How Antennas Work: From Moving Electrons to Radio Waves

Radio can seem like magic. A small piece of metal sends energy across a valley with nothing connecting it to the receiver. But the physics behind it starts with something simple: electrons moving in a wire. This guide follows that movement step by step, from a plain wire to a working dipole antenna and monopole antenna, then back again to how an antenna receives a signal.

Start with a wire: what are electrons doing?

A metal wire is already full of electrons that can move. In copper, roughly one electron from each atom is free to wander through the metal instead of being tied to a single atom. A battery does not need to supply these electrons. They are there before you connect anything.

Connect a power source and it creates an electric field along the wire. That field pushes on every free electron, and they all begin to drift in the same direction, on top of their fast random motion.

This drift is surprisingly slow, often only a fraction of a millimeter per second. A lamp still lights almost immediately because the electric field travels through the wire far faster than the electrons do, so electrons all along the wire start moving at nearly the same moment.

What makes electrons move back and forth?

A battery pushes in one direction. A radio transmitter does not. It produces an alternating voltage, one that changes smoothly, reverses polarity, and repeats.

When one end of the wire is positive, electrons are pushed toward it. When the polarity reverses, the electric force reverses too, and the electrons head the other way. They never make real progress along the wire. They just move back and forth around their local positions.

One detail often causes confusion. Electrons carry negative charge, so they move opposite to the direction we conventionally call electric current. The convention was settled before anyone knew about electrons, and it stuck. Current arrows and electron arrows therefore point in opposite directions.

Reading the wave: what does the graph actually show?

Radio signals are usually drawn as a sine wave. Four basic terms describe it:

• Cycle: one complete repeat of the pattern.
• Amplitude: how far the wave reaches from zero.
• Frequency: how many cycles happen each second, measured in hertz. A radio signal at 868 MHz repeats 868 million times every second.
• Phase: where the wave is in its cycle at a given moment.

The most important thing to remember is that the graph shows how a quantity such as voltage or current changes over time. It is not the physical path of an electron. Electrons do not travel along a wire in the shape of a sine curve.

The best way to read the wave is to follow one complete cycle. Current directly describes the collective movement of the electrons, so we will use it here. Each electron also keeps its fast random motion, so "moving" below means their collective drift. Remember that conventional current points the opposite way from that drift. Follow the numbers on the graph:

1. Far left. The electrons have shifted as far left as they will go. Their collective drift briefly stops, so the current is zero.
2. Moving right, at top speed. The electrons pass through their center position. Current is at its peak, pointing left, so the red curve is at its lowest.
3. Slowing, then far right. The electrons reach the other extreme, and their collective drift briefly stops again. Current is zero.
4. Moving left, at top speed. They pass through center going the other way. Current peaks pointing right, so the red curve is at its highest.
5. Back where they started. They slow to the far left again, and the next cycle begins.

Notice that position and current do not peak together. The electrons are furthest from center exactly when the current is zero, and the current is strongest as they pass through center.

This example is a simple resistive wire, where voltage rises and falls in step with current, so a voltage graph would have the same timing. An antenna is different. Electrons respond to the local electric field at their own location, and current and charge vary along the antenna, so a voltage graph alone does not always tell you the exact timing of the current.

The distances are also tiny. At typical radio frequencies and currents, the back-and-forth drift can be much smaller than an atom.

How does that movement produce a radio wave?

Three facts from physics come together here:

1. Electrons have an electric field. Any charge has one around it, and it reaches out into the surrounding space.
2. Electric current produces a magnetic field. Moving charges create a magnetic field around the wire.
3. Changing fields create each other. A changing electric field produces a magnetic field, and a changing magnetic field produces an electric field.

A steady current gives a steady magnetic field, and that field simply stays around the wire. But in an antenna, currents and charges change direction millions of times per second. The fields near the antenna cannot adjust instantly, because changes spread at a finite speed. Part of the changing field keeps moving outward as a self-sustaining pattern of electric and magnetic fields. That traveling pattern is an electromagnetic wave, and it carries energy away from the antenna at the speed of light.

The electrons stay in the metal. Only the wave travels away.

The wave has its own timing too. It comes from change, so the field it carries depends on how quickly the current is changing. Current changes fastest at the moment electrons reverse, which is when the current itself is zero. That effect then takes time to travel, so it arrives later at any distant point. Electron position, current, and the outgoing field therefore do not all peak together.

Why does antenna length matter?

A wavelength, often written λ, is the distance a wave travels during one cycle. Radio waves are very fast, but at 868 MHz each cycle is extremely brief, so a wave travels only about 345 mm before the next cycle begins.

Now picture a signal moving along an antenna. When it reaches the tip, the metal ends, so it reflects and travels back. At suitable lengths, the outgoing and reflected waves combine into a strong repeating pattern of current along the antenna. It's a little like pushing a swing at the right moment: each new push from the transmitter adds to the motion already underway. This is resonance, and it is why the timing of the signal and the length of the antenna need to work together.

Two lengths are especially common. A half-wave antenna is half a wavelength long from tip to tip. A quarter-wave antenna is one element a quarter of a wavelength long, working with a ground plane.

Once the idea is clear, the arithmetic is simple. Wavelength is the speed of light divided by frequency, so a quarter wavelength in meters is about 75 divided by the frequency in MHz. That gives about 86 mm at 868 MHz and 82 mm at 915 MHz, both common LoRa frequencies.

The dipole antenna: two arms working together

A dipole antenna is two conducting arms connected to a signal source at the center. The source pushes electrons into one arm while pulling them out of the other, then reverses. In this way, electrons move along the arms in the same overall direction, first one way and then the other, and charge is repeatedly pushed toward the ends.

In a typical half-wave dipole, this settles into a repeating pattern called a standing wave:

• Current is strongest near the center and falls toward zero at the tips, because electrons cannot flow past the end of the metal.
• Charge builds up near the ends, with one tip positive and the other negative, swapping each half cycle.

The result is the familiar doughnut-shaped radiation pattern. A dipole radiates most strongly to the sides and almost nothing directly off its ends.

How a dipole launches a wave, step by step

Charge and field lines give another useful way to picture the same process. An electric field line runs from positive charge to negative charge, so watch what the lines do during the first half cycle:

1. Charge starts to separate. The source pushes electrons down the antenna. The top arm is left with positive charge and the bottom arm gains negative charge, starting near the center. Short field lines connect them.
2. The field spreads. As charge spreads along the arms, the field lines grow outward with it. Changes in the field travel at the speed of light, so distant space doesn't feel them yet.
3. Charge peaks. A quarter of a cycle in, the most charge has built up near the tips, and the electrons' collective drift briefly stops. Current is zero, just as in the cycle we followed earlier.
4. The changes spread outward. As the electrons reverse direction, the electric and magnetic fields around the antenna change. These changes spread outward at the speed of light, carrying energy away. The curved lines in the diagram help us picture the electric field; they are not physical strings breaking away from the wire.

In the next half cycle the charge builds the opposite way round, and the pattern repeats with the field pointing the other way. That repeating pattern of changing electric and magnetic fields, spreading outward, is the radio wave.

The monopole antenna: one element and a ground plane

A quarter-wave monopole is one conducting element, a quarter wavelength long, working with a conducting surface called a ground plane. The signal is fed between the base of the element and that surface. Despite the name, a ground plane does not need to be connected to the earth.

Think of an ideal, flat, perfectly conducting ground plane. Charges in its surface rearrange in response to the element, and their effect above the plane is exactly the same as if a mirror image of the antenna existed below it. Positive charge at the tip of the element corresponds to negative charge at the tip of its reflection, and the current in the image flows in the matching direction. Put together, the element and its image have the same form as a half-wave dipole. The ground plane electrically supplies the missing half.

This means the radiation pattern of an ideal quarter-wave monopole matches the top half of a dipole's doughnut.

Real ground planes are not infinite, of course. A vehicle roof, a set of radial wires, or a circuit board can all serve, but a small or irregular surface changes the tuning and can tilt the pattern. The mirror model is a starting point, not an exact description of every device.

Dipole vs. monopole: a simple comparison

Here are the two designs side by side at the same frequency.

Neither design is automatically better. The choice usually depends on the space available, the mounting surface, and what the device is attached to.

Optional technical note: impedance and gain

In the ideal case, a half-wave dipole has a feed impedance of about 73 Ω and a gain of about 2.15 dBi. An ideal quarter-wave monopole on an infinite, perfectly conducting ground plane has about half that impedance, roughly 36 Ω, and a gain of about 5.15 dBi, because the same power goes into half of space. A real installation's peak gain depends on its geometry, losses, and radiation pattern.

How an antenna receives a signal

Now reverse the whole story. When an electromagnetic wave reaches a receiving antenna, its electric field pushes on the free electrons in the metal. As the field alternates, the electrons move back and forth, producing a small alternating voltage and current at the antenna's feed point. A receiver amplifies and decodes that signal.

Thanks to a property called reciprocity, an antenna generally has the same radiation pattern and gain whether it is sending or receiving. A dipole that transmits poorly off its ends also receives poorly from that direction.

A practical example: a monopole on a tracker

The Loko GPS tracker uses a monopole antenna, so everything above applies directly. Its pattern is weakest off the tip, so pointing the antenna straight at the other radio is usually worse than keeping it upright. It needs a conducting surface to work against, and nearby metal or a body can change its tuning and absorb energy. Antenna placement is only one part of a radio link, as these guides on radio line of sight and signal range explain.

Further reading

• What's the difference between a dipole and a ground plane antenna? (Electronic Design)
• Understanding antenna specifications and operation, part 2 (DigiKey)
• Dipole antenna and monopole antenna (Wikipedia)