488 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

Miniaturization of Slot Antennas Using Wire LoadingBratin Ghosh, Senior Member, IEEE, S. K. Moinul Haque, and Nageswara Rao Yenduri

Abstract—The design of a miniaturized slot antenna based onwire-loading is presented. The miniaturization can be achievedwith minimal increase in the lateral space along the slot width.Both the cases of a slot antenna on dielectric substrate and a slotantenna on ground plane are considered. For the former case, aminiaturization of 28.83% is achieved. The radiation characteris-tics of the size-reduced antenna are almost similar to the unloadedslot, with low cross-polarization levels. For the slot antenna on aground plane, the miniaturization is affected by a cavity-backeddesign with the loading wires penetrated into the cavity. A 45.52%reduction in resonant frequency is achieved relative to the un-loaded slot. The radiation characteristics in the upper hemisphereare almost unperturbed compared to the unloaded slot.

Index Terms—Cavity-backed, coplanar waveguide (CPW),L-probe, miniaturized, slot antenna, wire.

I. INTRODUCTION

S IZE reduction of antennas has been an active area of re-search in the past and present. This is particularly also

in view of the fact that antennas occupy the largest real estateamong all wireless system components. In view of its low-pro-file nature and easy integrability with planar elements, consid-erable research efforts have been directed toward the miniatur-ization of the slot antenna.The slit loading technique for the miniaturization of slot an-

tennas discussed in [1] and [2] achieves a significant size re-duction of the antenna. However, the inductive slits still occupyadditional area on the substrate along the slot width, increasingthe effective dimension of the antenna.In the following work, an alternate design topology is pre-

sented for reduction in size of the slot antenna. To affect thetransverse miniaturization of the slot, the reactive environmentaround the slot is compensated using loading wires penetratedinto the substrate. The configuration extends the slot area onlyminimally in the transverse direction. It is also seen that it is pos-sible to reduce the resonant frequency of the slot significantlyby about 28.83% using the above topology without significantdegradation in matching or radiation characteristics. The an-tenna configuration is readily amenable to fabrication or massproduction through the system on package/multilayer printedcircuit board (PCB) technologies [3], where the lateral space/real estate requirement on the substrate is much more stringent,

Manuscript received February 12, 2013; accepted March 27, 2013. Date ofpublication April 01, 2013; date of current version April 16, 2013.The authors are with the Department of Electronics and Electrical Commu-

nication Engineering, Indian Institute of Technology, Kharagpur 721302, India.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.2013.2255857

Fig. 1. Slot antenna on dielectric substrate with loading wires surrounding theslot. (a) Top view. (b) Expanded 3-D view around slot.

while the vertical direction is free for the outermost layer andcan be used for miniaturization.The above technique is also applied for the case of a slot in

a ground plane without a backing substrate. To affect the cou-pling between the slot and the matching elements in this case,the loading wires are penetrated into a cavity backing the slot.A very high reduction in resonant frequency by 45.52% is ob-served in this case. The radiation pattern in the upper hemi-sphere is also relatively unperturbed compared to an unloadedslot.

II. SLOT ANTENNA ON DIELECTRIC SUBSTRATE

The antenna structure is shown in Fig. 1. The loading wireson either sides of the slot are penetrated into the substrate toenhance coupling between the slot and the loading elementsthrough the substrate. The depth of penetration is the same as thesubstrate height. In order to minimize the effect of loading nearthe center of the slot where the electric field is strong, the lengthof the loading wires are gradually increased away from the slotcenter for the loading wires placed opposite to the feed side of

1536-1225/$31.00 © 2013 IEEE

GHOSH et al.: MINIATURIZATION OF SLOT ANTENNAS USING WIRE LOADING 489

Fig. 2. Measured and simulated return-loss characteristics of the loaded andthe reference slot antennas on dielectric substrate.

TABLE ICPW AND SUBSTRATE DIMENSIONS/PERMITTIVITY

the slot, as shown in Fig. 1(b). This also ensures that the patterncharacteristics are relatively unperturbed compared to the un-loaded slot. In Fig. 1(b), denotes the total length of the wire

( to 6). The wires farther from the center after areall of total length of 15.0 mm. The wires to on the feedside of the slot are also of uniform total length of 15.0 mm. Thewires are symmetrically placed with respect to the feed on bothsides of the slot. The coplanar waveguide (CPW) and substratedimensions/permittivity are shown in Table I, with and

being the dimensions of the quarter-wave matching sec-tion. The antenna structures were simulated using the High Fre-quency Structure Simulator (HFSS) [4]. A finite conductivity ofcopper of S/m for the ground plane and a dielectricloss-tangent of 0.0023 for the substrate material is also used inthe simulations.The simulated and measured return-loss characteristics of the

antenna are shown in Fig. 2. An excellent agreement is observedbetween the simulated and measured results. The measured res-onant frequency of the antenna is observed at 2.32 GHz. Thiscan be compared to the resonant frequency of 3.26 GHz of theunloaded reference slot antenna, of the same length as the loadedslot, corresponding to a reduction of 28.83% in resonant fre-quency of the loaded slot relative to the reference slot. Theminiaturization can be explained by the compensation of thecapacitive reactance of the slot below resonance with the in-ductance of the loading wires. The length of the wires in thiscase is important in providing the required inductance for re-duction in resonant frequency and at the same time providingadequate match, as also observed in the variation in return-losscharacteristics with wire length discussed later. The concept can

Fig. 3. Measured and simulated radiation patterns of the loaded slot antennaon dielectric substrate at resonance.

also be compared to the miniaturization effect produced by cou-pling to a helical strip placed in close proximity to a monopoleand cancellation of the capacitive reactance of the size-reducedmonopole with the inductance of the helix in [5, Fig. 8].The E- and H-plane radiation patterns for the antenna in

Fig. 1 are shown in Fig. 3. The patterns are almost similar to anunloaded slot, with low cross-polarization levels. It is observedthat the E-plane pattern shows a slight asymmetry with theH-plane pattern almost unaffected by the loading wires. Thegain of the loaded and the reference slot antennas were mea-sured at 0.6 and 0.1 dBi, respectively. The efficiency of theantenna was measured using Wheeler’s cap method [6]. Themeasured efficiencies of the loaded and reference antenna were71.20% and 88.30%, respectively, which agree well with thecorresponding simulated efficiencies of 72.66% and 90.59%.The loss can be attributed to the finite conductivity of theground plane and the loading wires, the substrate dielectricloss, surface wave loss, and edge currents on the ground plane.An excellent agreement is also observed between the simulatedand measured patterns in Fig. 3.The variation in the return-loss characteristics of the antenna

structure with wire length is shown in Fig. 4(a). In the figure,refers to the maximum wire height with the length of the

wires near the slot center being scaled accordingly to minimizeloading near the slot center. The return loss is observed to im-prove with decrease in wire length with a simultaneous increasein resonance frequency. The variation in return loss with wirespacing is shown in Fig. 4(b). The return loss dip is observed tobe below 15 dB with an increase in the resonant frequency toabout 2.5 GHz with an increase in to 1.75 mm. A similartrend is seen with the variation in the wire distance from the slotin Fig. 4(c). A photograph of the fabricated antenna prototypeis shown in Fig. 5.

III. SLOT ANTENNA ON GROUND PLANE

In this section, the reduction in resonant frequency for a sloton a ground plane without a backing substrate is investigated.The slot antenna structure is shown in Fig. 6. The slot antennain this case is backed by a cavity, and the loading wires are pen-etrated into the cavity for coupling with the slot. An L-shaped

490 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

Fig. 4. Variation of return loss of the loaded slot antenna on dielectric substratewith (a) wire length. (b) Wire spacing. (c) Wire distance from the slot. All otherparameters are the same as in Fig. 2 in each case.

Fig. 5. Fabricated antenna prototype for the loaded slot antenna on dielectricsubstrate.

probe directed along the width of the slot [7], [8] is used forexciting the cavity [Fig. 6(b)]. The location of slot is chosen tooptimally couple to the excited mode of the cavity. Allthe loading wires through in this case are of uniformtotal length of 13.50 mm, with a penetration depth of 3.5 mminside the cavity. The total length of the loading wire nearthe center of the slot is 9.5 mmwith no penetration in the cavity,to prevent interference with the feed below. It is found that thevariation in the wire length away from the slot center in this caseadversely affects the coupling to the slot. The photograph of thefabricated antenna is shown in Fig. 7.Fig. 8 shows the return-loss characteristics of the antenna.

The reference antenna in this case is a slot on a ground plane ofthe same dimensions as the slot in Fig. 6, fed by a coaxial cable.The resonant frequency of the loaded antenna is observed to beat 3.77 GHz, compared to the resonant frequency of 6.92 GHzof the reference antenna. A high reduction in resonant frequencyby about 45.52% of the loaded slot is thus observed relative tothe unperturbed case. The small dip noticeable around 5.44 GHzin the loaded antenna characteristics is due to the resonance of

Fig. 6. Slot antenna on ground plane backed by cavity with loading wires. (a)Antenna configuration. (b) L-probe feed.

Fig. 7. Fabricated antenna prototype for the loaded slot antenna on groundplane.

the loading wires. A very good agreement is also observed be-tween the simulated the measured results.The radiation characteristics of the antenna are shown in

Fig. 9. The front-to-back ratio of the antenna is observed tobe at 26.80 dB due to the presence of the backing cavity. Lowcross-pol levels are observed in both E- and H-planes. Thepattern nulls close to the horizon at and 270 in theE-plane, occurring for a typical unperturbed slot antenna alsonoticeable in the previous design in Fig. 3, are absent in thiscase. The pattern characteristics are also seen to be relativelyunaffected due to the loading wires.The measured gain for the loaded and reference antennas is at

2.3 and 0.6 dBi, respectively. The measured efficiency of the an-tenna is at 81.10%, which can be compared to the measured effi-ciency of 93.32% for the reference antenna. The correspondingsimulated efficiencies for the loaded and reference antennas are83.23% and 95.43%, respectively. It can also be observed that,

GHOSH et al.: MINIATURIZATION OF SLOT ANTENNAS USING WIRE LOADING 491

Fig. 8. Measured and simulated return-loss characteristics of the loaded andthe reference slot antennas on ground plane.

Fig. 9. Measured and simulated radiation patterns of the loaded slot antennaon ground plane at resonance.

compared to the slot antenna on dielectric substrate, the gainof the loaded antenna is better than the reference antenna in thecurrent case. This is due to the enhancement in radiation at bore-sight relative to the back-radiation for the loaded antenna in thecurrent case due to the presence of the backing cavity, whichsuppresses the back radiation.Fig. 10(a) shows the variation in return-loss characteristics

with wire length. It is observed that the return loss in this caseis optimum for mm. The return-loss variationwith wire spacing and wire distance from the slot, shown inFig. 10(b) and (c) respectively show the resonant frequency toincrease to about 4 GHz as the return loss reaches 10 dB withan increase in wire spacing/wire distance from slot.

IV. CONCLUSION

A transverse miniaturization technique for planar slot an-tennas using wire loading is presented in this letter. Compared

Fig. 10. Variation of return loss of the loaded slot antenna on ground planewith (a) wire length. (b) Wire spacing. (c) Wire distance from the slot. All otherparameters are the same as in Fig. 8 in each case.

to the slit loading technique for the miniaturization of slotantennas investigated earlier, the current technique requiresminimal space transverse to the slot to achieve miniaturization.For the slot antenna on dielectric substrate, a reduction in

resonant frequency by 28.83% is achieved relative to the un-loaded slot. The radiation pattern is also almost unaffected forthe miniaturized antenna relative to the unloaded slot, with lowcross polarization.The above technique was also used to affect miniaturization

of a slot antenna on a ground plane. A 45.52% miniaturizationwas achieved compared to an unloaded slot. The radiation char-acteristics in the upper hemisphere are relatively unperturbed bythe loading wires, with low cross-pol levels.

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