AD420 DGPS, DGPS/GPS, GPS/GLONASS comb filtered antenna

 

Introduction

A very robust, compact symmetrical antenna, the AD420 (figure 1) is designed for reception of low power satellite transmissions down to -130 dBm in areas where general purpose antennae are inoperable due to L-band interference.

 

 

Figure 1. AD420/AD430 antenna with integrated comb (AD420) or interdigital (AD430) filter and amplifier. The overall diameter of the base is 150mm (6").

 

 

Electrical Specification

The antenna element is a shaped dipole with right hand circular polarisation, an omni-directional radiation pattern in azimuth, and good reception down to 5 degrees in elevation.

The element is connected to a built in GaAs FET amplifier with a precision low-loss comb filter. A comb filter’s frequency response is shown in figure 2 for the band 1530 to 1560 MHz enabling DGPS reception. Other options are filters tuned for the band 1530 to 1575 MHz for DGPS and GPS reception (also figure 2) or 1570 to 1610 MHz for GPS and GLONASS reception.

The amplifier gain is factory preset within the range 30 dB to 45 dB with an overall noise figure of less than 2.5dB. The amplifier is phantom powered via the coaxial output lead with a 3.3V to 20Vdc supply at typically 35mA. The amplifier complies with EMC requirements and is CE marked.

 

 

Figure 2. Frequency responses of two comb filters with different bandwidths for DGPS reception (in red), and DGPS and GPS reception (in green).  The measurements were obtained over the frequency range 1500 to 1600 MHz with an Agilent HP8714ET network analyser. Comb filters are light-weight, very robust, and have low pass-band losses.

 

 

Mechanical Specification

The antenna body is in two parts, milled from aluminium, which are hard anodised and dichromate and nickel acetate sealed, giving an attractive, but extremely rugged green finish offering superb resistance to salt water corrosion. An ‘O’ ring is used to ensure a waterproof seal between the case halves.  A 4mm thick GRP pressure moulding, which can be painted to any colour required, provides a strong, but low-loss radome. The antenna is sealed to a minimum of IP67, the limiting factor being the seal between the N connector on the antenna and the N connector on the coaxial cable.

The antenna complies with EMC requirements and is CE marked, its mass is 1.0 kg, operating temperature range is -30°C to +70°C, and storage temperature range is -40°C to +100°C.

 

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