Some brief explanations
- This script computes the far-field radiation pattern from a reflector antenna illuminated by its feed antenna.
This is done by geometrical optics - which is a good approximation as long as the reflector is much
larger than a wavelength. The feed antenna is assumed to emit radio waves into a cone of equal intensity.
These waves are followed by ray tracing to the reflecting surface, from which they are sent into free space.
From the outgoing rays the radiation pattern of the antenna is computed, as well as
the maximum power and the width of the outgoing beam of radiation.
- Reflector: the shape of the reflecting surface can be chosen as a parabola or a circle, both centered
around the axis of symmetry, or as a parabolic or circular segment offset from the axis of symmetry.
Caveat: Note that in order to make the calculations rapid enough for an interactive tool,
the reflector is treated as having a curvature in one dimension only. Hence, the effects of rays
reflected by the sides in e.g. a rotational paraboloid are not taken into account. As a consequence, the
HPBW (Half Power Beam Width) of a parabolic reflector of diameter D at wavelength λ with
51° λ/D is narrower than that of a paraboloidal dish (59° λ/D).
- The dimensions of the reflector (diameter, focal length, position of the feed) may be entered either
in wavelengths or in metres. Hit the enter-key after typing your value in the appropriate field.
The corresponding fields then will have a light green background.
- Feed: The feed radiates with equal intensity in all angles, within the specified angles.
An angle of -90° points (vertically) downward, 0° means horizonally towards the reflector,
and +90° is straight upwards (in this plot).
- show dish illumination switches on/off a light blue area to mark the angle range over which the
reflector is illuminated by the feed. Note that while all blue lines indicate rays from the feed to
the reflector, not all may carry power. While the calculations are done with 20000 rays, the plot
shows only 20 representative rays.
- also show feed position adjusts the plot area to include the feed position, to facilitate the
display of the feed when it is placed below or above reflector's rims, e.g. in a offset dish antenna.
- Output beam: The right hand plot displays the far-field radiation pattern of the antenna, in which
directions the red rays in the left plot point. While for a rough overview, the plot limits may be taken
between -90° and +90°, it it recommended to adjust the plot limits to a range of several HPBWs
in order to obtain more accurate numerical results for
-- the angle of the main beam direction (indicated by a vertical magenta line in the plot)
-- HPBW, the Half-Power Beam Width of the main lobe (the horizontal magenta line indicates the half-power level)
-- max.power (in dB relative to a focused and fully illuminated reflector)
- show wavefront switches on/off the display of red dots on the outgoing rays which are at the same
phase, thus representung the wavefront.
- Mouse position: displays abscissa and ordinate values. For the left plot it also gives:
the direction angle of the outgoing ray, the direction of this point as seen from the
centre of the reflector, and from the feed.
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