An interesting and useful array is one in which the elements are Spaced …………… apart and fed 90 องศา out-of-phase. By the time the signal from the delayed element reaches the element with the leading phase, its phase is now 90 องศา later and The two signals are in-phase and add, The signal from the leading element, on the other hand, reaches the lagging element with a delay such that the lagging element is now 180 องศา behind its phase and thus they cancel in that dilection. The result is a very useful unidilectional pattern with a deep null to the rear, as shown in Fig 14-8. A transmission line section with an electrical length of …………. Can be used to yield the desired 90 องศา delay. In some configurations the feed line can be remotely switched between the elements to provide reversible directionality.