An ultra high frequency (UHF) aerial, more commonly called a UHF antenna, can come in many different forms for use with electronic equipment that receives or transmits radio signals between 300 megahertz (MHz) and three gigahertz (GHz). This type of aerial can come in almost any shape or size but falls into one of several categories of antennas: quarter wave antennas, half wave dipole antennas, folded dipole antennas, loop antennas, and Yagi antennas. Each has advantages and disadvantages.
Quarter wave UHF antennas, commonly called “whip” antennas, consist of a single metal rod or wire that is one-quarter of the wavelength desired to be received long. They are usually mounted vertically, akin to the types of antennas seen on cars. As a result, they are said to be “vertically polarized,” with one pole at the top of the antenna and the other at the bottom. Most UHF transmitters are polarized horizontally, and quarter wave UHF aerial antennas are the least efficient of UHF antennas; simplicity of construction, ease of attachment to equipment, and the ability to receive signals from any direction, or omni-directionally, often outweigh that shortcoming, however.
Some quarter wave antenna designs coil the wire of the antenna so that it appears as a long spring. Though the antenna is still vertically oriented and polarized, the horizontal aspects of the individual windings of the coil do manage to improve the antenna’s efficiency. Despite this improvement, quarter wave UHF aerial antennas are usually only used in equipment where cost, portability, and the need to receive signals from all directions are paramount.
The half wave dipole UHF aerial, the standard in UHF antenna design, consists of two metal rods or wires that extend straight, usually horizontally, with one end near the other. These two components form one-half of the wavelength to be received long. Though half wave UHF aerials are more efficient than their quarter wave cousins, they only get good reception of signals that strike the antenna’s two elements perpendicularly, requiring that these antennas be directed toward the signals they are to receive. As the standard UHF aerial, half wave dipoles are often incorporated into other, more efficient, UHF antenna designs.
Folded dipole antennas are very common. This type of UHF aerial is made from a single piece of metal or wire folded or bent into a shape that brings the two ends close to one another. In this way, they function like a half wave dipole antenna and have almost identical characteristics, though they tend to be less efficient. The prime advantage of folded dipole antennas is that they are easy to manufacture from inexpensive materials. The circular loop-type UHF aerial, often used with a television, is an example of a folded dipole antenna.
Despite the name, loop antennas are not the same as the round wire loops used as television antennas. Instead, loop antennas consist of a length of wire wound many times around a ferrite core, usually only a few inches long. These antennas have good reception characteristics but are not entirely omni-directional. Frequently called “stick” antennas, loop antennas are most often internal antennas hidden inside the cases of electronics equipment.
Yagi antennas consist of pairs of metal rods arranged in parallel to one another, and are commonly seen serving as rooftop television antennas. The metal rods of the Yagi antenna serve three different purposes. One pair of the rods is actually a half wave dipole antenna. Behind the dipole there are one to three pairs of rods that serve as reflectors sending signals that have passed the dipole back toward the dipole. In front of the dipole are pairs of rods called directors that guide incoming signals so they focus on the dipole.
Yagi antennas are extremely efficient and often used where UHF signals are very weak. They do have a significant disadvantage: they are extremely directional and frequently only capable of receiving signals in a beam 20-degrees wide. Yagi antennas often have a motor to rotate them in different directions because they can only “see” stations or transmitters that they are pointed almost directly at.