---
tags: [radio, ham-study]
---

> Antennas convert electrical signals into radio waves and radio waves into
> electrical signals

Different sized antennae are used for different
[frequency](./Frequency_modulation.md) bands. In general, **longer wavelengths
(hence lower frequencies) require larger antennas**.

There are five antenna that you need to know for the exam:

- the half wave dipole
- the quarter wave
- the five-eighth wave
- the end-fed long wire
- the Yagi (beam) antenna

## The half wave dipole

![](../img/dipole-a.jpg)

Its total length is equal to half a wavelength. This informs the length it needs
to be in order to pick up a certain frequency.

![](../img/dipole-b.jpg)

For example, if 15MHz has a wavelength of 20 meters, your dipole should be 10
meters long ($\frac{20/2} = 10$).

The signal from a dipole radiates as follows:

![](../img/dipole-radiation-pattern.gif)