 of up to about 2:1. Developed in 1955 at the University of Illinois it has found widespread acceptance in many areas including the military. The log periodic antenna has many similarities to the more familiar Yagi because it exhibits forward gain and has a significant front to back ratio. In addition to this the radiation pattern stays broadly the same over the whole of the operating band as do parameters like the radiation resistance and the standing wave ratio. However, it offers less gain for its size than does the more conventional Yagi. The most common form of log periodic antenna is known as the log periodic dipole array (LPDA). The basic format for the array is shown in Figure 4.26. Essentially it consists of a number of dipole elements of a size that steadily diminishes from the back of the beam where the largest element is a half wavelength at the lowest frequency. The element spacing also decreases towards the front of the array where the smallest elements are located. In operation there is a smooth transition along the array of the elements that form the active region as the frequency changes. To ensure that the phasing of the different elements is correct, the feed phase is reversed as shown in the diagram. The operation of a log periodic antenna can be gained by taking the example of operation around the middle of the operating band. It can be seen that there is a phase reversal between elements in the antenna and this is crucial to its operation. When the signal appears on the antenna itself it first sees the smaller elements at the 'front'. These are spaced quite close together in terms of the operating wavelength and this means that the fields from these elements cancel one another out as the feeder sense is reversed between the elements. Then as the signal progresses down the Antenna elements Balanced Figure 4.26 A log periodic array Antenna systems 107 antenna a point is reached where the feeder reversal and the distance between the elements give a phase shift of about 360 degrees. At this poin