Description

The Model 3151 Foreshortened Log Periodic Dipole Antenna features a convenient space-saving design with the use of top hat-loaded, low-frequency elements. This reduced-sized design has optimized impedance loading to simulate the moderate gain of a full-size LPDA. Without size reduction, the longest dipole element length would be approximately 7.5 m (24.6 ft). Instead, the Model3151’s entire width is only 3.5 m (11.5 ft).To enhance measurement repeatability, the Model 3151’s longest elements are supported with nylon straps to ensure consistent element positioning.

The 3151 antenna features a specially-designed feed to handle high power requirements of up to 5 kW for generating high electric field strength. A hairpin matching stub is used to improve the low-frequency VSWR, which minimizes mismatch loss between the amplifier and antenna.

The 3151 antenna is constructed of lightweight, corrosion-resistant aluminum, providing years of trouble-free service. All elements are attached with knurled knobs for easy assembly.

The included wheeled pedestal permits horizontal and vertical antenna polarization, and tilt-angle adjustments. A pneumatic cylinder is used to change polarization, and can be remotely controlled using an optional ETS-EMCO Model 2090 Controller or an ETS-EMCO 2190 Virtual Controller. The Model 2090 is IEEE-488 compatible, and uses fiber optic I/O for communication with the pedestal.

Electric Field distribution
The Model 3151 is used for both horizontal and vertical polarizations. The phase center /source position of the antenna shifts from the rear to the front end of the boom as frequency increases. To ensure a high field strength at the low frequency range(30 MHz – 45 MHz), the LPDA antenna needs to be placed at a close distance. However, the beam width of the antenna gets narrower at higher frequencies. Therefore, the antenna needs to be placed at a farther distance at higher frequencies to cover a larger uniform area. If the same distance is kept for the high and low frequency ranges, the field uniformity is compromised above 80 MHz.