a better antenna at the receiver end.

The Hertzian Dipole

The simplest possible antenna, from a theoretical standpoint, is the Hertzian dipole — named after Heinrich Hertz, who first demonstrated electromagnetic radiation in 1887. A Hertzian dipole is an infinitesimally short piece of wire carrying a uniform current along its entire length. It does not exist in practice (a real wire always has a current distribution that varies along its length), but it is enormously useful as a mathematical building block.

The Hertzian dipole radiates power in all directions except straight along its axis — in other words, it does not radiate at all toward the ends (along the wire direction) and radiates most strongly at right angles to the wire. If you could look at the radiation pattern three-dimensionally, it would look like a doughnut (a toroid) with the wire running through the hole. This doughnut-shaped radiation pattern is characteristic of all dipole-type antennas, though the exact shape changes as the antenna length increases.

Any practical antenna can be analyzed by thinking of it as a large number of Hertzian dipoles arranged end-to-end along the conductor, each carrying its local current, with their individual contributions added together. The resulting pattern depends on the current distribution along the antenna and the geometry of the arrangement. This mathematical approach — called the method of superposition — is how antenna simulation software like EZNEC works, breaking the antenna into small segments and summing the contributions of each.

The Hertzian dipole also gives us the first insight into antenna gain. Because it does not radiate toward its ends, all of the radiated power is concentrated into the doughnut-shaped pattern. Compared to a hypothetical isotropic radiator (which radiates equally in all directions, like a sphere), the Hertzian dipole has about 1.76 dBi of gain at its equator. A full half-wave dipole, because of its current distribution, has slightly more gain than a Hertzian dipole at its equator — about 2.15 dBi. These numbers appear constantly in antenna specifications, and you will understand them fully by the time you finish this module.

Radiation Patterns

A radiation pattern is a diagram that shows how an antenna distributes its radiated power in different directions. It is usually shown as a polar plot — a circular chart where the distance from the center represents the signal strength (in dB) in each direction. Understanding radiation patterns is essential because you want to know where your signal is going, and where you are sensitive to incoming signals.

Radiation patterns have two important views. The azimuth pattern (also called the horizontal pattern) shows the radiation looking down from above — it tells you which compass directions have the strongest signal. The elevation pattern shows the radiation in a vertical cross-section — it tells you what angle above the horizon the signal goes, which determines whether you are optimized for local contacts (high angle, almost straight up) or DX contacts (low angle, nearly horizontal).

For the half-wave dipole oriented horizontally, the azimuth pattern is a figure-eight. The signal is strongest broadside to the wire (at right angles to the wire) and is zero off the ends. If you hang a dipole oriented east-west, your best signal is to the north and south, and you have a null to the east and west. You can use this null deliberately — for example, pointing the null of a dipole toward an interfering station to reduce its signal.

The elevation pattern of a horizontal dipole depends critically on height above ground. A dipole at a height of one wavelength (about 20 meters at 14 MHz, or about 66 feet) has most of its radiation concentrated at a low angle — good for long-distance HF contacts. A dipole at just one-quarter wavelength height (5 meters, or 16 feet) has most of its radiation going straight up, which is good for short-range NVIS (near-vertical incidence skywave) contacts but poor for DX. Height matters enormously for HF dipoles.

Current Distribution on a Real Antenna

A real dipole antenna carries a sinusoidal current distribution — the current is highest at the center (where you connect the feedline) and drops to zero at the ends. This is not something you can choose or change through clever engineering for a simple wire dipole; it is a natural consequence of the boundary conditions at the free ends of the wire. At the end of a wire, there is nowhere for the current to go, so it must be zero. At the center, the current is maximum.

This sinusoidal current distribution has profound consequences. It means that the center of the dipole (where current is highest) contributes the most to radiation. The ends (where current is low) contribute the least. This is why the feedpoint impedance of a half-wave dipole is around 73 ohms — the impedance seen at the current maximum is relatively low. If you fed the dipole at the end (at a current minimum, which is a voltage maximum), the impedance would be many thousands of ohms.

The current distribution also explains why a dipole must be approximately a half wavelength long to work well as a resonant antenna. At the resonant length, the current distribution is exactly one half-cycle of a sine wave — from zero at one end, through maximum at the center, back to zero at the other end. The antenna is resonant, meaning the reactive components of its impedance cancel, leaving only a resistive load for your transmitter. You can certainly use a dipole that is not exactly resonant, but you will need an antenna tuner to compensate for the resulting reactance.

As antenna length increases beyond a half wavelength, the current distribution becomes more complex. At a full wavelength, you get two half-cycles of the sine wave, and the current actually reverses direction along the wire. This changes the radiation pattern significantly — instead of a simple figure-eight, longer antennas develop multiple lobes and nulls. These multi-lobe patterns can be useful or problematic depending on your goals.

The current in a half-wave dipole follows a sine wave — maximum at the feedpoint, zero at the tips. The strongest radiation is broadside to the antenna.

View Larger

The Principle of Reciprocity

One of the most useful and beautiful results in antenna theory is the principle of reciprocity: an antenna's transmitting pattern and its receiving pattern are identical. Whatever direction the antenna radiates most strongly when transmitting, it is also most sensitive in when receiving. Whatever direction has a null in the transmit pattern is also a null in the receive pattern.

This is not obvious and it deserves a moment of thought. When you transmit, your antenna is converting electrical power into electromagnetic waves. When you receive, electromagnetic waves from space are inducing a tiny voltage in your antenna conductor, which travels down the feedline to your receiver. These seem like completely different processes. Yet the reciprocity theorem, which follows rigorously from Maxwell's equations, guarantees they produce the same directional pattern.

Reciprocity has enormous practical importance. It means you only need to measure or model an antenna pattern once — it applies equally to transmit and receive. It means that any antenna that has a beam focused toward Europe will also be most sensitive to signals from Europe when receiving. And it means that a null you deliberately create to reduce an interfering station's signal when receiving will also cause you to radiate less power toward that station when transmitting (sometimes a useful side effect, sometimes not).

There is one important caveat: reciprocity applies to the antenna pattern, but not necessarily to the overall system performance. On transmit, your antenna pattern and transmitter power determine what you send. On receive, your antenna pattern works with the receiver noise figure and sensitivity. A very low noise preamplifier can improve receive sensitivity without affecting transmit — the system is not symmetric even though the antenna pattern is. This is why some stations use separate transmit and receive antennas, with a high-gain receive antenna (like a Beverage wire) that would be impractical for transmitting.

What This Means for Your Station

Understanding the physics of radiation translates directly into better antenna decisions. First: antenna efficiency depends on having significant current in the antenna conductor. Short antennas (much shorter than a wavelength) have very small currents relative to the RF voltage, which means poor radiation. This is why a 40-meter dipole stretched across a small yard is always going to be compromised at low power — physics demands it. Knowing this, you can make an informed trade-off rather than wondering why your signal is weak.

Second: the relationship between antenna length and wavelength is fixed by physics. A 14 MHz antenna needs to be roughly 10 meters (33 feet) long for a half-wave dipole, not because someone invented that number, but because the wavelength at 14 MHz is about 21 meters and the physics of resonance requires a length of approximately λ/2. You cannot shrink a 40-meter antenna to the size of a cellphone without severe efficiency penalties.

Third: reciprocity means that when you choose an antenna for DX work, you are simultaneously choosing it for receive sensitivity in that same direction. Pointing a beam at Japan makes you both a stronger transmitter toward Japan and a more sensitive receiver from Japan. This bidirectional advantage is what makes directional antennas so powerful — they help you simultaneously on both ends of the contact.

Fourth: the 1/distance fall-off of field strength means that transmitter power increases are far less effective than antenna improvements. To double the distance at which you can be heard, you need to increase power by a factor of four (6 dB), or alternatively you can add 6 dB of antenna gain. Adding a 3-element Yagi beam (about 8 dBd gain) is roughly equivalent to increasing your transmitter from 100 watts to more than 600 watts — without the cost of an amplifier, without licensing restrictions, and without burning extra electricity.