Antennas

The idea

An antenna is the bridge between electricity in a wire and waves in the air — a fishing rod for radio waves. Physically, it converts electric current wiggling in metal into electromagnetic waves flying through space, and back again. The same antenna both transmits and receives: it is a megaphone that works in both directions. Every wireless device has one — your phone, WiFi router, and car radio all use antennas to talk to each other.

Size matters, and it follows wavelength. The most common antenna, the half-wave dipole, is half a wavelength long. Since wavelength in meters is roughly 300 divided by frequency in MHz, you can predict antenna sizes on sight:

• AM radio (about 1 MHz, λ ≈ 300 m) → towers 75–150 m tall
• FM radio (100 MHz, λ = 3 m) → the classic 1.5 m whip on old cars
• WiFi (2.4 GHz, λ = 12.5 cm) → a 6 cm element hidden in your router
• mmWave 5G (28 GHz, λ ≈ 1 cm) → antennas smaller than a grain of rice

An antenna the wrong size for its frequency is like a guitar string tuned to the wrong note — it barely resonates, and most of your power never leaves the metal. The matching also has to be electrical: antennas and cables are designed around a standard impedance (usually 50 ohms) so power transfers cleanly instead of reflecting back.

The shapes you will meet: the whip or rod is simple and radiates all around (radios, routers); the dish is a mirror that focuses waves into a beam (satellite TV); the Yagi is the classic rooftop "fishbone" TV antenna that aims one way; the patch is a flat copper rectangle that hides inside phones and drones. Once you can estimate wavelength, you can look at any antenna — a tower, a dish, a phone — and guess what frequency it uses and what it is for.

The math

A half-wave dipole's length comes straight from the wave equation:

$$L = \frac{\lambda}{2} = \frac{c}{2f}$$

For FM radio at 100 MHz: λ = 300/100 = 3 m, so the dipole is about 1.5 m. In practice, end effects shorten the ideal length slightly, to about 1.43 m at 100 MHz.

Common misconceptions

• Myth: A bigger antenna is always a better antenna. Reality: An antenna needs to be the right size for its wavelength. An oversized antenna at WiFi frequencies is mistuned, not improved.
• Myth: Transmit antennas and receive antennas are different devices. Reality: Antennas are reciprocal — the same structure works identically in both directions.
• Myth: Antennas amplify signals. Reality: Antennas are passive. They can focus energy in a direction (gain), but they never add power.

Try it

Open the Antenna Playground and switch between antenna types: rotate the dipole's donut-shaped pattern in 3D, then change the frequency and watch the required element size scale with wavelength.