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A Survey of Possible Passive Antenna Applications of High-Temperature Superconductors Robert J

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A Survey of Possible Passive Antenna Applications of High-Temperature Superconductors Robert J 1498 IEEE TRANSACTIONSON MICROWAVE THEORY AND TECHNIQUES, VOL. 39, NO. 9, SEPTEMBER 1991 A Survey of Possible Passive Antenna Applications of High-Temperature Superconductors Robert J. Dinger, Member, ZEEE, Donald R. Bowling, and Anna M. Martin (Invited Paper) Abstract -A survey of possible applications of high-tempera- antenna, that exploit HTS material await further develop- ture superconductors (HTS’s) to antennas and antenna feed ment before their application to antenna systems can be networks is present?. The frequency range of consideration is 1 MHz to 100 GHZ: Three antenna application areas seem reasonably examined. appropriate for HTS material: electrically small antennas and Below, we discuss in some detail three application their matching networks; feed and matching networks for com- areas: electrically short antennas and their matching net- pact arrays with enhanced directive gain (superdirective arrays); works; feed networks and elements for superdirective and feed networks for millimeter-wave arrays. Preliminary ex- antenna arrays; and millimeter-wave array feed networks. perimental results are presented on YBaCuO and TIBaCaCuO In part to demonstrate the enhancement in radiation 500 MHz half-loop antennas that show an increase in radiation efficiency (compared with a copper antenna at the same temper- efficiency possible, we present results on two half-loop ature) by a factor of 5 for the HTS antennas. HTS antennas operated at 500 MHz. We then mention Keywords-High temperature superconductivity; antennas; the possibility of integrated passive feed networks of electrically small antennas; antenna feed networks; antenna components that provide signal processing in the spatial, arrays; superdirertivity temporal, and frequency domains. We end with a brief discussion of practical considerations. This paper is one in I. INTRODUCTION a lengthening series of papers examining the potential antenna applications of HTS’s [21-[4]. N this paper we examine the potential of high-temper- Iature superconductors (HTS’s) for antenna system ap- 11. ELECTRICALLYSHORT ANTENNAS plications. The reduced surface resistance, R,, of high- temperature superconductors has the potential to lower A. Antenna Efficiency the insertion loss of RF and microwave devices by many Fig. 1 is the equivalent circuit of an antenna connected dB’s. The loss suffered between the transmission line to a load. The resistance R, accounts for conductor and input/output port and the radiation “ports” of an an- dielectric losses in both the antenna and the matching tenna system can be substantial, and HTS components circuit. When conjugately matched for maximum power can lead to a performance enhancement that justifies the transfer such that XL= - X, and RL = R, + R,, the cost and complication of cooling. radiation efficiency is given by Our primary frequency range of interest is about 1 MHz = radiated power/input power to 100 GHz. Although the antennas that have been devel- q oped from low-temperature superconductors have oper- = P,/Pi,=R,/(R,+ R,). (1) ated at frequencies substantially below this range in the 3 to 300 Hz frequency band [l], these antennas are most The input power is calculated at the input to the match- properly viewed as a type of magnetometer. They rely on ing network, rather than the antenna terminals. the very low noise properties of Josephson junctions, Because HTS material reduces the conductor losses in which have yet to be demonstrated reliably in HTS mate- R,, the antennas that benefit from HTS materials are rial. Our focus is on the passive components in antenna those for which R, > R,. The only class of antennas for systems. Active components, such as variable phase which this is generally true are electrically small antennas, shifters, attenuators, and receivers integrated with the i.e., antennas with dimensions that are small compared with the wavelength of operation. Resonant and aperture Manuscript received December 18, 1990; revised April 30, 1991. This antennas usually have R, >> R,. work was supported by the Office of Naval Research and the Defense The bandwidth of the conjugately matched antenna Advanced Research Projects Agency. The authors are with the RF and Microwave Technology Branch, with center frequency wo is given by Naval Weapons Center, China Lake, CA 93555. IEEE Log Number 9101404. B = WO RL / X, = WgR ,/ X, + OoRd / X, . ( 2) US.Government work not protected by U.S. copyright Ill 11111 I11111 I lllllllllllll I I11111 111111111 lull 1111 DINGER et al.: SURVEY OF POSSIBLE PASSIVE ANTENNA APPLICATIONS 1499 Load An ten na reactance @ reactance Radiation Load A resistance resistance @ Loss Feed pointy resistance / I Twin-lead with dielectric jacket IS1 Fig. 1. Equivalent circuit of an antenna connected to a load. 0.01 x The quantity B, = woR,/Xa is the radiation bandwidth, which is the “intrinsic” bandwidth established by the 0.012 x configuration of the antenna and which is independent of the material from which the antenna is made. A feature w of electrically small antennas is that X, is large and R, Transmission line cross-section small, so that the radiation bandwidth is narrow. Combin- Fig. 2. Dipole antenna with single-stub matching network. The dielec- ing the definition of B, with (1) and (2) yields tric constant of the jacket is E‘ = 3.586,. 7B= B,. (3) Antenna length = 0.2 wavelength Hence, the product of the efficiency and the overall 1.0- . ’ .....’1 . ‘..‘..I ’ . ..... antenna bandwidth is a constant. Clearly, any increase in Loss Tangent efficiency brought about by using HTS material will pro- - o.ooo1 duce a narrowing of bandwidth. The radiation bandwidth 0.8 - ------I 0.oOoJ can often be increased for a given size antenna by such C ..1..1.1..1.. 0.001 techniques as the top loading of monopoles [51, but there 0 is a fundamental limit to B, established by the size of the antenna [61. c L---.”.---.-----..“.lh...~~ .-0 0.4 B. Example: A Short Dipole Antenna U and Matching Network - To assess the potential improvement in efficiency that 0.2 is possible by replacing the normal conductors with HTS’s, I . .....I . ....I. we have analyzed in detail the electrically small antenna 0.0 . shown in Fig. 2. A short dipole of overall length L is lo4 lo3 lo-* driven from a balanced twin-lead transmission line. A Surface Resistance (ohms) shunt short-circuit stub is used to match the impedance of the transmission line to the antenna. The twin-lead trans- Fig. 3. Radiation efficiency for the dipole and antenna and matching network of Fig. 2 for three dielectric loss tangents. mission line is enclosed in a dielectric, which is required in a practical application both for support and efficient cooling of the conductors. The transmission line parame- 0.2h, with the dielectric loss tangent as a parameter. The ters were selected to give a practical size in the 100 MHz three-decade range of R, shown encompasses the surface to 1 GHz frequency range and to produce a 50 1R resistance values that can reasonably be expected from impedance with a reasonable value of dielectric constant. improved HTS material. As R, is lowered, the efficiency The antenna and the matching network in Fig. 2 are improves as expected, but it levels off at the value of R, probably a feasible configuration for a practical HTS where dielectric losses begin to dominate. This value antenna; indeed, it is similar to the HTS antenna for occurs when the dielectric attenuation constant, ad,be- which measurements were presented in [7]. However, this gins to exceed the conductor attenuation constant, a,. configuration also serves as a surrogate for a wide variety Unless the loss tangent is lop4 or less, only a rather of electrically short antennas, such as a wire monopole modest improvement in radiation efficiency can be real- driven by coax line and a printed circuit dipole driven by ized. a microstrip transmission line. Fig. 4-is a plot of radiation efficiency as a function of We have discussed this calculation in [8]; here, we dipole length. A radiation efficiency approaching 100% summarize the results. In Fig. 3 we plot the efficiency as a appears to be possible with a dipole as short as O.lh. The function of the surface resistance for a dipole length of bandwidth-efficiency trade-off of (3) is demonstrated in - -1 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 39, NO. 9, SEPTEMBER 1991 Rs = 0.1 mR Loss tan 0.001 100 Ua -0 .-P cn U) 6 L 10 a 3 0 P -a e .-0 1 -0 a 4 I .. u.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4 Dipole Length L (wavelengths) Dipole Length L (wavelengths) Fig. 4. Efficiency versus dipole length. Fig. 6. The fraction of power dissipated in the indicated portions of the antenna and matching network. The portion labeled “line” refers to the section of transmission line between the stub and the antenna. C. Experimental Results: A Half-Loop Antenna Preliminary measurements have been taken at 500 MHz on two antennas with the design shown in Fig. 7. The antennas have an HTS film patterned on a LaAIO, sub- strate to form a half-loop over a ground plane. The half-loop on the top half of the substrate protrudes through a slot in a copper ground plane. The coupled-line section is a matching network that transforms the input impedance of the antenna (Zi, = 0.1 + j80 Q) to the 50 Q impedance of the feed line. Two different versions of the antenna have been mea- sured to date. One antenna is fabricated from YBaCuO, with its matching network ground plane fabricated from another film of YBaCuO positioned behind the antenna .U1 .1 1 substrate in a sandwich structure.
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