AD440 antenna with dual interdigital filter

 

Introduction

The AD440 Antenna (figure 1) is designed for reception of low power satellite transmissions at GPS and DGPS frequencies down to -130 dBm in areas where using a general purpose antenna is unreliable or impossible due to L-band interference. Incorporating a ground plane with optional extension, the AD440 supercedes the AD330. The antenna element directly feeds a compact dual low loss interdigital filter which in turn feeds a low noise amplifier. One filter is tuned for GPS reception whilst the other is tuned for DGPS reception.

 

Figure 1. AD440 antenna with integral 190 mm (7.5") diameter ground-plane. The dual interdigital filter and LNA are housed under the ground plane with an N type connector output. Fixing holes are provided for the mounting bracket.

 

Electrical Specification

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

The dual interdigital filter has two pass-bands of 12 MHz, which may be centred between the limits 1520 MHz to 1600 MHz. For this application, one band has been centred at 1575.42 MHz for GPS reception and the other at 1530 to 1542 MHz for DGPS. The filters’ band losses are <1.5dB (figure 2), resulting in an overall noise figure of <2.8dB when coupled to a built in GaAs FET amplifier with a nominal gain of 45dB. The amplifier is phantom powered via the coaxial output lead with a 5V to 20Vdc supply at typically 30mA.

 

Figure 2. Frequency response of dual interdigital filter used in AD440 antenna. The lower frequency pass-band is for reception of DGPS over the frequency range 1531 to 1541 MHz, whilst the higher frequency band is centred on the GPS frequency of 1575.42 MHz

 

Mechanical Specification

The antenna body is made from two pieces, 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.

In order to minimise insertion loss, the interdigital filter components are  machined from high conductivity copper.

The antenna complies with EMC requirements and is CE marked, its mass is 2.2 kg, operating temperature range is -30°C to +70°C, and storage temperature range is -40°C to +100°C.

A mounting bracket is supplied, which bolts to the base of the antenna, enabling attachment to vertical or horizontal poles up to 60mm in diameter using the included M8 ‘V’ bolts.

 

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