Polyrod antenna with flared notch feed

A polyrod antenna fed at an end by a flared notch antenna. The space required to feed the polyrod is reduced, and a broader operating bandwidth is achieved. Gain of the antenna can be increased by increasing the polyrod length. A number of the polyrod antennas can be packed together in an array configuration.

TECHNICAL FIELD
 The invention relates to polyrod antennas, and more particularly to a new
 technique for feeding polyrod antennas with flared notch antennas.
 BACKGROUND OF THE INVENTION
 Both polyrod and flared notch antennas are part of a larger family of
 antennas that exploit length in the endfire direction to achieve gain. A
 polyrod is a cylindrical shaped polystyrene rod a few wavelengths long.
 Other materials such as fiberglass may be used instead of polystyrene if
 desired. When properly excited, the polyrod acts as an endfire aerial.
 Traditionally, polyrods are fed with waveguide. A matching section is
 usually included to transition efficiently from the waveguide to the
 polyrod. As with other endfire antennas, like the flared notch, the gain
 of the polyrod may be increased by a corresponding increase in its length
 in the endfire direction, as described in The Antenna Engineering
 Handbook, H. Jasik, McGraw Hill, 1961, Chapter 16, pp. 16-1 to 16-24.
 The waveguide required to feed the polyrod has proven incompatible with
 some of the proposed applications for the polyrod. One application, adding
 a polyrod array to the face of a mechanically scanned slot array, would be
 severely constrained if not impossible to accommodate using the waveguide.
 Not only would the waveguide feeding the polyrods be a problem because of
 the aperture blockage but also the feeding network for the waveguides
 would require more space.
 Moreover, the waveguide is bulky and requires matching sections. Secondary
 to this is that the system, i.e., the polyrod and waveguide, is inherently
 narrow banded, i.e., limited at the low end by the cut-off frequency of
 the waveguide.
 It would therefore be advantageous to provide a technique for feeding a
 polyrod antenna which has reduced volume and higher gain and broader
 operating bandwidth than the conventional waveguide-fed antenna.
 SUMMARY OF THE INVENTION
 A polyrod antenna system comprises a polyrod antenna element and a feed
 system for feeding the polyrod element from an end thereof. In accordance
 with the invention, the feed system includes a flared notch antenna having
 a flared notch radiator section. The polyrod element is attached to the
 flare region of the flared notch so that the polyrod element is fed by
 signals as they travel along the flared notch. The antenna system operates
 in a first mode at which the combination of the polyrod element and the
 flared notch antenna act as a dielectric loaded flared notch, and in a
 second mode at a frequency band at which the polyrod element operates as a
 surface-wave antenna.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 FIGS. 1A and 1B show a radiating element 50 comprising polyrod element 60
 fed by a flared notch antenna 70 in accordance with the invention. The
 polyrod 60 is fed by energy traveling along the flared notch 72 of the
 antenna 70.
 FIGS. 2A-2C illustrate the tapered polyrod element 60. The element includes
 a long tapered section 60A and a short tapered section 60B, divided at
 plane 60C. Although the polyrod 60 in this example is shown with a uniform
 taper on both the long and short tapered sections, other configurations
 may also be employed, e.g., constant diameter or non-uniformly tapered
 polyrods.
 The flared notch 70 in this exemplary embodiment is attached to the polyrod
 60 by a press fit arrangement. The short tapered portion 60B of the
 polyrod 60 is fabricated with two triangular shaped sections of material
 removed therefrom, defining triangular open sections 64A and 64B, and
 leaving a center section 64C of solid polyrod material. The flared notch
 element 70 can be fabricated of a solid piece of metal such as aluminum,
 of thickness t and shaped to form the flare 72. The open sections 64A and
 64B are slightly undersized compared to the thickness t of the flared
 notch element 70, so that the polyrod element 60 can be wedged (for a
 press fit) into the flared notch 70. Other configurations are possible
 with more or less polyrod material removed from within the flare region.
 Also, other techniques for attachment, such as using bonding material, may
 be employed depending on specific design requirements.
 The antenna 70 includes a balun 74 fed by a coaxial-to-stripline connector
 76. As shown in FIG. 1A, a section 80 of the metal structure that makes up
 the flare 70 is hollowed out internally to form an internal channel 82 to
 accommodate a stripline transmission line 84 and balun section 74. The
 stripline transmission line 84 comprises a conductor line fabricated on a
 dielectric substrate suspended within the open channel 82. The stripline
 84 runs within the channel 82 from the coaxial connector 76 on one side of
 the slotline region 78 of the flare to the balun section 74 on the other
 side. The slotline region 78 is an open section formed in the solid metal
 structure of the flare 70. The stripline 84 is exposed at the slotline
 region 78. Other types of transmission lines may be used to feed the
 slotline region. Balun refers to the transition from an unbalanced
 transmission line (in this case the stripline 84) to a balanced
 transmission line (in this case the slotline). The function of the balun
 is to provide an efficient transition between the stripline and the
 slotline such that maximum power transfer is achieved at that transition
 over the frequency band of operation.
 Although the flared notch antenna 70 employs an exponentially tapered
 flared notch 72, other configurations may also be employed. For example, a
 linearly tapered flared notch, or a combination of linearly (or
 exponentially) tapered flare and constant width flare. In any case the
 balun structure would be identical to that used with the exponentially
 flared notch. Other connector and balun configurations may alternatively
 be employed.
 The shape of the long tapered section 60A of the polyrod is determined from
 traditional polyrod design techniques. The shape of the small tapered
 section 60B is selected to smooth the impedance from the slotline through
 the polyrod into free space.
 The exemplary polyrod-flared notch radiating element 50 is fabricated of
 separate polyrod and flare elements. Alternatively, the polyrod-flared
 notch can be fabricated from a single piece of material and the material
 may be plated or painted with a conductive coating in the area designated
 for the flare. FIGS. 3A-3C show a single piece polyrod-flared notch feed
 radiating element 100 in accordance with the invention. The polyrod
 plastic material is plated with metal in the areas needed to define the
 flare feed, and is otherwise exposed. The element 100 includes the polyrod
 portion 102, again having a long tapered section and a short tapered
 section, and a flared notch section formed of a flat rectilinear portion.
 One surface 104 of the flat portion is plated with metal to define the
 flared notch 106. Here again, the polyrod portion 102 extends into the
 flared notch 106, and is fed with energy traveling along the flare region.
 In this exemplary configuration, the flare is fed with a microstrip
 transmission line 110 and balun section 112 at the slotline region 114 of
 the flare. The line 110 and balun section 112 are defined by a
 metallization pattern on the opposed surface of the polyrod material from
 the plated area 104, as shown in FIG. 3C. The line 110 is connected to
 coaxial connector 116. Other feeding configurations using different balun
 sections may be used.
 FIGS. 4A-4D show the similarity between the radiating properties of the
 flare and the polyrod, by representations of the typical electric field
 lines as energy propagates along the polyrod, the flared notch, and the
 combination. FIG. 4A is a schematic transverse cross-sectional view of a
 polyrod, and illustrates the electric field lines transverse to the
 polyrod. FIG. 4B is a schematic longitudinal cross-sectional view of a
 polyrod, illustrating the longitudinally extending electric field lines.
 FIG. 4C illustrates the electric field lines for a flared notch radiator.
 FIG. 4D is a schematic diagram of the combined polyrod fed with a flared
 notch.
 Because of the symmetry with which a polyrod is constructed, it is possible
 to feed the polyrod with two flared notches and achieve circular
 polarization as shown in an exemplary system 150 in FIGS. 5A-5D. Here, the
 polyrod 152 is fed along one plane by a first flared notch radiator 160,
 and along a second orthogonal plane by a second flared notch radiator 170.
 The radiator 160 is fed by an embedded stripline 162 extending within an
 internal open channel formed within the body of the radiator 160 as in the
 flared radiator 70 of FIGS. 1A-1B, extending from coaxial connector 166 to
 balun 164 on opposite sides of a slotline region. Similarly, the radiator
 170 is fed by an embedded stripline 172 extending within an internal open
 channel formed within the body of the radiator 170. The polyrod 152 is
 formed with four relieved open areas at the short tapered end, two to
 receive the flared notch of radiator 160 and two orthogonally placed
 relative to the first two open areas to receive the flared notch of
 radiator 170. The respective radiators 160 and 170 are centered on the
 longitudinal axis of the polyrod.
 By replacing the conventional waveguide feed with a flared notch feed
 element in accordance with the invention, many advantages can be realized.
 First, the space required to feed the polyrod is reduced and a separate
 matching section is no longer necessary. Secondly, higher gain and a
 broader operating bandwidth are achieved using the flared notch--polyrod
 combination. Also, since the new feeding technique requires less space to
 accommodate the polyrod, the elements may be packed tighter together in an
 array configuration, as shown in FIGS. 6A-6B. Here the array 180 comprises
 rows of flared notch-fed polyrod elements 50 as in FIGS. 1-2, with the
 elements in the middle row offset from the elements of the adjacent rows
 to permit tighter packing of the elements.
 Another advantage of the invention lies in the ability to replace flared
 notch length with polyrod length. Ordinarily, to increase the gain of a
 flared notch antenna, the length of the flare must be increased. This can
 require more space. By lengthening the polyrod instead, the same gain may
 be achieved without having to increase the length of the flared notch,
 thus reducing the necessary amount of metal, weight and space.
 A mechanical advantage of the flare-polyrod combination lies in the fact
 that the flared notch structure stabilizes the base of the polyrod. For
 some applications, this same degree of mechanical stabilization may not be
 possible with a waveguide-fed polyrod.
 As a system, the flared notch-fed polyrod antenna 50 works in two modes.
 One mode is the frequency band at which the combination acts as a
 dielectric loaded flare, where the polyrod, fabricated of dielectric
 material, serves as the dielectric load. The other mode is the band over
 which the polyrod operates as a surface-wave antenna. The lower frequency
 bound of the first mode depends on the width of the flare. For efficient
 radiation, the electrical length of the flare width should be greater than
 or equal to one half of one wavelength. With the polyrod present, this
 condition will be altered somewhat because the dielectric material of
 which the polyrod is fabricated will tend to effectively increase the
 electrical length of the flare width, and thus lower the frequency bound.
 The transition from the first mode to the second mode will occur when the
 electrical length of the diameter, d (or largest cross-sectional
 dimension) of the polyrod is large enough, in wavelengths, to support an
 appreciable surface wave. This transition occurs at different frequencies
 for different polyrod materials, and can be determined through experiment.
 The upper frequency bound of the second mode occurs when the ratio of
 polyrod diameter to operating wavelength reaches a large enough value such
 that higher order waveguide modes are excited within the polyrod, at which
 point the antenna radiation pattern of the combination will have a null in
 the end fire direction.
 Depending upon particular application requirements, the dielectric material
 of the polyrod with its associated dielectric constant can be used as a
 tuning parameter. Since the wavelength inside the polyrod material (for
 materials with dielectric constants greater than unity) is effectively
 lowered, the starting frequency of the region over which the flare-polyrod
 combination works as a loaded flare is also lowered. Moreover, the
 transition point between the modes will also be moved down in frequency.
 By choosing the dielectric constant correctly through empirical testing
 and analysis, the frequency band of operation may be adjusted for desired
 performance.
 Recent EW passive listening, passive attack, and increased situational
 awareness requirements have called for antennas with high (12 dB to 16 dB)
 gain that must be co-located with the main radar antenna array, in most
 cases on the face of a slotted planar array, so that the capabilities of
 the existing gimbal may be exploited. Traditional solutions to this
 problem, such as a slotted array which uses aperture area to achieve the
 necessary gain, would not work in this case since there is no practical
 way to mount them on the front of the array without severely impacting the
 performance of the main array. An array of flared notch fed polyrods can
 be used in such an application. The unique feature of the flared notch fed
 polyrod that makes it useful in these systems is that it can provide the
 necessary gain using extent in length, rather than aperture area, without
 seriously impacting the performance of the main array. For example, a
 flared notch fed polyrod, 3.5 wavelengths long, has a gain of
 approximately 14.5 dB. To get the same gain using a slot array would
 require at least nine slots.
 FIG. 7A shows the outline of a slotted planar array 200 for use as a radar
 antenna. The planar slotted array 200 has radiating slot boundaries 210.
 Co-located with the radar antenna is a polyrod antenna system whose
 elements 220 are located at the radiating slot module boundaries 210. FIG.
 7B shows an elevation view to the slotted planar array. Among the
 advantages this feeding technique offers over the traditional waveguide
 fed polyrod are reduced volume, weight, components, complexity, and higher
 gain and broader operating bandwidth.
 It is understood that the above-described embodiments are merely
 illustrative of the possible specific embodiments which may represent
 principles of the present invention. Other arrangements may readily be
 devised in accordance with these principles by those skilled in the art
 without departing from the scope and spirit of the invention.