hiệu quả, Á, ga D ADH dv nơi mà các tài khoản hiệu quả đối với bất kỳ tổn thất ohmic trong dây chuyền thức ăn chăn nuôi và các yếu tố bức xạ. Hiệu quả có thể được thể hiện trong số tỷ lệ%. Hiệu quả có thể là một yếu tố quan trọng, tùy thuộc vào thiết kế hệ thống thức ăn chăn nuôi và thiệt hại liên quan. | 186 TRANSMITTING ANTENNAS FOR DIGITAL TELEVISION efficiency ga ndhdv where the efficiency accounts for any ohmic losses within the feed lines and on the radiating elements. Efficiency may be expressed in numeric ratio or percent. The efficiency can be a significant factor depending on feed system design and associated losses. For example a typical 12-bay turnstile antenna for the high-VHF band may have an efficiency of only or 83 . The RMS horizontal directivity may be as low as unity for an omnidirectional antenna to up to nearly 6 for some highly directive UHF antenna types. More common values are in the range of 2. In addition to its relationship to the antenna directional pattern choice of the antenna gain involves several other trade-offs. As discussed earlier the antenna beamwidth is inversely proportional to aperture length. Since decreasing beamwidth implies increasing directivity it follows that gain is proportional to length. The relationship between gain and length leads to many of these trade-offs. One consequence of greater vertical directivity and length is higher wind load. Figure 7-32 shows the wind shear for typical omnidirectional VHF and UHF transmitting antennas as a function of gain. The linear relationship is evident. Overturning moment is another important antenna structural parameter which rises even faster with increasing gain. This is a consequence of the overturning LB ẼH3HB tUJUHF Figure 7-32. Antenna wind load versus gain. ANTENNA IMPEDANCE 187 moment being the product of the mechanical center of pressure and shear. Since the shear of a uniform cylinder is proportional to length the overturning moment is approximately proportional to the square of length. In some cases the antenna gain may be limited by the available aperture space. Since at a specified channel gain is proportional to antenna length the available aperture space may become a key consideration. Also cost of the antenna is often proportional to length. Actual values .
