WO2016067269A1

WO2016067269A1 - Null forming in circularly polarized antenna patterns using reactive loading of multi-arm spiral antenna

Info

Publication number: WO2016067269A1
Application number: PCT/IB2015/058417
Authority: WO
Inventors: Waldemar Kunysz; Ronald H. Johnston; Michal M OKONIEWSKI
Original assignee: Uti Limited Partnership
Priority date: 2014-10-30
Filing date: 2015-10-30
Publication date: 2016-05-06

Abstract

Embodiments of antennas and systems for null forming in circularly polarized antenna patterns using reactive loading of multi-arm spiral antennas are described. In an embodiment, the antenna comprises a substrate, an antenna structure coupled to a surface of the substrate, the antenna structure being disposed in a region defined by a first boundary and a second boundary, the second boundary being confined within the first boundary, and one or more feed ports coupled to the antenna structure proximate to at least one of the first boundary and the second boundary.

Description

NULL FORMING IN CIRCULARLY POLARIZED ANTENNA PATTERNS USING REACTIVE LOADING OF MULTI-ARM SPIRAL ANTENNA

RELATED APPLICATION

This application claims priority to U.S. Provisional Application Serial No. 62/073,010 filed October 30, 2014, the entire text of which is specifically incorporated herein by reference without disclaimer.

Field

This disclosure relates generally to antenna systems, and more specifically, to null forming in circularly polarized antenna patterns using reactive loading of multi-arm spiral antennas.

Background

There have been significant developments of satellite-based commercial applications in the last decade. These, and other industries, are generating a need for re-configurable pattern, circularly polarized antennas that allow for the creation of deep nulls pointing towards an interfering signal or a multipath generated replica of the original signal. Satellite-based reception has many benefits, such as global reach; however the Achilles' heel of these systems is their susceptibility to RF jamming, self-interference and multipath. Proprietary digital processing has been used to maintain a lock on the signal in the presence of the interfering signal. Digital phased arrays have been used for null forming in the digital domain for military applications. Both approaches, however, do not work when the receiver RF front end is saturated in the presence of a very strong jamming signal. The only solution to such conditions is a reconfigurable pattern antenna that can create deep RF nulls towards the interfering signal and prevent receiver front end saturation. There are various methods of designing reconfigurable pattern antennas, such as phased array antennas. Typical designs are often large and expensive, requiring complicated RF feed systems connected to a multiple channel receiver where extensive digital signal processing is employed. Summary

Embodiments of antenna devices are disclosed. In an embodiment, an antenna device includes a substrate, an antenna structure coupled to a surface of the substrate, the antenna structure being disposed in a region defined by a first boundary and a second boundary, the second boundary being confined within the first boundary, and one or more feed ports coupled to the antenna structure proximate to at least one of the first boundary and the second boundary.

In one embodiment, the one or more feed ports include feed pins configured to extend in a direction that is perpendicular to the surface of the substrate. In some embodiments, the antenna structure is a spiral rotating from the first boundary into the region defined by the first boundary and the second boundary. The antenna structure may include a plurality of spirals winding from the first boundary to the second boundary.

In an embodiment, the antenna structure includes a first spiral antenna coupled to a first feed port at a first radial angle with reference to a center point of an area defined by the second boundary. In one embodiment, the antenna structure includes a second spiral antenna coupled to a second feed port at ninety (90) degrees from the first radial angle. The antenna structure may also include a third spiral antenna coupled the a third feed port at one hundred eighty (180) degrees from the first radial angle. Additionally, the antenna structure may include a fourth spiral antenna coupled to a fourth feed port at minus ninety (-90) degrees from the first radial angle. Various alternative embodiments may include more or less feed ports coupled at various alternative angles.

In an embodiment, the first spiral is coupled to the third spiral in a second region of the antenna structure, the second region being defined by the second boundary. In such an embodiment, the second spiral is coupled to the fourth spiral in a second of the antenna structure, the second region being defined by the second boundary. In an embodiment, the antenna structure comprises a plurality of feed ports, and wherein at least one feed port of the antenna structure is coupled to a reactive load, and at least one feed port of the antenna structure is coupled to an excitation source. .

The antenna structure may further include a ground plane disposed on a surface of the substrate opposite of the surface on which the antenna structure is disposed. In such an embodiment, the feed ports extend from the ground plane through the substrate to the antenna structure.

In one embodiment, the antenna structure includes a first spiral antenna coupled to the surface of the substrate, and a second spiral antenna spaced apart from the first spiral antenna. The antenna structure may also include a second region defined by a third boundary and a fourth boundary, the fourth boundary being confined within the third boundary, and the third boundary being confined within the second boundary. In such an embodiment, the antenna structure includes a first spiral antenna disposed within the first region, and a second spiral antenna disposed within the second region. Such an embodiment may also include a plurality of feed ports, at least one feed port being disposed proximate the first boundary, at least one feed port being disposed proximate the fourth boundary, and at least one feed port being disposed between the first boundary and the fourth boundary.

In an embodiment, the antenna structure is configured for dual frequency operation and dual polarized operation. The antenna structure may also be configured for dual frequency operation and single polarized operation. Additionally, the antenna structure is configured for dual frequency operation and multi-polarized operation.

In an embodiment, the antenna structure comprises a dual sinuous structure. The antenna structure may be configured for left hand circular polarization (LHCP). The antenna structure may be configured for right hand circular polarization (RHCP). Alternatively, the antenna structure is configured for both left hand circular polarization (LHCP) and right hand circular polarization (RHCP). In still another embodiment, the antenna structure is configured for left hand circular polarization (LHCP), right hand circular polarization (RHCP), and linear polarization.

Brief Description of the Drawings

The present invention(s) is/are illustrated by way of example and is/are not limited by the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity, and have not necessarily been drawn to scale.

Fig. 1 LHCP co-polarized outside -fed four arm spiral antenna (assume radiation coming out of the page) with a shallow metal cavity backing behind the spiral arms. Fig. 2A RHCP cross-polarized fields from open-ended inner arms.

FIG. 2B RHCP cross-polarized fields from short-ended inner arms.

FIG. 2C LHCP cross-polarized fields from a combination of open and shorted arms inside spiral arm ends, and LHCP co-polarized fields.

Fig. 3 is an LHCP polarized outside-fed, inside cross-joined four arm spiral antenna (Type 1).

Fig. 4 is an LHCP dual polarized outside-fed, inside joined four arm spiral antenna (Type

2).

Fig. 5 illustrates a cross-section view of the 4-arm spiral antenna type 1 shown in Fig. 3.

Fig. 6 illustrates an active region corresponding to Mode 1 of tightly wound 4-arm spiral at frequency of 2 GHz.

Fig. 7 illustrates normalized far field of four-arm spiral antenna excited using integer Mode 1,4 (gray curves) and non-integer Mode 1,8 (black curves), at frequency of 2 GHz.

Fig. 8 illustrates active regions of four-arm spiral antenna excited using N-mode 1, 4 (gray curve) and M-mode 1, 8 (black curve), frequency- 2GHz.

Fig. 9 illustrates an embodiment of a two-arm spiral antenna geometry.

Fig. 10A illustrates a discrete spiral array computed at frequency of 2 GHz.

Fig. 10B illustrates analog spiral antenna with phase gradient of S_m ^■ 0. 2 computed at frequency of 2 GHz.

Fig. 11 illustrates co-polarized (RHCP) vertical cut (φ=0) at 1 GHz of a four arm spiral antenna type 1, excited in non-integer Mode 1 (two adjacent arms) with reactive loading of 1 pF (3rd arm) and 4nH,8 nH,16nH, 32nH (4th arm).

Fig. 12 illustrates co-polarized (LHCP/RHCP) planar radiation pattern cut at 4 GHz of a four arm spiral antenna type 2, excited in non-integer Modes (1,8) to (7,8). Fig. 13 illustrates an embodiment of a top surface of the bottom ground plane (used to form the antenna cavity) of a four arm spiral antenna showing outside and inside ports excitation pins, where the feeding circuit is located on the underneath side of the PCB.

Fig. 14 illustrates a top PCB of four arm spiral antenna showing outside and inside ports excitation pins (solder points at the end of each spiral arm).

Fig. 15 illustrates anechoic chamber measurements with single inner arm (J7) excited, all other ports are open ended (main null depth of -45 dB located at theta=-51° and phi=-15°, freq=1575 MHz).

Fig. 16 illustrates an antenna circuit schematic of a circuit according to the present embodiments.

Fig. 17 illustrates anechoic chamber measurements with two inner arms (J6,J7) excited, Jl ,J4 terminated in 12 nH using a 3dB 90° coupler, J2-J5 and J3-J8 connected together, wherein the main null (depth is -48 dB) is located at theta=-66° and phi=+0°, freq=1600 MHz).

Fig. 18A illustrates an embodiment of a top layer of a four arm RHCP (excited from outside) spiral antenna type 1.

Fig. 18B illustrates an embodiment of a bottom layer of a four arm RHCP (excited from outside) spiral antenna type 1.

Fig. 19 illustrates characteristics of a four arm spiral antenna, outside fed with J3=3.3pf, J4=5.6 nH, at freq=1600 MHz.

Fig. 20A illustrates the Axial Ratio of a 4-arm standard spiral antenna excited from the inside at freq=1600 MHz.

Fig. 20B illustrates the Axial Ratio of a 4-arm standard spiral antenna excited from the outside at freq=1600 MHz.

Fig. 21 A illustrates the co-polarized radiation pattern of a 4-arm standard spiral antenna excited from the inside.

Fig. 21B illustrates the co-polarized radiation pattern of a 4-arm crossed-arm spiral antenna excited from the outside. Fig. 22A illustrates the cross-polarized radiation pattern of a 4-arm standard spiral antenna excited from the inside.

Fig. 22B illustrates the cross-polarized radiation pattern of a 4-arm crossed-arm spiral antenna excited from the outside.

Fig. 23A illustrates a top layer of dual sinuous antenna.

Fig. 23B illustrates a combined top and bottom layers of dual sinuous antenna.

Fig. 24 illustrates current distribution of dual sinuous antenna at 2 GHz excited from outside spiral arms (inner arms are joined using vertical vias)

Fig. 25 illustrates current distribution of single sinuous antenna at 2 GHz excited from inside spiral arms.

Fig. 26 illustrates current distribution of single sinuous antenna at 2 GHz excited from outside spiral arms.

Fig. 27 illustrates peak gain vs frequency for a single sinuous antenna excited from inside spiral arms.

Fig. 28 illustrates peak gain vs frequency for a single sinuous antenna excited from outside spiral arms.

Fig. 29 illustrates an embodiment of a dual frequency, dual polarized spiral antenna.

Fig. 30 illustrates an embodiment of a dual frequency, single polarized spiral antenna.

Fig. 31 illustrates an embodiment of a dual frequency, multi polarized dual-spiral antenna.

Detailed Description

The described embodiment include simple methods of creating small, wideband reconfigurable antennas for the reception of circular and/or elliptically polarized signals using a single channel receiver. The embodiments of the spiral antenna is used for satellite signal reception (circularly polarized) and have the ability to create narrow spatial nulls to suppress any intentional or unintentional interference signal. Such embodiments may allow for full control of null placement in both azimuth and elevation planes above the antenna horizon. The antennas may also allow a polarization sense change (from LHCP to RHCP or to linear polarization).

The position and depth of the null may change (rotate its position) due to scaling properties of spiral antenna with frequency. However the null position and its depth is tuned to a given frequency by adjusting the phase gradient of excitation ports and values of reactive termination ports.

Spiral antennas have a natural "built-in" handedness that allows for the transmission or reception of circularly or elliptically polarized signals. The presence of multiple spiral arms provides multiple design degrees of freedom. For example, a four arm spiral antenna has eight possible excitation ports. In normal applications only one to four ports need to be excited leaving the remaining "entry" ports for other usage. One such usage is to employ them as "reactively" loaded passive ports to establish a new antenna behavior (i.e. modified antenna pattern). The current flow on the outer circumference of the spiral antenna is varied by applying various loads along the antenna arm. In some embodiments, the antenna beam is steerable in various directions depending upon the location of the loads, with the gain staying uniform within 1.5 dB for most of the tested configurations. However, the method of tilting the antenna pattern was more targeted to moving the main beam than generating deep nulls, which also caused major degradation of the Axial Ratio. In our proposed antenna configuration, the reactive loads are placed at the outside ends of spiral arms. This reduces the complexity of the antenna structure while preserving a good Axial Ratio for various null positions.

The described spiral antenna structures allow reduction of cross-pol radiation as improved axial ratio through the entire radiation pattern, including the null area. Some spiral antennas may radiate a circularly polarized field along its axis with a polarization sense corresponding to the winding sense of the spiral. However, the presence of a current wave on the spiral arms flowing in the opposite direction to the desired one increases the cross- polarization level and so may change the polarization of the antenna to be elliptical and even linear. Such a current can occur from reflections at the ends of spiral arms. Various methods have been used to suppress the reflected energy. In one such method a lossy material (such as a resistive card or other resistive loading) is placed at the end of each spiral arm in order to absorb incident currents in both the radiation mode and transmission line mode [6] . Another method is the combination of serrated ground plane pattern and ferrite loading [7]. In order to create deep nulls off the antenna's boresight, various non-conventional excitation methods need to be employed. Most spiral antenna designs can exhibit in certain configurations a relatively high level of cross-polarization levels that can be enhanced when adding "reactively" loaded ports. To reduce these effects various embodiments of spiral antenna structure designs are described herein.

In an embodiment, the described methods and systems for null forming in circularly polarized antenna patterns use reactive loading of multi-arm Archimedean spiral antennas. The present embodiments describe multi arm wideband spiral antenna structures with reconfigurable radiation pattern that can be configured to transmit or receive signal with various polarizations, wide range of frequencies and applications. Embodiments may also generate very deep spatially narrow nulls, in otherwise omnidirectional radiation pattern, is described. The nulls can be steered by a combination of various phase excitation and reactive loading of spiral arms. The described antenna structures allow reduction of cross-pol radiations as improved Axial Ratio through entire radiation pattern (including the null area). The satellite based communication/location systems have many benefits (i.e. global reach); they are however susceptible to RF jamming, and interference. The presence of a deep null in the antenna radiation pattern (30-50 dB) can significantly reduce the level of the interfering signal and prevent the receiver front-end saturation.

Beneficially, the described embodiments provide the ability to generate nulls using single receiver and antenna architecture allowing for savings in size, cost, weight and power consumption. Additionally, such embodiments may provide simplified null steering. Also, the described embodiments may provide improved Axial Ratio for all cases (with null or without null). Additional benefits may include: the ability to have large number of spiral arms (N>=8) that was not practically achievable with classical approach of inner arm excitation, increases gain at low frequencies (or extends the bandwidth and hence it can be reduced in size for the same BW), ability to feed the spiral antenna from the outside (currently antenna are fed from inside in most applications), RF interference and multipath mitigation, and enhanced antenna gain at the lower edge of operating bandwidth (due to doubling of electrical length while constrained in the same space). One of ordinary skill may recognize additional benefits and advantages of the present embodiments. Most spiral antennas have been designed with excitation ports locates at the center (inside ends of spiral arms) of the antenna. In our designs we use external spiral arms for all port locations (excitation and reactive loading). The outside fed spiral antenna exhibits the same problem of reflected energy at the truncated spiral arm ends as the inside-fed spiral arms do.

In certain embodiments a multi-arm spiral antenna structure 100 is presented, such as a 4-arm spiral antenna 102, which may be fed from the outside using 0°, -90°, 180°, +90° feed ports 104a-d respectively, in a phase progression to create a Mode-1 LHCP radiation pattern that radiates out of the page, as seen in Fig. 1. In an embodiment, the arms 102 may be disposed in a region defined by a first boundary 106 and a second boundary 108. The first boundary 106 and second boundary 108 may not be tangible objects, but rather a logical separation of areas defined by features of the antenna structure 100. In an embodiment, the feed ports may be disposed proximate the first boundary 106. In alternative embodiments, the feed ports may be disposed proximate the second boundary 108. In various other embodiments, reactive loads may be disposed proximate either the first boundary 106 or the second boundary 108 as described in Figs. 2-4.

Open inner ends 202 or shorted inner ends 204 of spiral arms 102 may reflect any incident energy causing the reversal of the phase gradient across the spiral arms as shown in Figs. 2A-2C. In some embodiments, the phase gradient of the reflected wave is the same regardless whether the spiral ends are open-ended or shorted.

However, in some embodiments reflected energy can still have the correct phase gradient to contribute to the co-pol radiation instead of unwanted cross-pol radiation if one pair of spiral arms are shorted while the other arms pair is left open-ended. This provides a simple method of improving the Axial Ratio and efficiency of these spiral antennas.

Table 1 Reflected phase gradient between spiral arms (open and shorted-ended arms).

There are two additional methods to avoid the phase gradient reversal that cause unwanted RHCP radiation. The embodiment of the antenna shown in Fig. 3 (which is referred to herein as Type 1) has two external ports excited in 0°, -90° phase gradient with inner opposite arms joined together. This arrangement causes the back-travelling energy to have the same phase gradient as the forward-travelling energy. The remaining two external arms may be connected to passive ports that can be used to either absorb the incoming reflected energy or reflect again using reactive loading. In some embodiments, reactive loading allows dynamic radiation pattern changes. This arrangement introduces an antenna that has better polarization purity than if the reactive loads are connected at the center of the antenna (i.e. connected to the inner arm ends).

Another approach is to connect two opposite sense spiral antennas in the center while again exciting them at the outer ends, as seen in Fig. 4, which we may refer to as Type 2. The sense of handedness is left-handed for the top layer (black color) spiral antenna when excited from outside and also left-handed for the lower layer (red color) when excited from the inside. This arrangement reduces the reflected current's cross-polarization level at the arm ends, since the spiral arm handedness and phase gradient are matched for a given polarization excitation. The spiral arm handedness and phase gradient coincides in a normal spiral antenna configuration, therefore providing efficient radiation of a co-polarized signal.

The two spiral antennas are separated by a small vertical distance h {i.e. substrate thickness) and their inner arms are connected together using vertical vias. This configuration allows multiple polarization diversity similar to the sinuous antenna. The difference is that the sinuous antenna is of an equiangular type while this is an Archimedean type and provides lots of space for multiple ports. In our case we have eight available ports on the outside perimeter of the spiral antenna.

A. Antenna Construction Details

The concept of shallow lossless cavities (up to XI 50) has been demonstrated in [8-10]. Shallow lossless cavities allow increasing the net antenna gain at the expense of an increased Axial Ratio. A ring absorber 508 used in [11] allows for mitigation of some resonant effects induced by the antenna cavity. Shallow lossless cavities with a thin RF absorber (MT-30) placed at the perimeter 510 of the cavity (as shown in Fig. 5) may be used for various commercial multi-arm (N>12) antenna designs [12] . A 4-arm spiral antenna 102 was designed to operate from 1-4 GHz above a ground plane 502. It was simulated and tested with a shallow cavity, 15mm deep, spiral antenna radius of 58.9 mm and a substrate radius of 75mm.

A thin double copper clad substrate (1.6mm) with low dielectric constant (E_r=1.5) is etched to make a 4-arm spiral Archimedean antenna, feed lines 104 and reactive load circuits 506. The spacing (dr) between the arms is 1mm, trace width 0.5mm and number of turns set to 15. The spiral arms are joined in the middle. Two outside ends of four spiral arms are connected to a pair of reactive loads while the other two outside ends are used to excite the antenna. See the cross section in Fig. 5. Although the above example has been given for illustrative purposes, one of ordinary skill will recognize that a variety of different design parameters may be used in accordance with the present embodiments.

In certain embodiments, the antenna structure is disposed in a first region, the first region being defined by a first boundary and a second boundary. As used herein, the term "boundary" means an edge of an area in which the antenna structure is disposed. The boundary need not be a physical or tangible object, but rather is defined by a logical or extrapolated edge of the area based on the positioning of the physical elements of the antenna structure.

B. Reactive Loads

The concept of employing reactive loads is based on exciting only a subset k of all available N arms (k<N). The remaining arms would be terminated in reactive components (L, C) which, in turn, allow of control of the phase shifts of the reflected waves. These phase shifts, in turn, may allow steering of the pattern's main beam/null to a different spatial location. The "excited" and "reactively loaded" arms are connected together in order to increase the mutual coupling between them. Such a connection is easily implemented in the center of the spiral arm antenna, which necessitates feeding the antenna from the outside. A couple of such arrangements for a four-arm spiral antenna are shown in Figs. 3 and 4.

Table 1 displays the phase gradient when spiral arms are open-ended and short-ended. Terminating open spiral arms with reactive loads provides a wide range of phases of the reflected waves. Phase progression adjustment of the reflected currents allows for fine steering of the nulls created in the antenna radiation pattern while the coarse steering is achieved by changing phase progression value between adjacent arms using M-mode excitation described in Section IV. Capacitive loads between 0.5pF-15pF allow almost a -180° to 0° reflection angle and inductive loads between 0.5nH-30nH allow almost a 0° to +180° reflection angle range of adjustment of the phase of the reflected energy.

II. N-INTEGER MODES

The concept of modes on a spiral antenna was first appreciated by early inventors (see e.g.: Edwin Turner and John Dyson [13]), who designed a two-arm spiral antenna with a single mode of operation. Early researchers found that exciting a 2^nd mode proved to be impossible with a two-arm spiral antenna. The introduction of multi-arm spiral antenna designs by Paul

Shelton in 1960 allowed an antenna to be excited in higher order modes [14].

The number of modes of operation is directly proportional to the number of spiral arms. The phase gradient at the antenna input terminals for a given transmission mode n is given by:

where N is the number of spiral arms. The radiation pattern for a given mode n is identical to mode N-n except for the reversal of antenna pattern polarization. The fundamental modes n=l and n=N-l provide an omnidirectional pattern with peak directivity on antenna boresight but with opposite polarization. The remaining modes have a deep narrow null on the antenna boresight. The highest mode is generally not considered useful since it provides an unbalanced system (vector summation of initial phase does not end up with zero) that causes power to be reflected back into the antenna feed line system.

The presence of a null for modes n=2...n=N-2 provides an attractive opportunity for many applications e.g.: target tracking, interference cancelling, etc. The method of controlling the null typically requires a complicated passive (lossy) or active circuit. The null angle resolution of passive, lossy circuits is very limited (an i.e. complex hybrid circuit offers at best 22.5° angular resolution).

III. NON-INTEGER MODES

The N-arm spiral antenna can also be excited with a slightly unbalanced higher order mode m, normally associated with M-arm spiral antenna, where M is greater than N.

2mn

Ψ_™ =—ΓΓ m = Ι, , . Μ - Ι , Μ > N (2) The non-integer mode is associated with the fact that the phase gradient

is a fraction of ψ_η.

This approach takes advantage of the fact that the effective radiation area (also referred to as active region) around the circumference of m*wavelength (rrik) is quite large and can be truncated. As shown in Fig. 6 and 8, the maximum radiation for the fundamental mode (m=l) associated with a 4-arm spiral antenna does indeed occur at an antenna circumference of 1 *λ. However, there is also a significant radiation within the circumference area of a=0.5 λ to 1.5 λ. A tightly wrapped spiral antenna may radiate out of a circular band of circumference λ. A simulated current distribution is shown in Fig. 6. The active region is clearly visible when relative currents in all arms are close to zero degree (light green color). The active region forms a ring whose location depends on the operating frequency and excited mode. We can observe that the active region occupies a significant portion of antenna aperture.

It can be observed that the shape of the active region is symmetrical around antenna circumference (1*λ in Fig. 8). If the active region is truncated on one side, then the radiation contribution may be "uneven" resulting in a (nonsymmetrical) pattern squinting. This can be implemented in two ways: first by exciting the antenna with an unbalanced higher order mode or, second, by truncating the antenna aperture. There may naturally be some gain degradation (i.e. a 50% reduction of radiation area may translate to 3 dB gain loss), however we may gain the ability to steer the main beam or null off boresight to another elevation angle Θ. This is evident in Fig. 7 which shows various planar cuts of a normalized radiation pattern of a 4-arm spiral antenna excited in two different modes. N-mode 1,4 is the 1^st fundamental mode associated with 4-arm spiral (phase gradient of 0°-90^ο-180^ο-270°) while a higher order, non- integer M-mode 1,8 is the 1^st fundamental mode normally associated with 8-arm spiral (phase gradient of 0°, -45°, -90° and -135°).

Each line in Fig. 7 represents a separate vertical cut at various phi angles. The red curves correspond to the fundamental mode and show no nulls in the pattern. In contrast, the black curves indicate that at some phi angles a deep null (-47 dB) is present at theta angle near ±40°.

Fig. 8 shows that exciting a 4-arm spiral with non-integer Mode 1, 8 causes the active region to be shifted towards the center of the antenna. Mode 1, 4 has an active region in the expected area of circumference of one wavelength. IV. NULL STEERING

An algorithm to compute the necessary phase gradient across the spiral arms to steer a null to a desired frequency is given in [9]. The phase excitation values used for non-integer mode excitation described in this paper are in the same range as values computed in [9]. The precise phase excitation required [9] to satisfy this algorithm might be difficult to implement practically over wide frequency range. The present embodiments allow one to achieve this by generating a course phase excitation (M-mode) coupled with "finer" null steering using reactive loads located at spiral arm ends.

It can be shown that the Array Factor (AF) of an M-arm discrete spiral antenna consisting of N isotropic radiators is (see Fig. 9):

M N

A = ^ ^ J_{n 6}} [1ιτ(φ_η) 5ίη(θ) οο5( -φ_η-β_γη)+α_η+δ_γη] ^

m=l n=l

2π 27rm 2nm

9

Where s is the mode number, dL_n is a distance along the spiral arm, a_n is the excitation of the n-th element, β_ΉΙ is the angular separation between adjacent arms and S_m is the phase gradient between the spiral arms.

The necessary null condition is satisfied when the desired phase gradient between spiral arms satisfies the following criteria ρπ

a_n + kr{(p_n) sin(0_Q) cos(0_o - <p_n - jff_m) + 8_m =— ,

p = ± 1

In the digital array case, the null steering is achieved by controlling coefficients a_n. In our case of analog (continuous) spiral array the only parameter that we can control is the phase excitation between spiral arms— 6_m (N-modes and M-modes used in previous sections). It can be shown (see Figs. 10A-10B) that adjusting S_m also produces desired null location control as in the case of the discrete spiral array.

V. SIMULATION RESULTS

A commercial 3D electromagnetic software was used to simulate various spiral-antenna topologies. The main design focus was on a planar four-arm Archimedean type spiral topology with tight winding (spacing of 1 mm and trace width of 0.5mm) and 15 turns. A. Crossed Spiral Antenna (Type 1)

As the first example let us consider an antenna of Type 1 , described previously in section II (Fig. 3 and 5). In the simulation, the first two adjacent arms are excited with a 90° phase gradient (to create Mode 1 type RHCP pattern) while the third outside arm is terminated in 1 pF load and the fourth outside arm is terminated sequentially with reactive loads of 4nH, 8nH, 16nH and 32nH. See Fig. 11. The null depth is in the order of 25-35 dB and occupies a relatively small conical angle, hence significantly attenuating the interfering signal while maintaining good signal reception everywhere else. Fig. 11 Co-polarized (RHCP) vertical cut (< >=0) at 1 GHz of a four arm spiral antenna type 1, excited in non-integer Mode 1 (two adjacent arms) with reactive loading of 1 pF (3^rd arm) and 4nH,8 nH,16nH, 32nH (4^th arm)

B. Dual Polarized Spiral Antenna (Type 2)

A second example is a four arm spiral antenna of Type 2 (shown in Fig. 4). All four arms are excited from the outside, using various M-mode values of phase gradient between adjacent spiral arms. The simulated radiation pattern at 4 GHz is shown in Fig. 12.

Fig. 12 demonstrates the ability to change antenna polarization while maintaining the same vertical position (θ=±25°) of the null in the radiation pattern. The peak antenna gain of Mode 7, 8 is lower than Mode 1, 8 due to the fact that the handedness of Mode 7,8 is opposite to the physical handedness of the spiral antenna itself.

VI. MEASURED RESULTS

Antenna simulations were performed to demonstrate the initial concept of creating deep nulls in the radiation pattern using novel feeding and termination techniques. The antenna feeding circuits were designed for various types of connections to further explore the concepts described in previous sections. Two types of antennas were designed and manufactured. The first antenna is a four-arm equiangular spiral structure with the ability to excite inner arms as well as outside arms. It also allows for various connections between inner and outer arms. The second antenna is a four-arm Archimedean structure which allows excitation of only the outside arms. Each pair of inner arms that are separated by 180° is joined together.

A. Four-Arm Equiangular Spiral Antenna An equiangular four-arm spiral antenna was used first to assess the feasibility of creating nulls while maintaining the handedness of circular polarization. The spiral antenna has a maximum radius of 60 mm and is backed by a shallow metal ground plane (reflector) at a distance of 15mm (λ/17 at lower end of the band). An RF ring absorber is placed on the perimeter of the cavity filling the space between antenna and ground plane, see Fig. 5. All measurements were performed in an anechoic chamber (10'χ20'χ10') designed for 500MHz- 18 GHz bandwidth.

Two quadrature arms are driven with a 90° phase gradient in order for the antenna to be predominately circularly polarized. The feeding circuit was designed to have an option of feeding spiral arms from the outside ends or inside ends of spiral arms. See Fig. 13. The cavity bottom PCB with eight pins, is used for spiral arm excitation or termination.

The other two arms (parasitically coupled to the driven arms) are loaded with reactance on one or both ends of the spiral. The antenna was designed to support a minimum frequency of 1.0 GHz. Antenna PCB layout (Fig. 14) suggests LHCP handiness for outside fed arms and RHCP handiness for inside fed arms, however either polarization can be achieved by using a proper phasing gradient between driven spiral arms.

Fig. 15 represents the upper hemisphere radiation pattern where an excitation port is connected to a single inner arm at port J7. The remaining inner and outer ends of spiral arms were left open ended. A significant dip in the amplitude of the radiation pattern can be observed via the presence of two nulls reaching a level between -40 to -55 dBic.

The scaled bar shown in Fig. 15 and subsequent figures represents the measured antenna gain in dBic (circularly polarized with respect ideal isotropic antenna).

Fig. 16 is a block diagram illustrating one representation of the spiral antenna system. The spiral antenna is treated as an 8-port network with 4 ports on the left hand side representing the outside spiral arms connections while 4 ports on the right hand side represent the inside arms connections. The box in the middle represents the spiral antenna itself.

The 3 dB 90° hybrid coupler (attached to J6, J7 ports) is necessary to generate a 90° phase gradient necessary to excite a circularly polarized non-integer mode 1 in the spiral antenna. The second 3 dB 90° hybrid coupler (attached to Jl, J4 ports) serves a double purpose. The main goal is to introduce phase reversal of the reflected energy. It can be shown that with two identical reactive loads connected to the 3-dB hybrid, substantially all energy may be reflected with 180° phase reversal. The phase reversal of the reflected energy is re-radiated as a co-polarized component since the arm handiness is also reversed in that case. Secondly, the hybrid provides good isolation between different impedances as encountered in this case (between antenna and reactive loads).

The logic behind the connections of J2 to J5 and J3 to J8 is to re-launch the energy from two "excited" outer arms back to two "parasitic or passive" inner arms. This causes uneven current distribution within the spiral antenna and hence formation of a sharp deep null as shown in Fig. 17.

The location of the null is stable, with respect to frequency, since the phase gradient of the reflected current is the same because both ports Jl and J4 are terminated with equal inductances. Using the internal ends of spiral ports is difficult due to space constraints and mutual coupling between pins that connects the arms with reactive loads or excitation circuitry. This may cause undesired effects such as additional unwanted nulls being present in the radiation pattern. The crossed arms spiral antenna discussed in the next section solves this problem since no pins are required for the inner arms. The null is stable over a limited frequency range of 10-20 MHz due to the frequency scaling properties of the spiral antenna. A fixed location of the null over a wider frequency range is accomplished by the use of reactive termination control and the phase excitation of active ports.

B. Crossed Arms Spiral (Type 1)

A planar crossed arm spiral antenna is shown in Figs. 18A-18B. Fig. 18A illustrates a surface of the substrate on which the spiral antenna 102 is disposed, and Fig. 18B illustrates an opposing surface of the substrate with ports for coupling either a reactive load or an excitation source. The trace width and the spacing between the spiral arms are both 1mm. One pair of spiral arms are joined on the top of the PCB while the other pair on the bottom of the antenna PCB as shown in Fig. 18 (right side). This corresponds to the simulated topology shown in Fig. 4, Section II.

The RHCP radiation pattern of the four-arm antenna, excited in non-integer Mode 1,4 (two ports - Jl, J2) with other two ports (J3 and J4) terminated in reactive loads, is shown in Fig. 19. C. Axial Ratio

An introduction of a null in the radiation pattern can cause severe degradation of the Axial Ratio in the overall radiation pattern (not just in the close proximity to the null area). Figs. 20A-20B show that the cross-spiral antenna excited from the outside perimeter of the antenna (Fig. 20B) has a superior Axial Ratio performance when compared to the classical spiral antenna excited from the antenna's inside ports (Fig. 20 A).

The cross-pol radiation pattern in the case of the classical spiral antenna excited from inside is relatively high due to reflections from the outside spiral arm ends and it does not change much with the presence of the null. Due to the nature of this novel antenna approach, the cross-polarization level for the cross-spiral antenna is much lower, hence the introduction of a null in the main co-polarized pattern has a corresponding lower cross-pol level in the same region. Refer to corresponding Figs. 21A through 22B. Fig. 21A illustrates the co-polarized radiation pattern of a 4-arm standard spiral antenna excited from the inside. FIG. 21B illustrates the 4-arm cross spiral antenna excited from the outside. The frequency of excitation is 1600 MHz in both cases. Fig. 22A illustrates the cross-polarized radiation pattern of a 4- arm standard spiral excited from the inside, and Fig. 22B illustrates the 4-arm cross spiral antenna excited from the outside.

D. Impedance

The spiral antenna impedance can vary with the type of excited radiation mode and chosen spiral design parameters [15]. We have found the measured value to be around 100 Ω, which is in agreement with [15]. The impedance converges to 100 Ω at higher frequencies; however at low frequencies it exhibits large oscillations, likely caused by the shallow cavity and increased excitation of higher order modes. We would expect that increasing the number of arms should reduce how many higher order modes are excited and hence provide more uniform impedance with frequency.

The antenna structures described provide methods of creating and steering a deep null in radiation patterns. The method relies on excitation of only a subset of spiral arms in a given mode while reactively loading the remaining arms. Sharp nulls (in order of 30-50 dB) in circularly and elliptically polarized patterns are achieved. Steering the null in the azimuth plane (phi) in increments of 2π/Ν is relatively easy by means of sequential rotation of the pair of spiral arms that are excited. Additional "fine" steering is obtained by reactive loading of spiral ends. Null steering in elevation angle is more constrained. In normal configurations, the elevation angle range is limited to locations near the antenna horizon (theta angle between 80°-90°). Non-integer excitation in Mode 2 (i.e. two arms out of four excited) with the addition of reactive loading allows further steering of the null off the antenna boresight.

VII. ADDITIONAL EMBODIMENTS

The dual sinuous antenna can be printed on two layers with 2nd spiral antenna unit being a mirror image of 1st one as shown in FIGs. 23A-23B, for example. In some embodiments, the first antenna element is spaced apart from the second antenna element. For example, in some embodiments a spacer or spacing layer is disposed between the first spiral antenna and the second spiral antenna.

Such antenna have a double physical length compared to a single spiral antenna and there is no cross-pol radiation associated with backwave travelling current since the handedness of the 2nd structure support the co-pol radiation. Embodiments of such antenna are excited from inner arms to have as wide possible bandwidth as possible. However if used in application where multiple arms are required (more than six) the outside excitation can be an acceptable compromise for reduced bandwidth.

Fig. 24 shows a current distribution on such antenna excited from the outside with inner spiral end connected together through vertical vias. This is not a typical current associated with spiral antenna and resembles more of a ring antenna.

Figs. 25-26 show that there is a substantial difference in a current distribution between two cases: outside and inside excitation of spiral arms. The performance converges (for both cases) at lower frequencies where the "effective" radiation region is getting close to the outside excitation port region.

Figs. 27 and 28 demonstrate that the spiral antenna behaves the best at lower frequencies (<1.5 GHz) and has more gain. Adding a second spiral below further improve this performance. Increasing the gain without changing the physical aperture of the antenna is one of the most- sought aspect of modern antenna design due to limited space allocated to a typical antenna platforms

A. Dual frequency, dual polarized spiral antennas.

In some application there is no need for wideband performance but operation at distinct two frequencies (transmission and reception) and quite often at two different polarization. This can be accomplished with a spiral antenna excited in the middle of the spiral structure with a corresponding antenna winding that matches the desired polarization, see Fig. 29. In some embodiments, the first spiral antenna is disposed in a first region and the second spiral antenna is disposed in a second region. The first region may be defined by a first boundary and a second boundary, the second boundary being contained within the first boundary. The second region may be defined by a third boundary and a fourth boundary, the fourth boundary being contained within the third boundary, and the third boundary being contained within the second boundary.

Fig. 29 illustrates a first set of spiral arms 102 disposed in a first region defined by the first boundary 106 and the second boundary 108, and a second set of spiral arms 102 disposed in a second region defined by a third boundary 2202 and a fourth boundary 2204. Similarly, the third and fourth boundary need not be physical components, but may be logically defined in response to the dispositions of elements of the antenna structure, such as the feed ports and reactive load ports, or the physical boundaries of the spiral arms 102.

The solid black dots represents excitation ports while empty circles represents possible locations of reactively loaded terminations for null steering. The outside spiral will correspond to lower frequency of operation and inner spiral to higher frequency of operation. Note the spiral handedness is the same from the point of view of excitation ports point of view that will support different circular polarization. In case when same polarization is required the structure would have to be implemented as shown on Fig. 30

We can have another combination with dual spiral structures printed on both side of the PCB substrate as shown in Fig. 31. This approach mitigates the problem associated with the previous dual-spiral antenna in cases where excitation ports were further apart than half wavelength (required to prevent 'grating ' lobes to form in the visible space).

Although the invention(s) is/are described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention(s), as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention(s). Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims.

Unless stated otherwise, terms such as "first" and "second" are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The terms "coupled" or "operably coupled" are defined as connected, although not necessarily directly, and not necessarily mechanically. The terms "a" and "an" are defined as one or more unless stated otherwise. The terms "comprise" (and any form of comprise, such as "comprises" and "comprising"), "have" (and any form of have, such as "has" and "having"), "include" (and any form of include, such as "includes" and "including") and "contain" (and any form of contain, such as "contains" and "containing") are open-ended linking verbs. As a result, a system, device, or apparatus that "comprises," "has," "includes" or "contains" one or more elements possesses those one or more elements but is not limited to possessing only those one or more elements. Similarly, a method or process that "comprises," "has," "includes" or "contains" one or more operations possesses those one or more operations but is not limited to possessing only those one or more operations.

REFERENCES

[l] R.F. Harrington, "Reactively controlled directive arrays", IEEE Trans. Antennas

Propag., vol. AP-26, No.3, pp. 390-395, 1978.

[2] K. Gyoda and T. Ohira, " Design of electronically steerable passive array radiator

(ESPAR) antennas", Proc. IEEE Antennas Propag. Soc. Int. Symp_^. vol. 2, pp. 922-

925, Salt Lake City, UT, USA, July 2000.

[3] H.Kato and Y. Kuwahara, "Novel ESPAR antenna", Proc. IEEE Antennas Propag.

Soc. Int. Symp.^ vol. 4B, pp. 23-26, Washington, DC, USA, July 2005.

[4] A. Mehta and D. Mirshekar-Syahkal, "Spiral antenna with adaptive radiation pattern under electronic control", Proc. IEEE Antennas Propag. Soc. Int. Symp. , Vol. 1, pp. 843-846,

Monterey, CA, USA, June 2004.

[5] A. Mehta, D. Mirshekar-Syahkal, and H. Nakano, "Beam adaptive single arm rectangular spiral antenna with switches", LEE Proc. Microwave, Antennas Propag., Vol. 153, No. 1, pp. 13-18, 2006.

[6] T.T. Wu and R.W.P. King, "The cylindrical antenna with nonreflecting resistive

loading", IEEE Trans. Antennas Propag., vol. 13, No.3, pp.369-373, 1965. [7] A. Kramer, S. Koulouridis, C.-C. Chen, and J. L. Volakis, "A novel reflective surface for an UHF spiral antenna", IEEE Antennas Wireless Propag. Lett., vol. 5, No.l, pp. 32-34, 2006.

[8] J. Kaiser, "The Archimedean two- wire spiral antenna", IRE Trans. Antennas Propag., Vol.

8, No. 3, pp. 312-323, 1960.

[9] M.J. Radway, D.S. Filipovic, "Wideband pattern nulling with multiarmed spiral antennas", IEEE Antennas Wireless Propag. Lett., Vol. 12, pp. 864-867, 2013.

[10] M.J. Nurnberger, J.L. Volakis, "New termination and shallow reflecting cavity for ultra wide-band slot spirals", Proc. IEEE Antennas Propag. Soc. Int. Symp., Vol. 3, pp.1528-

1531, Salt Lake City, UT, USA, July 2000.

[11] J.J.H. Wang, V.K. Tripp, "Design of multioctave spiral-mode microstrip antennas", IEEE

Trans. Antennas Propag. , Vol. 39, No.3, pp. 332-335, 1991.

[12] http://www.novatel.com/products/gnss-antennas

[13] J. Dyson, "The equiangular spiral antenna," IRE Trans. Antennas Propag. , vol. AP-7,

No.2, pp. 181-187, 1959.

[14] J. Shelton, " Four arm spiral direction finding system", Private communication at

Radiation Systems Inc., Alexandria, VA to Langthorne Sykes, U.S Naval Ordnance Test

Station (NOTS) China Lake, CA., 23 Dec. 1960.

[15] J.A. Huffman and T. Cencich, "Modal impedances of planar, non-complementary, N- fold symmetric antenna structures", IEEE Antennas Propag. Mag., Vol. 47, No. 1, pp.

110-116, 2005.

Claims

1. An antenna device, comprising:

substrate; an antenna structure coupled to a surface of the substrate, the antenna structure being disposed in a region defined by a first boundary and a second boundary, the second boundary being confined within the first boundary; and one or more feed ports coupled to the antenna structure proximate to at least one of the first boundary and the second boundary.

2. The antenna of claim 1 , the one or more feed ports comprising feed pins configured to extend in a direction that is perpendicular to the surface of the substrate.

3. The antenna of claim 1, wherein the antenna structure is a spiral rotating from the first boundary into the region defined by the first boundary and the second boundary. .

4. The antenna of claim 3, wherein the antenna structure comprises a plurality of spirals winding from the first boundary to the second boundary.

5. The antenna of claim 1, wherein the antenna structure comprises a first spiral antenna coupled to a first feed port at a first radial angle with reference to a center point of an area defined by the second boundary.

6. The antenna of claim 5, wherein the antenna structure comprises a second spiral antenna coupled to a second feed port at ninety (90) degrees from the first radial angle.

7. The antenna of claim 6, wherein the antenna structure comprises a third spiral antenna coupled the a third feed port at one hundred eighty (180) degrees from the first radial angle.

8. The antenna of claim 7, wherein the antenna structure comprises a fourth spiral antenna coupled to a fourth feed port at minus ninety (-90) degrees from the first radial angle.

9. The antenna of claim 8, wherein the first spiral is coupled to the third spiral in a second region of the antenna structure, the second region being defined by the second boundary.

10. The antenna of claim 8, wherein the second spiral is coupled to the fourth spiral in a second of the antenna structure, the second region being defined by the second boundary.

11. The antenna of claim 2, wherein the antenna structure comprises a plurality of feed ports, and wherein at least one feed port of the antenna structure is coupled to a reactive load, and at least one feed port of the antenna structure is coupled to an excitation source.

12. The antenna of claim 1, further comprising a ground plane disposed on a surface of the substrate opposite of the surface on which the antenna structure is disposed.

13. The antenna of claim 12, wherein the feed ports extend from the ground plane through the substrate to the antenna structure.

14. The antenna of claim 1, wherein the antenna structure further comprises a first spiral antenna coupled to the surface of the substrate, and a second spiral antenna spaced apart from the first spiral antenna.

15. The antenna of claim 1, wherein the antenna structure further comprises a second region defined by a third boundary and a fourth boundary, the fourth boundary being confined within the third boundary, and the third boundary being confined within the second boundary.

16. The antenna of claim 15, wherein the antenna structure further comprises:

a first spiral antenna disposed within the first region; and a second spiral antenna disposed within the second region.

17. The antenna of claim 16, further comprising a plurality of feed ports, at least one feed port being disposed proximate the first boundary, at least one feed port being disposed proximate the fourth boundary, and at least one feed port being disposed between the first boundary and the fourth boundary.

18. The antenna of claim 1, wherein the antenna structure is configured for dual frequency operation and dual polarized operation.

19. The antenna of claim 1, wherein the antenna structure is configured for dual frequency operation and single polarized operation.

20. The antenna of claim 1, wherein the antenna structure is configured for dual frequency operation and multi-polarized operation.

21. The antenna of claim 1, wherein the antenna structure comprises a dual sinuous structure.

22. The antenna of claim 1, wherein the antenna structure is configured for left hand circular polarization (LHCP).

23. The antenna of claim 1, wherein the antenna structure is configured for right hand circular polarization (RHCP).

24. The antenna of claim 1, wherein the antenna structure is configured for both left hand circular polarization (LHCP) and right hand circular polarization (RHCP).

25. The antenna of claim 1, wherein the antenna structure is configured for left hand circular polarization (LHCP), right hand circular polarization (RHCP), and linear polarization.

26. The antenna of claim 1 , further comprising one or more reactive loads coupled to one or more of the feed ports, the reactive loads configured to cause null steering of radiation emitted by the antenna.