AD221 and AD251 standard GPS antennae



Very robust and compact antennae principally for the offshore market, the AD221 and AD251 (figures 1 and 2) are designed for reception of low power satellite GPS transmissions at 1575.42 MHz.


Figure 1. AD221 standard GPS antenna with central stud mounted TNC connector. The base is 90 mm (3.5") in diameter.

Figure 2. AD251 standard GPS antenna with N type connector and mounting bushing. The base is 90mm (3.5") in diameter.


Electrical Specification

The antenna element is a shaped crossed 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 dual ceramic filters. The amplifier gain is factory preset within the range 30 dB to 48 dB with a total noise figure of typically 2.5 dB including filters. The amplifier is phantom powered via the coaxial output lead with a 3.3V to 20Vdc supply at typically 40mA.

Both antennae comply with EMC requirements and are CE marked.


Mechanical Specification

The antennae cases are machined from aluminium blocks, hard anodised and dichromate and nickel acetate sealed, giving an extremely rugged green finish that offers superb resistance to salt water corrosion. The antennae are sealed to a minimum of IP67, the limiting factor being the seal between the connectors on the antenna and coaxial cable. The mass of the AD221 is 0.5 kg and the AD251 is 0.65 kg.

A 5/8" UNC stud carrying a TNC is used to mount the AD221 (figure 1), whilst the AD251 with N type connector (plate 2) has a plastic bushing threaded to accept  a 5/8" UNC bolt, allowing easy repositioning of the antennae. Alternatively, for longer term installations, an optional 'L' shaped aluminium mounting bracket, which bolts directly to the aluminium base of the AD251, can be used in place of the bushing.

4mm thick, 70mm high, GRP pressure mouldings, painted to any required colour, provide strong and electrically low-loss radomes.

Operating temperature range for the antennae is -30C to +70C and the storage temperature range is -40C. to +100C.


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