UNDERSTANDING DIGITAL TV - Ro.Ve.R. Laboratories S.p.A.

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PROPAGATION EFFECTS REFLECTIONS FROM THE GROUND At the point of reflection the incoming and outgoing angles are the same If you vary the antenna position, the point of reflection changes! 26
Another situation where the antenna is shown to be the most important component for improving reception is that of “reflections from the ground.” This system is useful for many towns located close to large water surfaces, e.g. lakes or the sea, where reflections are strong and there are rather large angles, given the short distances from the transmitters. In the case of very distant transmitters, however, the angles are small and force long distances between the antennas. With professional antenna towers, you can make systems work with distances in the order of 30 or 40 meters. For example, a transmitter 1000 meters high at a distance of 10 km, is seen under an angle of about 5 degrees, which requires a distance of about 3 meters. To see if there are reflections from the ground, measure the field and slowly vary the height of the antenna. If you notice any dB variations, there are sure to be ground reflections (see picture above). In this case positioning the antenna at the maximum point may not be enough to solve the problem, because reflection conditions vary with weather conditions. It is not the reflected signal’s change in amplitude that causes problems, but the variation of the point and the reflection phase, which takes you from a maximum to a minimum point. A very effective system is to shield the antenna from the reflected beam, by lowering it and using the roof of buildings to shield it from the reflected signal. 27
Page 1 and 2: UNDERSTANDING DIGITAL TV • Radio
Page 3 and 4: SUMMARY 6 Radio frequency spectrum
Page 5 and 6: The difference between the two sign
Page 7 and 8: The function that allows you to fin
Page 9 and 10: The power of the digital signal is
Page 11 and 12: The modulation is the same type for
Page 13 and 14: We know that signals can be “poll
Page 15 and 16: The DVB-T signal has this behavious
Page 17 and 18: Here you can see a schematic block
Page 19 and 20: When it is said that a DVB-T system
Page 21 and 22: Let us assume that the electromagne
Page 23 and 24: We will illustrate a laborious, but
Page 25: This chart is useful, even if it do
Page 29 and 30: The back to front antenna ratio is
Page 31 and 32: The reflections caused by a mismatc
Page 33 and 34: Since the system is immune to refle
Page 35 and 36: Operation is based on being able to
Page 37 and 38: Nearly every instrument has an SFN
Page 39 and 40: Here are a number of rules to impro
Page 41 and 42: This picture shows an example of an
Page 43 and 44: As an example, we have superimposed
Page 45 and 46: The diagram clearly shows the trans
Page 47 and 48: To obtain minimum reception stabili
Page 49 and 50: These predictions are verified when
Page 51 and 52: The electromagnetic field is measur
Page 53 and 54: Even if the MER is a good indicator
Page 55 and 56: The diagram shows once again the th
Page 57 and 58: This table is calculated for system
Page 59 and 60: Micro-interruptions are not always
Page 61 and 62: So ripple (waves) in the spectrum p
Page 63 and 64: In Figure 3 and 4 you can see the s
Page 65 and 66: Figure 5 shows DVB-T signal ch.8 th
Page 67 and 68: Using this case study you can see h
Page 69 and 70: Directive 2004/108/EC requires that
Page 71 and 72: The diagram shows the real case of
Page 73 and 74: MPEG4 REAL TIME SPECTRUM 6-HOUR BA
Page 75 and 76: mod. HD COMPACT STC THE MOST ADVANC

UNDERSTANDING DIGITAL TV - Ro.Ve.R. Laboratories S.p.A.

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