Cornelis G.M. van ’t Klooster, Vladimir V. Parshin, Evgeni A. Serov, A.B. Smolders
IMAGE LICENSED BY INGRAM PUBLISHING
System requirements can dictate the use of antenna reflector materials that are not electrically optimum and clearly not “perfectly electrically conducting.” This makes loss measurements necessary. A loss that is not acceptable for one application might be acceptable for another application. Instrument or radiometer antennas require a low loss, but, sometimes, this could be less critical for telecommunication antennas. We investigated less conventional materials, including a nickel-based material, for a radio telescope antenna (Atacama Large Millimeter/submillimeter Array [ALMA]) and other materials as needed in certain space antenna applications. A sensitive open resonator was used. The data collected provide an understanding of and insight into the loss properties for the respective applications. This has led to useful models confirmed by the data.
A low reflection loss is needed for radio telescope and space antenna reflectors. Structural precision, thermal, and space environmental constraints can dictate specific materials. Then, the loss must be known. Investigations were carried out with the Institute of Applied Physics (IAP), Nizhny Novgorod, Russia, in a collaboration that has led to joint publications over a period exceeding 10 years, with presentations at various locations.
The IAP is involved in advanced research and the production of high-power microwave devices, comprising gyrotron and related high-power guided-wave and quasi-optical components. A low loss is mandatory for quasi-optical mirrors (reflection) and windows (transmission); e.g., at the output port of a gyrotron, a fraction of a percent as loss can be disastrous for materials in megawatt applications. The IAP has developed sensitive test facilities including an open resonator for material testing and research at low, ambient, or high temperatures. Such equipment also allows spectral line research and atmospheric science as well, to emphasize a sensitivity. The IAP facility, test methods, and applications are described in [1], [2], and [3]. For reflection loss measurements, the open resonator has been used in a reflection modus.
The principle goes back to early laser developments and resonators [4], [5], [6], [7], [8], [9], [10]. A familiarity is assumed with quasi-optical techniques. A basic paraxial mode (the Gaussian “ground” mode) is exploited as a mode in a Laguerre expansion between two spherical mirrors with a very low reflection loss, providing a high-quality-factor (high-Q) resonator. Higher order modes are not desired. Adaptation for reflection or for transmission is possible to inspect a sample under test. The reflection loss is derived from a quality comparison for an unloaded and a loaded resonator based on frequency measurements only. This is elaborated in the supplementary information (available at https://doi.org/10.1109/MAP.2022.3203295). A use of dry nitrogen ${(}{N}_{2}{)}$ gas as medium under a plastic cover is convenient (Figure 1). It reduces effects due to other atmospheric components, although an atmospheric model can be used to assess the effects based on pressure (p), temperature (T), and water vapor pressure (e) [1].
Figure 1. An open resonator facility. (Source: IAP; used with permission.)
The band for testing was 100–200 GHz initially. Later, it was extended to 360 GHz, as indicated in the results. The model information discussed can be of further interest for metalized reflectors; a thin-film aluminum (Al) layer on top of carbon fiber-reinforced polymer (CFRP); or, in general, for two-layer metal combinations.
The first material was for an ALMA reflector; it is based on nickel (Ni). This led to investigations of other less conventional materials for space applications.
The Ni electroformed material for high-precision sub-millimeter-wave (sub-mm-wave) reflector panels is new. A ${3}{\times}$ higher resistivity ${(}{\rho}{)}$ of pure Ni compared to pure Al combined with an ultrafine surface roughness and metal coating made reflection loss tests valuable to support ALMA panel developments. A compliance with electrical system allocations for loss was provided by the Max Planck Institute for three frequencies in the ALMA band (at 300, 600, and 1,200 GHz); see [11]. High-frequency resolution and an accuracy of ${<}{0.5}\,{\times}\,{10}^{{-}{3}}$ was exploited in the 100–200-GHz band available at the IAP. The supplementary information (available at https://doi.org/10.1109/MAP.2022.3203295) provides more detail. Today, a Ni technology is used in 25 ALMA antennas and allows observation in directions very close to or into the sun.
Mass, high structural accuracy, and strength suggest CFRP for space reflectors. However, CFRP has a resistivity ${\rho}$ that is higher than that of the usual metals and, thus, a higher loss. Different woven CFRP fiber patterns and variations in ${\rho}$ give a variability in the surface resistance and, thus, in the loss. Wavelengths below a few centimeters suggest a metal coating to reduce loss. Different methods exist, for example, thin vacuum-deposited aluminum (VDA), thick VDA, or a metal film. Models are elaborated on and compared with the data.
An instrument sub-mm-wave antenna can be mounted inside a baffle. The baffle material must have a low loss at (sub-)mm-waves to prevent additional thermal noise due to the loss. A baffle material used for the Infrared Space Observatory (ISO) [12], a European Space Agency (ESA) mission, appears to cause high loss at (sub-)mm-wave frequencies.
The ultrastable CFRP reflector for the Planck mission [13] has a thick Al coating (a VDA process). Precise tests as a function of temperature provide the reflection loss. An equivalent surface resistivity ${(}{\rho}_{\text{equiv}}{)}$ is informative in a model (as for ALMA). The Planck reflector has operated at a low temperature in space at ∼70 K. A precise assessment of the test facility and the tests are demanding. The accuracy has to be assessed and depends in part on the sample under test. See [1] and the supplementary information (available at https://doi.org/10.1109/MAP.2022.3203295).
Pure titanium (Ti) has a larger ${\rho}$ value than Ni. A Ti-6Al-4V alloy has an even higher ${\rho}$, with, moreover, a nonlinear function of temperature [14]. Ti-6Al-4V is used for a high-gain antenna reflector for the ESA Bepi–Colombo mission. CFRP cannot be used at the high temperatures predicted for Bepi–Colombo [15]. A good agreement has been found with a model, which was not expected for a metal alloy. Ti-6Al-4V has also been used for the ESA mission Solar Orbiter, which uses only the X band (not the X/Ka band as for Bepi–Colombo).
Table 1 summarizes the materials, which are identified in the first column. The last column indicates earlier publications, now further elaborated. The model information is revisited here to make the data survey more complete and useful for other users and modelers [16]. A VDA process gives a controlled Al metal layer; its thickness is important. Novel loss models for a Ti alloy and for the coating are given with thickness as a parameter. An equivalent resistivity ${(}{\rho}_{\text{equiv}}{)}$ derived from the test data is informative as well. Space missions of the ESA are mentioned; https://www.esa.int gives more information.
Table 1. The types of materials investigated.
The “General Loss Aspects” section introduces the classical loss estimation for a metal. The “Tests of Various Reflector Materials” section gives discussions along with data and model information. The “Thin Film on Top of CFRP” section elaborates on models, in particular, with a second layer. This is followed by the “Concluding Remarks” section.
The supplementary information (available at https://doi.org/10.1109/MAP.2022.3203295) outlines how Ni-based panels were developed and selected for ALMA antennas, how reflection loss is found from resonator tests using precise frequency measurements only, and how a ground mode can be discriminated. It gives valuable notes for accuracy and model aspects. It is suggested that attention be given to Figure 7S in the supplement and the suggestions for potential further activities.
Maxwell’s equations provide an ideal estimate for the loss [26] when ${\rho}$ is known. It is quantified as the power ${(}{1}{-}{R}{)}$ dissipated in a sample upon reflection. We have a resulting loss relation, which is also referred to as the Hagen–Rubens law [1], [27], with R as the reflected power: \[{\text{Reflection loss}} = {1}{-}{R} = \frac{4{R}_{s}}{{Z}_{0}} = \sqrt{{8}{\epsilon}{\omega}{\rho}} = \sqrt{\frac{{8}{\epsilon}{\omega}}{\sigma}}{.} \tag{1} \]
To maximize Q at resonance, a resonator like that at the IAP has low-loss silver-coated (Ag) spherical mirrors (Figure 1). A use of a dry ${N}_{2}$ gas environment helps to handle the impact of a medium.
The diffraction loss and the loss due to the thin dielectric film for coupling must both be kept small and calibrated; see the supplementary information (available at https://doi.org/10.1109/MAP.2022.3203295) and [1]. Units from the International System of Units (SI) are preferred, but centimeter–gram–second (CGS) units are also used to describe electromagnetic relations in convenient formulas [28]. The conductivity ${(}{\sigma}{)}$ would require a conversion factor ${1} / {4}{{\pi}{\varepsilon}}_{0}$. The Hagen–Rubens law (SI units) is then given in (1) [26], assuming ${\epsilon} = {\epsilon}_{0}$ and ${\mu} = {\mu}_{0}$, with SI and CGS units for ${\sigma}$ in (2) [27]: \[{\text{Reflection Loss}} = {1}{-}{R} = {2} \sqrt{\frac{{2}{\epsilon}{\omega}}{{\sigma}_{\text{SI}}}} = {2} \sqrt{\frac{f}{{\sigma}_{\text{CGS}}}}{.} \tag{2} \]
Conductivity data in CGS units are large numbers and not normally found in engineering data: a preference for SI units is noted. The loss is for a normal incidence of the field on the sample. A modified setup allows measurements at a 45° incidence angle [29], convenient for inspecting transverse electric and transverse magnetic fields and differences in polarization, but it is not considered here. In an ideal world, (2) is useful. However, it also helps to derive an effective resistivity ${\rho}_{\text{eff}}$ from the test data, which can contribute to further insight into the loss mechanism.
Investigations at the IAP of the electroformed Ni panel developed for ALMA are first addressed. Results, discussions, and model aspects are provided for the other materials in Table 1.
The Max Planck Institute (Bonn, Germany) has tested samples for the European Southern Observatory [30] at 300, 600, and 1,200 GHz. It has shown a compliance with the ALMA requirements [11] (≤2% with ± 1% error).
Here, we discuss the development tests. A small-scale roughness is used, described with roughness parameter Ra, to scatter sunlight (visual and infrared). Ni has a thermal absorption ${\alpha}$, which can lead to a temperature increase and, thus, a thermal deformation. A metal coating reduces an absorption ${\alpha}$. Three sets of samples were investigated with the IAP. We discuss only the results for the first and third sets, which are considered the most important. The second set is not discussed; some information can be found in [17].
Figure 2 shows the data for 10 Ni samples [17], [18]. Two sample pairs (Ni2 and Ni3 as well as Ni8 and Ni9) are shown with one curve for each pair. A theoretical loss is added (1) for Ni, Al, and an Al–Mg–Si alloy. The latter alloy is used in Al constructions, and it has a higher ${\rho}$ than pure Al. Ag or pure Al is preferred for resonator mirrors, not Al alloy 6082, leading to a lower Q. Roughness (Ra) is a measurable quantity used in machining—an arithmetic average over a path length or area. A quantity Rq is encountered as the average of a root of a sum of quadratic deviations (root mean square [RMS]). Rq is not related to Ra and usually has a larger value than Ra. We have ${Ra}\,{<}\,{0.3}{\mu}$ to scatter visual wavelengths at the ALMA panel. It is ${Ra}\,{<}\,{(}{1} / {1,000}{)}{\lambda}$ at the highest ALMA frequency, which is smaller than the usual tolerance for antenna reflectors at the operational frequency. The IAP has shown that Ra should not exceed a skin depth ${\delta}$ [20] (copper [Cu] samples).
Figure 2. The reflection loss for 10 samples to support ALMA panel development [17].
A modeling tool (CST) uses Rq (RMS); recall that Rq > Ra. The CST model is based on the work in [31]. The smallest value for Rq (RMS) as used in the CST model exceeds a skin depth of ${5}{\times}$ at 100 GHz; thus, ${Rq} / {\delta}\,{\approx}\,{5}$ at 100 GHz [32]. This CST model cannot be used for a small roughness as encountered here. Further work is needed to secure the handling of a very small roughness in CST. Data obtained with an open resonator have provided a nearly linear relation when ${Ra}\,{<}\,{\delta}$. Resonator test data with ${Ra}\,{>}\,{\delta}$ can show either a higher or a lower loss value [20].
The data in Figure 2 do not correlate with the physical sample parameters. The nonplanarity of a sample is noted from its frequency response (Ni1 and Ni6): the data do not follow an expected square root tendency (1). An Al coating gives a loss reduction for Ni2, Ni3, Ni7, and Ni10 as expected, compared to Ni (Ni8, Ni9, Ni4, and Ni5). An Al coating is less durable than the preferred Rh coating. Figure 3 shows data for Al samples with a loss higher than expected, possibly due to the use of another type of Al material. The curve of an Ni sample with the highest loss (Figure 2) is shown in Figure 3: a blue curve for Ni5 with extrapolation up to 250 GHz. The sample with the highest loss (worst case) would have a loss of ${\approx}\,{0.5}{\%}$ at the lowest test frequency in [11] (at 300 GHz). Note that it is ${2}{\times}$ less than the final value reported in [11, Table 2] for the worst sample. Predictions are given for pure Al (dotted blue) and Ni (dotted red) in Figure 3, using (1). A comparison was made with an ESA–European Space Research and Technology Centre (ESTEC) facility [33]. However, at ESTEC, the error, ${\sim}\,{\pm}{0.4}\,{\cdot}\,{10}^{{-}{3}}$ at that time, was too large compared to the value to be measured.
Figure 3. The ×10–3 reflection loss for four samples: Al(a), Al(b), Al(c), and Ni5. Ni5 has been extrapolated from [17].
A panel technology was selected with an adapted surface roughness Ra and thin Rh coating. Rh has good thermal–optical properties and gives protection over the lifetime for an environment at an altitude of ${\approx}\,{5,000}{\text{ m}}$ in the Atacama desert in Chile. The ${\rho}$ of Rh is lower than the ${\rho}$ of Ni; this implies some surface resistance reduction. However, there is a small increase in the loss due to the surface roughness. Figure 4 shows the results for a subset of samples. An equivalent ${\rho}_{\text{eff}}$ (Figure 5) is derived from the curves in Figure 4. Note that ${\rho}_{\text{eff}}$ is rather constant with frequency, as it should be (1). The ${\rho}_{\text{eff}}$ in Figure 6 is derived from Figure 7 for a second subset of the third set. The roughness Ra is smaller than the skin depth ${\delta}$ for Ni at the highest test frequency. Now, ${\rho}_{\text{eff}}\,{\approx}\,{2}{\times}$ ${\rho}$-{nominal} for pure Ni. The curves are nearly flat in Figure 6. A slight increase is noted for Ni9 and Ni10. A loss of ${≤}\,{\sqrt{2}}{\times}$ the expected loss from (1) is better than a factor of ${2}{\times}$, as mentioned in [8]: our factor is ${<}\,{\approx}\,{\sqrt{2}}{\times}$ for our data. The results for all ALMA samples are good indeed (for 100–200 GHz). Extrapolation for a “worst” Ni sample has provided a single-point comparison with [11]. A loss of ${\approx}\,{0.5}{\%}$ at 300 GHz for our worst sample is ${2}{\times}$ better at 300 GHz than the value in [11] for the final ALMA samples.
Figure 4. The loss values for the first subset of six samples (the third sample set).
Figure 5. The effective ${\rho}$ derived from the loss values for six samples. Two calculated results for Ni and Al are added.
Figure 6. The ${\rho}_{\text{eff}}$ values from Figure 7 and for pure Ni (dotted). Note that the scale is at 10–7.
Figure 7. The measured reflection loss for the second subset of five samples from the third sample set, supporting ALMA panel development.
CFRP samples with and without metallization were tested. The back side of a CFRP sample gives data for bare CFRP. A higher loss occurs for CFRP with thin metallization for a polarization perpendicular to the top-layer fibers, and a lower loss occurs for a polarization parallel to the fiber direction. The blue and red curves in Figure 8 show such an effect. The cause is that the metallization (VDA) is too thin. The red curve shows a lower loss toward a higher frequency. The skin depth decreases for a higher frequency; thus, there is a thicker conducting layer in terms of the skin depth. However, the surface impedance is complex, with anisotropy. The green and purple curves in Figure 8 show the loss data for bare CFRP with the polarization in the fiber direction measured at two different test positions on a sample. Formally, (1) cannot be used accurately. It is used for plane surfaces with some plane symmetry and for a polarization direction perpendicular and parallel to the fibers.
Figure 8. The metalized CFRP (with polarization perpendicular and parallel to the fibers) and bare CFRP (with polarization parallel to the fibers). A dependence on the fiber direction is noted. Not shown is the response for a polarization perpendicular to the fibers, which approaches ${\approx}\,{30}{\%}$ for bare CFRP [19] in this frequency range.
The two directions (perpendicular and parallel) were tested; this led to indicative data for ${\rho}_{\text{eff}}$. The result is shown in Figure 9. For bare CFRP, we have ${\rho}_{\text{eff}}\,{\approx}\,{1,800}\,{\cdot}\,{10}^{{-}{8}}{\Omega}{\text{ m}}$ derived from Figure 8, qualitatively comparable to the results in [34], for a polarization parallel to the CFRP fiber direction. The ${\rho}$ for CFRP is higher than that for metals. We can still neglect a dielectric displacement current compared to a conductive current. A prediction can be made for CFRP coated with a thin metal layer on top [22] for a polarization parallel to the fiber direction using a simple model, which is elaborated in the next section. CFRP composites can be different; thus, ${\rho}$ also differs: one needs to measure the loss.
Figure 9. The derived ${\rho}_{\text{eff}}$ for bare CFRP with polarization parallel to the fiber direction.
Figure 10 shows ${\rho}_{\text{eff}}$ derived from Figure 8 for metalized CFRP using (1). Note a dependence on the direction of polarization (perpendicular or parallel to the CFRP fibers). Such effects have also been reported in [35]. The slope is negative (red curve) because the thickness of a (thin) metal coating increases with the frequency in terms of the skin depth ${\delta}$; thus, there is lower loss at a higher frequency. Thicker metallization is needed for a more equal response for two orthogonal polarizations.
Figure 10. The ${\rho}_{\text{eff}}$ for a metalized CFRP sample with polarization perpendicular to (red) and parallel to (black) the fiber direction.
Potential baffle materials were tested [17]. These were used in the past for optical satellite instruments. Samples “Oa” and “Ob” differ. (There were two of each; no further public information is available). Such baffle (or “shroud”) materials for an optical instrument show loss in the microwave regime, much higher than that for pure Al. Figure 11 shows a higher loss for type Oa than for type Ob. Thus, Oa would induce more noise than Ob [17]. Samples Oa1 and Oa2 differ for the two Oa-type samples. Type Ob samples (Ob1 and Ob2) show better agreement and lower loss.
Figure 11. The baffle material indicated with Oa and Ob (sample size each 70 × 70 mm).
A third baffle material is from the ISO [12]; it is very shiny and has a thin coating on top. The reflection loss is shown in Figure 12 for the shiny surface. The presence or absence of a thin coating makes a very small difference. The high loss for the shiny side is noted in Figure 12. A measurement of the back side of the ISO baffle sample led to an unexpected result. The blue curve in Figure 13 shows a low loss for the back side (which has a dull gray visual appearance). The ${\rho}_{\text{eff}}$ follows with (1). It is close to the ${\rho}$ of pure Al. The ISO material is very suitable for a baffle for a radiometer antenna (but for 100–200 GHz), provided its back side is used. This can easily conflict with thermal–optical requirements. (The thermal absorption ${\alpha}$ is potentially higher due to its “gray” appearance; it needs to be tested.) For cold temperatures, Al must be pure to avoid a “rest” resistivity at low temperatures [36], [37]. The ISO material has also been tested at the University of Stuttgart (with a low loss [21]).
Figure 12. The reflection loss for the ISO baffle material, shiny side, originally used for the optical regime.
Figure 13. The ISO baffle material, back side (Al), and CFRP samples with VDA.
Figure 13 shows the early results for metalized CFRP. Other Planck samples have been measured for 110–200 GHz. More signal sources are needed (synchronized backward-wave oscillators) to cover a wider band [1], [3]. The data in Figure 13 are for an earlier technology for 110–200 GHz. The results for samples of a related technology are in Figure 14 for a wider band (110–360 GHz). The samples have been “guiding” samples in a vacuum chamber during the VDA process of the first Planck reflector for monitoring the VDA process. The thickness of VDA is not known. One sample is indicated as being “a bit yellow” ${(}{S}_{1}{)}$. Two other samples showed a nearly identical result (identifier ${S}_{2})$ in Figure 14. Curve ${S}_{4}$ is for an accurate Ag-coated bronze mirror of the IAP [1], [20]. Ag could be used on top of a CFRP reflector, but Al gives a lower loss below ∼90 K, and it would require protection to prevent Ag oxidation.
Figure 14. The Planck–Herschel technology for three VDA CFRP samples (S1 and S2). Two samples showed a comparable loss (S2). S4 is a low-loss Ag sample from the IAP.
An automated test process gives data for consecutive resonance curves [3] from which the “dots” are derived as shown in Figures 13 and 14 (now with an accessible wider frequency band). A small systematic variability is noted for the curves for ${S}_{1}$ and ${S}_{2}$ close to 183 GHz. This could be due to proximity to an H2O line, but its effect is small. Statistical deviations have to be accounted for. Sufficiently thick, pure Al is needed in VDA to mimic the property of pure-Al bulk material (Figure 13).
The medium, usually ${N}_{2}$, is monitored. It is important to know the impact of the medium in resonator tests. The use of Ag-coated spherical mirrors gives a high Q. An Al alloy 6082 or Al alloy 6061 mirror would reduce the sensitivity of a resonator compared to low-loss Ag-coated mirrors by nearly ${2}{\times}$ at room temperature. This impacts the results; an example is found in [38], where an Al alloy was used, thus leading to a lower Q. The following references are informative: [1], [3], and [39].
A sample is positioned in the resonator with an access on its back side. In addition to the sample support, the back side can be used for cooling or heating; thus, tests over a temperature range are feasible. A thermally conductive support developed by the IAP allows the sample to be cooled with liquid nitrogen for a loss measurement at low temperatures. A low temperature has been reached at 123 K on the reflecting side of the sample. The low thermal conductivity of CFRP causes a thermal gradient (from the cold back side to the reflecting front side). Also, a lateral temperature gradient is present on the reflecting side between a clamped sample perimeter and the active reflecting zone, as the Al layer is thin. The support (Cu) on the back side reaches 85 K. It is with a cold finger in a liquid ${N}_{2}$ vessel. The edge of the sample is at 113 K, and the reflecting center is at 123 K.
In Figure 15, results are shown for three locations—T1, T2, and T3—using three calibrated thermocouples. Figure 16 shows curves derived for the reflecting side of a sample (T3) and also for an Al sample with high purity. An Al sample has good thermal conductivity. It reaches a lower temperature when cooled from its back side. A slight flattening is noticed in the loss curve for Al at low temperatures; this is due to impurities in Al. A parameter to describe the impurity is the residual resistivity ratio (RRR) value, which is a quotient of ${\rho}$ at 300 K and ${\rho}$ at 4 K. The so-called Bloch–Grüneisen law describes it with a ${T}^{5}$ temperature dependency at low temperatures [41]. An open resonator can be used to derive surface resistivity data for good conducting samples, as in Figure 5. The observed effect of RRR has been investigated by the IAP [39], [40]. It can be noted in Figures 16 and 17 that the temperature is <100 K for a particular grade of Al material.
Figure 15. The measured loss of the Planck sample as a function of temperature for 140 and 340 GHz. T1, T2, and T3 are the thermocouple locations.
Figure 16. The reflection loss of the Planck reflector sample and the Al sample as a function of temperature for 141 and 340 GHz. The loss for an Al sample with 0.9985 purity is also shown [40].
Figure 17. The derived effective ${\rho}$, assuming Hagen–Rubens law (1). An Al sample with a purity of 0.9985 is shown, as are data for pure Al and Ag based on [36].
Figure 17 shows derived ${\rho}_{\text{eff}}$ data for samples with ${\rho}$ curves from [36]. The measured data for an Al sample approximate a curve for pure Al. The two Planck CFRP samples may have had a peculiar Al but show a slightly higher loss. The effective ${\rho}$ for the Planck material is different for the two samples, as shown by the red and black curves in Figure 17. The pure, thick Al coating on top of CFRP has a protective coating, which can also have a small influence on the reflection loss. The testing demonstrates that one can observe small deviations. The dotted green and red curves in Figure 17 show the ${\rho}$ values for Ag and Al from [36]. They show that Ag has a lower loss than Al at room temperature but not below 100 K. One could consider Ag for metallization, but only for temperatures above 100 K. Also, Ag requires surface protection as well to prevent oxidation, which could give increased emissivity otherwise due to oxidation (black).
The difference is noted for pure Al and thick VDA in Figure 17. This underlines a need to measure the reflection loss of CFRP with high-quality VDA. Differences can be due to the VDA process, the purity of Al, or a protective coating. It is assumed that pure-grade Al is used in the VDA process. This is important for future missions after Planck. The difference in the loss at room temperature and at 120 K is shown in Figure 18. The data were used for the Planck mission [13]. A test at the end of the mission by the Planck team did confirm our results in Figure 18 during a gradual warming up of the Planck telescope in space: the thermal noise increase in the radiometer front end was monitored in a creative test [42] for this purpose.
Figure 18. The reflection loss of the Planck reflector material as a function of frequency at 120 and 296 K.
The external devices of the Bepi–Colombo satellite are exposed to high temperatures [15]. A high-gain reflector antenna is one of them. It must provide stable communication and the stable transfer of coded time series for scientific purposes with Earth, irrespective of the spacecraft attitude with respect to Mercury and the sun. CFRP cannot be used at a high temperature, so Ti-6Al-4V was selected. Ti-6Al-4V has a high ${\rho}$ that is nonlinearly dependent on the temperature. The ${\rho}$ has a negative slope at high temperatures due to a state change of the Ti alloy [14]. Knowledge of ${\rho}$ as a function of temperature [14] allows the loss to be predicted as a function of temperature. A heating capability was designed by the IAP for testing in a resonator [25], [43].
Figure 19 shows four out of five samples (50 mm in diameter) from the same material batch. The colors have remained more than a decade after testing. The temperature increase of a Ti alloy in an N2 environment leads to the formation of TiN (titanium nitride, of nanometer thickness), which has a yellow color. Ti in contact with oxygen becomes purple to black (TiO3) at elevated temperatures. Different color gradations occur depending on the temperature and energy level. The effect on the reflection loss due to a thin TiN or TiO3 layer is so low that it could not be measured [25], [43]. Figure 20 shows data for five samples at 70 GHz with a prediction using [44]. The loss is quite constant up to 500 °C (the maximum test temperature): from ${\sim}\,{7.2}\,{\cdot}\,{10}^{{-}{3}}$ to ${\sim}\,{7.7}\,{\cdot}\,{10}^{{-}{3}}$. A negative slope for the ${\rho}$ of Ti-6Al-4V as a function of temperature causes the effect (see page 498 from [14]). The loss is ${\approx}\,{2}{\times}$ higher than for ALMA samples. A minor deviation is noted for sample Nr4 in Figure 20. Sample “4” was tested separately; it is from the same batch [43].
Figure 19. The Ti-6Al-4V 50-mm-diameter samples for open resonator measurements.
Figure 20. The reflection loss of Ti-6Al-4V at 70 GHz as a function of temperature. **Sample 4 was tested separately.
A nearly constant loss is noted in Figure 21 at 70 and 158 GHz for Nr4 with a prediction. Figure 22 shows a derived ${\rho}_{\text{eff}}$ for four samples except Nr4 and a curve for ${\rho}$ from [14] and [44]. Small differences might be due to the alloy composition of Ti-6Al-4V. Figure 23 shows the loss for Ti-6Al-4V at ambient temperature. Two backward-wave oscillator sources were used for two overlapping bands (red: 60–120 GHz and black: 105–160 GHz). Extrapolation downward gives the loss in the deep space frequency band of 32–34 GHz for the Bepi–Colombo antenna. A 0.5% loss at 35 GHz is stable over a temperature range.
Figure 21. The Ti-6Al-4V predicted and measured data for 70 and 150 GHz. The prediction uses (1) and data from [44].
Figure 22. The Ti-6Al-4V effective ${\rho}$ (70-GHz data) derived from test data using one. Textbook data are shown (from page 498 in from [14]).
Figure 23. The reflection loss of Ti-6Al-4V as a function of frequency at room temperature (23 °C). The green dots relate to the frequencies found in Figure 21.
Data for a thin metal layer on CFRP are shown in Figure 8. A thin metalized film is used on top of CFRP to reduce the loss for telecommunication or radiometer applications [23]. A metalized film reduces the loss, but the loss is higher than for CFRP with a thick VDA, as used for Planck.
A model for a conductive layer on top of a conductor is used with known ${\rho}$ data. The model is ${\approx}\,{>}{75}$ years old [45], [46]. It was used in [22] to explain test data: consider a metal “1” on top of a conductor “2”, with, respectively, conductivity ${\sigma}_{1}$ and ${\sigma}_{2}$. A surface impedance ${Z}_{s}$ follows as \[{Z}_{s} = {R}_{{s}1}\,{\cdot}\,{(}{1} + {j}{)}\frac{{\sinh}\,{(}{\tau}_{1}{d}{)} + \frac{{R}_{{s}2}}{{R}_{{s}1}}{\cosh}\,{(}{\tau}_{1}{d}{)}}{{\cosh}\,{(}{\tau}_{1}{d}{)} + \frac{{R}_{{s}2}}{{R}_{{s}1}}{\sinh}\,{(}{\tau}_{1}{d}{)}} \tag{3} \] with ${\tau}_{1} = {(}{1} + {j}{)}\,{\cdot}\,\sqrt{{\pi}{f}{\mu}_{1}{\sigma}_{1}}$ in “1” with thickness d, skin depth ${\delta}_{1}$, and surface resistance ${R}_{{s}1} = {1} / {\sigma}_{1}{\delta}_{1} = \sqrt{{{\pi}{\mu}}_{1}{f}{\rho}_{1}}$. The carrying conductor “2” has skin depth ${\delta}_{2}$ and surface resistance ${R}_{{s}2} = {1} / {\sigma}_{2}{\delta}_{2}$. An assumption is made that we have a relative permeability ${\mu}_{r} = {1}$ or ${\mu}_{1} = {\mu}_{2} = {\mu}_{0}$. This is the case for many metals [26] except for ferromagnetic materials. Few data exist for Ni, but this is also correct [47] for frequencies above a few gigahertz and, thus, for ALMA frequencies. The real part of ${Z}_{s}$ is the surface resistance ${R}_{s}$, which gives a loss value using (1). Predictions in [22] were made for a coating on CFRP with published data from [22], [23], [24], [40], and [48]. Figure 24 shows measured data. An increase was recommended in [22] for the thickness d to reduce the loss. It is observable in Figure 25 that it leads to a lower loss. The ${\rho}$ of the carrying conductor is complex (CFRP). The loss for a coated CFRP is higher than for a pure metal, depending on d.
Figure 24. A thin metal coating (Al film) on CFRP. The results for three lateral positions, a variability (low, middle, and high), are shown. A test at 88 K is also shown.
Figure 25. The reflection loss for a thin Al film on top of CFRP. The ${\rho}$ of CFRP is assumed to be ${2,000}\,{\cdot}\,{10}^{{-}{8}}{\Omega}{\text{ m}}$. The horizontal scale is in ${\text{meters}}\,{\cdot}\,{10}^{{-}{7}}$.
A test at 88 K shows a ${2}{\times}$ lower value (IAP). It is important to increase d, but it depends on the applications. This would be important for a reflector for a radiometer antenna. A loss reduction is also obtained at cold temperatures, as shown in Figure 24. It is still a higher loss than for pure Al at that temperature. A thickness increase is of interest for the thin coating on a CFRP reflector.
An example concerns the silver plating of Ti-6Al-4V. Ag reduces the loss, as shown in Figure 26 and as expected from (3). A loss reduction results in ${5}{\times}$ for a top layer of ${0.25}{\mu}$ for an Ag coating at a frequency of 32 GHz. For the ALMA material, it has been partly applicable. A small roughness term gives a comparable increase of loss as an achieved reduction with a thin coating—at least, this is the case for an Rh coating on top of Ni. An Al coating with its lower ${\rho}$ reduces the loss, but an Al coating is not preferred over the durable Rh coating.
Figure 26. The reflection loss at 32 GHz of Ti-6Al-4V coated with an Ag layer as a function of the thickness of the coating layer.
We obtained a loss value of 0.008 in measurements of a thin film on top of CFRP in Figure 24. A predicted quantity for the loss is comparable using (3). A more steep curve is noted when the coating becomes thinner. A prediction for Al on top of CFRP is based on a ${\rho}$ for CFRP of ${2,000}\,{\cdot}\,{10}^{{-}{8}}{\Omega}{\text{ m}}$. Figure 25 shows the results. The curves are quite constant as a function of the frequency already at some thickness d. A steep behavior is observed when it is thinner than ∼50 nm. A stable trend is observed in the data in Figure 24. The results for three different coating thicknesses are shown in Figure 27 for 100–200 GHz. A predicted loss for d = ∼30 nm compares with the measured data in Figure 24. The loss does not change much as a function of frequency.
Figure 27. A thin Al film coating on top of CFRP at three different thicknesses.
Accurate loss data for antenna reflector samples were discussed. Such data have served the ALMA, Planck, and Bepi–Colombo projects as well as other space developments. The data can be supportive for other developments. A metal coating was discussed for CFRP with supportive analyses. Other applications can benefit for space antennas or radio telescope antennas [38].
Material effects can be investigated related to the purity, the thickness of a coating, or a protective coating at a low loss level and over a temperature range. This could be a subject for further investigations as well as for new materials, like synthesized or artificial coatings. Other coatings on top of a conductor can be modeled using multiple layers (more than the two layers considered here). For that, one has to go back to Maxwell’s equations. The ${\rho}_{\text{eff}}$ is useful in assessments; Figures 5 and 17 are examples. The roughness aspects observed with samples for ALMA deserve further investigations. Recall that the roughness of the ALMA samples is well below current modeling capabilities—smaller than what could be modeled with a CST model. Modeling and testing go hand in hand. For critical low values in applications, one should measure samples, taking note of the material, purity, and temperature effects.
The European Space Agency is acknowledged for supporting the research over the years more than a decade ago. It provided results as published with references indicated in Table 1 and formed the basis for this article. The reviewers are acknowledged for their comments. This article has supplementary downloadable material available at https://doi.org/10.1109/MAP.2022.3203295, provided by the authors.
Cornelis G.M. van ’t Klooster (kvtklooster@gmail.com) has been affiliated with Toegepast Natuurkundig Onderzoek (TNO), The Netherlands and the European Space Agency–European Space Research and Technology Center (ESTEC, The Netherlands) and is now retired and works part-time with the University of Technology in Eindhoven (TUE) Department of Electrical Engineering, 5600MB Eindhoven, The Netherlands. He graduated from TUE in 1978 and the University of Technology in Delft in 2001. He is a Senior Life Member of IEEE.
Vladimir V. Parshin (parsh@ipfran.ru) is a senior researcher at the Institute of Applied Physics of the Russian Academy of Science, Nizhny Novgorod 603950, Russia, where he has worked since 1977. He graduated from the Radio Physical Faculty of Gorky State University, USSR, in 1972.
Evgeni A. Serov (serov@ipfran.ru) has been with the Microwave Diagnostics Lab of the Institute of Applied Physics of the Russian Academy of Science, Nizhny Novgorod 603950, Russia, since 2005. He graduated from Nizhny Novgorod State University in 2009 and received the Ph.D. degree in physics in 2013.
A.B. Smolders (a.b.smolders@tue.nl) is a full professor in electromagnetics at the University of Technology in Eindhoven (TUE) Department of Electrical Engineering, Eindhoven 5600MB, The Netherlands and dean of the electrical engineering faculty of TUE. He graduated from TUE in 1989 and received the Ph.D. degree in 1994 from TUE. He is a Senior Member of IEEE.
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Digital Object Identifier 10.1109/MAP.2022.3203295