[MLB-WIRELESS] Orientation of antennas

[Tony Langdon, VK3JED]

At 06:01 PM 15/07/2002 +1000, Ryan Abbenhuys wrote:

>I have an antenna orientation question too.....not quite as dumb I hope ;-p
>
>Okay, the waveguide on my AP is horizontally polarised, so of course if
>you're connecting with a galaxy or similar dish you have to swing it around
>90 degrees from the usual orientation.  However, one of my client
>connections is using one of my commercial 24's, which allows you to just
>spin the dipole 90 degrees instead of the whole dish, thus you get the full
>use of the adjustments on the dish mounts.
>.....however, after putting up with a mysterious shite signal for a few
>months I pulled it down to try something else, span the dipole back to the
>original orientation and instead spun the whole dish........and whola!!
>Connection jumps from something like -85 to -70.
>So my question is, does the reflector itself somehow have an effect on the
>polarisation??  I thought it was just the dipole itself?

The reflector on a Galaxy antenns is highly polarised.  This is due to the 
fact that there is a narrow spacing of the grid in one direction and  a 
wide spacing in the other plane.  If the feed has the same polarisation as 
the grid, then the reflector will do its job.  However, if you rotate the 
feed 90 degrees, it won't reflect off the dish, but pass straight 
through!  There are two possible solutions:

1.  Make sure the feed is of the correct polarisation.

2.  Cover the dish surface with chicken wire, wire mesh or aluminium foil, 
to remove the large gaps in the grid spacing.

73 de Tony, VK3JED
http://vk3jed.vk.irlp.net