مركز برايت للغات والترجمة وتقنية المعلومات
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مركز برايت للغات والترجمة وتقنية المعلومات
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مركز برايت للغات والترجمة وتقنية المعلومات

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مُساهمةموضوع: طلب ترجمة    طلب ترجمة  I_icon_minitimeالخميس يوليو 21, 2011 3:27 am

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هذا المقطع هو مقطع من كتاب الاتصالات الفضائية



This consists of a receiving antenna feeding directly into a low-noise amplifier/converter combination. A
parabolic reflector is generally used, with the receiving horn mounted at the focus. A common design is to have the focus directly in front of the reflector, but for better interfer¬ence rejection, an offset feed may be used as shown.


Huck and Day (1979) have shown that satisfactory reception can be achieved with reflector diameters in the range 0.6 to 1.6 m (1.97-5.25 ft), and the two nominal sizes often quoted are 0.9 m (2.95 ft) and 1.2 m (3.94 ft). By contrast, the reflector diameter for 4-GHz reception can range from 1.83 m (6 ft) to 3 m (10 ft). As noted in Sec. 6.13, the gain of a parabolic dish is proportional to (D/Al. Comparing the gain of a 3-m dish at 4 GHz with a 1-m dish at 12 GHz, the ratio D/'A equals 40 in each case, so the gains will be about equal. Although the free-space losses are much higher at 12 GHz compared with 4 GHz, as described in Chap. 12, a higher-gain receiving antenna is not needed because the DBS operate at a much higher EIRP, as shown in Table 1.4.
The downlink frequency band of 12.2 to 12.7 GHz spans a range of 500 MHz, which accommodates 32 TV/FM channels, each of which is 24-MHz wide. Obviously, some overlap occurs between channels, but these are alternately polarized left-hand circular (LHC) and right-hand circular (RHC) or vertical/horizontal, to reduce interference to accept¬able levels. This is referred to as polarization interleaving. A polarizer that may be switched to the desired polarization from the indoor con¬trol unit is required at the receiving horn.
The receiving horn feeds into a low-noise converter (LNC) or possibly a combination unit consisting of a low-noise amplifier (LNA) followed by a converter. The combination is referred to as an LNB, for low-noise block. The LNB provides gain for the broadband 12-GHz signal and then converts the signal to a lower frequency range so that a low-cost coaxial cable can be used as feeder to the indoor unit. The standard fre¬quency range ofthis downconverted signal is 950 to 1450 MHz, as shown in Fig. 8.1. The coaxial cable, or an auxiliary wire pair, is used to carry dc power to the outdoor unit. Polarization-switching control wires are also required.
The low-noise amplification must be provided at the cable input in order to maintain a satisfactory signal-to-noise ratio. An LNA at the indoor end of the cable would be of little use, because it would also amplify the cable thermal noise. Single-to-noise ratio is discussed in more detail in Sec. 12.5. Of course, having to mount the LNB outside means that it must be able to operate over a wide range of climatic con-ditions, and homeowners may have to contend with the added prob¬lems of vandalism and theft.


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