# Printed Antenna Arrays with High Side Lobe Suppression

Authors: Aleksandar Nešić; Ivana Radnović; Zoran Mićić
Journal: Active and Passive Electronic Components
 (2008)

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## Abstract

The problem of side lobe suppression (SLS) in printed antenna arrays has been investigated in the paper. Influence of several factors that make difficult design and realization of antenna arrays with relatively high SLS has been analyzed. We introduced a new type of printed antenna array with symmetrical pentagonal dipoles and symmetrical tapered feed network with Chebyshev distribution enabling SLS better than 34 dB in E-plane. Agreement between simulated and measured results is very good. The antenna is suitable for integration with other microwave circuits. Presented antenna is low cost and very simple for realization.


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## Body

## 1. Introduction

Microwave antenna
arrays are usually used in telecommunication systems such as indoor and outdoor
wireless LANs, point to point and point to multipoint and also in radar microwave and
millimeter systems. One of the main antenna characteristics is SLS in radiation
patterns which is defined for telecommunication systems (usually for microwave
links) by international standards and recommendations [1]. In conventional
radar systems, requirements for SLS are much severe since responses from side
lobes practically are false targets. Depending on the antenna class, required SLS
in telecommunications systems is about 20 to 40 dB. For example, in AWACS and
F-16 radar systems, this suppresion is better than 50 dB.Such impressive SLSs are hardly achievable with conventional microstrip
antenna arrays (with patches). In microstrip antenna arrays presented in
literature, side lobe levels are suppressed 25 dB (related to main lobe) at
best. A relatively small number of publications
dealing with this issue are available [2–4].


## 2. Limiting Factors in Realization of Printed Antenna Arrays with High Side Lobe Suppression

There are
several problems in realization of printed antenna arrays with relatively high SLS.
The main of them are toler-  ances in fabrication, mutual coupling between radiating elements, limitations
in feasibility of feed network realization, and surface wave effect as well as
parasitic radiation from a feeding network.Due to effects mentioned above, it is quite difficult to
obtain sidelobe suppression better than 25 dB in microstrip antenna arrays [2].
Tolerances affect various parameters on which sidelobe suppression depends are as
follows: (1) dimensions of radiating element, (2) dimensions of feeding lines
(width and length) including branches that represent impedance transformers, and
(3) distance between radiating elements.(1)
and (2) influence tolerances of phase and amplitude of radiating elements on
which SLS depends directly. As it is known, microstrip patches have relatively narrow
bandwidth, that is, quick change of impedance with its dimensions change. The
consequence of this is notable phase as well as amplitude deviation from
optimized values. Another factor is tolerance of feed network dimensions
(microstrip lines and impedance transformers) on which amplitude and phase
deviations depend directly.Another group of factors that affect
side lobe suppression includes (4) surface wave effect, (5) parasitic radiation
from a feed network, especially when the feed network is printed on the same
dielectric substrate as the antenna elements, and (6) mutual coupling between
radiating elements. However, mutual coupling effect can be easily incorporated
into design of the feed network. The above factors are the most noticeable in
conventional microstrip antenna arrays with patches as radiating elements that
are printed on a dielectric   substrate and are fed by conventional (unbalanced)
micros-  trip lines.


## 3. Side Lobe Suppression

In
order to decrease side lobe levels, various tapered distributions are used in
antenna arrays: cosine, cosine-squared, Gaussian, Taylor, and Chebyshev. These distributions
are being chosen depending on required
side lobe attenuation, possible pedestal in distribution (Imax⁡/Imin⁡ ratio), desired position of radiating elements, desired position, that is, distribution of side lobes, distance
between radiating elements, number of
radiating elements, and expected
tolerances in realization.Pozar
and  Kaufman in [2] show that variation in frequency of 1% causes, in patch
antenna arrays, the phase change of 12.5∘ which dramatically decreases SLS from
40 dB (in ideal case) to 26 dB. Only due to parasitic radiation from a microstrip
feeding line, SLS decreases from 40 dB to about 30 dB.  Other parasitic factors that degrade side lobe suppression in
patch antenna arrays such as surface wave effect and difraction are also
significantly influenceable, so one cannot expect SLS better than 25 dB. The
exception is case when each radiating element is fed through a separate tunable
phase shifter and atennuator and when the feed network is not on the same dielectric substrate with
elements [2].Beside conventional printed antenna
arrays with patches fed by conventional asymmetrical microstrip line, there are
printed antenna arrays with printed dipoles, usually of pentagonal shape (one
half of them on one side and another half on the opposite side of the
substrate). These dipoles operate on the second resonance and are fed by a
symmetrical (balanced) microstrip line, Figure1,
[5].Figure 1
Pentagonal dipole as a basic element of the antenna array
printed on dielectric substrate.Behind the array, there is a
reflector plate [6] or the linear array is placed between two plates which form
a corner reflector [5]. Figure 2 shows real and imaginary parts of pentagonal
dipole impedance versus frequency, printed on dielectric substrate of 0.254 mm
thickness end εr = 2.1. It is obvious that impedance
variation with frequency is very slow which is of crucial importance for arrays
with high SLS. Also, due to the fact that feed network is symmetrical and
consists of symmetrical microstrip lines, parasitic radiation from it is
practically eliminated. Majority of factors that make difficult realization of
printed antenna arrays with high side lobe suppression has been eliminated in
these printed arrays.Figure 2
Real and imaginary part of
pentagonal dipole impedance versus frequency (εr =  2.1, h=0.254mm, and S=λ0/4)
when aperture angle (α)
is 180∘.Because of tolerances in
photolithographic process as well as dilatations caused by temperature change,
deviations from projected values of position, amplitude, and phase of radiating
elements in the array occur. We will investigate influence of these tolerances
in the case of printed antenna array with 8 broadband pentagonal dipoles
operating on second resonance and with mutual distance of 0.85λ0. Dipoles are fed by feed network
enabling Dolph-Chebyshev distribution of the second order with pedestal (Imax⁡/Imin⁡)
of 17 dB.Realizable values of relative
tolerances have been assumed at operating frequency of 26 GHz:(i)
deviations in distances between radiating elements in the array: 1% or 2% of λ0,(ii)
amplitude deviations along tapered lines: 1 dB or 2 dB,(iii)
phase deviations:2∘ or 4∘ (corresponds to
about 40 μm
and 80 μm
tolerances in length of the feeding line).Using [7], SLS has been calculated: (a) in ideal case; (b) in case of
higher deviation only in position of radiating elements; (c) in case of
greater amplitude errors only; (d) in case of greater phase errors only; (e)
in real cases when all minor errors exist; and (f) when all existing errors
are of greater value. Results are presented in Table 1. Errors were randomly
distributed in the simulation process (Monte Carlo method).Table 1
Radiating elements positioning errorAmplitude errorPhase errorSide lobe suppression of the highest lobe
[dB]a00040.72b0.02λ00038.56c02 dB038.87d004∘32.66e0.01λ01 dB2∘35.05f0.02λ02 dB4∘32.02Figures3(a), 3(b), 3(c), 3(d),
3(e), and 3(f) show
simulated radiation patterns for the cases given in the table above.(a), (b), (c), (d), (e), and  (f) Simulated
radiation patterns for several cases of deviations (amplitude, phase, and
radiating elements positioning deviations).
(a)(b)(c)(d)(e)(f)For the
purpose of better insight in the assumed values resulting from tolerances in
realization, we give the absolute values of assumed errors at 26 GHz: tolerances
in radiating elements positioning are 230μm
Figures 3(b), 3(f); phase errors result from tolerances
in feed lines’ lengths that are 40 μm
Figure 3(e) and 80 μm
Figures 3(d), 3(f). Amplitude errors occur as a consequence of tolerances in impedance
transformer lines widths.


## 4. Concept, Design, and Realization Ofprinted Tapered Linear Antenna Array with Corner Reflector

Proposed
antenna array consist of three parts: (1) axial array of eight printed
pentagonal dipoles (Figure4, Detail B) (2) feeding network printed on the same
dielectric substrate with the pentagonal dipoles (Figure 4, Detail A) and (3)
corner reflector consisting of two metal plates. Distance between the dipoles
(at the center frequency) is chosen in such a way to obtain relatively high-array
gain with sufficient SLS in tapered array. In our case, the distance between
axial dipoles is 0.85λ0. Also, with such distance between axial
dipoles, mutual coupling is very low making the design and optimization of the
antenna array relatively easy. Pentagonal dipoles’ dimensions were optimized
with program package WIPL-D [8]  so to obtain impedance of 100 Ω at the center
frequency of 26 GHz. During the optimization the influence of symmetrical
microstrip feeding line of Zc=100⁢Ω was taken into account. In this case, we
have adjusted dimensions of pentagonal dipoles in printed array to obtain
impedance of 100 Ω at the center frequency of 26 GHz, taking into consideration
the reflector influence. Since the dipoles are electrically symmetrical
elements, the feeding network with tapering is realized with symmetrical
(balanced) microstrip lines.Figure 4
Printed antenna array and tapered feeding network integrated on the same
dielectric substrate (εr = 2.1, h=0.254mm, and tgδ = 4×10−4).
Detail A: tapered feeding network. Detail B: Pentagonal dipole with a symmetric
microstrip feeding line of Zc=100Ω.By using
LINPLAN program package [7], we calculated distribution coefficients that could
be achieved with a printed feeding network. Dolph-Chebyshev distribution of the
second order with Imax⁡/Imin⁡ of 17 dB has been chosen.
Distances between dipoles are 0.85λ0. Under these conditions, we
have obtained distribution coefficients enabling highest SLS of 40.72 dB (@ Θ
= ±18∘). In order to attain desired
distribution, we designed feeding network in symmetrical microstrip technique
with λ/4 transformers,  T1−4 and Ta,b (Figure 4, Detail
A). With obtained structure, we carried out full-wave analysis [9] in the
frequency range from 24 GHz to 28 GHz. Corrections
of phase deviations were accomplished by changing the lengths of particular
branches in the feeding network, while amplitude deviations remained
uncorrected. After these corrections, we achieved symphase feeding of all
dipoles in the array.Corner
reflector is designed using the results from [10], which contains very detailed
experimental results obtained by variation of length (L), width (W), aperture
angle between corner reflector plates (α),
and distance of radiating element from apex (S). Suitable radiation pattern
with relatively high gain and high SLS in H-plane is obtained with L=W=4λ0,
α = 45∘, and S=0.7λ0.Axial
array with feeding network and BAL-UN [11] is placed between two metallic
plates forming a corner reflector with α = 45∘. Beamwidth in H-plane (azimuth)
depends mainly on the angle between the metallic plates and length of the reflector
plates (L), while SLS in E-plane depends only on the linear antenna array,
Figure 5.Figure 5
Realized antenna array with corner reflector compared to US quarter.Feeding
lines for dipoles penetrate the junction of two reflector plates. In the place
of this junction, there are holes through which symmetrical microstrip lines of
the feeding network pass. Influence of the metallic plate on the microstrip
lines is minimized by selecting the sufficient holes’ diameter (2 mm).


## 5. Obtained Results and Comments

Simulated
and measured results are presented in Table2 and
Figures 6–8. The
discrepancy between simulated and measured SLS is due to tolerances in
photolithography and mounting process because relatively small inaccuracies can
significantly influence precise distribution. The return loss measured at SMA
connector is presented in Figure 8. The measured gain of the antenna is about 1 dB smaller than the simulated because the feeding network, BAL-UN, and
transition from microstrip to SMA connector were not taken into account.
Photograph of the realized antenna array with tapered distribution and 45∘ corner reflector operating in 26 GHz
range is shown in Figure 5.Table 2
Simulated and measured results
at 26 GHz. G: gain; HSLSE, HSLSH: highest side
lobe suppression (E-plane, H-plane); F/B: front to back ratio; HPBWE:
E-plane 3 dB beamwidth, HPBWH: H-plane 3 dB beamwidth.
G [dBi]HSLSE[dB]HSLSH[dB]F/B[dB]HPBWE[∘]HPBWH[∘]Simul.21.438.4353210.523.8Meas.20.834.732.635.410.723.8Figure 6
Normalized simulated
E-plane radiation pattern at 24 GHz, 26 GHz, and 28 GHz.Figure 7
Simulated and measured
radiation pattern in H- and E-plane at 26 GHz (highest side lobe is
suppressed about 35 dB in relation to the main lobe).Figure 8
Measured return loss of the antenna array.


## 6. Conclusion

The paper investigates possibilities
and limitations in printed arrays side lobe suppression. All relevant factors on which SLS
depends are particularly analyzed: phase deviations, amplitude deviations,
radiating elements positioning deviations, mutual coupling between elements,
and parasitic radiation from the feed network. It is shown that SLS,
satisfactory for most microwave telecommunication and especially radar systems,
is hardly achieveable with conventional microstrip antenna arrays with patches
due to their narrow bandwidth that is, quick variation of impedance with
dimensions change, parasitic radiation from the feed network, and surface wave
effect.Antenna structure with printed
pentagonal dipoles forming the array is proposed. The dipoles operate on the
second resonance and are fed by a symmetrical (balanced) microstrip line. Array
consisting of 8 axially placed dipoles is fed through the feed network with
impedance transformers enabling Chebyshev distribution with pedestalImax⁡/Imin⁡=17⁢dB. We have
analyzed effects of particular parameters with assumed tolerances on which SLS
depends. Linear antenna array realized in this way is placed between two
metallic plates forming a corner reflector which enables achieving relatively
high SLS and narrow beamwidth even in H-plane. Measured beamwidth in E-plane at
26 GHz is 10.7∘. Experimentally obtained E-plane SLS at 26 GHz, which depends
only on the antenna array, is better than 34 dB and is, to the authors’
knowledge, the best result published so far. The array is realized using
standard photolithographic process with moderate precision of ±10 μm. Simulated and measured results are in very
good accordance.



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*Source: 542929-2008-05-25.xml*