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236 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010

Bandwidth-Enhanced Electrically Small PrintedFolded Dipoles

Yanyan Zhang and H. Y. David Yang , Fellow, IEEE 

 Abstract—This letter presents a wire folding scheme to enhancethe bandwidthof an electrically smallresonantmicrostrip antenna.As an example, meandered two-wire slow-wave lines spread onboth sides of a dielectric slab short-circuited at both ends form avertically folded printed dipole. It is found that the radiation re-sistance increases by about a factor of 4, and the energy storageincreases by about a factor of 2, using a folded dipole. It is shownthrough both measurement and simulation that the -factor is re-duced by about 50%throughantenna folding. A 0 1 4  -long foldeddipole resonant at 2.43 GHz shows a bandwidth of about 4.8%.The proposed folding method facilitates planar antenna miniatur-ization with sufficient bandwidth.

 Index Terms—Electrically small antenna (ESA), folded dipole,microstrip, slow-wave structures.

I. INTRODUCTION

IN WIRELESS communications where the entire system

may reside on a small circuit board, the area reserved for

an integrated antenna is often much smaller than a wavelength.

There is significant renewed interest recently in electrically

small antennas [1]–[4]. A fundamental issue that limits antenna

size reduction is the impedance bandwidth (or the quality factor

). In addition to the necessary gain, an antenna should be

resonant with sufficient surface for radiation.Common integrated antennas in wireless systems are sus-

pended monopoles on a substrate surface with a vicinity ground

to provide a current return path [5]. In many systems, such

as RFID, a dipole antenna (two symmetric monopoles) that is

self-balanced with a differential input is preferred [6]. Resonant

wire antenna size can be much reduced by winding the traces

into a multilayer structure within the content of metamaterials.

An electrically small antenna (ESA) can be made from a section

of metamaterial slow-wave transmission line [7]. Although the

miniaturized antennas (or ESAs) are self-resonant, a common

problem is insufficient radiation resistance due to the small an-

tenna area that leads to a large -factor. Antenna bandwidth isstill limited by the overall antenna volume.

An approach to improve the -factor is to distribute effec-

tively radiating currents into two or three dimensions with arm

currents adding up in phase. Folded dipoles or monopoles pro-

vide such a solution. Folded dipoles had been known for many

Manuscript received March 10, 2010; accepted March 15, 2010. Date of pub-lication March 29, 2010; date of current version April 08, 2010.

The authors are with the Department of Electrical and Computer Engi-neering, University of Illinois at Chicago, Chicago, IL 60607 USA ( e-mail:yzhang40@uic.edu; hyang@ece.uic.edu).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LAWP.2010.2046875

decades as an approach to increase the dipole impedance by a

factor of 4 for impedance matching purpose [8]. Lately, it was

proposed to enhance the radiation resistance of a meandered

folded monopole in free space above a ground plane [9], [10]. It

was demonstrated that the radiation resistance, bandwidth, and

efficiency of a meandered monopole increase substantially by

antenna folding. A nonplanar folded left-handed metamaterial

monopole was also designed and tested recently [11]. The im-

provement in efficiency and bandwidth over an unfolded coun-

terpart is also observed.

This letter presents the investigation of a vertically folded

meandered dipole on a circuit board. An aim is to demonstrateits dramatic bandwidth enhancement capability. For a vertically

folded dipole (in contrast to a coplanar folding), the two radia-

tion arms are at the top and bottom dielectric surfaces, respec-

tively, connected through vias at the end of the dipoles. It basi-

cally maintains the same antenna area of a printed dipole since

the clearance is necessary on its opposite side.

A meandered folded dipole was proposed earlier [12], which

demonstrated how to reduce antenna size by meandering and to

obtain sufficient radiation resistance by folding. This letter is a

step further that addresses the reduction in stored energy such

that the bandwidth is enhanced significantly. The way to accom-

plish it is to space the folded dipole with sufficient distance sothat the two arms in a folded dipole are more or less decoupled.

Both simulated results using HFSS software and measured

data are presented. The -reduction (and bandwidth enhance-

ment) through the use of antenna folding is also quantified.

II. PRINTED MEANDERED DIPOLE FOR SIZE REDUCTION

Fundamental issues of an ESA are high (small bandwidth)

and impedance tuning. A quarter- or a half-wavelength slow-

wave line, which corresponds to a miniaturized resonant an-

tenna, provides a size reduction solution. General antenna band-

width is limited as the antenna volume is small. As the an-

tenna effective length is smaller, in order to maintain resonance,longer traces are needed. For example, a meander dipole is in ef-

fect a much longer dipole compressed in a smaller length. As a

result, ohmic loss is higher (longer current path) and radiation

resistance is smaller (a smaller effective radiation length) than

those of a straight resonant dipole.

As an example to demonstrate this common problem for

an electrically small integrated antenna, the cases of mean-

dered microstrip dipoles on a 3.17-mm FR-4 ( and

) dielectric surface are considered. For

comparison, the cases of four-cell, six-cell, and eight-cell me-

andered dipole (strip width 0.2 mm) are investigated. All three

cases are designed resonant at 2.2 GHz with height 4.65 mm,

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ZHANG AND YANG: BANDWIDTH-ENHANCED ELECTRICALLY SMALL PRINTED FOLDED DIPOLES 237

Fig. 1. A six-unit-cell meandered dipole on a dielectric surface. Strip width is0.2 mm.

TABLE IANTENNA CHARACTERISTICS OF A PRINTED MEANDERED DIPOLE

and the length varies depending on the unit cell length. The top

view of a six-cell meandered dipole (three unit cells on each

side of the dipole) is shown in Fig. 1. The total trace length for

the six-cell case is about 72 mm, while a straight dipole is about

43.5 mm, both resonant at 2.2 GHz. The input impedance,

radiation efficiency, and radiation patterns are obtained from

HFSS simulation. The -factor including the material and

metal losses are derived from the dipole input impedance [13]

as

(1)

The radiation (including surface wave power) is the total

divided by the radiation efficiency. At the same resonant fre-

quency 2.2 GHz, the dipole is meandered to reduce its physical

length. The smaller the unit cell size ( ) is the smaller resonant

dipole effective length is and more unit cells are needed.

Table I shows that as the dipole resonant length reduces, boththe radiation resistance (not including resistance due to losses)

and efficiency decrease due to the longer length trace length

(ohmic loss) and less effective radiating area. At the same time,

the radiation increases substantially. Overall antenna perfor-

mance becomes worse as the resonant dipole length decreases.

The small antenna is not an effective radiator, although it is

self-tuned to resonance. A meandered two-wire line is an ex-

ample of a slow-wave transmission line, which is basically a

transmission line periodically loaded with distributed inductors

or capacitors. The transmission line wavelength could be much

smaller than a TEM line [14]. In spite of the fact that a mean-

dered line is not an ideal slow-wave line for space, losses, and

-factor consideration, the observation in Table I pertains to anelectrically small antenna in general.

Fig. 2. A meandered folded dipole with the second arm on the board backsideconnecting through vias.

III. FOLDED MEANDERED DIPOLE FOR REDUCTION

A folded dipole is basically a two-wire transmission line res-

onator with a short at both ends. The feed is at the center opening

of one of the wires. There are two modes of operation, the

even mode (the dipole mode) and the odd mode (transmission

line mode). The case considered here is a symmetrically folded

dipole. According to the theory of a folded dipole [8], antenna

input impedance is given as

(2)

where is the input impedance for half of  

the shorted slow-wave lines (odd mode phase constant and

impedance ) and is the unfolded dipole input impedance

when the other half of the two-wires is removed. In the free-

space case, when the lines are half-wavelength long, both the

dipole mode and the (TEM) transmission line mode resonate

simultaneously , and the folded dipole radiation re-

sistance is four times of an unfolded dipole.A careful look of the folded dipole reveals that the antenna

mode is basically for a one-wavelength dipole with the outer

half of each monopole arm bent backward so that the radiating

currents in the first and the second half of the arm add in phase

and in effect increase the radiated power by a factor of 4 as com-

pared to a half-wavelength dipole. As an example, a meandered

dipole and its folded version are designed and fabricated on a

3.17-mm FR-4 slab. The dipole is the same as the one shown in

Fig. 1. Both folded and nonfolded antennas are 16.3 mm long

and 4.65 mm wide. The traces are 0.2 mm wide. For the folded

dipole case, the second arm is on the backside of the slab. The

photograph of the prototype including the feed line is shown inFig. 2, where the circle marks are the locations of the vias.

The comparison of a folded and nonfolded meandered dipole

based on simulation without the feed line is shown in Table II.

As observed in Table I, the meandered dipole (nonfolded) is

designed resonant at 2.2 GHz, and it is far from resonant at

2.43 GHz, as shown in Table II. However, when two such

dipoles are folded together, the resonant frequency shifts to

2.43 GHz. This is due to the fact that the wavelengths of the

antenna (even) and transmission line (odd) modes are quite

different in integrated circuits. Nevertheless the radiation resis-

tance increases by almost four times [as predicted in (2)], and

the radiation (excluding losses) reduces by about half, while

the stored energy is about double, with a folded dipole. Thestored energy increase (two times) is the result of two identical

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238 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010

TABLE IICOMPARISON BASED ON HFSS SIMULATION OF A FOLDED AND A NONFOLDED

PRINTED MEANDERED DIPOLE

TABLE IIICOMPARISON BASED ON MEASUREMENT (INCLUDING FEED LINE) OF A

FOLDED AND A NONFOLDED PRINTED MEANDERED DIPOLE

dipoles with the same current (even mode) with sufficient

distance in between. The total dipole line length (end to end) is

about 14% of a free-space wavelength at 2.43 GHz.

The feed line of the prototype is tapered coplanar strips. The

slot width is 0.2 mm, the same as the feed gap of the dipole.

The strip width of the slot line is tapered from 0.76 mm at the

dipole to 4.3 mm at the board edge for an SMA connector.

This feed line is necessary to push the dipole away from theboard edge. The simulation and measurement comparison of 

the folded dipole and the nonfolded dipole including the feed

lines are shown in Table III. The length of the feed line from

the dipole to the SMA is about half of the guided wavelength.

With the loss of the coplanar lines included, the input impedance

at SMA is slightly different from the input impedance at the

dipole input. Table III shows good agreement between simula-

tion and measurement in -factor (or BW). Note that -factor

is about half using the folded dipole as predicted. For compar-

ison purposes, the dipole results (nonfolded) are shown for both

2.2 and 2.43 GHz. Interestingly, the resonant resistance of the

folded dipole is about four times of a dipole resonant resistance,although compared at different frequencies (their resonant fre-

quencies). This example demonstrates clearly that the use of an-

tenna folding could enhance significant (almost double) band-

width of an electrically small antenna. The only possible down-

side is the use of the board area on the opposite side of the an-

tenna surface.

Simulated and measured reflection coefficients versus fre-

quency for the folded dipole shown in Fig. 2 are shown in Fig. 3.

As seen from the return loss curves, the folded dipole without

the feeding line (based on simulation) and loss has 10 dB

impedance bandwidth of 100 MHz from 2.38 to 2.48 GHz

at the center frequency of 2.43 GHz. Adding the feeding

line and losses widens the 10-dB band by about 10 MHz,shifting slightly toward a lower frequency range. Return loss

Fig. 3. Reflection coefficients of the meandered folded dipole shown in Fig. 2.

Fig. 4. Bandwidth comparison of a meandered dipole and its folded versionbased on full-wave and circuit simulations, both tuned to 50   at 2.43 GHz.

measurement and simulation agree very well. The impedance

bandwidth is related to according to [13] as

(3)

where is the voltage standing wave ratio (VSWR).

For 10 dB impedance bandwidth and(fromTable III), the bandwidth is about 4.8%, while the mea-

surement shown in Fig. 3 shows a bandwidth about 4.9%

(120 MHz divided by 2.43 GHz). In contrast, for a meandered

dipole of the same length (no folding), the bandwidth is only

about 2.7% (simulation) and 2.4% (measurement) based on

(2). Direct bandwidth comparison is not feasible, as the two

antennas resonate at different frequencies with different res-

onant resistance. A better approach is to assume lossless LC 

impedance matching to 50 at 2.43 GHz for the two antennas.

The 10-dB bandwidth comparison based on the reflection co-

efficient is shown in Fig. 4. The results show 70 MHz in-band

(2.9%) for a dipole and 120 MHz in-band (4.8%) for a folded

dipole. The use of antenna folding has a clear advantage of en-hancing the bandwidth within the same board volume, although

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ZHANG AND YANG: BANDWIDTH-ENHANCED ELECTRICALLY SMALL PRINTED FOLDED DIPOLES 239

the actual antenna area is twice as much counting the dielectric

surface area on both sides. Antenna folding offers a bandwidth

enhancement method by effectively utilizing the circuit board

area and volume and could be useful in personal wireless sys-

tems where the antenna area is of the primary concern.

Antenna gain calculations and measurements show that the

folded dipole and the unfolded dipole carry similar radiationcharacteristics. When tuned at resonance, both have about 2 dB

maximum gain, and the patterns are similar to a dipole antenna

pattern (a donut shape).

IV. CONCLUSION

Previously, it had been shown that the use of a meandered

folded dipole could reduce antenna size while maintaining suf-

ficient radiation resistance. Normally, reducing antenna length

also reduces the radiation resistance in an ESA. This letter ad-

dressed the reduction in stored energy such that the -factorcan be reduced by two times, resulting in significant bandwidth

enhancement. The way to accomplish it is to space the folded

dipole sufficiently (arm distance) so that the two arms in a folded

dipole are more or less decoupled. It was found that by using

two identical meandered dipoles and folding them together with

proper spacing, the folded meandered dipole has half of the

-factor.

The meandered dipole is a special case of slow-wave lines

for size reduction of a resonant antenna within the content of 

metamaterial slow-wave structures. It was found that folding

(short-circuit both ends) two electrically small resonant dipole

could enhance the bandwidth as much as 70%–100%. It was

also shown that this bandwidth enhancement is coming from in-creasing radiation power by almost four times and stored energy

by almost two times, as is evident from the input impedance in-

formation. The proposed scheme can be extended to multiple

folded antennas for further bandwidth enhancement. The con-

cept could also be applied to folded monopoles, where half of 

the dipole is replaced by a ground plane.

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