1 Introduction

Among the scheduled experiments withthe Engineering Test Satellite VIII (ETS-VIII),the Communications Research Laboratory(CRL) is planning high-speed data communi-cation experiments for S-band multimediamobile satellite communications, using mobileand portable earth stations [1]. The CRL hasproposed several antennas for the portableearth stations and is in the process of conduct-ing the relevant R&D.

It is desirable that a portable earth stationoperate efficiently with low power, in light ofthe required mobility and flexibility in opera-tion. Accordingly, a high-gain antenna isrequired. A parabolic antenna is one example;it features a simple structure and aperture areamay be freely selected to obtain the necessarygain. This type of antenna is therefore oftenused in portable earth stations. However, therigid metal structure and the relationshipbetween antenna size and gain present limita-tions in terms of dimensions.

In particular, when the parabolic antenna

is used for emergency communications in theevent of disaster, the need for an antenna thatprovides both the necessary gain and portabili-ty is heightened, as it must be easily transport-ed in all kinds of conditions. In order to meetthis need, we have proposed and produced aprototype parabolic reflector antenna that canbe mounted in a folding case. This paper dis-cusses the structure and electrical characteris-tics of the parabolic reflector antenna devel-oped.

2 Desired specifications of theantenna

The ETS-VIII will be equipped with onboardbase-band switches for high-speed data com-munications [2]. The switches control trans-mission signals at a transmission rate of 1024kbps without forward error correction. To uti-lize the transmission power of the satellite andearth stations, the combination of Convolutioncoding and Viterbi decoding is adopted as aforward error correction (FEC) technique. Anautomatic repeat request (ARQ) technique is

JANG Jae-Hyeuk et al. 183

4-6-3 Parabolic Reflector Antenna Mounted InsideFolding Case

JANG Jae-Hyeuk, TANAKA Masato, and HAMAMOTO Naokazu

We developed a folding parabolic reflector antenna for a portable earth station to con-duct high speed data communication experiments on the Engineering Test Satellite VIII(ETS-VIII). This antenna is mounted inside a thin folding case, designed to be the antennacarrier, and can easily be folded and unfolded because the reflector is made of a flexiblereflector material. We performed experiments to verify the electrical performance of theantenna. Even though the antenna has a flexible reflector, the experimental results showthat the expected antenna performance can be achieved for the communication experi-ments for the ETS-VIII satellite.

Keywords Engineering Test Satellite VIII, High speed data communication experiments, Portableearth station, Folding parabolic reflector antenna

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also applied as error correction in addition tothe FEC in order to reduce the received signallevel degradation due to the shadowing effecton the mobile satellite channel.

The earth stations intended for the high-speed data communication experimentsinclude vehicle-mounted mobile stations andportable earth stations. The portable earth sta-tions will be designed to transmit power of 10W in the S-band. Calculating the link budgetwith this transmitting power and with a trans-mission rate of 1024 kbps before error correc-tion, available up/down link margin of severaldecibels is expected when Tx and Rx antennagain is greater than 12 dBi. We thereforeestablished a desired TX and RX antenna gainof 12 dBi or greater for the parabolic reflectorantenna discussed herein. In addition, the fre-quency bands required for communicationexperiments with the ETS-VIII are2655.5–2658.0 MHz for Tx and2500.5–2503.0 MHz for Rx. The requiredantenna polarization is left-hand circularpolarization (LHCP) both for Tx and Rx.

3 Outline of the prototype antenna

Fig.1 shows pictures of the prototypeantenna. In Fig.1 (a) the antenna is folded inthe case and in Fig.1 (b) the antenna isdeployed. The antenna is the front-fed spheri-cal reflector antenna that can be mounted in afolding case and consists of a folding case, areflector made of conductive woven fabric, theprimary radiator of a microstrip patch antennawith parasitic element, and an RF diplexer.Here we will discuss the structure and charac-teristics of each component.

3.1 Folding caseFig.2 shows a schematic drawing of the

exterior and interior structure of the prototypeantenna. The folding case is 400 mm wide,300 mm long, and 30 mm thick. It has a han-dle for easy portability. Two RF SMA—con-nectors are placed beside the handle (indicatedin the upper left in Fig.2 (a)). These connec-tors are connected to the Tx and Rx connec-

tors of the diplexer inside the case and facili-tate connection of the Tx and Rx cables fromthe portable earth station to the antenna. Fig.2(b) shows the internal structure of the casewhen the case is open. The diplexer is fas-tened in the upper right of the case and the pri-mary radiator is stored in the upper left of thecase. The conductive fabric reflector is foldedin half for storage in the remaining space.

3.2 Reflector deployment method Fig.3 illustrates the deployment of the

reflector, a process similar to the opening of around folding fan. First, open the case anderect the reflector, still folded in half, orientedin the direction indicated by arrow (1) inFig.3. Next, unfold the reflector in the direc-tion indicated by arrow (2) in Fig.3, using therotating mechanism attached to both sides ofthe reflector, as if opening a round folding fan.

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.3/4 2003

(a) Folded antenna

(b) Deployed antenna

Pictures of prototype parabolic reflec-tor antenna

Fig.1

Then, tilt the case cover in the direction indi-cated by arrow (3) while orienting the reflec-tor in the direction indicated by arrow (4).Hook the tip of the reflector to the case coveras shown in Fig.4 (b) to fix the reflector sur-face toward the satellite radio-wave source.

3.3 Structure of the reflectorFig.4 shows the structure of the deployed

prototype antenna. Fig.4 (a) is a front view ofthe antenna observed in the line of sight paral-lel to the ground. Fig.4 (b) is a side view

observed in a similar manner.The reflector features an aperture diameter

of 350 mm. The reflector surface is made ofconductive woven fabric. Seven parabolicmetal frames support this material in a para-bolic shape. The back of the reflector has reg-ularly-spaced slender pouches for the insertionof the metal frames. Two frames are insertedinto the pouches to form the circumference ofthe reflector and five frames are inserted intothe pouches to form the paraboloid surface ofthe reflector.

The reflector rim and the tips of the metalframes are attached to an ABS board 2 mm inthickness (Acrylonitrile Butadiene Styrene, aradio-transparent material) to hold the entirestructure in place. The ABS board is designed

JANG Jae-Hyeuk et al. 185

(a) Exterior structure of the case

(b) Interior structure of the case

Exterior appearance and interiorstructure of the prototype antenna

Fig.2

Deployment mechanism of the proto-type parabolic reflector antenna (sideview)

Fig.3

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to fold in half. This structure allows the userto fold the reflector into the case simply byfolding the ABS board. This also renders thereflector wind and water resistant, allowingfor outdoor use.

A circular hole with a diameter of 61.2mm is cut in the center of the ABS board; theprimary radiator is screwed into this hole.This method simplifies the installation of theprimary radiator and allows it to be held in asteady position. The angle from the feed axisto the reflector rim is 90 degrees, that is, theF/D ratio is 0.25.

3.4 Conductive woven fabricThe conductive woven fabric used for the

developed antenna has excellent softness and

flexibility and is usually used as an electro-magnetic interference (EMI) shield. It is ofPET/Cu+Ni (base/metal) construction with athickness of 125μm, weight of 72 g/m2, and asurface resistance of 0.05Ω/sq. The reflectioncharacteristics of the fabric were investigated;results indicate a return loss of －0.03 dB anda transmission loss of －70 dB in the S-band.These values were measured by connectingthe coaxial-waveguide adapter (for the S-band) to the input/output ports of the networkanalyzer, inserting the conductive woven fab-ric between the adapters, and wrapping theadapters and the fabric with copper tape toprevent surface-current leakage.

3.5 Primary radiatorIn view of the limited storage capacity

within the case, a microstrip patch antennahaving the features such as a small size, low-profile and lightweight is used for the primaryradiator. Fig.5 shows a picture of the primaryradiator and its structure.

The primary radiator uses a circularlypolarized single-fed circular patch antennawith parasitic element. The parasitic elementis mounted at a height of 12 mm from the feedpatch and fitted with a plastic spacer. Theradii of the feed patch and the parasitic ele-ment are 20.9 mm and 18.45 mm, respective-ly. The radius of the feed patch antenna sub-strate is 29.5 mm. The substrate is of Arlon’sPTFE with a thickness of 1.7 mm and adielectric constant of 2.6. A similar substrateis also used for the parasitic element. Thetruncated segment of the parasitic element isdetermined by optimizing the axial ratio andthe V.S.W.R characteristic when attached tothe reflector. The ground of the patch antennais enclosed with ABS resin for protection fromwind and water.

3.6 RF diplexerThis antenna unit also contains a diplexer

(indicated in the upper right of Fig.2 (b)). TheRF diplexer is 200 mm wide, 63 mm long, and40 mm thick. The pass band is ±15 MHz forTx and ±25 MHz for Rx at the center frequen-

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.3/4 2003

(a) Front view

(b) Side view

Structure of the deployed prototypeantenna

Fig.4

cy (2657.5 MHz for Tx and 2502.5 MHz forRx). Insertion loss is 1 dB or less in the Txand Rx frequencies, V.S.W.R is 1.3 dB or less,and isolation is 75 dB or greater. The maxi-mum allowable input power is 20 W.

4 Antenna electrical characteris-tics

Fig.6 shows the radiation patterns of the

prototype parabolic antenna. The solid curvein the figure corresponds to the radiation pat-tern at a frequency of 2.5025 GHz, and thedashed curve corresponds to the radiation pat-tern at 2.6575 GHz.

As the reflector of the prototype parabolicantenna is made of flexible conductive wovenfabric, random reflector surface errors areexpected, despite the seven reflector supports.This random surface error causes phase fluctu-ation at the aperture surface and impairs theelectrical performance of the antenna, includ-ing reduced gain and an increase in the side-lobe. The asymmetry observed in the sidelobein Fig.6 is considered to be due to this randomsurface error.

Table 1 details the electrical performanceof the prototype parabolic antenna. The meas-ured electrical values satisfy the specificationsalready discussed, with verified target gain of

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(a) Picture of the primary radiator

(b) Schematic diagram of the primary radiator

Configuration of primary radiatorFig.5

Radiation patterns of the prototypeparabolic antenna

Fig.6

Performance specifications of theprototype parabolic antenna

Table 1

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12 dBi or greater, as required for use in theETS-VIII high-speed data communicationexperiments, and confirmed compliance withremaining specifications (such as those relat-ing to frequency band and polarization).Aperture efficiency obtained based on the gainindicated in Table 1 is approximately 30%.

In accordance with the feed antenna radia-tion pattern characteristics, the aperture effi-ciency of parabolic antennas highly dependson the tradeoff relationship between the aper-ture taper efficiency and the spillover efficien-cy. The maximum aperture efficiency isachievable when the primary radiator is set foran edge illumination of about －11 dB [3].The maximum achievable aperture efficiencythus obtained for a single reflector with anearly rotationally symmetric feed pattern isapproximately 75%. If an open-ended wave-guide is used as the primary radiator, the max-imum aperture efficiency is approximately60% [3].

Aperture efficiency obtained with the gainindicated in Table 1 is lower than thatachieved with an open-ended waveguide.This may be due to various causes, includingthe random surface errors caused by the flexi-ble reflector surface and the blocking lossattributable to the primary radiator. However,considering the mechanical structure of theantenna system under study (for example, 90-degree angle between the feed axis and the

reflector rim), a certain trade-off in electricalperformance is inevitable. We will study thisproblem in further depth.

5 Summary

We have reported on the structure and per-formance of a prototype parabolic reflectorantenna mounted inside a folding case,designed as a Tx and Rx antenna for portableearth stations to be used in the high-speed datacommunication experiments planned for theETS-VIII.

We have demonstrated that the use of aflexible conductive woven fabric for theantenna reflector material allows for a light-weight antenna capable of storage in a thincase that is easily carried. The electrical per-formance of the prototype was measured, withresults indicating that the model satisfies thedesired values and specifications of the ETS-VIII experiments, although aperture efficiencyis somewhat low.

This antenna will be used in the communi-cation experiments in the basic experimentalphase following the launch of the ETS-VIII.We are proceeding with further preparations toensure smooth execution of post-launch exper-iments, including studies on improving anten-na performance and demonstration experi-ments using portable earth stations.

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.3/4 2003

References1 N. Hamamoto, Y. Hashimoto, M. Sakasai, Y. Tsuchihashi, and M. Yoneda, "An experimental multimedia

mobile satellite communication system using the ETS-VIII satellite" AIAA-98-1301, A collection of technical

papers of the 17th ICSSC, pp. 408-416, Feb. 1998.

2 Y. Hashimoto, T. Ide, M. Sakasai, T. Tejima, Y. Matsumoto, and N. Hamamoto, "The project of mobile-satel-

lite broadcasting and communication experiments using ETS-VIII, — Multimedia communication experi-

ments for mobile stations — ," IEICE Conf., B-234, Mar. 1996. (in Japanese)

3 Warren L. Stutzman and Gary A. Thiele, "Antenna Theory and Design" John Wiley & Sons, pp.342-347,

1998.

JANG Jae-Hyeuk et al. 189

JANG Jae-Hyeuk, Ph. D.

Research Fellow, Mobile SatelliteCommunications Group, WirelessCommunications Division

Antenna, Radio Wave Propagation,Satellite Communications

HAMAMOTO Naokazu

Research Supervisor, Wireless Commu-nications Division

Satellite Communication Engineering

TANAKA Masato, Dr. Eng.

Chief Researcher, General AffairsDivision (present Senior Researcher,Research Planning Office, StrategicPlanning Division)

Satellite Communications, Antennas

190 Journal of the National Institute of Information and Communications Technology Vol.50 Nos.3/4 2003