ROTATABLE ANTENNA HEIGHT SELECTION BY ANDREW CORPORATION
Introduction
The radiation pattern; i.e., the power radiated in any direction relative to that in the direction of maximum radiation, in the elevation (vertical) plane, is strongly influenced by the presence of ground beneath the antenna. This is because the radiated signal is equal to the sum of the signal radiated directly from the antenna and that reflected from the ground. The relative phase of these components changes with the antenna's height above ground, electical characteristics of the ground and polarization, either adding to or cancelling the field due to the direct ray.
Fixed station static versions of the Horizontally Polarized Log Periodic Antenna (HLPA) such as Andrew models 2701 and 2702 are configured to provide uniform elevation plane (take-off angle) coverage as a function of frequency. This is achieved by arranging the antenna such that for any given frequency the corresponding active region of the antenna is at a constant electrical height above ground with the apex at or very near ground level. These antennas are typically used on short to medium range point-to-point circuits.
For Rotatable Log Periodic Antennas (RLPA) such as Andrew Models 2004 and 2731 mounted on a single, central support structure; such an asymmetric configuration would cause undue physical stress to the rotator. To alleviate this problem, the apex of the antenna is elevated to a point where the whole antenna is at a constant physical height above ground. This results in a variable take-off angle as a function of frequency.
Applications
Results of ionospheric propagation analyses show that for medium/long range point-to-point circuits this phenomena is desirable in order to achieve optimal system performance. Other applications for the RLPA are in ground/air and shore/ship long range circuits where the variable take-off angle versus frequency characteristic is obviously required because of the variable range nature of the circuits.
Andrew Model 2004/2731
The effects of varying the apex height are discussed in the article entitled "A Guide to Apex Elevation." The elevation plane coverage of Andrew Model 2731 mounted on a 100 ft (30 m) ground mounted tower is similar to that depicted in Figure 3A of this article.
The compact Model 2004, on the other hand, was designed primarily for roof mounting applications. As previously discussed, the ground in front of the antenna plays an important part in the formation of the vertical plane radiation pattern; this distance increasing rapidly as the take off angle decreases. This area is known as the first Fresnel zone and is that required in front of an antenna in order to effectively form the lowest ground reflection lobe. The zone is elliptical in shape with the major axis oriented in the direction of propagation.
Because the roof immediately below the antenna is small and on the near edge of the Fresnel zone, its contribution to the formation of the reflected wave is minimal and it is the actual height above the foreground which determines the elevation plane pattern.
Examples of elevation plane coverage for various effective heights above ground, corresponding to the range of standard tower heights available are shown in Figures 1-6. The antenna consists of a Model 2004 with heights of 23 ft (7 m), 33 ft (10 m), 43 ft (13.1 m), 63 ft (19.2 m), 78 ft (23.8 m) and 103 ft (31 m) (which would, in the roof mounted application, include the building).
