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CHAPTER ONE: ANTENNA SELECTION
Basically, a receiving antenna is a device for intercepting the electromagnetic waves or signals, sent from a transmitter. Some antennas are simple vertical poles; others are small wire loops attached to the back of a TV set.
In this manual, we will discuss the outdoor TV antenna design with which most of us are familiar: a central horizontal boom with small elements attached at right angles. The main receiving element of an antenna is called the dipole (Fig. 1-1). All of the other antenna parts are designed primarily to help the dipole do its job. The dipole consists of two half-elements to which the transmission line is attached. It is the element around which the other antenna parts are designed and positioned.
BASIC TV ANTENNA TYPES
TV antennas can be grouped in four major functional categories: VHF/FM, UHF, UHF/VHF/FM, and FM only combinations.
VHF and FM Antennas LIKE CM CROSSFIRE
Most VHF (Very High Frequency) antennas are engineered to receive TV channels 2 through 13. They also will receive the FM radio band, which is located between TV channels 6 and 7. FM only antennas are available also (Figure 1-6). Channels 2 through 6 are known as the low band. Channels 7 through 13 are referred to as the high band. Some VHF antennas are designed to receive only one band, either the low or high band. Antennas designed to receive both the low band and the high band are called VHF/FM broad-band antennas (Figure 1-2).
TV channel 1 "disappeared" in the early days of TV because of a change in frequency assignments by the Federal Communications Commission (FCC). A reshuffling of FM, Amateur, and other bands removed TV from the frequencies previously reserved for TV channel 1. Because the other 12 TV channels were already numbered 2 through 13, the channel 1 designation simply was dropped.
UHF Antennas LIKE CHANNEL MASTER UHF ANTENNAS
UHF (Ultra High Frequency) antennas are designed to receive TV channels 14 through 69, the UHF TV band. (Most of the New off air HDTV channels will be on UHF band) The UHF TV band originally extended from channel 14 to channel 83. However, the FCC now has reassigned channels 70 through 83 (also known as the translator band) to mobile communications use. Although there still are many antennas capable of receiving all 82 channels, the translator band (former TV channels 70-83) is useless to the TV viewer. UHF TV antennas come in a wider variety of shapes and sizes than VHF/FM antennas. The wider variety of UHF designs (Figure 1-3) is possible because they don't require the long elements that VHF/FM antennas do.
Combination UHF/VHF/FM Antennas
LIKE C.M. UHF ANTENNAS CROSSFIRES, ADVANTAGE
Combination UHF/VHF/FM TV antennas are designed to receive both the UHF and the VHF/FM bands. Although the use of separate antennas for each band is ideal for peak reception, high-performance UHF/VHF/FM combination models are readily available and are becoming increasingly more popular. Examples of combination UHF/VHF/FM antennas are shown in figures 14 and 1-5.
HOW TO SELECT THE RIGHT ANTENNA
There are many bands and hundreds of TV antenna models available. However, choosing the right one is relatively easy if you are aware of a few basic reception and TV antenna characteristics.
First, "good reception," or production of a snow-free color TV picture, requires a signal level of about 1000 microvolts (pV) = 1 millivolt (mV). To deliver this signal level to the receive, the antenna requires a certain antenna gain. The amount of gain required is dependent on the distance between the station's transmitting antenna and the receiving antenna. The required type of antenna therefore depends on the channels to be received and the distance and direction of the customer's home from the transmitting antennas. These facts are readily available for any area but be sure your information is accurate and complete. Call a local TV station if you have questions. Most TV stations are willing to help TV antenna installers because they also benefit from the improved reception to the station's viewers.
Be sure to discuss with your customers the number and types of channels that are receivable. This may sound basic, but your customers may not understand the difference between VHF/FM and UHF. They also may not be aware that with the right equipment they may be able to receive out-of-Town channels, some of which may carry sports programs that are locally blacked out. The most important points to remember when selecting an antenna are its gain, sensitivity classification, directivity, and front-to-back ratio.
Gain
The gain of an antenna indicates the relative strength of signal it can deliver to a receiver. Most manufacturers list the gain of their antennas in decibels (abbreviated "dB"). The higher the antenna gain, the stronger the signal at the antenna output terminals. ln most cases the larger the antenna, the higher the gain.
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