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October 14th 09, 12:39 PM
posted to rec.radio.amateur.homebrew
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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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