Horizontally Polarized Log Periodic Antennas (HLPA) have found wide application in HF communications circuits because they provide good electrical characteristics (radiation pattern coverage, power gain and low VSWR) over the complete HF spectrum. In addition, it is possible to control and optimize the elevation plane coverage obtained from HLPAs by proper placement of the antennas relative to the ground.
Well known and commonly used examples of elevated apex HLPAs are the Rotatable Log Periodic Antennas (RLPAs), such as Andrew Models 2004 and 2731, and transportable log periodic antennas such as the Andrew Model 747CD. The extensive use of these antennas is due in large measure to the variable elevation plane cover age as a function of frequency being near optimum for their applications.
The stationary version of the HLPA, such as the Andrew Models 2701 and 2702, are typically used on point-to-point circuits and are designed in a manner which provides a constant elevation plane (take-off angle) coverage as a function of frequency. This characteristic is achieved by placement of the apex of the antenna at, or very near ground level.
The constant take-off angle as a function of frequency characteristic provided by economically practical HLPAs meet the requirements of point-to-point circuits for ranges up to approximately 930 miles (1500 km).
Long Range Point-to-Point Circuits
Results of ionospheric propagation analyses show that for longer range circuits, a variable take-off angle as a function of frequency is desirable in order to optimize system performance. It is possible to achieve the required elevation plane radiation coverage by appropriate elevation of the apex of a HLPA.
Figure 1 illustrates the range of take off angles at frequencies for which the ionospheric propagation analysis was performed for a 2175 miles (3500 km) range circuit. Figure 1 also illustrates the location of upper and lower half-power points in the elevation plane radiation pattern from an elevated apex HLPA (Model 2702E) supplied by Andrew to optimize circuit coverage.
Ground/Air and Shore/Ship (Long Range Coverage)
Other applications of elevated apex HLPAs are in ground/air and shore/ship long range circuits. The variable take-off angle versus frequency characteristic is obviously required for the applications because of the variable range nature of the circuits. In many cases, the RLPA finds application in this type of circuit because it readily accommodates the variable elevation plane and azimuth plane requirements of the applications. However, cases arise when it is more economical to provide the coverage required with a single, or tower sharing arrangement of elevated apex HLPAs. Figure 2 illustrates a tower sharing arrangement of elevated apex HLPAs used in a shore-to-ship HF communications network.
Andrew Model 2701E/2702E
The Model 2701E/2702E series of Stationary Elevated Apex Horizontally Polarized Log Periodic Antennas consist of the well known and field proven Model 2701/2702 series of antennas, with apex elevated to optimize circuit coverage.
Specifications (other than elevation plane coverage and dimensions) for the Model 2701E/2702E remain the same as those for the different versions of Model 2701/2702 series of antennas.
Examples of elevation plane coverage from a particular Model 2702E are shown in Figures 3A, 3B, 3C and 3D. The antenna consists of a Model 2702-104 (4 MHz low frequency cut-off) with apex elevated 100 feet (30 metres), 75 feet (23 metres), 50 feet (15 metres) and 25 feet (8 metres), respectively. Other versions are avaiable on request.