Ví dụ, một điển hình 12-bay cửa quay ăng-ten cho ban nhạc highVHF có thể có hiệu quả chỉ có 0,83 hoặc 83%. RMS ngang directivity có thể thấp như sự thống nhất đối với một ăng-ten đa hướng lên đến gần 6 chỉ thị cho một số đánh giá cao các loại ăng-ten UHF. Giá trị phổ biến hơn trong phạm vi của 2. | 214 RADIO-WAVE PROPAGATION LINEAR DISTORTIONS The expression for relative signal strength due to multipath may be used to estimate the propagation induced linear distortions across a digital TV channel. From this result the degradation of EVM C N or the tap values for an equalizing filter may be estimated. This effect is similar in nature to that of a mismatched transmission line. To compute frequency response and group delay the starred equation in the Multipath section is written in rectangular form r V1 Re TT _V0_ V Im V- _V0_ 1 Y A oURn 1 C An cos------ c n 1 _ i oURn y .L sin c n 1 The amplitude of the frequency response is simply the magnitude of the vector Re V Vo C j Im V V0 or Mag V V 0. - V 2 V 2 1 2 Re V0 C Im V0 The phase is the angle of this vector V Ph T7-_V0. 1 Im V V0 tan Re V V0 Both amplitude and phase are proportional to echo magnitude. If the direct signal is obstructed the echo magnitudes may be greater than unity. Recall from Chapter 4 that group delay GD is the negative first derivative of phase with respect to angular frequency. Also recall from calculus that 1 u du dx dx 1 C u2 For the present calculation let u Im V V0 Re V V0 and x o. It is now straightforward to find the derivatives of the real and imaginary parts from which the group delay may be computed d Re V V0 do d Im V V0 do n 1 y- URnAn sin oURn c c n 1 yp URnAn cos oURn c LINEAR DISTORTIONS 215 The group delay after considerable manipulation is GD y a yRn ẵRnAn cos wỗRn c _ L An . - c N N An sin 5Rn c ỖRnAn c sin ỖRn c _ _ 2 N ỵ An sin ịfù8Rn c _n 1 2 N 1 An cos 5Rn c n 1 This complex expression has the dimensions of seconds as expected. The group delay is proportional to both echo magnitude and delay. The components of this expression are similar in form to a Fourier series with coefficients equal to the amplitudes of the interfering waves. The periods are proportional to the frequency and the incremental distance traveled by the waves. To visualize the effect of multipath
