Dear Group:

A popular antenna for LPFM stations is an antenna that I call an OI
antenna (because of its appearance). It consists of about 0.25 wave
lengths (WL) of thick wire shaped into a circle [the O] in the
horizontal plane with about 0.125 WL of thick wire in the vertical plane
connected to each end of the circle [the I]. In other words, it is
essentially a 0.5 WL dipole with one leg extending from the center to
the left in a horizontal circle that then points down while the other
leg extends to the right in a horizontal circle that then points up.

Commercial examples are the Telecom TFC1K and the well built
Dielectric DCR-L. Most often, two of the antennas are used vertically
spaced by 1 WL and driven in phase. All venders characterize the
antenna as being circularly polarized.

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

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?

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.

Thanks in advance for your insight.
73 Mac N8TT
--
J. Mc Laughlin - Michigan USA
Home:

"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

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
Home:




"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

"J. McLaughlin" wrote:
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.
___________________

True, the FCC is concerned only with the RMS gain of an "omni" FM broadcast
antenna, no matter how bizarre its real patterns. The antenna OEM publishes
his antenna gains based on perfectly omni patterns in the horizontal plane,
and those values must be used when determining the ERP of an FM station --
no matter if the true patterns have multiple nulls in the horizontal plane
that may be only 10 or 15% of that gain. The problem is in knowing what
those true patterns are from an installed antenna.

Operating and installation details such as the tower width used, tower
construction details, FM frequency, antenna azimuth/elevation/offset on the
tower, presence and location of conduits and transmission lines and other
metallic structure in and near the antenna aperture, tower guys etc all can
have a significant, often unpredictable affect on the final radiation
patterns from the installed antenna.

The safest way to know what the patterns of the installed antenna are likely
to be is to have them measured on the OEM's test range, using a replica of
the tower or pole on which the antenna will be installed including all the
"stuff" in and near the antenna aperture, and the pick a mounting
arrangement that produces the most omni patterns. For c-pol (and v-pol
only) antennas, that may require adding vertical parasitics near each
element to fill in the natural v-pol null caused by the antenna supporting
structure, which is essentially vertical. ( N.B. The patterns from FM
transmit antennas mounted on towers of large cross-section are difficult to
impossible to fully optimize using parasitics.)

R. Fry

Visit http://rfry.org for FM broadcast system papers.

"J. McLaughlin" wrote
With reasonable offset distances from a 12 inch diameter
metal pole (the tower that is likely to be used), the driving
impedance changes dramatically.
_________________

The input impedance of any/all sidemount antennas (ring-stub or otherwise)
is affected by the electrical environment in which they operate. A common
input spec for a sidemount FM transmit antenna without on-site matching is
~1.5:1 VSWR. FM transmit antennas with input VSWRs greater than about
1.15:1 across the 200kHz bandwidth of the channel can noticeably reduce
stereo and SCA performance.

A transformer can be installed at the junction of the antenna input and main
transmission line to provide a way to optimize the match after antenna
system installation. But this takes some rather good test equipment, not
just the typical in-line wattmeter (Bird, etc) or the VSWR meter in the
transmitter.

Note that installing and adjusting an input matching transformer on an FM
transmit antenna does not affect antenna radiation patterns. Final patterns
are a function of the antenna design, and the electrical environment around
the antenna (within several wavelengths).

R. Fry