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Ideally, an antenna should be mounted at the pointwhere the signals are the strongest, though often I this is impractical or impossible. Also, the antenna should be mounted where it can be easily serviced should repairs or adjustments be required in the future. So carefully "aiming" (orienting) the antenna and adjusting its height often can overcome the problems created by installing it in a slightly weaker signal area.
In an area where there is a relatively short, unobstructed signal path between the installation and the transmitting towers, you can mount the antenna just about anywhere and receive sufficient signal. As you move deeper into the fringe areas however, there may be significant differences in signal strength at various points on the roof. This is where careful antenna selection, precise orienting, and accurate adjustment of the height become essential. The careful installer should be able to locate a spot where there is both a usable signal and sufficiently easy access to the antenna.
Large trees can present reception problems and nearby high-voltage power lines can cause interference that cannot be completely eliminated. Consequently, the further the antenna is from large trees and high-voltage lines, the better the reception will be.
WALKING THE ROOF FOR OPTIMUM SIGNAL STRENGTH
"Walking the roof" is usually the best way to find the spot on the roof where the strongest signals are present. First, attach a small TV antenna to short mast (4-6 feet). Attach one end of a short transmission line to the antenna terminals. Attach the other end of the transmission line to the input jack of a field-strength meter. Then walk to the most suitable mounting areas. Point the front of the test antenna in the general direction of the transmitter tower(s). Slowly, rotate the antenna while carefully watching the field-strength meter. (Most field strength meters are equipped with shoulder straps that free both of your hands for manipulating the antenna.)
USING A FIELD-STRENGTH METER
A small, battery-powered portable TV is helpful for determining general reception quality. However, there is no better way to determine actual signal strength than to use a field-strength meter (Figure 3-1). It is one of the best investments you can make. The meter will save you so much time and trouble that it will quickly pay for itself.
Field-strength meters come with detailed operating instructions. Below are general characteristics and operating procedures that apply to most fieldstrength meters.
A field strength meter has an input impedance of 75ohms. Most television antennas have an output impedance of 300 ohms. For accurate measurements these two impedances must be "matched." Attach a balun (matching transformer) to the antenna terminals and run coaxial cable from the balun to the meter input jack. CTV coaxial cable has an impedance of 75 ohms.)
Most field-strength meters will measure the strength of both VHF and UHF signals and will separately measure each channel. Most field strength meters will also separately measure the picture and sound signals of each channel.
The ideal is to get the strongest signal possible. When this is not possible and you need to sacrifice a little strength, connect the test antenna output to a battery operated, portable TV set and check the picture and sound to ensure that the signal levels are adequate for acceptable reception. The TV will also help you better explain reception possibilities and problems to your customers.
A field-strength meter indicates signal strength in microvolts (uV) and decibels relative to one milli-volt (dBmV). One microvolt is one-millionth of a volt, or 0.000001 volt. The microvolt reading is probably the easiest to use. As a general guideline, a signal level of 50 uV at the antenna output terminals is the weakest usable signal; however, it will require strong preamplification to provide acceptable TV reception. Antenna output signals of 500 uV and above are considered adequate. Any signal level below 1,000 uV will require preamplification.
To use the field-strength meter, orient your test antenna in the direction of incoming signals and activate the meter. To orient the actual installed antenna in an area where several channels are coming from the same general direction, aim the antenna to obtain the highest reading with the field-strength meter set on the highest channel to be received. (Channel 13 is the highest VHF channel, and 69 is the highest UHF channel.) As a result, this will orient the shortest antenna element as well as properly align remaining elements. Of course, if the channels are coming from different directions, you will have to take separate readings for each and ad3ust the antenna(s) accordingly.
ADJUSTING THE HEIGHT FOR OPTIMUM SIGNAL STRENGTH
Field Strength Meter.
Just as the received signal strength varies with the mounting site and the direction in which the antenna is pointed (oriented), the height at which the antenna is installed will also affect signal strength. A few feet up or down may make a big difference in the level of the received signal(s), especially on UHF channels. Watch your field~strength meter as you slowly raise and lower the test antenna.
The height at which you ultimately mount the antenna depends on two things: signal strength and practicality. You should mount the antenna only as high as necessary, even if your meter indicates that the signal level continues to increase as you raise the antenna. Choose the height at which you get the highest signal level without having to resort to guyed mast and other more expensive and difficult to install equipment. Your objective should be to get as close as possible to an unobstructed "line of sight" between the transmitter tower(s) and the receiving antenna so that the signal clears all large buildings and other obstructions.
In some cases, obtaining adequate signals may require the installation of a multiple array. A multiple array is two or more antennas set at different heights on the same mast ~igure 3-2). Ideally, these antennas should be spaced seven feet apart so that they won't interact with each other. If this is not practical, try to mount them at least five feet apart. When installing more than one antenna on a mast, try to mount the largest (heaviest) at or near the mast support. This will help prevent excessive strain on the mast during high winds.
Fig. 3-2. A vertically stacked antenna array.
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