An antenna system including a phasing circuit for producing balanced, phase isplaced, signals for connection to an antenna. The antenna comprises, for each set of balanced phase signals, a pair of antenna elements disposed serially along a helical path. A transmission line, connected to each of the phasing circuit terminals, drives each antenna element pair at a center location by being connected to the proximate ends of each pair. The antenna has a omnidirectional radiation pattern, a wide band width, a good front-to-back ratio and can be constructed in a compact form.

STATEMENT OF GOVERNMENT INTEREST 
The invention described herein may be manufactured and used by or for the 
Government of the United States of America for governmental purposes 
without the payment of any royalties thereon or therefor. 
BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
This invention generally relates to antennas and more specifically to 
antennas characterized by omnidirectional radiation patterns. 
(2) Description of the Prior Art 
Numerous communication networks utilize omnidirectional antenna systems to 
establish communications between various stations in the network. In some 
networks one or more stations may be mobile while others may be fixed land 
based or satellite stations. Omnidirectional antenna systems are preferred 
in such applications because alternative highly directional antenna 
systems become difficult to apply, particularly at a mobile station that 
may communicate with both fixed land based and satellite stations. In such 
applications it is desirable to provide an omnidirectional antenna system 
that is characterized further by a wide band width, a good front-to-back 
ratio, right- or left-handed circular polarization and a compact size. 
Some prior art omnidirectional antenna systems use an end fed quadifilar 
helix antenna for satellite communication and a co-mounted dipole antenna 
for land based communications. However, each antenna has a limited band 
width and collectively their performance can be dependent upon antenna 
position relative to a ground plane. The dipole antenna tends to have a 
low front-to-back ratio that can cause heavy reflections when the antenna 
is mounted on a ship, particularly over low elevation angles. These 
co-mounted antennas also have spatial requirements that can limit their 
use in confined areas aboard ships or similar mobile stations. 
The following patents disclose helical antennas that exhibit some, but not 
all, the previously described desirable characteristics: 
U.S. Pat. No. 3,623,113 (1971) Faigen et al. 
U.S. Pat. No. 4,644,366 (1987) Scholz 
U.S. Pat. No. 5,134,422 (1992) Auriol 
U.S. Pat. No. 3,623,113 to Faigen et al. discloses a balanced, tunable, 
helical mono-pole antenna that operates independently of a ground plane. 
This antenna utilizes a centrally fed, multiple-turn, helical antenna with 
a single element. End winding shorting means in the form of "top hat"- or 
"can"-type housings tune the antenna by changing the active electrical 
length of the antenna. A feed loop is centrally disposed to the helical 
mono-pole antenna winding to provide a balanced input to the antenna. 
Although this antenna is compact and can be tuned through a wide band 
width, it does not provide an omnidirectional radiation pattern. 
U.S. Pat. No. 4,644,366 to Scholz discloses a miniature radio transceiver 
antenna formed as an inductor wrapped about a printed circuit card. A 
peripheral conductor on one side of the card provides distributed 
capacitance to the end of the antenna that cancels inductive effects and 
broadens band width. A peripheral conductor on the opposite side of the 
card provides a capacitance to ground to tune the antenna to frequency. An 
unbalanced transmission line connects between one end of the antenna and a 
tap or feed point to provide impedance matching and tuning. This antenna 
has a limited band width for a given connection point. Moreover it does 
not produce an omnidirectional radiation pattern. 
U.S. Pat. No. 5,134,422 to Auriol discloses an antenna with helically 
wound, equally spaced, radiating elements disposed on a cylindrical 
surface. Antennas identified as prior art antennas in this reference 
include helically wound, end driven antenna elements. The other ends of 
the elements terminate as open circuits. These antennas provide circular 
polarization, an omnidirectional radiation pattern and a good 
front-to-back ratio. The Auriol patent is particularly directed to a 
structure that uses a conductive, meandering strip to connect the driven 
ends and establish various phase relationships and tuning. This antenna is 
designed to produce high quality circular polarization, an omnidirectional 
radiation pattern and a good front-to-back ratio, but only over a narrow 
frequency band. 
The following patents disclose center-fed spiral antennas that exhibit 
some, but not all, of the previously described desirable characteristics: 
U.S. Pat. No. 4,243,993 (1981) Lamberty et al 
U.S. Pat. No. 5,053,786 (1991) Silverman et al 
U.S. Pat. No. 4,243,993 to Lamberty et al discloses broad band antennas 
comprising center feed, spiral antenna arms arranged on planar and conical 
surfaces. Each antenna arm includes one or more choke elements that 
resonate at a predetermined operating frequency to eliminate or minimize 
undesired radiation and reception characteristics and provide sum and 
difference mode operations with both right-hand and left-hand circularly 
polarized radiation characteristics. Feeding an antenna as disclosed in 
the Lamberty et al patent with a phased sequence of signals produces a 
radiation pattern that exhibits a null along an antenna bore sight axis 
and a maximum field along a cone of revolution about the bore sight axis. 
Although this antenna has a broad band width and provides circular 
polarization, it does not provide an omnidirectional radiation pattern. 
U.S. Pat. No. 5,053,786 to Silverman et al. discloses a broad band 
directional antenna in which two contiguous conductive planar spirals are 
fed at their center. The antenna is positioned near a cavity to absorb 
rear lobes in order to improve the front-to-back ratio. Even with this 
improvement in the front-to-back ratio, the antenna provides a relatively 
narrow beam pattern having both horizontal and vertical polarization. 
Apparently, this antenna is designed to operate with a linearly polarized, 
high gain, narrow beam. Thus the antenna does not provide an 
omnidirectional radiation pattern or circular polarization. Moreover, by 
absorbing the rear lobes, the power transmitted into the reserve lobes is 
lost making the antenna less efficient in radiating during a transmitting 
mode. 
SUMMARY OF THE INVENTION 
Therefore it is an object of this invention to provide a broad band 
omnidirectional antenna. 
Another object of this invention is to provide a broad band omnidirectional 
antenna with good front-to-back ratio. 
Yet another object of this invention is to provide a broad band 
omnidirectional antenna that operates with circular polarization. 
Yet still another object of this invention is to provide a broad band 
omnidirectional antenna that operates with a circular polarization and 
exhibits a good front-to-back ratio. 
Yet still another object of this invention is to provide a broad band 
omnidirectional antenna that is simple to construct. 
In accordance with this invention, an antenna extends along an axis normal 
to a ground plane and includes a plurality of sets of axially coextensive, 
serially placed, elongated conductive elements. The serially placed 
elements in a set lie along one of a plurality of substantially, equally 
spaced, right helical paths. Individual transmission lines attach to the 
elements at centrally located, proximate ends for centrally feeding the 
elements in a set.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 depicts, partially in perspective and partially in schematic form, a 
communications system 10 that includes a signal processor 11 and an 
antenna system 12 that embodies this invention. As with most communication 
systems, the signal processor 11 can operate in a transmitting mode, a 
receiving mode, or alternatively, in a transceiving mode alternately 
transmitting and receiving. Therefore the signal processor 11, although 
shown as a block in FIG. 1, is intended to represent appropriate 
transmitting, receiving or transceiving apparatus. Such equipment is well 
known in the art so a detailed description of such apparatus and its 
operation in conjunction with the antenna system of this invention is not 
necessary for understanding this invention. 
Still referring to FIG. 1, the antenna system 12 includes a phasing circuit 
13 and an antenna structure 14 that includes a plurality of antenna 
element pairs. The specific embodiment disclosed in FIG. 1 includes four 
antenna pairs designated by reference numerals 15, 16, 17 and 18. Each 
antenna element pair includes two axially spaced elongated conductive 
antenna elements. For example, the antenna element pair 15 includes an 
upper antenna element 15A and a lower antenna element 15B; antenna element 
pairs 16, 17 and 18 include elements 16A, 16B, 17A, 17B, 18A and 18B 
respectively. This particular embodiment utilizes four such antenna 
element pairs; the number of pairs, N, can vary so long as the antenna 11 
includes at least two pairs (i.e., N.gtoreq.2). As a practical matter, the 
number of pairs generally will be between 2 and 16 (i.e., 
2.ltoreq.N.ltoreq.16). 
Each antenna element pair, such as antenna element pair 15 lies along the 
path of a right helix for some number, M, turns on a cylindrical support 
19. In this particular embodiment antenna element pair 15 lies along a 
helical path of 1/2 turn over the overall length of the antenna 14 so 
M=0.5. If it is desired to produce an antenna with an omnidirectional 
pattern, the number of turns should be one or less (i.e., 
1/8.ltoreq.M.ltoreq.1). If M&gt;1, then the antenna becomes more directional. 
Such operation would be beneficial for applications in which the angle of 
reception or transmission was fixed. 
Each antenna element pair has a central feed point for connection to the 
other circuitry. Specifically, a balanced feed transmission line 20 
connects by means of a conductor 21 to the antenna element 15A and by 
means of a conductor 22 to the antenna element 15B. The feed points are 
located at the central proximate ends of the elements in an antenna 
element pair, such as elements 15A and 15B. The other end, or free end, of 
each antenna element terminates in an open circuit. In essence the antenna 
element pair 15 is a helical dipole, i.e., a dipole laid along a helical 
path. 
Similar connections are made to the other antenna element pairs. For 
purposes of clarity only one additional element connection is shown in 
FIG. 1. That is a connection provided from a transmission line 23 by which 
a conductor 24 connects to the upper antenna element 18A while another 
conductor 25 connects to the center point of the lower element 18B. 
FIG. 2A also discloses connections between the conductors 21 and 24 and 
antenna elements 15A and 18A. A conductor 26 of a transmission line 27 
connects to antenna element 16A; a conductor 30 of a transmission line 31, 
to the antenna element 17A. As previously indicated, these connections are 
made at the mid point of each antenna element pair (i.e., at the bottom of 
the upper antenna elements 15A, 16A, 17A and 18A of FIG. 1). FIG. 2B 
depicts the connection of the conductors 22 and 25 to the antenna elements 
15B and 18B. A conductor 32 from the transmission line 27 connects to the 
antenna element 16B; and a conductor 33 of the transmission line 31, to 
the antenna elements 17B. 
Thus the antenna structure shown in FIG. 1 comprises four helically wrapped 
dipole antennas and four separate transmission lines that centrally feed 
each dipole. Stated generally, the antenna comprises dipoles along N 
helical paths for being driven by N transmission lines. As will be more 
apparent by reference to FIGS. 2A and 2B, the spatial angle of .phi. is 
determined by the number, N, of antenna element pairs. Specifically: 
##EQU1## 
This spatial angular spacing also corresponds to the phase difference of 
signals applied by the phasing circuit 13 to the various antenna element 
pairs. In the specific embodiment shown in FIG. 1, the phasing circuit 
produces the fundamental signal on a transmission line 20 and phase 
signals delayed by 90.degree., 180.degree. and 270.degree. on conductors 
27, 31 and 23 respectively. Transmission lines 20, 23, 27 and 31 are 
typically unbalanced lines, such as coaxial conductors. Baluns 40 through 
43 are utilized with the transmission lines 20, 27, 31 and 23 respectively 
to produce a balanced feed at the connection of each transmission line to 
its corresponding antenna element pair. Baluns are well known in the art 
for providing unbalanced to balanced signal conversion. Although baluns 
can take many forms, it has been found that a balun formed by wrapping at 
least one turn of a coaxial cable around an annular ferrite core provides 
an appropriate unbalanced to balanced conversion. 
The balanced signals from the phasing circuit are applied in sequence to 
the various element pairs in a direction corresponding to the rotation of 
the conductors. That is, while the fundamental signal from the 0.degree. 
balun 40 on the transmission line 20 is applied to the antenna element 
pair 15, the phase delayed signals from baluns 41, 42 and 43 for the 
transmission lines 27, 31 and 23 are applied to the antenna element pairs 
16, 17 and 18 respectively in sequence. When viewed in the position shown 
in FIG. 1, both the rotation of the helix and the application of phase are 
to the left, or clockwise with respect to a helix axis 45. 
With appropriate sizing of the various components, the resulting antenna is 
characterized by an omnidirectional radiation pattern, a wide frequency 
band, a good front-to-back ratio and good structure. A further 
understanding of the advantages of this invention can be more fully 
attained by referring to the design and construction of a specific antenna 
embodiment utilizing this invention. In the particular embodiment shown in 
FIG. 1, the antenna comprises four element pairs (i.e., N=4) and each 
antenna element pair revolves by 1/2 turn about the axis 45 (i.e., M=0.5). 
Typically the antenna will have an overall length along the axis of about 
1/2 wavelength to one wave length at the center frequency. 
An antenna for operating in the frequency band from 240 Mhz to 400 Mhz has 
been constructed with a nominal axial length of 18 inches, that 
approximates 0.5.lambda. where .lambda..sub.0 =36.9 inches for a mid 
frequency, f.sub.0 =320 Mhz. If the antenna has a greater length, gain 
will improve as will the size of the antenna structure. It is anticipated 
most antennas will be constructed having an axial length of 
0.5.lambda..sub.0. 
Increasing the number, N, of elements will increase the gain of the 
antenna, but decrease its band width. 
The antenna diameter, D, normally is selected to be less than 0.3 
.lambda..sub.0. In this particular embodiment the overall diameter is 
selected to be 0.15 .lambda..sub.0. The radius, "a" of the elements is 
also selected to be less than 0.01.lambda..sub.0. In this particular 
application the antenna 14 has an overall diameter of 5.5 inches and each 
of the elements has a diameter of 0.5 inches.: 
EQU a=0.25&lt;0.01.lambda..sub.0 (2) 
With these particular dimensions the antenna system 12 has a diameter of 
6.5 inches. The value of "a" has no effect on the overall performance of 
the antenna, but does change the impedance of the antenna. The diameter, 
D, will change the pitch angle and this can impact gain and band width. 
The pitch angle for the helix is given by: 
##EQU2## 
The length of an antenna element pair for a given antenna, T, is given by: 
##EQU3## 
FIG. 3 depicts the performance of the antenna constructed in accordance 
with these dimensions for .lambda..sub.0 =320 Mhz when the axis 45 is 
positioned as shown in FIG. 1 to be vertical in space. The top of the 
antenna, formed by the free ends of the antenna elements 15A through 18A, 
constitutes the front of the antenna. As will be apparent from viewing 
FIG. 3, the radiation pattern has a substantially equal gain for 
essentially the hemisphere above the ground plane. This particular plot 
depicts performance when the antenna is located approximately 9 feet above 
sea water. The power gain for the antenna only varies by about 3 dB when 
the frequency varies from 240 to 400 Mhz. Moreover, the variations remain 
within 3 dB over that band width as the antenna position is displaced from 
9 through 12 feet above a ground plane, such as sea water. Consequently 
the antenna shown in FIG. 1 and constructed in accordance with this 
specific dimension described above meets all the objectives of this 
invention. The antenna covers a broad frequency band. It is 
omnidirectional in a hemisphere above the earth as a ground plane. It has 
a good front-to-back ratio with essentially all power radiating forwardly 
of the antenna. Finally, it is relatively insensitive to its position or 
displacement relative to a ground plane. 
This invention has been described in terms of a specific embodiment. 
Various modifications can be made with respect to the number of antenna 
element pairs, the antenna diameter, length and other features. The effect 
of varying each of the important parameters has been established. 
Therefore, it is the intent of the appended claims to cover all such 
variations and modifications as come within the true spirit and scope of 
this invention.