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THE NERC MST RADAR FACILITY AT ABERYSTWYTH WINDPROFILER BEAM FORMATION


The polar diagram of the mainlobe for a windprofiler radar
beam typically has a Gaussian form. The beam width is inversely
proportional to both the frequency of operation and the
horizontal dimensions of the antenna; for a width of the order
of a few degrees, dimensions of the order of 100 m are required
for lowerVHF radars and of the order of a few metres for UHF
radars. The different conventions for describing the beam width
are described below. In order to reduce the
sidelobe levels, it is common to use an antenna array which
allows the transmitted and received power to be tapered off away
from the centre. This is illustrated for the case of the NERC
MST Radar's antenna array which is composed of a 10 × 10
grid of 'quads' (each quad being composed of a 2 × 2
subgrid of 4element Yagi aerials); the division of the antenna
array into quads has an additional significance in terms of beam
steering. Equal power weighting is given to the central section
of 4 quads (green), the two middle sections of 16 quads each
(red and blue), and the two outer sections of 32 quads each
(yellow and magenta). In the transmit mode, each of the five
sections is connected to a separate 32 kW peakpower
transmitter. 

It is common to see the (twoway) polar diagram of the mainlobe for a (vertically directed) radar beam described as: where θ is the angle from zenith and θ_{0} is the e^{1} halfwidth, i.e. the halfwidth of the beam at which P_{2W} drops to 0.368 of its peak value; the term twoway refers to the fact that the same antenna is used for transmission and reception. The beam width can alternatively be described in terms of the (twoway) halfpower halfwidth, θ_{2WHPHW}, i.e. the halfwidth of the beam at which P_{2W} drops to 0.5 of its peak value; the relationship between &theta_{0} and θ_{2WHPHW} can be derived from the ratio of P_{2W}(θ_{2WHPHW}) to P_{2W}(0) through the following steps: However, the beam width is often described in terms of the ONEWAY halfpower halfwidth, θ_{1WHPHW}, i.e. for the antenna used only for transmission OR reception. The oneway polar diagram of the mainlobe is simply the square root of the twoway case, i.e: and it can be shown, using the same steps as above for the twoway case, that &theta_{0} and θ_{1WHPHW} are related through the expression: Finally, it is useful to define the twoway polar diagram in terms of standard Gaussian notation: i.e. with a factor of 2 in the denominator of the exponent so that the measure of beam width, σ_{0}, is the standard deviation, i.e. the e^{1/2} halfwidth or the halfwidth of the beam at which P_{2W} drops to 0.607 of its peak value; &sigma_{0} and θ_{0} are related through the expression: so that: This description of beam width is required for correcting (standard deviation) radar return spectral widths for the effects of beam broadening. Note, however, that the spectral widths in the existing files of NERC MST Radar data are defined in a nonstandard way; the values must therefore be multiplied by a scaling factor of 1.25 (explained in the file format page). The NERC MST Radar has a oneway halfpower halfwidth of 1.5°, i.e. sin^{2}σ_{0} = 2.47 × 10^{4}. Boundarylayer windprofilers tend to have oneway halfpower halfwidths of the order of 5°.


Page maintained by
David Hooper Last updated 17th December 2002 