Before the internet age, antennas were commonplace extensions of most TV and radio sets, but these days they seemed to have disappeared. In reality, they have not disappeared, but rather we have come to rely upon devices that use smaller antenna. This growing reliance upon new forms of wireless technology has made the physics of antenna alignment more important than ever. Each of the following ideas can be used for a lesson that illustrates one of the important properties of antenna alignment.
Polarization and Orientation
Electromagnetic waves oscillate perpendicularly to the direction of propagation, with the electric and magnetic field components oscillating perpendicular to each other. This means that radio signals will have some sort of polarization, and as a result, there may be some optimal orientation between the transmitting and receiving antennas. A monopole, or “whip,” antenna is a single metal rod or wire that is most commonly used for AM radio and WiFi devices, and can be used to demonstrate the vertical and horizontal component of a linearly polarized radio signal by simply tilting a receiving antenna and measuring the change in signal strength.
Dipole antennas are composed of two metal rods, and can be used in circularly polarized radio signals. Such dipole antennas form the basis of more sophisticated antenna designs.
The directivity of a radio signal is used to distinguish a collimated radio beam from an omnidirectional one. Directional antennas have the benefit of focusing the power of a transmitted signal into a single preferred direction, while omnidirectional antenna radiate a signal whose power is uniform in all directions.
Placing a USB WiFi dongle in a Pringles or coffee can is often used as a cheap and easy way to turn an omnidirectional WiFi antenna into a directional antenna, and can serve as a practical demonstration of directivity. Parabolic WiFi antennas, such as the WindSurfer design, are also popular as they can be made using printouts from the internet and a little aluminum foil.
Wavelength and Antenna Size
Antennas have an optimal size, known as the electrical or phase length, which depends on the wavelength of the transmitted signal. Typically, the length of an optimal antenna will be the wavelength of the transmitted signal, but depending on the polarization and directivity of the signal, antenna components may be placed a one-half or one-quarter intervals of the wavelength. This effect can be observed by comparing the signal strength of antenna of various sizes.