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From: haynes@ucscc.UCSC.EDU (99700000)
Newsgroups: rec.ham-radio,sci.electronics
Subject: Re: Circular polarization vs omnidirectionality
Keywords: antenna
Message-ID: <5023@saturn.ucsc.edu>
Date: 7 Oct 88 05:44:53 GMT
References: <17770@glacier.STANFORD.EDU>
Sender: usenet@saturn.ucsc.edu
Reply-To: haynes@ucscc.UCSC.EDU (Jim Haynes)
Organization: California State Home for the Weird
Lines: 38

I have an antenna on VHF that might have the properties you're looking
for.  It is from FAA surplus and is labeled circularly polarized, so I
guess it is.  It's rather hard to describe, but here goes.

The elements are swastika shaped.  Imagine a swastika made out of solid
metal.  Now take the outer, hooked part of the swastika arms and bend
them up from the open ends so they make an angle of about 30 deg. to
the plane of the swastika.  This is one "pole" of the antenna.  The
other is another swastika with the arms hooked in the opposite direction
and bent down from the plane.  The plane of the swastikas (inner arms) is
horizontal.  I don't know if there's anything (like a matching device)
between the coax connector and the elements.  The outer conductor of
the coax is clearly connected directly to the lower swastika, but there
may be something between the inner conductor and the upper swastika.

In case that description is totally confusing I'll try a different one.

Imagine two thick disks, horizontal, one above the other like an Oreo
cookie. (Must be time for my bedtime snack.)  Sticking out from the
edges of the disks at four points 90 degrees apart are four parallel
conductor transmission lines (made of about 3/4" diameter rods at VHF).
At the end of each of these transmission lines is a dipole of the
same diameter.  The dipoles, rather than being horizontal are tilted
(rotated) at about 30 degrees to the plane of the disks and transmission
lines.  So if you look horizontally at the antenna the near dipole
is tilted one way and the far one is tilted the other way.

I assume the FAA wants this kind of antenna to receive equally well
from airplanes in all directions; and circularly polarized so it
doesn't care about the orientation of the (linearly polarized) antenna
in the plane.

haynes@ucscc.ucsc.edu
haynes@ucscc.bitnet
..ucbvax!ucscc!haynes

"Any clod can have the facts, but having opinions is an Art."
        Charles McCabe, San Francisco Chronicle