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VC 2014 Wiley Periodicals, Inc.

ACTIVE FOLDED LOOP ANTENNA WITHRECONFIGURABLE FEED AND GROUNDPOINTS

Abhishek Singh and Seng Thail SronEthertronics, 5501 Oberlin Dr. Suite 100, San Diego, CA 92121;Corresponding author: asingh@ethertronics.com

Received 17 September 2013

ABSTRACT: By utilizing the inherent symmetry of a folded loop struc-ture, a novel active antenna solution is presented that can optimize theperformance of the antenna in terms of efficiency for various user cases

like the mobile phone in free space, in the hand or close to the head—right or left side. The principle is to excite two distinct modes, and we

can note as much as 1.5 dB improvement in efficiency while maintainingall the bandwidth requirements for 3G operation. The structure can beimplemented with relative ease on most smart phones and shows poten-

tial for meeting the bandwidth requirements of a 3G handset. VC 2014

Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1437–1441,

2014; View this article online at wileyonlinelibrary.com. DOI 10.1002/

mop.28365

Key words: folded loop antenna; reconfigurable antenna; active

antenna

1. INTRODUCTION

Handset manufactures are faced with the challenging task of

developing handsets that need to cover various protocols like

GSM, UMTS, and now LTE, which are associated with different

bands of the frequency spectrum which can range from 700

MHz to 2.7 GHz. In addition to this, handsets also need to sat-

isfy the various specifications imposed by cellular operators for

different real world scenarios in terms of total radiated power

and total isotropic sensitivity. These specifications are set for

different cases like when the handset is placed besides user’s

head or in user’s hand.

In recent years, these factors have greatly influenced the

position, size, and the type of antenna used in mobile phones.

The cell phone antennas have transitioned from the external

stubs, to various monopole, inverted F antenna (IFA) and planar

IFA-based embedded designs, which have paved the way for

more innovative and conformal antenna structures that are used

in the modern smart phones.

Different variants of embedded antennas like the folded loop

and folded dipole have become popular in recent years [1–3]

because of their wide band response owing to the multiple

resonances in high cellular band (between 1.7 and 2.7 GHz) and

especially due to the peculiarity of these resonances correspond-

ing to the symmetry in the antenna generating common and dif-

ferential modes. One of the high band resonances provides a

unique differential mode that can reduce the contribution of the

ground plane of the phone in the radiation mechanism of the

antenna. This is in turn helps reduce the degradation in the radi-

ation efficiency of the antenna when the phone is next to the

head with the hand.

However, folded loop or folded dipole type antennas cannot

completely overcome the effect of mismatch losses when the

antenna is placed next to the head or hand. There are modes

that utilize the ground plane as part of the radiating structure

and will inherently suffer from a mismatch when the configura-

tion changes from a free space case (FS) to one where the

phone is besides user’s head (BH); in user’s hand (H) or in

user’s hand and placed besides user’s head (BHH).

The loading effect from the head or hand typically detunes

the antenna and forces the antenna to be tuned lower in fre-

quency compared to the FS case. These mismatches result in a

reduction of the power accepted by the antenna and in turn

degrades the efficiency of the antenna. A classic way to counter

act this frequency offset is to try to enhance the antenna band-

width to allow taking into account possible shifts in frequency.

Different passive techniques have been published [4] to

expand the frequency bands of these antennas, but the proposed

solutions, such as adding a second folded loop [3] or the use of

feed mechanism by capacitive coupling [5], have many physical

implementation constraints which make their use difficult and

extremely complex in a real product. Another challenge is also

introduced from some vertical integration as typically; the area

under the antenna is reused for the speaker, microphone, and

micro USB connectors and does not allow these solutions to be

implemented easily.

The solution presented in this article offers a new way to

implement and reconfigure a folded loop type antenna to

improve the performance of the phone in its various real world

scenarios. The proposed design concept and measured results in

terms of reflection coefficient and efficiency for the various

user cases are presented in this article.

2. PROPOSED CONCEPT DESCRIPTION

The concept combines the use of a symmetric passive folded

loop type antenna element and an active circuit built around a

double-pole double-throw (DPDT) switch (Fig. 1).

This type of antenna structure [6] typically generates multi-

ple resonances in the high band. The symmetry of the loop

structure generates a balanced mode. In addition to this, unbal-

anced modes are generated by the air gap enclosed in the loop

structure and also due to coupling between the radiating arm of

the loop and the ground plane.

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 6, June 2014 1437

The DPDT switch is controlled by a typical voltage of 1.5 V

and can be characterized by its insertion loss ranging from 0.2

dB at 0.7 GHz and 0.4 dB at 3 GHz [7]. The overall improve-

ment in efficiency for the system is calculated by the summation

of the radiated efficiency of the antenna and the insertion loss

(from the DPDT switch) incurred in the feed network because

of switching between the feed and ground points. It is, therefore,

important to select a DPDT switch with optimum insertion loss

in the frequency band of interest as the global benefit on effi-

ciency is directly linked to it.

When the high band resonance is from an unbalanced mode,

the ground plane acts as a part of the radiating structure along

with the loop and the current distribution is no longer symmetrical

around the loop structure. Therefore, the coupling of the antenna

with the environment is not the same when the phone is posi-

tioned to the left or right of the head. Similarly, the impact of a

right hand (HR) or left hand (HL) on the phone is not the same.

The concept presented here is based on the utilization of the

symmetry of the radiating element relative to the long axis of the

ground plane. This offers a significant advantage when the feed

and ground points of the antenna (Fig. 1) are swapped, the reso-

nance frequencies of the new configuration of the radiating ele-

ment, remain close to the original configuration with a slightly

offset, which allows us to offer antenna reconfigurability and can

be exploited to maximize performance in different use cases.

In the architecture shown in Figure 1, when the left leg of

the loop is used as feed and right leg is used as ground for the

antenna Mode 1 (noninverted mode) is generated and a second

mode in the form of Mode 2 (inverted mode) can be generated

by swapping the feed and ground connection points.

3. CST SIMULATION

The antenna structure was simulated in CST Microwave Studio.

For the simulations, the typical dimensions of a smart phone were

used (Fig. 2). The conductive elements are made of copper and

the support for the antenna makes use of PC-ABS plastic. In addi-

tion, the radiator has been designed so that it can be produced in

a simple manner using flexible printed circuit. The antenna, which

covers GSM, DCS, and UMTS bands, was simulated in FS for

the noninverted mode (Mode 1) and inverted mode (Mode 2).

4. PROTOTYPE

To verify the simulation results, a prototype was fabricated.

Reproducing the antenna modeled in CST, the prototype (Fig. 3)

was built with a FR4 substrate and antenna support was made

of PC-ABS plastic. The return loss (dB) and the efficiency (dB)

of the antenna were measured for Modes 1 and 2 in the follow-

ing configurations: (1) FS case; (2) besides phantom head left

and right side (BHL and BHR); (3) in HL and HR; and (4)

besides the head with the HL and HR (BHHL and BHHR).

5. RESULTS

5.1. Comparison between Simulated and Measured Results forFS ConfigurationThe measurement results show good level of agreement with the

simulated results. The comparison of results between simulations

and measurements (Fig. 4) shows that there is a very minor shift

in frequency in measurements compared to the simulations. This

minor shift can be attributed to the variation in the dielectric

Figure 2 Antenna simulation model. [Color figure can be viewed in

the online issue, which is available at wileyonlinelibrary.com]

Figure 3 Antenna prototype. [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]

Figure 4 Simulated and measured return loss plot (dB) in FS: configu-

ration for noninverted mode (Mode 1) and inverted mode (Mode 2)

Figure 1 Proposed concept. [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com]

1438 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 6, June 2014 DOI 10.1002/mop

constant of the PC_ABS plastic material used in simulation

compared to the prototype.

As expected owing to the symmetry of the structure in FS,

the two modes generated by swapping the feed and ground

points are almost identical as can be seen from the measured

return loss and efficiency plots.

Figure 5 shows the simulated and measured return loss plots

(dB) for Mode1 and Mode 2 in FS. The resonance at 915 and

1800 MHz and are, respectively, due to the half-wavelength

mode and full wavelength mode of the antenna, respectively.

With the swapping of feed and ground points (mode 2), the res-

onance at 915 MHz hardly moves, while at 2150 MHz there is

minor shift in frequency between the modes.

Figure 6 reveals that at 2150 MHz because of the symmetry

of the structure, the direction of the current distribution is simi-

lar. However, by swapping the feed and ground points it is pos-

sible to alter the region of strong current distribution from the

right (Mode 1) to the left side (Mode 2) with reference to the

axis of the ground plane. This behavior is especially useful

when the environment around the phone changes, for example,

the phone is placed in the HL or HR.

Simulations as well as measured results on the prototype

reveal an average improvement of 1.5 dB in efficiency when the

phone is moved from left to right side by selecting the appropri-

ate mode in the hand held case and besides head and hand case.

5.2. Besides Head Configuration ResultsThe besides head use case was measured using a specific

anthropomorphic mannequin (SAM) Phantom model and was

tested by placing the antenna prototype on the left (BHL) and

right side (BHR) of the head. The loading from the SAM Phan-

tom model typically introduces around a 4 dB drop in efficiency

for antenna compared to the FS case. The typical efficiency

spec for a 3G handset in besides head configuration is 29 dB

for the low band and 26 dB for the high band.

Figure 6 Current distribution plot for antenna in FS for noninverted

mode (Mode 1) and inverted mode (Mode 2). [Color figure can be viewed

in the online issue, which is available at wileyonlinelibrary.com.]

Figure 7 Return loss plot (dB) for besides head left and right configu-

ration for noninverted mode (Mode 1) and inverted mode (Mode 2).

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

Figure 8 (a). Measured low band efficiency (dB) and (b) measured

high band efficiency (dB) for the two modes in the besides head

configuration

Figure 5 Simulated and measured efficiency plot (dB) in FS: configu-

ration for noninverted mode (Mode 1) and inverted mode (Mode 2)

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 6, June 2014 1439

The return loss (dB) and efficiency (dB) were measured and

are shown in Figures 7 and 8, respectively. The return loss plots

reveal that there is not much variation from the free pace case in

terms of detuning the antenna for both Mode 1 and Mode 2. There

is some variation in the efficiency of the modes as is shown in

Figures 8(a) and 8(b). By switching to the appropriate mode the

efficiency spec for the besides head case can be satisfied.

Figure 8(a) shows the low band efficiency plots for besides

head left and besides head right configurations. For the BHL

case, Mode 1 exhibits better efficiency on an average especially

at around 850 MHz as compared to Mode 2 whereas Mode 2

provides better efficiency overall for the BHR case. Figure 8(b)

shows that for the high band case Mode 1 generally exhibits

better efficiency for both BHL and BHR cases. Figure 9 shows

the return loss.

5.3. Hand Held Configuration ResultsThe previous use case with the phantom head did not have a big

impact on the reflection coefficient of the two modes for the

antenna but the use case wherein the antenna is placed in user’s

hand either left or right has a major effect on the reflection

coefficients as well as the efficiency.

The detuning of the antenna for the hand held case can be

attributed to the close proximity of the hand model to the

antenna which causes the antenna to detune and also degrades

efficiency due to absorption losses.

The effect of swapping the feed and ground points is more

pronounced at the high band as compared to the low band. At

high band [Fig. 10(b)], it can be clearly seen that the efficiency

improves by almost 2dB by switching to Mode 2 for the HL

case and efficiency is better for Mode 1 for the HR case. Figure

11 shows the return loss for BHHL and BHHR.

Figure 10 (a) Measured low band efficiency (dB) and (b) measured

high band efficiency (dB) for the two modes in the left and right hand

configurations

Figure 11 Return loss plot (dB) for BHHL and BHHR: configuration

for noninverted mode (Mode 1) and inverted mode (Mode 2). [Color fig-

ure can be viewed in the online issue, which is available at wileyonline-

library.com.]

Figure 9 Return loss plot (dB) for antenna in HL and HR: configura-

tion for noninverted mode (Mode 1) and inverted mode (Mode 2).

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

Figure 12 (a) Measured low band efficiency (dB) and (b) measured

high band efficiency (dB) for the two modes in the BHHL and BHHR

configurations

1440 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 6, June 2014 DOI 10.1002/mop

5.4. Besides Head and Hand Configuration ResultsThe besides head and hand configurations shows results similar

to the hand held case with a further reduction in overall effi-

ciency due to losses incurred by absorption from the hand and

head Phantoms. Similar to hand held case it is easy to see the

improvement in the high band by implementing the switching

circuit between the feed and ground points. For the whole high

band from 1700 to 2200 MHz [Fig. 12(b)], Mode 2 exhibits bet-

ter efficiency for the BHHL case and Mode 1 exhibits higher

efficiency for the BHHR case.

It should be noted that the possibility of switching feed and

ground points can achieve a gain of at least 1 dB, which compensates

for the insertion loss from the DPDT switch used for switching.

The fabricated antenna prototype was tested for various use

cases that are close to the real world implementation of this

type of antenna in a mobile phone. The results for the various

measurements are summarized in Table 1.

Thus, the measured results confirm that there is an improve-

ment in efficiency by utilizing this antenna architecture for the

besides head, hand held, and besides head and hand cases.

6. CONCLUSION

A novel implementation for adding reconfigurablity to the folded

loop type antenna by utilizing an active circuit to swap the feed

and ground points was evaluated in this article. This architecture

utilizes the symmetry in the loop structure in addition to the mul-

tiple modes generated in the high band for the antenna. Simula-

tions and experiments exhibit good agreement and show that we

can gain as much as 1.6 dB over majority of the high band for

the BHHL and BHHR cases by switching to the optimum mode.

The measurement results presented in this article are limited

to the implementation of this type of active antenna in a cell

phone form factor; however, it can be implemented in other

devices which can utilize the symmetry of radiating elements.

REFERENCES

1. C. Di Nallo and A. Faraone, The folded inverted conformal antenna

(FICA), In: 2005 IEEE Antennas and Propagation Society Interna-

tional Symposium, Vol. 4B, Washington, DC, 2005.

2. I. Szini, C. Di Nallo, and A. Faraone, The enhanced bandwidth

folded inverted conformal antenna (EB FICA) for mufti-band cellu-

lar handsets, In: 2007 IEEEAntennas and Propagation Society Inter-

national Symposium, Honolulu, HI, 2007.

3. C.-H. Ku, H.-W. Liu, and S.-Y. Lin, Folded dual-loop antenna for

GSM/DCS/PCS/UMTS mobile handset applications, Antennas Wire-

less Propag Lett 9 (2010).

4. A. Kajitani; Y. Kim, H. Morishita, and Y. Koyanagi, Wideband charac-

teristics of built-in folded dipole antenna for handset,In: 2007 IEEE

Antennas and Propagation Society International Symposium,

Honolulu, HI, 2007.

5. M. Zheng, H. Wang, and Y. Hao, Internal hexa-band folded monop-

ole/dipole/loop antenna with four resonances for mobile device,

IEEE Trans Antennas Propag 60 (2012).

6. J. Sun, Q. He, H. Yin, and Z.D. Milosavljevic, An enhanced multiple-

mode penta-band antenna(EMMA) for mobile applications with

improved beside head and hand performance, In: 2013 IEEE Antennas

and Propagation Society International Symposium, Orlando, FL, 2013.

7. Peregrine Semiconductors, PE42128x UltraCMOSVR SP7T/SP7T

MIPI Dual-Feed Antenna Switch For 4G Mobile Wireless

Applications.

VC 2014 Wiley Periodicals, Inc.

EXPERIMENTAL ANALYSIS OFDIFFERENT MEASUREMENTTECHNIQUES FOR CHARACTERIZATIONOF MILLIMETER-WAVE MIXERS

Itziar Maestrojuan,1 Simon Rea,2 I~nigo Ederra,1 andRam�on Gonzalo1

1 Antennae Group, Public University of Navarra, Campus Arrosadias/n, 31006, Pamplona, Navarra, Spain; Corresponding author:itziar.maestrojuan@unavarra.es2 Millimetre Technology Group, STFC Rutherford AppletonLaboratory, Didcot, United Kingdom, OX11

Received 18 September 2013

ABSTRACT: This article compares for the first time three well-known

procedures for characterizing the noise temperature and conversion loss

of a millimeter wave mixer. To carry out this study, a 183 GHz subhar-

monic mixer has been measured using three common procedures, that is,

the “attenuator,” the “gain,” and the “noise injection” procedures. Fur-

thermore, for every measurement procedure, three different detection

methods have been used; a broadband power meter, a yttrium iron garnet

filter working together with a broadband power meter, and a spectrum

analyzer. The “gain procedure” has turned out to be the most stable one

in terms of less variation between consecutive values and flatter results

along all the frequency range. Results obtained with every detection

method are consistent with each other, showing a similar performance of

the mixer independent of the detection method used. In addition, the effect

of the inclusion or omission of an isolator between mixer and intermedi-

ate frequency pre-amplifier has also been evaluated. For the first time,

clear conclusions about its influence are driven. The inclusion of the iso-

lator has culminated in more reliable results, although the measured NT

values are increased. VC 2014 Wiley Periodicals, Inc. Microwave Opt

Technol Lett 56:1441–1447, 2014; View this article online at

wileyonlinelibrary.com. DOI 10.1002/mop.28364

Key words: mixer measurement techniques; noise temperature; conver-

sion loss; millimeter-wave mixers

1. INTRODUCTION

There are several procedures commonly used for measuring the

performance of millimetre-wave mixers [1, 2]. In general, they

all require the de-embedding of the mixer performance from Y-

factor measurements. The performance of a mixer can vary con-

siderably depending on the measurement technique used. How-

ever, to the best of our knowledge, the already known different

procedures used to characterize millimeter-wave mixers have

not previously been studied in depth and compared using a ref-

erence mixer/receiver test set-up. Furthermore, as there is no

standard approach adopted by the millimetre-wave community

for mixer characterization, it is difficult to contrast the perform-

ance of different mixers presented in the literature and in manu-

facturer Websites.

TABLE 1 Summary of Measured Results DetailingImprovement in Efficiency by Switching between Modes 1 and2 for Different Configurations

FS

BHL

and BHR

HL

and HR

BHHL

and BHHR

High

Band (dB)

Low

Band (dB)

High

Band (dB)

High

Band (dB)

Proposed benefit �0.3 �0.5 �1.7 �2

Circuit losses

(from DPDT

switch)

0.4 0.2 0.4 0.4B

Net benefit �20.1 �0.3 �1.3 �1.6

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 6, June 2014 1441