An antenna is nothing more than a piece of metal. Its job is to convert electrical energy into an electromagnetic wave for transmitting a radio signal, and vice versa for receiving. In this lesson, we’ll cover the key concepts you need to choose the right antenna, whether it’s for an SDR project or anything else.

To choose an antenna, you first need to decide on the frequency you’re working with. But what exactly is frequency?
When you plug into a wall outlet, the current is alternating, meaning it regularly switches direction (it goes from + to – and back). This change happens smoothly, forming a neat sine wave.

For example, in France, the current oscillates at 50Hz, meaning it changes direction 100 times per second. That’s what we call frequency, measured in Hertz.
A radio is also an oscillator; we refer to a VFO (Variable Frequency Oscillator), a fancy way of saying you can change the frequency.
For instance, in your car you might turn the dial to tune into 102.4MHz (NRJ), which means there are 102.4 million oscillations per second!

The most important aspect of an antenna is its length, which should be chosen according to the frequency you’re interested in. We say an antenna is resonant at a specific frequency, which means it converts electrical energy to radio waves (and vice versa) efficiently.
For an antenna to be in resonance with a frequency, its length must correspond to a specific fraction of the signal’s wavelength.

The wavelength is calculated with a simplified formula, knowing that we work mostly with frequencies in MHz and the speed of light:

Once you’ve calculated the wavelength of the signal you want to use, you can choose a fraction of that value to size your antenna, typically a half-wave (λ/2) or a quarter-wave (λ/4).
For example, suppose you want an antenna for the 433MHz band. First, calculate the wavelength: 300/433≈0.70cm. If you decide on a half-wave antenna, its length should be half the wavelength, so about 34.5cm.

Let’s keep it simple. For effective signal transfer between the antenna and your transceiver, they must be well matched. This matching is determined by a property called impedance.
Think of it as the resistance in an electrical circuit, but for alternating currents. In RF, you want the antenna’s impedance to match that of your device. A common standard is 50Ω, for example, SDR receivers typically use this value. In short, you’ll always aim for an antenna close to 50Ω.

There are tons of antenna types, so we won’t cover them all, but here are three important ones.

The simplest antenna is a doublet. It comes in a folded version (often called a trombone) and the classic dipole; we’ll focus on the latter, though both work on the same principle.
In its basic form, it’s a half-wave antenna (λ/2), with each arm measuring λ/4. It is fed at its center and is installed isolated in space, away from the ground.

The angle between the 2 arms can be adjusted to fine-tune the impedance. When they’re straight as shown, the impedance is 75Ω. With a 120° angle, you achieve a 50Ω match as seen in this project.
Also, a dipole can be mounted horizontally or vertically, depending on the polarization of the waves you want to capture.
Lastly, this type of antenna focuses its energy in two directions, making it very effective when you know the precise direction of your signal.

A ground plane antenna features a vertical conductor of quarter-wave length (λ/4) mounted above a conductive surface, known as the ground plane, hence the name. This ground plane acts like a mirror, “completing” the antenna by reflecting waves, effectively creating an equivalent to a dipole.
To form this ground plane, you can use metallic rods called radiators arranged evenly around the base, giving the antenna a spider-like look 🕷️. Similar to the dipole, if you want a 50Ω system, the radials should be spaced at about 120°.
Otherwise, you can use the ground, the sea, a car roof, anything conductive.

Yagi antennas, which work from HF to UHF, are directional. Also known as “rake” antennas, they’re the ones usually seen on rooftops for terrestrial TV.
The idea is to start with a half-wave doublet (λ/2), then add non-powered parasitic elements to focus the energy in a specific direction. You have director elements, which are shorter than the doublet and placed in front, and reflector elements, which are longer and positioned behind.

By increasing the number of elements, the impedance decreases and the gain increases ↗️. These antennas are great when you need to capture a signal coming from a particular direction, as they concentrate the energy toward it.

You should now have a better understanding of how to choose your antenna and adjust its size according to the signal you want to receive :)