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 ?

Consider the following scenario:

======

Take a 10GHz RF source, and attach an appropriate feed horn to fully
illuminate a 2m dia dish.

Adjust focus so that the wave front out of the dish is parallel (2m
diameter ?)

At a distance, arrange the same setup, except the RF source is
replaced by a detector.

Make a note of received signal strength.

======

Repeat the same experiment at the same frequency, but with a
3mdiameter dish at each end, and appropriate feed horns .

Make a note of received signal strength.

======

Also, see http://www.idesignz.org/misc/dishproblem.pdf

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