A robust, compact, symmetrical antenna, the AD430/3141 or AD430/7080 (figure 1 and plate 1) is recommended for reception of low power satellite transmissions at either DGPS or GPS frequencies down to -130 dBm in areas where general purpose antennae are inoperable due to L-band interference.

Figure 1. General arrangement of the antennae AD 420 and AD 430 with mounting bracket. The diameter of the ground plane is 150 mm (6"). Three M4 fixing holes are provided on the ground plane for an optional extension disc.

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 via an interdigital filter. The amplifier gain is factory preset within the range 30 dB to 48 dB, with a total noise figure of less than 3 dB (figure 2) including the filter. In order to suppress L band interference, the filter has a narrow pass-band, with significant attenuation of out of band interference: An example for DGPS reception within the band 1531 to 1541 MHz is shown in figure 2, whilst for GPS reception the pass-band is centred on 1575.42 MHz. Other bandwidths and pass-band centres are available on request. In order to minimise insertion loss, the interdigital filter components are  machined from high conductivity copper.

The amplifier is phantom powered via the coaxial output lead with a 5V to 20Vdc supply at  typically 30mA. The antenna complies with EMC requirements and is CE marked.

Figure 2. Gain and noise figure of a low noise amplifier with an interdigital filter tuned for DGPS reception. The response is characterised by a low filter loss and hence low noise figure (<3dB) in the required pass-band, with rapid 'roll-off' either side. The measurements were obtained with an HP 8970B noise figure meter.

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 electrically low-loss radome. The antenna is sealed to a minimum of IP67, the limiting factor being the seal between the N connectors on the antenna and coaxial cable.

The mass of the antenna is 1.4 kg. Dimensions are shown in figure 1.  Operating temperature range for the antenna is -30° C to +70° C and storage temperature range is -40° C to +100° C.