Abstract—A novel dual-polarized ultrawideband planarmonopole antenna operating in the microwave frequency bandis presented. The novel antenna is designed around a disc-loadedmonopole antenna with a single microstrip feed and three polar-ization controlling switches. The antenna is designed on low-costFR-4 substrate with 1/2-oz copper cladding. The overall size ofthe antenna is 53 70 mm . The dual-polarized antenna exhibitsover 97.6% 10-dB return loss at 3.1 GHz, a gain of 1 dBi acrossthe band of operation with a polarization isolation better than15 dB across the band.
Index Terms—Dual-polarized antenna, monopole antenna, po-larization diversity, wideband.
I. INTRODUCTION
W IRELESS technology and communications have expe-rienced tremendous growth and development in the past
two decades. For every wireless communications system, thechoice and design of the antenna is crucial andmust meet systemspecifications [1], [16]. In the past several years, dual-polarizedantennas have been more commonly used in order to enhancechannel capability by solving the issues of multipath fading andcoupling between adjacent antennas of a multiple-input–mul-tiple-output (MIMO) system. Besides the requirement of dualpolarization capability, antennas with large operating bandwidthequal to or greater than 50% are required in order to cover mul-tiple bands of operation worldwide.The most common type of dual-polarized antennas reported
in literature are those comprising two antenna elements that arefed independently [2]–[5]. The most commonly used antennasare patch, slot, and disc-loaded monopole antennas due totheir simple and compact layouts. However, patch antennastend to have a restriction on bandwidth ( 2%) when directlyfed, although this can be extended by means of coaxial andaperture slot feeding techniques [6]. Nevertheless, the use oftwo independent antenna elements and feeds increases the sizeof the antennas. A comprehensive review of single-elementdual-polarized monopole antennas has been presented in [7].However, these antennas do not allow for the separation of thesignal received in different polarizations due to the single-feedsetup. Another type of dual-polarized patch antenna based
Manuscript received February 06, 2014; revised April 04, 2014; acceptedApril 25, 2014. Date of publication April 29, 2014; date of current version May08, 2014.The author is with Nitero Pty Ltd., Melbourne, Vic. 3000, Australia (e-mail:
[email protected]).Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LAWP.2014.2320891
Fig. 1. Layout of the dual-polarized monopole antenna.
on metamaterials has been reported in [8]. The patch antennauses metamaterial parasitic elements in order to enable polar-ization diversity, however the main limitation of this antennais bandwidth. Similar to the patch antenna, wideband anddual-polarized characteristics can also be achieved in a slot an-tenna. Several dual-polarized slot antennas have been reportedin recent years [9]–[14]. However, these antennas exhibitbandwidths less than 50% and therefore can have limitations incovering multiple wireless technology bands.Monopole antennas are good candidates for wideband
achievement since their wideband operation is attributed to theoverlapping of multiple antenna modes that are distributed overthe spectrum [15]. They also exhibit figure-of-eight radiationpatterns with the null located in the direction of the antenna’spolarization.In this letter, we present a novel dual-polarized antenna ar-
chitecture based on a single-monopole antenna element. The an-tenna is designed on low-cost FR-4 substrate with 1/2-oz coppercladding. The overall size of the antenna is 53 70mm and ex-hibits 100% bandwidth and polarization isolation greater than20 dB. This architecture allows for the development of ultraw-ideband disc monopole antennas based on single elements withhigh polarization isolation and omnidirectional radiation patterncoverage by using a dual feed controlled via RF switches. Theantenna is the first of its kind to use a disc monopole fed andcontrolled in two orthogonal polarizations.
II. DESIGN
The layout of the antenna is presented in Fig. 1. The antennais manufactured on 1.5-mm-thick FR-4 substrate ( ,
) with 17 m of copper cladding on the top andbottom of the substrate. It comprises a copper disk and 50-feed network in the topmetal layer (brown color) and the ground
Fig. 2. Surface current distribution in the monopole antenna for (a) horizontalpolarization and (b) vertical polarization stimulation.
Fig. 3. Biasing and control of the RF SPDT switch.
plane (yellow color) in the bottom metal. The antenna com-prises three single-pole double-throw (SPDT) RF switch chipsPE4259 to control the polarization diversity and to disable exci-tation of the microstrip feedline for the polarization that we donot wish to excite. Chips 1 and 2 are active for vertical polar-ization, and chips 1 and 3 are active for horizontal polarization.Fig. 2 shows the surface current distribution for two different
polarization modes. It is clear from Fig. 2(a) that when the hor-izontal polarization feed network is excited (chips 1 and 3 areON, chip 2 is OFF) that the surface current is predominantly inthe horizontal direction, therefore creating a linear polarized ra-diation field in the horizontal plane for the antenna. The samecan be said from Fig. 2(b) except that it is in the vertical plane.We can see that the antenna radiates mostly from the part of thedisc closest to the ground plane from which it is fed. This couldallow for certain parts of the disc to get truncated without greatlyinfluencing the antenna performance in order to further reducethe area occupied by the antenna.The polarization diversity of the antenna is controlled via
three SPDT RF switches; see Fig. 3. The RF switches are 3.3V compatible and allow for the RF signal to flow from and toRF1 to and from RFcmn when the CTRL signal is set to “0.” Inthis case, the RF2 port has a high impedance. Alternatively, theRF switch enables the RF signal to flow from/to RF2 to/fromRFcmn when the CTRL signal is set to “1.” In this case, RF1port has a high impedance.Fig. 4 shows the manufactured dual-polarized monopole an-
tenna on FR-4 substrate. The antenna has a flat cable attached toit so that biasing and control signals can be supplied to the RFSPDT switch chips. The total power consumption of the antennacircuit is W. Table I shows the value of the antenna di-mensions presented in Fig. 4.
Fig. 4. Photograph of antenna with dimensions manufactured on FR-4.
Fig. 5. Simulated and measured return loss of the antenna in vertical and hor-izontal polarization modes.
TABLE IANTENNA LAYOUT PARAMETERS
III. RESULTS
Fig. 5 presents the simulated and measured one-port -pa-rameters of the prototype antenna for two different polarizationmodes of the antenna.Fig. 6 shows that the RF switching chips operate up to
4 GHz, and their return loss and insertion loss deteriorate be-yond 4.5 GHz, therefore limiting the bandwidth of the antennaas presented in Fig. 5. This can be avoided by using RF switcheswith a wider band of operation if required. The simulated andmeasured 10-dB return losses (RLs) of the antenna are exhib-ited when the vertical polarization mode is set by configuringRF switch chips 1 and 2 on and keeping RF switch chip 3 off.The simulated and measured 10-dB return losses of the antennaare exhibited when the horizontal polarization modes is setby configuring RF switch chips 1 and 3 on and keeping RFswitch chip 2 off. In vertical polarization mode, the antennareturn loss is measured to be below 10 dB from 1.5 to 4.5 GHz,resulting in 100% bandwidth. In horizontal polarization mode,the antenna return loss is measured to be below 10 dB from
858 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014
Fig. 6. Measured insertion loss and return losses of the RF switches.
TABLE IICOMPARISON OF BANDWIDTH FOR DUAL-POLARIZED ANTENNAS
1.65 to 4.8 GHz, resulting in 97.6% bandwidth. The antennamatching in the two polarizations is not symmetrical due to themicrostrip line feeds not being identical in length. It is clearthat the vertical polarization feed provides a better match inthis case than the horizontal polarization feed. A comparisonof achieved RL bandwidth between dual-polarized antennaspreviously reported and this prototype is presented in Table II.Considerable improvement in bandwidth is achieved by usingmonopole antennas instead of slot antennas and by using widebandwidth switches. An increase in bandwidth of 51.8% hasbeen achieved at a reduction of the total area of the antennafrom 225 [9] to 37.1 cm .Figs. 7 and 8 show the simulated and measured copolar and
cross-polar radiation pattern components of the antenna in E-and H-planes at 2.5 and 3.5 GHz, respectively. These frequencypoints were chosen to present the radiation pattern of the an-tenna across the operating frequency band. The monopole an-tenna is operating in the fundamental mode, hence we can ex-pect the radiation patterns to remain very similar to the onespresented in Figs. 7 and 8 up to 4 GHz. From Fig. 7, it is clearthat the cross-polar component suppression is better than 15 dBalong the majority of the angular space. From Figs. 7 and 8, wecan see that the simulated and measured results agree reason-ably well with each other. The copolar E-field patterns exhibit afigure-of-eight shape as expected. The H-field patterns exhibit amore omnidirectional radiation pattern that is also as expected.The copolar radiation patterns also show that there is a tilt in thepatterns away from the grounds. This tilt is usually not presentin radiation patterns of monopole fed with a single feed. How-ever, in this case, the antenna is dual-fed with a ground planeon the bottom and right side of the antenna (Fig. 4, top), whichresults in the antenna’s E- and H-field copolar radiation patternto tilt to the left in the vertical polarization mode, while in thehorizontal polarization mode, the tilt is in the right direction.Fig. 9 shows the simulated and measured gain of the antenna
in both polarizations. The gain of the antenna is measured to be
Fig. 7. Simulated and measured copolar and cross-polar radiation patterns ofthe antenna at 2.5 GHz of (a) E-plane in vertical polarization mode, (b) H-planein vertical polarization mode, (c) E-plane in horizontal polarization mode, and(d) H-plane in horizontal polarization mode.
Fig. 8. Simulated and measured copolar and cross-polar radiation patterns ofthe antenna at 3.5 GHz of (a) E-plane in vertical polarization mode, (b) H-planein vertical polarization mode, (c) E-plane in horizontal polarization mode, and(d) H-plane in horizontal polarization mode.
between 1 and 1.8 dBi gain across the band in both polarizations.Above 5 GHz, the gain drops mainly due to the increase in inser-tion loss of the RF switches as shown in Fig. 6. The simulated
Fig. 9. Simulated and measured gain of the antenna as well as the simulatedisolation between the feeds.
Fig. 10. Simulated time-domain response of a first-order Rayleigh pulseprobed in the antenna far field.
Fig. 11. Simulated group delay of the antenna in both polarizations.
and measured results agree well. The isolation of the antennais shown in Fig. 9. Only simulated results have been presenteddue to the limitation of measuring the isolation between the twofeeds. The drop in isolation at the higher frequency band is inlarge due to the reduction of the RF switch insertion loss as itapproaches the upper frequency limit of its operation. The sim-ulated efficiency of the antenna from 1.5 to 4.8 GHz in bothpolarizations is 94%. The antenna was simulated by activatingone polarization at a time with chip 2 being modeled as an opencircuit to achieve horizontal polarization and chip 3 being mod-eled as an open circuit to achieve vertical polarization.Figs. 10 and 11 show the time-domain performance of the
antenna, which is important for pulsed-based systems. Fig. 10shows the distortion of a first-order Rayleigh pulse in the farfield of the antenna. Fig. 11 shows the simulated group delay ofthe antenna in both polarizations. The variation of the antennagroup delay across the frequency band is 350 ps.
IV. CONCLUSION
In this letter, a novel dual-polarized monopole antenna ispresented. By applying two individual feeds to a disk-loadedmonopole antenna at orthogonal points controlled by RFswitches, a dual-polarized antenna with high isolation ( 20dB), wide bandwidth around 3.1 GHz, and better than 15 dBcross-polar component suppression is achieved. The prototypeantenna is built and tested, and it is reported to achieve betterthan 97.6% bandwidth at 3.1 GHz. The gain of the antenna isapproximately 1 dBi across the band. With these features, theantenna can be used in various cellular and wireless networkingfrequency bands to enhance the data throughput at lower cost.Future work on this antenna could involve redesigning theantenna in order to exhibit circular polarization by adjusting thetwo feed paths so that there is a quarter-wavelength differencein length and by replacing chips 2 and 3 with short circuits.
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