AD500 combined GPS/DGPS and UHF antenna

 

Introduction

The AD500 antenna (figure 1) is designed for reception of low power satellite transmissions at L1 GPS and DGPS frequencies in areas where using a general purpose antenna is unreliable or impossible due to L-band interference; it also incorporates an UHF element tuned to 458 MHz for use with a radio modem linked to the GPS receiver.

A single 'N' type connector is provided on the antenna for both the DGPS/GPS and UHF signals, enabling the use of one coaxial cable. An optional multiplexer is used to separate the signals at the GPS/DGPS receivers and UHF transmitter/receiver.

 

 

 

Figure 1. AD500 combined GPS/DGPS and UHF antenna

 

Electrical Specification

The DGPS/GPS 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, which directly feeds a compact dual low loss interdigital filter connected to a low noise amplifier. One filter is tuned for GPS reception whilst the other is tuned for DGPS reception. The DGPS/GPS dipole is integrated into the UHF antenna, without compromising the electrical performance of either.

The dual interdigital filter, machined from high conductivity copper, has two pass-bands of 12 MHz, which may be centred between the limits 1520 MHz to 1600 MHz. For this application, one band is 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, resulting in an overall noise figure of <2.8 dB 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. 3.3Vdc operation is available on request.

 

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 extremely rugged, attractive, 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, painted to any colour required, provides a strong, 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 3 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 two M8 ‘V’ bolts.

 

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