The dish problem
Paul Keinanen wrote:
On Mon, 12 Oct 2009 21:45:39 -0700 (PDT), MarkAren
wrote:
Hi All,
Modern engineering text indicates that for the same frequency, a large
fully illuminated dish will provide more gain than it’s smaller
equivalent. Why is this ?
...
In both cases all of the TX energy is transmitted in a parallel beam,
whose diameter is the same as the respective dish.
****
A parallel beam is not formed, rather a slightly expanding beam due to
diffraction.
Diffraction also limits the resolving power of an astronomical
telescope, so when looking at some binary stars, a small telescope
will show only a single blob, a slightly larger telescope will show an
elongated figure (e.g. figure of 8) and an even larger telescope will
show two separate stars.
The diffraction limit is defined as 1.22*wavelength/diemeter radians
or about 70*wavelength/diameter degrees.
Diffraction also controls the beam spreading from a parabolic disk or
laser. For this reason, a laser (with an aperture less than 1 cm) can
not be used to illuminate the reflectors on the moon, but typically
the laser beam is transmitted through a telescope with typically 1 m
diameter. The beam is 100-1000 times narrower than the beam from the
laser alone. The area illuminated on the moon is 10,000-1,000,000
times smaller and hence reflected power that much stronger than with a
bare laser.
Paul OH3LWR
Great response!
Brian W
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