Re: [amsat-bb] S-Band Antenna Comparisons, etc.

[Joseph V Murray <k0vty@xxxxxxxx>]

Hi Jon

If you have a copy of "Antennas" 2nd by John Kraus,    On page 282 there
is a picture of a helical antenna.    It has a flat circular open grid
ground plane .     The statement by Kraus is "  Note that the open grid
of the ground plane has both circular and radial conductors.    Both are
essential. "    
I chose to add a ground plane for a helix to the parabolic case for what
is essential for circular polarization .  The dish or the ground plane
have to have  conductors mounted correctly .     The opposite is also
important for the case where a dish is required for linear polarization
so that a reduction of all other polarizations can be managed by building
the dish with no circular conductors.   

Placing a linear dipole feed antenna on a linear wire dish ( Bar-B-Que)
where the polarity of the dipole is counter to the wires in the dish will
indicate much less functionality.
Taking a linear dish with a correctly aligned Linear feed and rotating
the dish for the purpose of detecting polarization is very positive. 

Kraus has more recently removed the flat solid ( or flat open grid )
ground plane and added a one turn or a two turn loop ground plane  for
the typical axial mode helical form .   The single turn is close to the
first turn feed point of the helix and the second reflector is mounted
1/3 to 1/2 WL from the feed point.     

Another point for you folks with these antennas.    The only way to check
VSWR on a feed antenna is when it is mounted properly at the dish focal
point .    The refractive index of the dish surface changes the VSWR of
the feed antenna if it is not mounted first.
 
R 

Joe  K0VTY
=======================
On Mon, 08 Jan 2001 07:16:17 -0600 Jon Ogden <na9d@mindspring.com>
writes:
>on 1/7/01 11:10 PM, Joseph V Murray at k0vty@juno.com wrote:
>
>> Circular polarization requires a solid dish surface.
>
>I really have a hard time with this.  As Dave pointed out the surface 
>is
>seen by the RF signal as a solid, not the grate that we see.
>
>Now I will admit that a parabolic section will tend to pick up signals 
>in
>the plane its laying in so that would make it look more horizontal or
>vertical depending on how its placed.  But I would think that if you 
>have a
>full parabolic dish made up of a grate of rods that it should not 
>matter
>which direction they run.
>
>73,
>
>Jon
>NA9D
>
>-------------------------------------
>Jon Ogden
>NA9D (ex: KE9NA)
>
>Member:  ARRL, AMSAT, DXCC, NRA
>
>http://www.qsl.net/ke9na
>
>"A life lived in fear is a life half lived."
>

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