[Smaug] [Wireless] dish antennas?

Jim Warner warner at ucsc.edu
Sun May 1 13:41:20 PDT 2005


On Sat, 30 Apr 2005 18:46:55 -0700
 William Estrada <mrumunhum at popdial.com> wrote:

>
>So I will need an many antennas as I can get my hands on.  Free is best.
>I can disassemble them if need be.  I got a 10 foot dish out of Scott's
>Valley two weeks ago for the Sierra link.  That's a start.
>
>I would appreciate any pointers.
>
==================
UC Santa Cruz had such an antenna for our 18 GHz radio
link across Monterey Bay.  It's gone now.  The mount for
the antenna was very big, very expensive and with its
ballast, weighed around 600 pounds.  Even if the antenna
is free, mounting it so it can't move around in the wind
is an expensive endeavor for a big antenna.

Another important factor is that the positioning of the
feed horn or radiating element at the focus of the
parabolic dish cannot be done casually or your won't
get good focus and you'll be wasting the capability of
your antenna.  The larger the antenna, the greater the
requirements for accuracy so you don't defocus and
spoil the gain.  Repurposing an antenna to a new
frequency should be done with care.  Ideally, your
test mount would permit you to scan the antenna while
observing the signal strength from a distant source
so you can map out the antenna's spacial characteristics
and be sure it is working the way you hoped.

When you make an antenna 2X in diameter, it gains at
least 4X in weight while requiring 2X better stability
because of its smaller angular aperature.  That means
the mount has to be 8X better.

Eventually you're paying more for the antenna+mount
than for the radio, or maybe you passed that point a
long time ago.

I have heard Jack Ungar speak and I understand his point
that a well-focussed antenna is better than brute force
using excessive power and pointlessly spraying RF
energy in unwanted directions.  And I've got no problem
with 2 ft antennas and 6 degree wide beams.  But before
you jump whole hog into the market for used earthstations,
I think you should look at things other than 'how can
I make my WRT go 30 miles' and make sure you've got the
most cost effective solution.




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