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Antennas and Microwaves

The picture on the left shows a quadrifilar helix together with its feed network. The antenna, as its name suggests, consists of a 4-start helix fed in phase quadrature ( 0,-90,-180,-270 ). The principal use for this type of antenna is to produce a circularly polarised, hemispherical radiation patterns, perfect for GPS/Satcom applications.

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On the right is a 64-element vertically polarised X-band microstrip patch array. An antenna such as this shows exactly why the patch is such a popular choice for array design.

The whole feed network and all the radiating elements can be etched onto a single substrate using conventional printed circuit technology , making it very cheap and reliable to produce. This antenna was for a doppler radar.

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Left is a stripline feed for a 6-element dual polarised array. Similar technology to the X-band array except in this arrangement the conductor is sandwiched between 2 ground-plane / dielectric layers. This geometry reduces the risk of spurious radiated power from the feed network itself. This example was for a cellular basestation antenna.

This image shows the Inmarsat3 L-band antennas being tested at Matra Marconi’s facility in Portsmouth, UK. The transmit and receive antennas are 2 meter aperture parabolic reflectors fed by 22-element arrays of helices.

Beamforming networks allow the array to be excited with several different power distributions, altering the illumination of the dish and generating spot beams on the earth’s surface.

Mechanical alignment of high gain spacecraft antennas is critical for geostationary satellites such as Inmarsat. The geostationary orbit radius is 42164km. At this distance the entire earth disk subtends an angle of only 17.4 Deg.

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Left is the prototype quasi-optic subsystem for the Microwave Humidity Sounder, currently in orbit aboard NOAA 18.

This is really more of a channel filter than antenna. It uses dichroic plates and PTFE lenses to separate and focus the received signals into small corrugated horns at 89, 157 and 183 Ghz respectively.

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The images on this page show a few different antennas that I have been lucky enough to work on. For me the appeal of antenna design is the variety in their physical construction and the range of skills that are required to realise them.

Antenna design can get you involved in electromagnetic theory, finite element modelling, programming, RF test and measurement, materials and manufacturing processes to name but a few.

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