Abstract—A dual-function circuit carrying out both the RFpower-division and single/multiband filtering action is presented.It is used as building element to develop highly frequency-selectivepower-distribution networks for planar array-type antennas.The operating principle of this filtering/feeding circuit, exploitingtransversal signal-interference techniques, is described. Also,formulas and guidelines helping in its theoretical design are given.As proof of concept, microstrip prototypes of four-way single- andtriple-band filtering/feeding networks are built and tested.
Index Terms—Array antennas, bandpass filters, feedingnetworks, microstrip, multiband antennas, multifunctionalcircuits, power dividers, signal interference, ultrawidebandtechnology.
I. INTRODUCTION
N OWADAYS, a lot of attention is being paid to microwavemultiband circuit research. This is because of the need
for advanced multifunction/multifrequency RF front ends formodern radiocommunications and radar applications. High-di-rectivity antennas are no exception [1], [2].
Regarding planar array-type multiband antennas, differentapproaches have been proposed during the last decade [3]–[6].They have a common basis of transferring the design task tothe radiating element to achieve the multifrequency capability.Thus, several array-antenna solutions for dual-band systemshave been devised by only acting on the patch geometry, e.g.,through fractal-shape or perforated radiator arrangements.However, to the best of the authors’ knowledge, no effort hasbeen put into developing multifrequency feeding networks fora more efficient multiband performing of the overall antenna.
The aforementioned limitation is overcome here. A novelconcept of single/multiband feeding network for planar arrayantennas with intrinsic filtering capability is described. It isbased on an original dual-function power-divider/filteringcircuit exploiting feed-forward signal-combination princi-ples. This class of feeding network allows high-selectivitysingle/multiband filtering actions to be incorporated to theantenna. As a consequence, the necessity of an additionalhigh-frequency filter carrying out the signal-band selection iscircumvented.
This letter is organized as follows. The theory of the two-waysingle/multiband filtering/feeding network is presented inSection II. Section III shows the experimental results of two
Manuscript received November 29, 2010; accepted January 03, 2011. Dateof publication January 13, 2011; date of current version January 24, 2011. Thiswork was supported by COST Action IC803 “RF/Microwave CommunicationSubsystems for Emerging Wireless Technologies.”
Digital Object Identifier 10.1109/LAWP.2011.2106192
Fig. 1. Detail of the proposed two-way filtering/feeding network.
developed microstrip multistage filtering/power-distributioncircuits with single- and triple-band operation. Finally, the mostrelevant conclusions of this work are set out in Section IV.
II. THEORETICAL FOUNDATIONS OF THE TWO-WAY
FILTERING/FEEDING NETWORK
The detail of the proposed two-way filtering/feeding networkis shown in Fig. 1. It consists of a “T-type” power splitter endedin transversal signal-interference filtering sections at its outputs.A matching line segment is also inserted at its input. The elec-trical lengths of the lines shaping the matching section and thepower splitter must be selected to be at the de-sign frequency , whereas their characteristic imped-ances remain as design variables. The charac-teristic impedances and the electrical lengths of the line seg-ments making up the transversal filtering sections are referredto as and , respectively. The referenceimpedance is denoted as .
The key element in the described circuit concept to attainthe single/multiband filtering capability is the transversal sec-tion. This type of transversal network, shaped by two in-par-allel transmission-line segments, was previously reported andapplied to microwave single/dual-passband filter design [7], [8].In this letter, a generalized version of this filtering section toconform an arbitrary number of passbands is employed. Its op-eration philosophy is the same as that of its single/dual-pass-band counterparts, as follows: The overall filtering action is de-rived from the feed-forward interaction of the two signal compo-nents coming from the input signal, after propagating by its twobuilding transmission lines. Specifically, to obtain single/multi-band bandpass transfer functions, constructive signal interac-tions for passband shaping and out-of-band signal-energy sup-pressions to create transmission zeros must be produced.
1272 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010
Fig. 2. Illustrative examples of the synthesized power transmission response��� � � �� �� for the two-way filtering/feeding network �� � � � � ����� � � �
��� � � ���� � � � ��� � � � �. (a) � � �� �.
(b) � � �� �.
Fig. 3. Control of the operation bandwidths of the two-way filtering/feedingnetwork for � � � �� � � � � � ���� � � �
��� � � ����.
A. Design Equations
Formulas and guidelines to synthesize the devised two-wayfiltering/feeding network can be deduced from basic theory ofmicrowave circuits. These design rules are as follows.
• Frequency-selective profiles with first-order pass-bands within the periodic spectral range are fea-sible through the action of the transversal filtering section.To this aim, the following design equations must be used:
(1)
(2)
Here, the condition has been considered withoutany loss of generality. Moreover, spectral symmetry rel-ative to of the overall transfer function within the in-terval has also been forced. The analytical expres-sions for the center frequencies of the synthesized pass-bands, , are
(3)
Fig. 4. Frequency-asymmetrical power transmission ��� � � �� ��and reflection ��� �� responses of the two-way filtering/feeding network(� � �� ��� � � ���� � � �
with being a transmission- or a reflection-zerofrequency for even or odd, respectively.
• A symmetrical power division between branches with noinput power reflection at —and reasonably preserved at
—is set for and .1Some illustrative examples of the power transmission re-
sponse of the two-way filtering/feeding network fulfilling theobtained design formulas are detailed in Fig. 2. As shown,single/multiband bandpass filtering characteristics with trans-mission zeros at both sides of each passband are achieved.
B. Control of the Transfer-Function Performances
The design variables of the proposed two-way fil-tering/feeding circuit enable the control of its transfer-functionproperties. Regarding this capability, some rules are as follows.
• Under the above design equations, the degree of freedomresulting from (1) allows to modify the single/multibandpassband bandwidth. This is verified in Fig. 3 for .As can be seen, the passband spectral widths get broaderas higher values for the variables are selected.
• By modifying the electrical-length parameter values fromthose provided by the theoretical formulas, moderatelyfrequency-asymmetrical responses with regard to
1Note that, although not addressed in this letter, two-way filtering/feedingnetworks with asymmetrical power division could also be designed throughthis approach. This would by done by using different characteristic imped-ances � �� � �� �� � � for the lines of the “T-type” powersplitter. Indeed, a power ratio of � between the output ports #2 and #3with no input power reflection is achieved at —and nearly maintained at � � � � � —for � � � � � �� and � � �
�� � .
GÓMEZ-GARCÍA et al.: PLANAR ARRAY-ANTENNA FEEDING NETWORK WITH SINGLE/MULTIBAND FILTERING CAPABILITY 1273
Fig. 6. Developed microstrip four-way single-band filtering/feedingnetwork.
Fig. 7. Simulated and measured power transmission (from port #1 to port#2, �� �), reflection (at port #1, �� �), and group-delay (from port #1to port #2, � ) responses of the developed four-way microstrip single-band filtering/feeding network. Similar performances are obtained for theremaining output ports. (a) �� �. (b) �� �. (c) � .
become feasible.2 This is confirmed in Fig. 4. As shown,second-order equiriple dual passbands with different band-widths and close-to-passband selectivity are obtained.
Also, it must be remarked that are key parameters toproperly adjust the in-band flatness and matching in multistagecircuits, as the prototypes designed in Section III prove.
III. EXPERIMENTAL RESULTS
Using the cascade of identical versions of the proposedtwo-way filtering/feeding circuit, two microstrip single- and
2In this case, the generation of a transmission or a reflection zero at � for �even or odd, respectively, cannot be assured. This is because (2) could not besatisfied at � .
Fig. 8. Power transmission ��� � � �� � � �� � � �� �� and reflec-tion ��� �� responses—with the out-of-band attenuation mask to be met—ofthe ideal synthesized four-way triple-band filtering/feeding network. The designparameter values are as follows (� � ��� GHz, � � �� �): � � �� ��� � �� � � � �� � � �� � � ��� �� � � ��� � � �� ��� � �.
Fig. 9. Developed microstrip four-way triple-band filtering/feeding network.
triple-band frequency-selective power-distribution networks forfour-element array antennas have been built and tested. Theyhave been synthesized both through the design equations andrules previously given and by optimizing their four-way powertransmission profiles. The obtained results are reported.
A. Single-Band Prototype
The first design example consists of a broadband bandpass fil-tering/feeding circuit at 2.5 GHz. For this prototype, a 3-dB ab-solute bandwidth of 500 MHz (i.e., 3-dB relative bandwidthof 20%) has been chosen. Moreover, to avoid adjacent out-of-system and cochannel interferences, a power rejection level forthe stopbands higher than 25 dB throughout a spectral widthbroader than that of the main passband was imposed.
The four-way power transmission and input power reflectioncurves of the ideal synthesized ultra-broadband bandpass fil-tering/feeding network are drawn in Fig. 5. As can be seen,a maximally flat bandpass transfer function with sharp cutoffslopes resulting from transmission-zero generation is attained.
A photograph of the developed microstrip four-waysingle-band filtering/feeding network prototype is shownin Fig. 6. For circuit manufacturing, a low-loss organic-ce-ramic microstrip substrate CER-10 of Taconic has been usedwhose parameters are the following: relative dielectric constant
, dielectric loss tangent ,dielectric thickness mm, and cooper metallizationthickness m. Further circuit-size reductions are achiev-able by using fractal geometries or other layout miniaturizationtechniques [9]. Note that this could be a mandatory issue indense array antennas made up of a large number of patches tominimize undesired coupling effects arising between adjacentfeeding lines.
The simulated and measured power transmission (from port#1 to port #2), input reflection (at port #1), and group-delay(from port #1 to port #2: noisy and smoothed curves) responses
1274 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010
Fig. 10. Simulated and measured power transmission (from port #1 to port#2, �� �), reflection (at port #1, �� �), and group-delay (from port #1 to port#2, � ) responses of the developed four-way microstrip triple-band filtering/feeding network. Similar performances are obtained for the remaining outputports. (a) �� �. (b) �� �. (c) � .
of this prototype are depicted in Fig. 7. Here, the results of botha circuit model and an electromagnetic (EM) simulation car-ried out with the software packages HP-Eesof Libra and HFSSver. 10.0 of Ansoft, respectively, are included [10], [11]. Ascan be seen, a fairly close agreement between measurementsand predictions has been obtained. Regarding the in-band outputport isolation, comparable performances to those of its classicalcounterpart were observed.
B. Triple-Band Prototype
The second design example corresponds to a triple-bandbandpass filtering/feeding network covering the ultrawidefrequency interval 1.5–3.5 GHz. In this case, an absolute3-dB bandwidth of 185 MHz has been selected for its pass-bands. Besides, as in the previous example, the obtaining of25-dB rejected bands with a bandwidth at least equal to thefixed 3-dB spectral width for the passbands has been forced.
The four-way power transmission and input power reflec-tion profiles of the ideal synthesized triple-band bandpass fil-tering/feeding circuit are plotted in Fig. 8. As proven, a triple-passband frequency-selective response exhibiting sharp-rejec-tion stopbands with transmission nulls is achieved.
A photograph of the manufactured microstrip four-way triple-band filtering/feeding network prototype is shown in Fig. 9. Thesame substrate as in the first example was used.
The simulated and measured power transmission (from port#1 to port #2), input reflection (at port #1), and group delay(from port #1 to port #2: noisy and smoothed curves) parame-ters of this circuit are represented in Fig. 10. Again, the agree-ment obtained between these theoretical and experimental re-sults is reasonable enough to validate the concept. Some devia-tions existing between these results consist of the appearance ofsome spurious-resonance peaks at the passband flanks. As con-firmed by the EM predictions, these effects are imputable to thegeneration of undesired interline electromagnetic coupling notcontemplated by the circuit-model simulator and caused by thehigh dielectric thickness of the microstrip substrate. To a minorextent, the manufacturing tolerances may also have some influ-ence on this disagreement.
IV. CONCLUSION
This letter has reported a class of planar single/multibandfiltering/feeding network for array antennas. It uses, as con-stitutive circuit, a new concept of two-way signal-interferencepower divider configuration with added filtering functionality.The theoretical principles of this power-divider approach andboth equations and rules for its synthesis have been detailed.Moreover, two microstrip prototypes of four-way single- andtriple-band filtering/power-distribution networks have beensuccessfully constructed and characterized. Advantages ofthis feeding network approach are design simplicity and spu-rious-radiation minimization by avoiding the presence of EMcouplings to perform the filtering action. Further research workis the development of filtering/feeding circuits with strong fre-quency-asymmetrical selectivity by using stepped-impedancetransmission lines and phase-Schiffman sections.
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