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wireless hacking projects for wifi enthusiasts phần 8

Ngân sách tính toán. (Một máy tính trực tuyến có thể được tìm thấy Đường dẫn bị mất, số tổn thất trong dB xảy ra khi một tín hiệu vô tuyến đi qua không gian miễn phí (không khí), còn được biết đến như mất không gian miễn phí (FSL). FSL có thể được tính bằng cách sử dụng công thức sau: | Antennas Chapter 10 235 Budget calculation. A good online calculator can be found at www.afar.net RF_calc.htm and www.qsl.net pa0hoo helix_wifi linkbudgetcalc wlan_budgetcalc.html Path loss the amount of loss in dB that occurs when a radio signal travels through free space air is also known as Free Space Loss FSL . FSL can be calculated using the following formula FSL isotropic 20Log10 Freq in MHz 20Log10 Distance in Miles 36.6 Additional factors you should consider when determining your link s requirements Radiation pattern propagation angle The propagation angle is given in degrees and denotes how much area in degrees an antenna broadcasts its signal. Example Vertical angle 45 degrees Horizontal angle 7 degrees. Search the Internet for various antenna manufacturers to find examples of Smith charts that represent various propagation angles. Polarity All antennas have a pole short for polarity which can be horizontal vertical or circularly polarized. Polarity indicates the angle of the RF wave s propagation in reference to an H V C plane.You must insure that all Wi-Fi systems you want to communicate with have antennas on the same pole. The difference in H V poles if for example one antenna is horizontally polarized and the other is vertically polarized is a loss of 30 dB. Vertical horizontal beam width This is the angle of the RF beam referenced to the horizontal or vertical plane. Typically the higher the gain the more focused narrow the beam. Example A 24 dBi antenna commonly has an 18-degree beam width vs. a 9 dBi antenna which will have a 45- to 60-degree beam width. Fresnel zone The Fresnel zone is the propagation path that the signal will take through the air. The Fresnel zone can be determined using the formula below. The Fresnel zone is important when installing Line-of-Site equipment because if the Fresnel zone or any part of it is obstructed it will have a direct and negative effect on the system connectivity. Fresnel Zone Calculation 72.1 SqrRoot Dst1Mi