"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,

Thank You,
Mike

The DirecTV dishes are not prime focus, but rather offset. If it came with
the original horn mount, use it.

Yes, it is an offset feed and it did come with the mount and feed (LNA?)
Actually it came with two LNAs, but they didn't match each other. I don't
remember what junk pile this came from and don't have any info about it's
working condition.
I did measure the distance from the front of feed to the dish center and
it is 18" as calculated, however as I said I don't know if this worked as I
got it.

You can use:
http://www.w1ghz.cx/software/hdl_3b4.zip
to find the focal length,. Your formula assumes prime focus.

This link doesn't look right.

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.

So I could use a biquad or patch as the feed antenna without much
difference?


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

I'll look at the w1ghz link.
I was suprised this dish is made of plastic. Does the EM wave reflect off
of the plastic or does the plastic have a coating?
Thanks, Mike