Assignment 13

TYPES OF ANTENNAS

Introduction

This report discusses the common types of antennas available, their design and their attributes, which includes the radiation patterns, the gain and bandwidth characteristics.
Antenna Types
There are basically three classifications of antennas: Dipole, Reflector and Yagi. But the aggregate difference is in the design, as will be discussed.

Yagi Antennas
A Yagi antenna has several elements arranged in echelon. They are connected together by a long element, called the boom. The boom carries no current. If the boom is an insulator, the antenna works the same.
The rear-most element is called the reflector. The next element is called the driven element. All the remaining elements are called directors. The directors are about 5% shorter than the driven element. The reflector is about 5% longer than the driven element. The driven element is usually a folded dipole or a loop. It is the only element connected to the cable, yet the other elements carry almost as much current. The more directors added, the higher the gain becomes. Gains above 20 dB are possible. But the Yagi is a narrowband antenna, often intended for a single frequency. As frequency increases above the design frequency, the gain declines abruptly. Below the design frequency, the gain falls off more gradually. When a Yagi is to cover a band of frequencies, it must be designed for the highest frequency of the band. An antenna has an aperture area, from which it captures all incoming radiation. The aperture of a Yagi is round and its area is proportional to the gain. As the leading elements absorb power, diffraction bends the adjacent rays in toward the antenna.
The formula for the aperture area of any TV antenna is A=Gl2/4p where l is the wavelength and G is the gain factor over an isotropic (having physical properties that do not vary with direction) antenna (not dB). The bandwidth of a Yagi can be increased by sizing the reflector for the lowest frequency of the band while sizing the directors for the highest. But this decreases the best gain of the antenna. A better way to increase the bandwidth is to replace the reflector element with a corner-reflector assembly.
This boosts the performance on the lower numbered channels without hurting the high channels. Although the Yagi/Corner-Reflector might not be the best antenna, it is the most common UHF TV antenna, mainly because it can be mounted on the front of a VHF antenna without degrading the VHF antenna. This is a very directional antenna. If miss-aimed by 5° one can lose 1 dB of signal. If the horizon is more than 5° above horizontal, one should tilt the antenna up to point at the horizon. The overhead view shows nulls at 30° and 90° to both sides. These can be used to eliminate multi-path (ghosts) or interference. One should simply rotate the antenna until the offending signal is in one of the nulls.


Reflector Antennas
Radio waves will reflect off of a large conducting plane as if it was a mirror. A coarse screen will serve as well. Reflector antennas are very common.The double bow-tie screen reflector shown above has an average gain of 6 dB. With a bigger screen it would have more. The parabolic reflector focuses the signal onto a single dipole, but its bandwidth is a little disappointing. The corner reflector has a little less gain but much greater bandwidth. The corner reflector has roughly the gain of three dipoles. It is a good medium gain antenna, widely used for UHF. Log-Periodic Dipole Arrays (LPDA)
The LPDA has several dipoles arranged in echelon and criss-cross fed from the front. The name comes from the geometric growth, which is logarithmic.
This is a very wideband antenna with a gain of up to about 7 dB. For any frequency, only about three of the elements are carrying much current. The other elements are inactive. As frequency increases, the active elements “move” toward the front of the array. Most VHF TV antennas are LPDAs. TV LPDAs come in two types: straight and Vee. The Vee type (LPVA) has a very slightly higher gain for channels 7-13.







The Dipole
This is the simplest TV antenna. Variations on the dipole are the bowtie, the folded-dipole and the loop (a difference on the folded dipole). All four have the same gain and the same radiation field: a torroid (doughnut shape). The gain is generally 2.15 dB.The dipole has positive gain because it does not radiate equally in all directions. To get more gain, an antenna must radiate in fewer directions. If one rotates an antenna about the forward axis (a line from the transmitting antenna) the signal strength will vary as the cosine of the angle. In other words, when the antenna elements are vertical, no signal is received because TV signals have horizontal polarization
Stacked Dipoles
N number of dipoles will take in N times as much RF power as one dipole, provided they are not too close to each other. Thus a 4-dipole antenna would have a gain of 8.15 dB. Dipoles are commonly stacked horizontally (collinearly), vertically (broadside), and in echelon (end-fire).
When dipoles are stacked horizontally, the horizontal beam width becomes very narrow. This is because they do not add in-phase for directions not straight ahead. Similarly, when stacked vertically, the vertical beam width becomes narrower. A lot of dipoles stacked vertically would give the gain you needed. The vertical narrowness of the resulting beam is of little importance, but the horizontal broadness of the beam means no rotor needed.

Reference:

• Common Antenna Types, a HDTV primer © http://www.hdtvprimer.com/ANTENNASAll adapted materials are protected by the named copyright. All rights reserved.

.