High Temperature Permittivity Measurements of Alumina Enhanced Thermal Barrier (AETB-8) Material for CEV Antenna Radomes

Carl H. Mueller(1) and Félix A. Miranda*(2)

(1) Mission Solutions Group, QinetiQ North America Operations, Cleveland Ohio 44135 (2) NASA Glenn Research Center, Cleveland, Ohio 44135

*E-mail: Felix.A.Miranda@nasa.gov Abstract. Alumina Enhanced Thermal Barrier (AEBT-8) material is currently being considered as a potential radome material for phased array antennas that will be conformally mounted to the outer surface of NASA’s Orion Crew Exploration Vehicle. Accordingly, there is a need to establish if the dielectric properties of the AEBT-8 material change as a function of temperature. In this paper we discuss the measurements of the real (İr’) and imaginary (İr’’) parts of the dielectric constant of AEBT-8 at temperatures from 25- 900oC and in the frequency range from 2-18 GHz. Our results show that neither İr’ or İr’’ showed a measurable temperature dependence when tested in the aforementioned temperature range. Comparison at room temperature to measurements taken using the waveguide technique showed good agreement for εr’, but the εr’’ values for the AETB-8 material are so low that diffraction phenomena limits the accuracy of the results obtained using the free space technique.

Introduction

In anticipation of the retirement of the current Space Transportation System (better known as the Space Shuttle) at the end of 2010, NASA is actively working towards the development of the Crew Exploration Vehicle (CEV) also known as Orion. Phased array antennas, conformally mounted to the outer surface of the Orion vehicle, will be part of the communication systems supporting the CEV. Thermal protective systems (TPS) materials (i.e., radome materials) will be used to protect the aforesaid antennas. A material known as Alumina Enhanced Thermal Barrier (AEBT-8) is being considered as one of the TPS materials. Consequently, there is a need to establish if the dielectric properties of AEBT-8 change as a function of temperature. In this paper we discuss the measurements of the real (İr’) and imaginary (İr’’) parts of the dielectric constant of AEBT-8 at temperatures from 25-900oC and in the frequency range from 2-18 GHz. The experimental approach to perform these measurements as well as the obtained results is discussed below.

Experimental The dielectric properties of AEBT-8 are measured using a High-Temperature Free-Space Dielectric Permittivity Measurement Test Bed (HTFS-DPTB). Pictures of the abovementioned test bed are shown in Figure 1. The operational principle of the HTFS-DPMP-TB is based on the creation of a free-space microwave signal propagation pathway, wherein the signal that is transmitted from a microwave source or transmit antenna (TxA) (e.g., a double-ridged waveguide horn antenna) is focused to a narrow beamwidth midway between two lens antennas, and collected at a receiving antenna (RxA)[1]. By measuring the phase and attenuation caused by the material under test (MUT) positioned at the focal points of the two lens antenna, the dielectric properties of

the MUT can be determined. The test bed measures the magnitude and phase of the transmission scattering parameter (S21) from which İr’ and İr’’ are then calculated. The presence of a surrounding furnace provides for measurements to be taken while the material is exposed to elevated temperatures. The engineering specifications of the HTFS-DPTB are as follows. The lenses are 30.5 cm diameter and 25.4 cm thick at the center; the separation distance between lenses is 125 cm and the distance from the TxA and RxA from each lens is 33 cm. The 30.5 cm long furnace is a commercial Watlow model HVS108A06-set semi-cylindrical ceramic fiber heater, with a 16.5 cm diameter aperture through which the signal propagates [2]. Heat is contained in the furnace using two disks fabricated from AETB-16, which is the same material as AETB-8, but with higher alumina fiber content [3, 4]. The input and return microwave signals, from and to the HP 8510-C Automatic Network Analyzer, respectively, are transmitted through coaxial cables (2-18 GHz range). Prior to taking HTFS-DPTB measurements, room temperature dielectric measurements of AETB-8 were taken using the waveguide measurement technique [5]. The advantages of the waveguide technique are that the microwave signal propagation pathway is completely defined, and samples can be machined to precisely fit the waveguide. It is also straightforward to perform transmission/reflect/load (TRL) calibrations over the entire frequency range of the waveguide system. As such, highly accurate S-parameter data are generated, and εr’ and εr’’ can be precisely calculated. The key disadvantage is that the waveguide approach is not amenable to high temperature measurements.

Measurement Results Using a WR430 waveguide system to perform measurements in the 1.7 – 2.6 GHz range, εr’ and εr’’ values of 1.13 and 0.0009 were measured at room temperature. Measurements were also performed at room temperature using a WR90 waveguide system in the frequency range 8.4 – 10.6 GHz, and εr’ and εr’’ values of 1.13 and 0.0010 were obtained. These data indicate that at room temperature, εr’ for AETB-8 is very low, and does not vary with frequency. For temperatures above 25oC, the S21 response of the HTFS-DPTB system is calibrated from 2 – 18 GHz. A benefit derived from the wide measurement bandwidth is that narrow time domain gating can be employed to minimize errors caused by multipath reflections. The permissible time domain gating is calculated using:

B2.1=τ (1)

Where τ is the minimum time domain gating span, B is the measurement bandwidth, and 1.2 is a constant applicable to the minimum time domain filter[6]. For these measurements, the bandwidth is 16 GHz, hence τ = 75 picoseconds. The τ value used is 100 picoseconds. The transmitted signal (T) is given by:

)(exp dT γ−= (2)

Where γ is the propagation constant (γ=α+jβ; α is the attenuation constant and β is the phase constant), and d is the sample thickness. Since the sample is non-magnetic, γ = (εr’ – j εr’’)1/2. Since we have established that AETB-8 is a low-loss material with εr’ close to that of free space, it was also assumed that the reflection coefficient (Γ) = 0 at the free space/sample boundary. Consequently, S11 data are not needed to calculate εr’ and εr’’. Because it is difficult to perform wideband S11 calibrations using the HTFS-DPTB system, the Γ=0 simplification is fortuitous and a key step towards using very narrow time domain gating to minimize multipath reflections. The calculated values of εr’ as a function of frequency, taken at temperatures from 25 – 900oC, are shown in Figure 2 (a). Date taken using the HTFS-DPTB system, show no evidence that altering the temperature has any impact on εr’. The data corresponding to the magnitude of the transmission coefficient (S21) as a function of frequency, at 25 – 900°C, are shown in Figure 2 (b). Temperature has no discernible impact on the magnitude of the transmitted signal. It’s noteworthy that the magnitude of S21 is slightly greater than 1.0 at all measurements. This phenomenon was repeatedly observed in a number of samples with similar or smaller sample diameters, and is attributed to diffraction near the sample edges[7, 8]. Figure (1.b) is a photograph showing the sample mounted in the furnace. The diameter of the sample is considerably smaller than that of the entry and exit ports (11.5 cm versus 16.5 cm), hence diffraction of microwave power near the sample edges is possible. Larger diameter samples that completely fill the furnace aperture do not show S21 greater than 1.0. Data showing phase as a function of frequency, at various temperatures, are shown in Figure (2.c). The linear phase versus frequency profiles indicates that the dielectric properties of AETB-8 do not change over the frequency and temperature ranges measured.

Conclusions The dielectric properties of Alumina Enhanced Thermal Barrier (AEBT-8) material have been investigated in the temperature range from 25-900oC and at frequencies from 2-18 GHz. Our results show that neither İr’ or İr’’ showed a measurable temperature dependence, when tested in the aforementioned temperature range. Comparison at room temperature to measurements taken using the waveguide technique showed good agreement for εr’, but the εr’’ values for the AETB-8 material are so low that diffraction phenomena limits the accuracy of the results obtained using the free space technique.

References

1. V.V. Varadan, R.D. Hollinger, D.K. Ghodgaonkar, and V.K. Varadan, “Free-Space, Broadband Measurements of High-Temperature, Complex Dielectric Properties at Microwave Frequencies,” IEEE Trans. Inst. and Meas., vol. 40, p. 843-846 (1991).

2. Watlow Electric Manufacturing Company (http://www.watlow.com). 3. http://www.nasa.gov/centers/kennedy/pdf/167473main_TPS-08.pdf 4. http://www.tpub.com/content/nasa2000/NASA-2000-tm210289/NASA-2000-

tm2102890046.htm. 5. Measuring Dielectric Constant with the HP 8510 Network Analyzer: The

Measurement of Both Permittivity and Permeability of Solid Materials, HP Product Note No. 8510-3.

6. Time Domain Analys(Agilent, 2007)

7. R. Grignon, M.N. AfsaComplex Dielectric PerMeas. Tech. Conf., p. 8

8. K.M. Hock, “Error CorMicrowave Measureme(2006).

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