My antenna question only stimulated Mr. Fry, a continuing, constructive
contributor to these proceedings. Here are some of the conclusions from
our correspondence and our modeling:
1 "Ring-stub" is an appropriate name for this antenna and was found
to be used by others.
2 Contributions to vertically and horizontally polarized far-field
signals is proportional to the diameter of the ring (for H) and height
of the stub (for V). [The first antenna that I looked at had
disproportionally tall stubs.]
3 At 94 MHz, it appears that a diameter of something like 300 mm and
a stub of about 230 mm about balances V and H. An optimum set of
dimensions was not found. Too many parameters exist to allow quick
convergence.
4 Radiation from the customary Gamma match used with this antenna
significantly distorts the FS pattern. Placing the "arm" directly under
one branch of the ring reduces the pattern distortion.
5 This type of antenna is usually mounted off to the side of a tower.
Manufactures often speak of towers no larger that 3 or 4 inches in
diameter. With reasonable offset distances from a 12 inch diameter
metal pole (the tower that is likely to be used), the driving impedance
changes dramatically.

This hitherto unknown (to me) antenna seems resistive to rational
design and is a candidate for heuristic design - if at all. Once again,
I ask if anyone has dimensions of a well working example of this
antenna.

Mind, for LPFM stations, the FCC is concerned with the average (or
rms) antenna gain. That is little changed with large changes in the
shape of the pattern. The University wishes to know where the gain is
going. Remember the first law of antennas: all antennas work. The
second law is: some antennas work better in certain directions than
other antennas.

73 Mac N8TT
--
J. Mc Laughlin - Michigan USA
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"German NG Server" wrote in message
...
"J. McLaughlin" wrote

1. What is (are) the conventional name for this antenna? I have
not
been able to find it in any of my references.

Commonly called a "ring-stub" element. It was one of the earliest
methods
of adding v-pol to an omni h-pol antenna of the "ring" design, popular
before CP was authorized by the FCC.

2. When I model one bay of the antenna I find that almost all of
the
radiation is vertically polarized. This is not at all surprising in
view of the symmetry of the horizontal element [the O].
What might I be doing wrong?

There is a considerable h-pol component from the portion of the
element in
the horizontal plane. But as h-pol and v-pol do not have the same
radiation
center from this design AND the horizontal part of the radiator
generates a
v-pol component for elevation angles not in the horizontal plane, the
axis
of net maximum v-pol radiation in a ring-stub element is not located
in the
vertical plane.

The PDF slide show (Paper 10) at http://rfry.org includes NEC2 pattern
studies of this and three other types of FM broadcast transmit
elements,
showing the differences in their free-space surface patterns without
the
effects of the tower.

Speculation: I may not have the dimensions correct though the
antenna I
modeled is resonant at 94 MHz. It is possible that venders are
equating
omni-directional-in-the-horizontal-plane with circularly polarized.

I have an EZNEC file that I will send to anyone who wishes it.

The impetus for these questions is a determination of an
appropriate
offset of these antennas from a large diameter tower.

Paper 6 at http://rfry.org shows some of the affects of two nearby
tower
structures on sidemounted FM broadcast transmit elements for one set
of
conditions. Unfortunately the patterns cannot be applied to any other
conditions.

The best approach is to have the patterns measured by the antenna OEM,
using
one or more mounting configurations preferred or possible in the final
installation.

R. Fry