Design Considerations for LTCC based UWB

Antennas for Space Applications

B. Hussain, I. Kianpour, V. Grade Tavares,

H. S. Mendonca

INESC-TEC

Faculty of Engineering, University of Porto

Porto, Portugal

[bhussain,ikian,vgt,hsm]@fe.up.pt

G.Miskovic*, G. Radosavljevic

*,V.V.Petrovic

†

*Institute of Sensor and Actuator Systems

*Vienna University of Technology, Vienna, Austria

†TES Electronic Solutions, Stuttgart Germany

*[goran.miskovic,Goran.Radosavljevic]@tuwein.ac.at

†vladimir.petrovic@tes-dst.com

Abstract—This paper presents a planar antenna using low

temperature co-fired ceramics (LTCC) substrate for extreme

environment applications. An ultra wideband (UWB) elliptical

patch antenna was designed and fabricated using an LTCC

Ceramtec GC substrate to demonstrate the capabilities of the

technology for wideband applications. The simulated results were

further validated experimentally. The fabricated antenna

provides a peak gain of 5dB over a bandwidth of 4 GHz (3 GHz –

7 GHz) with return loss better than -10dB. The radiation pattern

is omni-directional in the horizontal plane (θ=90⁰) over the whole

frequency range.

Keywords—LTCC, UWB Antennas, High Dielectric Constant

Substrate

I. INTRODUCTION

Planar antennas offer a compact, cost effective and efficient

mean to integrate transceiver chips. Conventional antennas

such as horn, parabolic dish and dipoles are effective, but the

need for portability and miniaturization on hand-held devices

is not compatible in volume with such antennas. Thus, by

resolving some of these problems, planar antennas have

become by far the most widely used antennas in today’s

telecom world.

Planar antennas are usually realized on FR4 Epoxy

substrates with copper metallization. Although it is a low-cost

and efficient solution for most current communication

systems, epoxy substrate does not perform well over a large

bandwidth due to the fact that the dielectric constant is not

strictly controlled. The durability of epoxy substrates is also

very limited in harsh environments. Extreme heat conditions

and high-pressure restrict the use of these antennas. In

environments such as Venus (500°C, 90 Atmospheric) or deep

underwater sensor applications, the epoxy based substrates

cannot be used. Therefore, there is a strong need to find

substrates that can replace epoxy and provide better

performance.

Ceramics are used in different industries due to their high

durability and resistance to extreme environments. However,

their high-firing temperatures strongly limit the use of

ceramics with electronics. The extreme heat involved, during

fabrication, in general is not suitable for metallization and chip

integration. The alternative solution is to use instead the so-

called low temperature co-fired ceramics (LTCC). This

substrate technology is fired at relatively lower temperature,

which has made them suitable for metallization with gold or

silver. As LTCC is ceramic in nature, it has high durability

against heat, moisture and other degrading factors. Another

interesting feature of LTCC is that multiple independent layers

can be prepared separately and subsequently fired together.

Thus, a high degree of integration with electronic circuits can

be achieved.

On one hand, dielectric losses in LTCC are quite negligible

due to low-loss tangent (0.001-0.005) [1], and on the other

hand, LTCC has a relatively high dielectric constant (6 to 9)

[1]. This provides the possibility for having a smaller antenna

foot-print.

In the past decade a good amount of research was directed

towards the achievement of LTCC substrates that could be

used for high-frequency applications. Traditionally, DuPont

LTCC tapes became preferred in such cases [2]. These tapes

possess excellent characteristics for RF design, such as low-

loss, stable dielectric constant and silver/gold compatibility.

However, it becomes rather difficult to deposit large

metallization surfaces without deformation. Alternatively,

Ceramtec GC tapes offer almost identical characteristics to

DuPont, but provide more stability for large deposition of

metal structures.

This paper then presents an elliptical patch antenna

fabricated from Ceramtec GC tapes. The antenna is designed

using commercially available EM tools (HFSS) and simulated

results are validated using anechoic chamber measurements.

The remaining of the paper is organized as follows. In

Section II, design challenges of LTCC are discussed. Section

III presents the antenna design and simulation results. The

experimental verification is provided in Section IV. The

analysis of results and conclusions are finally presented in

Section V.

II. DESIGN CHALLENGES WITH LTCC SUBSTRATES

From previous section, it becomes evident that LTCC

provides exciting properties for RF design and applications.

But LTCC also presents some design challenges. Fig.1 shows

the LTCC fabrication process for reference. The different

manufactures (DuPont, Ceramtec GC, Kerafol and others)

commercially supply green or LTCC tapes in form of a roll.

These tapes are cut according to the required size and then

metallization is deposited on these tapes, using screen-printing

techniques. In order to achieve the desired thickness, several

layers are stacked up. All these layers may or may not contain

metallization, depending on the design. After stacking up,

these layers are fired at a peak temperature of 900⁰ C. Firing

at this high temperature causes the tapes to shrink. The heating

profile is strictly controlled and monitored. For antenna

design, the maximum area of the antenna is determined not

only by the manufacturer specifications, but also by the size of

the metallic mould used for stacking up LTCC sheets. This

area will also shrink after the firing process, so a strict

maximum limit is placed on the antenna size. This

consequently also imposes a limit on the antenna gain.

Another important design consideration is the choice of

planar design, such as microstrip, stripline or coplanar

waveguide (CPW). Stripline and microstrip requires extra

metallization processes in order to form ground planes. CPW

provides a more simple and cost effective way to implement

the RF design. As the ground plane and radiating patch is on

the same layer, metallization can be done in a single step.

Also, as the conductors used for LTCC are mostly gold and

silver, CPW can also help in reducing the cost.

Stability of LTCC tapes is an important design

consideration, and the thickness of substrate on itself is also

defined by a minimum value. Different tapes have different

minimum thickness requirements determined by their material

composition. For RF design, it can be a critical parameter,

contrary to Epoxy substrate where thicknesses less than 100

µm are achievable.

The co-fired and post-fired processes can also bring some

design constrains. In co-fired processes, the metallization layer

is applied before firing. The consequent shrink hinders the

matching between the designed dimensions of the CPW

topologies and the actual resulting pattern after fabrication.

For such processes, extra area is provided to better fit the

designed dimensions after firing. Such dimension fitting

results in non-uniformities around the edges of the surface and

can cause diffraction at high frequencies. Moreover,

metallization is highly dependent on substrate deformation. In

post-fired processes, LTCC substrate is fired first and then

metallization layer is applied and fired again. This results in a

smoother surface but requires additional fabrication steps.

Fig .1: LTCC fabrication process flow chart (taken from [3])

III. ANTENNA DESIGN AND SIMULATIONS

Planar antennas offer a wide variety of shapes that includes

rectangular, triangular, circular and elliptical. A patch with

smoother edges, such as circular or elliptical, is usually wider

in bandwidth due to the absence of corners. For this reason,

elliptical antennas are today widely used for UWB

applications [4]-[7]. They offer a wide bandwidth response

with reduced size as compared to circular antennas. An

elliptical antenna is then adopted here due its optimum

response. Since the connector needs to be mounted on the

same face of the antenna, in order to avoid drilling holes

through the substrate, a 50 Ω CPW line was designed for the

signal feed, as Fig.3 shows.

The type of LTCC tape and metallic mould used, as referred

in previous section, determines the length and width of the

final structure. In the current design, the length ‘L’ and width

‘W’ (Fig 3) was taken as the maximum allowed size for the

fabrication process defined in section II (Fig.1). Three

ellipsoids were used to design the wideband antenna. As seen

in Fig .2, ellipse 1, with diameter ‘D1’, determines the

separation between the ground plane and the radiating patch.

Ellipse 2, with diameter ‘D2’, is the actual radiating patch.

Finally, ellipse 3 with diameter ‘D3’ is used to improve

matching. The sizes of these ellipses were calculated using the

following equations [5],

√

(1)

√

(2)

√

(3)

where ‘c’ is the speed of light, ‘f1’ is the lowest frequency and

‘Ɛre’ is the effective dielectric constant for CPW, calculated

using the following equation [5]:

For Cermatec GC tape, the minimum thickness required is

450 µm, thus the antenna was designed to fit this requirement.

The final dimensions of the structure can be found in

Table 1. Initially, the antenna was designed to provide gain

over a large frequency bandwidth (3-11GHz), with a substrate

thickness of 1.6mm. However, the final dimension of this

antenna was beyond the maximum size allowed by the

fabrication process. In order to fit the dimensions, the

radiating patch had to be scaled down. It resulted in a

reduction of bandwidth and gain. The simulated results

(together with experimental) can be observed in Fig 4.

Table 1: Antenna Dimensions for Fig 2

Dimension Symbol Value (mm or mm/mm)

W 36

L 41

D1/d1 27/32.40

D2/d2 18.4/12.32

D3/d3 4.46/3.52

a/s 1.7/0.6

IV. MEASUREMENTS

Antenna measurements were performed in an anechoic

chamber with the help of a Vector Network Analyser (Agilent

E8363b). The distance between transmitting and receiving

antennas was 4.4 meters, with the receiving antenna mounted

on a motorized platform with a 360º rotation in ɸ.

To analyse the effects of the fabrication process, two

antennas, Antenna 1 and Antenna 2 shown in Fig.3, were

fabricated using co-fire and post-fire processes, respectively.

In order to estimate the gain and radiation patterns of

fabricated antennas, three-antenna method [17] was used

(Antenna 1, Antenna 2, and a standard horn). From the Friis

Formula,

(

) (

)

(4)

the gain and radiation pattern of fabricated antennas were

calculated and plotted in Fig 4. The measurements were

repeated using gain comparison method and were found to be

in very good agreement with previous method. The results of

the two fabricated antennas are also shown in Fig.4.

Fig. 3: (a) Antenna 1(co-fired).(b) Antenna 2 (post-fired)

V. DISCUSSION AND CONCLUSIONS

From the results, it may be concluded that LTCC can be

used to fabricate RF antennas for wide bandwidths. Computer

Simulations were able to predict the behaviour of the antenna

over the bandwidth. Antenna 1 (co-fired) presented a

deformed silver layer (Fig.3). This imperfection of surface has

resulted in a frequency shift. On the other hand, Antenna 2

was post-fired and presents a smoother metal surface, but

contains a large non-metalized area on the sides. This has

introduced some extra insertion-loss at lower frequencies. The

radiation pattern is almost omni-directional over the whole

operating bandwidth. Such behaviour is ideal for Impulse-

radio communications, as the shape of wideband pulse will not

be affected by antenna. Thus, it demonstrates that Cermatec

Fig .2: Antenna Structure (the gray area represents metal)

W

D2

a

s

L

D1

d1

D3

d2

d3

(a)

(b)

GC tapes can be used for high frequency RF design. Table 2

presents a comparison of the current design with the state-of-

the-art using LTCCs. It can be easily observed that the antenna

built from ferrite based tapes are smaller in size but introduces

more design difficulties due to magnetic properties of

substrate. Conversely, non-Magnetic substrates, such as

Ceramtec GC and DuPont, require larger substrate sizes.

Introducing LTCC into RF domain opens opportunities for

more robust and more demanding applications. Integration of

sensors in LTCC technology [14]-[16] can help to build

efficient systems. Harsh environments such as space, alien

planets, engine chambers, nuclear reactors, deep underwater

sensors, and so on, can now be wirelessly monitored, and

communicated with, using LTCC based components.

ACKNOWLEDGEMENT

This work is funded from the European Union's Seventh

Framework Programme for research, technological

development and demonstration under grant agreement no.

289481. - project SENSEIVER.

Table 2: Comparison of the antenna from this work with similar

antennas from literature

Ref BW

(GHz)

Gain

(dB)

Dimensions

(L×W×H)mm

LTCC

Tape

Antenna

Type

This Work

3.0-7.0

5 41×36×0.48 Ceramtec

GC Elliptical

Patch

[8] 3.5-

6.5 0 30×25×1.2

Ferro

A65

Triangular

Patch

[8] 6.65-10.0

5 50×25×1.2 Ferro A65

Vivaldi Patch

[9]* 4.9-

5.9 6 30×24×1

DuPont

951

Circular

Patch

[10] 3.0-

5.0 4.5 19.25×12×0.18

Ferro

A6M

Monopole (Folded

Ground Plane)

[11] 3.0-

11.9 5 25×25×0.58

Ferro

A6M

Half

Octagon

[5]* 2-11.8 10 75×78×1 GL-550 Circular

Path

[12] 25.5-

29.5 5 12×12×1.136

DuPont

DP943

Rectangular

Patch

[13] 4.35-10.5

4.7 66×50×1 DuPont

951 Triangular

Patch *Simulated Only

Fig.4: (a) Measured and Simulated Results of Antenna 1.(b) Measured and

Simulated Results of Antenna 2.(c) Radiation Pattern of Antenna 1 & 2 at 3.5 GHz.(d) Radiation Pattern of Antenna 1 & 2 at 6.5 GHz

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