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Figure 3-2 Yagi pattern
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Beamwidth in degrees 3-dB points Main lobe
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Connections 20 dB Six-element yagi 10 0 c dB
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Back lobe Directivity pattern
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Figure 3-3 Log periodic antenna
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Ln Xn Xn X2 X1
Side lobes (2)
Back lobes
L1 Ln
1 wavelength of lowest antenna frequency 2 038 wavelength of highest antenna frequency
antenna beamwidth in degrees (at Antenna radiation pattern No elements in same plane L1 L2 L 3
L2 X1 X2 L2
Angle
3 db points)
L3 X2 X3
L4 X3 X4
tan
Figure 3-4 Corner re ector and parabolic antennas
Bow-tie driven element
Corner reflector
Parabolic reflector, driven element at focal point
Low gain broad band
High gain narrow band
proximity of antennas can interfere with the antenna beam patterns of each Proper spacing, depending on the wavelengths of the antennas, is required to overcome this problem 3112 Placement on the tower between VHF antennas is often the main consideration Since UHF stations operate at shorter wavelengths, the
Figure 3-5 Antenna mounting location on tower
A A (ft) 2 1 Channel 2 lowest frequency A (ft) 984 frequency (MHz) 984 178 ft 5525
3
Channel 3 example
1 The antenna for the weakest station should occupy the topmost section of the tower 2 If the receiver signals are equal, place the highest frequency (smallest) on top so as not to weigh the top unnecessarily 3 Follow the procedures outlined for proper minimum allowed spacing Vertical separation of antennas (a)
Top view 90-degree difference in bearing Antennas back-to-back (180 degrees apart) (b) Amin A1 0 180
(Antennas 0 degrees apart) 178 1 178 1 90 180 1 2
1 2
89 ft (or half what it was before) Antennas 90 degrees apart (c)
separation between antennas can be closer Also, the location of antennas depends on the pointing direction For example, antennas placed at directions on opposite tower legs can be spaced closer vertically, as shown in Figure 3-5 Notice that antennas pointing in a direction 180 from one another can be placed on different tower legs at the same level When working on the
Headends and Signal Processing
antenna placement problems on a tower, it is key to know the amount of tower height needed for the required antennas Once this measurement is known, the next issue is the amount of added height needed for proper signal strength The usual method involves an on-site signal survey, which, of course, involves some expense First and foremost is obtaining the broadcast station signal contour maps, which are often available from either the antenna manufacturers or from a consulting rm specializing in performing such on-site signal surveys Data taken from these maps can indicate whether a full signal survey is necessary In many instances, the cable system technical staff can simply go to the site and erect a temporary wideband rooftop antenna and take signal-level and picture-quality measurements with a signallevel meter and a TV set If any interference is evident, further signal tests with a spectrum analyzer may be necessary 3113 If some of the desired stations are weak because of the distance from the station, a preampli er may solve the problem In some instances, the preamp may be placed in the headend building before the signal processor, or if the signal is extremely weak, the preamp can be placed on the tower at the antenna Tower-mounted preamps usually are fed power through the coaxial cable down lead To prevent corrosion problems, care must be taken to maintain good coaxial connections with no chance of moisture ingress The signal-to-noise ratio is the most important parameter for the preamp placement problem; an example is shown in Figure 3-6 For UHF stations, the down-lead loss can become a signi cant parameter for particularly tall towers or where the headend off-air building is separated a long distance from the tower If this loss is signi cant, a tower-mounted converter can be used that converts the UHF signal to a VHF signal, which has a lower cable loss Often, these converters contain a preamp that improves the signal-to-noise ratio An example comparing the results of the use of a converter and the results without one is shown in Figure 3-7 Cable systems requiring tower-mounted converters and/or preampli ers are usually in remote areas, far from the broadcast stations For metropolitan cable systems, often a tall building s roof can be used to circumvent the multipath reception problem For such systems, the signal level is not a problem, so small, more elementary antennas are suf cient Again, an on-site signal survey will tell the story and indicate the needed equipment
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