Re: home-brew cantenna

[John Denker <jsd@AV8N.COM>]

I wrote:
> There will need to be some rules for the contest, for starters:
>  -- passive-only
>  -- within a size constraint (perhaps a 1'x1'x3' box) ... lest
>     somebody try to build an Arecibo-sized dish
>  -- within budget (perhaps $10.00 in parts, max)

I just thought of another rule that I like a lot:  the judging
of the contest ought to depend on *off-axis* performance.  For
instance, suppose we stipulate in advance that 12dB of antenna
gain is enough to get the job done.  Then let us compare two
antennas:
  a) Has >=12dB of gain (but not much more) for all angles up to 6
     degrees off-axis.
  b) Has huuuge amounts of gain on-axis but falls off steeply so that
     it has less than 12dB when you are less than 6 degrees off axis.

Then we declare (a) to be the winner.  Definitely.

This rule is important because without it, designers are tempted
to use overkill -- Arecibo-sized dishes et cetera -- and the
biggest advantage goes to the biggest smart-aleck.

In contrast, this rule means overkill doesn't pay ... which is
the typical real-world situation.  The advantage goes to clean,
elegant designs, with minimal standing-wave-ratio and minimal
parasitic dissipation.  It also means students get to think about
Liouville's theorem, which is always a good thing IMHO.

I'm a big believer in real-world physics.  An antenna that works
super-well in theory but is impossible to set up and aim sufficiently
accurately will fail in the marketplace.