"amdx" wrote in message
...
I have a Directv dish 35 inches wide x 20 inches tall 4inches deep.
I calculated the focus as F=D*2 / 16d where D =diameter and d=depth
F=18 inches Ok so far.
How do you find beamwidth? I think the 35" dimensiom would have a tighter
pattern than the 20 ", but how do you calculate,

Second, The feed antenna, There can be different feeds.
A simple monopole, dipole, biquad, patch, yagi, helix.
How does the feed antenna affect the gain of the dish, if at all?
Yea, it my be difficult to light the yagi or helix, so if you like throw
those out.
Thank You,
Mike

The DirecTV dishes are not prime focus, bu rather offset. If it came with
the original horn mount, use it.
You can use:
http://www.w1ghz.cx/software/hdl_3b4.zip
to find the focal length,. Your formula assumes prime focus.

The gain of the dish is not a function of the feed, but the system gain is
because one must take into account surface illumination, feed efficiency
etc.

The subject of feeds for dishes is complex if you really want to optimize
G/T etc. Paul W1GHZ has a teriffic on-line text about parabolic surfaces and
feeds:
http://www.w1ghz.org/antbook/preface.htm

Dale W4OP