.CHAPTER ONE: TV ANTENNA SELECTION

    Basically, a receiving tv antenna is a device for intercepting the electromagnetic waves or signals, sent from a transmitter. Some tv 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 atright angles. The main receiving element of an antenna is called the dipole  All of the other  TV antenna parts aredesigned 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

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  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.    TV Antennas designed to receive both the low band and the high band are called VHF/FM broadband TV antennas. 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  TV Antennas

UHF (Ultra High Frequency)  TV antennas are designed to receive TV channels 14 through 69, the UHF TV 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 is possible because they don’t require the long elements that VHF/FM antennas do.

Combination UHF/VHF/FM  TV Antennas

Combination UHF/VHF/FM TV antennas are designed to receive both the UHF and the VHF/FM bands. Although the use of separate TVantennas for each band is ideal for peak reception, high-performance UHF/VHF/FM combination models are readilyavailable and are becoming increasingly more popular. 

HOW TO SELECT THE RIGHT TV 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-freecolor TV picture, requires a signal level of about 1000 microvolts (µV) = 1 millivolt (mV). To deliver this signal level to the receiver, 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 TV antenna are its gain, sensitivity classification, directivity, and front-to-back ratio.

Gain

The gain of an TV antenna indicates the relative strength of signal it can deliver to a receiver. Most manufacturers list the gain of their  TV antennas in decibels (dB). The higher the  TV antenna gain, the stronger the signal at the antenna output terminals. In most cases, the larger the TV antenna, the higher the gain.

If you are in doubt about the amount of gain required, select an TV antenna that is slightly larger than you think is actually needed. The performance of all components deteriorates slightly during the years they are exposed to rain, sun, wind, and corrosion. Consequently, to ensure quality reception for a longer period, choose an antenna with a little more gain than is necessary. (Note: Excessive gain may cause overloading. Therefore, choosing the antenna with the highest gain might create more reception problems than it cures.) Sensitivity classifications are used to indicate the type of reception area for which the TV antenna is designed. Certain ranges of gain are best suited for certain types of areas. The appropriate range of gain for a specific area is what a manufacturer is indicating when labeling an antenna.

DIRECTIVITY

Direcitivity is the ability of an antenna to intercept signals from only one direction and reject those from other directions. Directivity indicates the  TV antenna’s ability to intercept signals arriving at its front and reject signals coming from the sides and rear. Generally, the more highly directive an antenna, the better it can reject signals from the sides and rear. The front-to-back ratio of an antenna can be helpful when attempting to determine its directivity. Front-to-back ratio is expressed in decibels (dB) and can be found in the literature accompanying a new TV antenna. This ratio indicates an antenna’s ability to reject signals coming from the rear (rear rejection). For example, an TV antenna with a front-to-back ratio of 25 dB will receive about 18 times more signal strength from the front than from the back. In most cases, an acceptably accurate estimate of an TV antenna’s directivity can be made by comparing its specified front-to-back ratio with the relative sensitivity.

Other factors, such as the antenna’s beamwidth, can affect its directivity. Beamwidth is related to an TV antenna’s overall gain and indicates how wide or narrow the antenna’s reception area is. For example, if two TV antennas have the same front-to-back ratio, the one with the highest overall gain will have the narrowest beamwidth and consequently, will be the most directive. An antenna with a relatively narrow beamwidth generally is best suited for areas where interference from sides is a problem. An antenna with a broad beamwidth is best suited for areas where a broad beam is needed to capture the signals from widely separated stations, and where interference is minimal. Beamwidth information, if included by the manufacturer, is usually displayed by use of polar plots. Selecting the most suitable TV antenna becomes easy with experience, but selecting quality materials should be the first decision to make before starting any installation. High-performance color reception usually requires highly sensitive equipment. In the long run, it pays to use the finest equipment available, from the TV antenna down to the receiver end of the transmission line. The slight additional cost should be considered an investment in longer systemlife and optimum performance. 

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