Radio Science Bulletin 313 - June 2005 - URSI

An Ultra-Compact Impulse-Radiating AntennaL.H. BowenE.G. FarrD.I. LawryJ.S. TyoAbstractWe describe the development of an ultra-compactimpulse-radiating antenna (UCIRA), intended for spaceapplications in low Earth orbit. Because a UWB (ultrawideband)antenna was desired for this application, an IRA(impulse-radiating antenna) was selected as the startingpoint for this development. The standard IRA configurationhas a single linear polarization, and has a very narrowbeamwidth at high frequencies. A dual-polarization antennawas desired with a larger beamwidth, so we modified theexisting design to support dual polarization, and we used ahyperbolic reflector to increase the beamwidth. The antennaalso had to survive the harsh space environment, had to belightweight and very compact for launch, and had to beremotely deployable.A deployable IRA with dual polarization using aflexible twinline feed has been developed that has verygood RF characteristics. Also, the theory for IRAs withhyperbolic reflectors and defocused feeds has been advanced.The UCIRA-2 developed here met many of the requirements.1. IntroductionWe summarize here our efforts to build an Ultra-Compact Impulse Radiating Antenna (UCIRA) for spaceapplications. This may be considered an extension of ourearlier work to develop an IRA for space applicationsfabricated from an inflatable membrane [1]. We subsequentlydecided that the inflatable technology was only appropriatefor very large antennas. For the smaller antennas in whichwe were interested, a mechanically deployable antennawould be more durable. An example of such an antenna isthe commercially available CIRA-2 [2, 3], with a diameterof 1.22 m (48 in.). However, the stowed size of the CIRA-2 was too large for satellite applications. This led to thedevelopment of the Ultra-Compact IRA or UCIRA, whichis described here.The UCIRA differs from the CIRA-2 in five respects.First, we reduced the collapsed length from 81 cm (32 in.)in the CIRA to 48 cm (19 in.). This was accomplished byreducing the F/D (focal length / diameter) ratio from 0.4 to0.3, and by adding a folding joint to each of the 12 radialsupport rods in the reflector. Second, we defocused thebeam with a hyperboloidal reflector, so we could see alarger field of view from low earth orbit (LEO). Third, weconfigured the antenna to operate with dual polarity. Fourth,we made the antenna automatically deployable with springsand electro-mechanical elements. Fifth, we used materialsthat could qualify for space applications.The UCIRA is intended to provide as wide a coveragearea as possible from a satellite platform in LEO. In orderto receive signals from horizon to horizon, the antennaneeds to have a beamwidth of 100° to 120°. Since IRAs areinherently highly directional (narrow beamwidth) at highfrequencies, the UCIRA must be defocused using ahyperboloidal reflector instead of a parabolic reflector asdescribed in [4, 5]. We also wanted the bandwidth to be verybroad, and a design based on an IRA is well suited for thisdue to its approximately 2 decades of bandwidth. TheCIRA-2, on which the UCIRA is based, is usable from150 MHz to 10 GHz [3]. In addition, we wanted the antennato have dual polarity, so twice the information is availablefrom a single antenna.To study dual polarity IRAs, we experimented witha 46 cm (18 in.) diameter solid aluminum IRA which hadtwo feeds, one for each polarity, vertical and horizontal.This antenna was the IRA-1D. The results of this dualpolarity experiment were reported previously in [6].It has been noted previously by Carl Baum [7] thata hyperboloidal reflector with a circular rim may have aproblem in its radiated frequency spectrum on boresight.This occurs at the frequency where the difference in ray pathlengths from the center and edge of the reflector is half awavelength. To address this problem, he proposed using aLeland H. Bowen is with Farr Research, Inc., 8 Ridge Road,Placitas, NM 87043 USA; Tel: +1 (505) 867-2731; Fax: +1(505) 867-9125; E-mail: lhbowen@farr-research.com.Everett G. Farr is with Farr Research, Inc., 614 Paseo DelMar NE, Albuquerque, NM 87043 USA;Tel: +1 (505) 293-3886; Fax: +1 (505) 323-1886; E-mail:efarr@farr-research.com.Dean I. Lawry is with the Air Force Research Laboratory/DE, 3550 Aberdeen Ave. SE, Kirtland AFB, NM 87117-5776 USA; Tel: +1 (505) 853-3392.J. Scott Tyo is with the University of New Mexico, EECEBuilding, Room 125, University of New Mexico,Albuquerque, NM 87131-1356 USA; Tel: +1 (505) 277-1412; Fax: +1 (505) 277-1439, E-mail tyo@ece.unm.eduTheRadio Science Bulletin No 313 (June, 2005) 39