The graphs below show the typical input and output beam power patterns.
The left-hand graph shows the fundamental mode (HG00) input, and the right-hand graph shows the output which approximates to the LG10 mode (i.e. radially, the fundamental mode; circumferally, the second mode).
Click on a graph to see an larger view.
Axial cut through the HG00 mode beam, overlaid with a theoretical Gaussian profile. The beam clearly has a high modal purity.
Axial cut through the focussed LG10 mode beam. This displays the deep axial null, low sidelobe levels and excellent symmetry.
The elevation patterns of different configurations of the antenna were measured, using a Boonton 4220 Power Meter fitted with a corrugated feedhorn (w0 = 4mm) at a range of about 1.5m.
This graph shows the elevation pattern obtained from the antenna without the spiral phaseplate fitted - i.e. HG00 illumination.
Severe scattering from the cone apex is evident at low elevation angles and the pattern is generally ragged.
The optics were 40mm diameter and the focussed beamwaist was 3mm.
This graph shows the elevation pattern of the antenna with an l = 2 spiral phaseplate fitted - i.e. LG20 illumination.
The use of the spiral phaseplate has virtually eliminated any scattering problems at low elevation angles and the beam shape is generally very smooth. The gradual tail in response at high angles is probably due to leakage round the top of the cone. Fitting a collar of absorbing material round the top of the cone should clean up the high angle response.
In this case the optics were 88mm diameter and the beam was not focussed down onto the cone, hence the rather narrow beamwidth.
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Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
using HTMLEdit3 on a StrongARM powered RISCOS machine.
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.