RESULTS OF REFLECTION LOSS MEASUREMENTS OF SAMPLE MATERIAL FOR RADIO ASTRONOMY TELESCOPE ANTENNA FOR PLANCK PROJECT

May 22, 2012 by admin Комментировать »

Van’t Klooster Kees (1), Myasnikova S. E., Parshin V. V.(2), Kasparek W.(3) (1) ESA Estec EEA, PO Box 299, 2200 AG, Noordwijk, The Netherlands (Kvtkloos@estec.esa.nl) (2> Applied Physics Institute RAS, 46 Ulyanov Str. Nizhny Novgorod 603950 Russia (parsh@appl.sci-nnov.ru) (3> Universitaet Stuttgart, Institute fuer Plasmaforschung, Germany (kasparek@ipf.uni-stuttgart.de)

Abstract – Reflectivity measurements have been carried out on high technology samples for the PLANCK radio telescope. High accurate results have been obtained at Applied Physics Institute in Nizhny Novgorod and an independent measurement with a total different setup at the university of Stuttgart confirmed, that one of the samples showed a strange behaviour. Moreover, it confirmed the high accuracy of the testing method.

I.  Introduction

Advanced radio telescope antennas for space applications are realised with application of stabile composite materials, which are lighter in general than various metal realisations. Reduction of reflection loss is of paramount importance and also – once exposed to harsh space conditions – the thermal design and with that the ther- mal-optical parameters are important to control. In an earlier publication [3] test results have been discussed, but that was more aiming at terrestrial application (ALMA sub-millimetre wave telescope). The accuracy of the method was presented in earlier papers [2,3,4]. Here the latest results are presented for sample measurements. A very interesting result is presented, by introducing data as obtained with another test facility at the University of Stuttgart. Although the latter result is only for one frequency, it is interesting to note so good comparison for data obtained with two different facilities. This is even more interesting, because the sample under test was behaving somewhat un-expected as described below, (‘baffle material’).

The reflection loss of a set of samples (reflector material for the Planck reflector) and of baffle material has been measured in a frequency band between 135 and 165 GHz. Earlier it was measured over a frequency range from 110 to 200 GHz. The size of the CFRP samples for the last testing was only 19*19 mm. This size has created a difficulty for the test set-up, compared to the test-set-up used for larger samples reported earlier.

The baffle material was measured before with larger sample size. The vacuum deposited Aluminium CFRP samples discussed here have been used as ‘guiding or monitoring’ samples during the coating of the full-size CFRP Planck reflector. One of the three samples had a yellow tint. It did show a higher reflection loss.

The differences are outlined for the necessary changes in the test-facility in order to measure such small size samples. A short-length resonator has been used.

The test set-up has been described before [2,3] and for recalling the good results obtained when testing representative ‘Planck’-reflector samples of 50*50 mm and to recall the importance of doing reflection loss measurements.

The results reported here are of general interest for a variety of applications:

We demonstrated, that reflection loss of reflectors, treated with a coating process as for Planck (or also for a reflector metallised with vacuum deposited aluminium, as shown in earlier measurements) is much lower than what is usually allocated in loss budgets for – for example – reflectors for telecommunication applications. Jt however depends on the coating process and its parameters. Therefore, measurement of reflection loss is important and the frequency range as reported here demonstrates this perfectly.

II.  Test Facility

The test approach and the set-up have been described in earlier publications [1, 2, 3, 4]. Just to recall: we discuss small values of reflection loss, around or below 0.2 % or around 0.01 dB.

The results showed earlier, that the reflection loss for the coated CFRP samples was slightly higher than the reflection loss for an aluminium sample.

The tests reported here include results for 19*19 mm size samples but only over a narrow frequency range and in despite of the small size the agreement with earlier results is good within that (narrower) available frequency band.

The test facility had to be adapted for these smaller samples – with consequences for the testing capability: not the full frequency range of 110 to 200 GHz could be covered. There could only be carried out a test within the frequency range of 135 to 165 GHz.

The resonator test set-up has been modified by changing mirrors with a large radius of curvature into a test set-up with two mirrors with smaller radius of curvature. In this way the beam-waist size is smaller (to fit on the 19*19 mm sized sample).

This is important in view of the potential spill-over (diffraction losses), when the small sample is placed in the beam-waist area.

The consequence however is, that the sensitivity for positioning error is increased and that the modal response within the measurement facility is more weak.

Moreover the stability of the Laguerre mode (used in the testing) is more sensitive to asymmetries and this could lead to less accurate response, when coupling in the excitation or coupling out (for testing observation) a linear polarised fraction of the signal with a coupling membrane as used in this facility. It all results in a narrower frequency band for this so-called short length resonator.

While the RF instrumental capability is able to handle 110 to 200 GHz, the testing of these small samples in this short resonator has only been possible over a frequency range from 135 to 165 GHz.

The accuracy for the available frequency range was degraded due to above points and due to the weaker signal after the test set-up modification.

III.Samples Considered for Testing

Two type of samples have been tested with a short resonator:

1. Three guiding samples (Planck reflector technology). Three samples were provided for testing purposes by the Planck project team. The three samples have been located as ‘guiding samples’ near to the main reflector during the metallisation process. One of the samples appeared to have a bit of a yellow tint (fig. 1).

2.Sample of the Baffle Material as proposed for the Planck Satellite. The reflection loss of a sample of the baffle material has been measured.

Fig. 2. Test Results coated CFRP samples, (Planck reflector material). Top curve is for yellow sample

The latter sample consists of an Aluminium plate protected by a thin plastic foil. It is noted, that the material had a slightly rough side and a shine side (below plastic protection foil) . Reflection loss of both sides has een measured already earlier and it was redone, because of un-expected response (see attachment). The shiny surface of that material might look attractive in the optical domain, possibly also attractive for a – £ (alpha or absorption – epsilon or emissivity).

Fig. 1. Three samples of a technology as applied for the Planck reflector. The central sample had a deviating colour: somewhat yellow

The shiny side appears to have a higher microwave reflection loss than the ‘non-shiny’side on the back. The backside has lower reflection loss and is close to a performance as expected from pure Aluminium. The latter side sample has been tested in the long and in the short resonator to compare the two resonator configurations (long and short resonator in fig.2).

IV.              Three Guiding Samples (Planck Reflector Technology)

Frequency [GHz]

Fig. 3. Test results for two (+ and x) Al-coated PLANCK reflector guiding samples and a ‘baffle’sample. (‘backside’of ‘baffle’ AI sample)

The following results have been obtained: In fig.2 the reflection loss is reported with the frequency (horizontal axis) and reflection loss (vertical axis) multiplied by 10′3 thus the values are in ‘promille’:

-1-‘Pink crosses’for a 19*19 mm sample, the reflection loss as measured in the short resonator is comparable to the result measured on 50*50 mm sample in the more accurate resonator. The results reported earlier are repeated in the fig below: ‘blue diamonds’ and ‘black crosses’ respectively sample ‘5 ‘ and ‘6’.

-2-‘Red squares’for a 19*19 mm sample with a shiny yellow appearance. The loss of the shiny yellow sample is about 40 to 45 % higher than sample T.

-3-‘Yellow circles’ for a 19*19 mm sample, the reflection loss as measured in the short resonator is comparable to the result measured on 50*50 mm sample as for sample ‘1’.

-4- ‘Red circles’ are results for the Aluminium reference sample in the short resonator. The comparison with sample ‘7’ below is a direct comparison for the two resonator facilities. The short resonator is used for for testing between 135 and 165 GHz. The long resonator (see Appendix) is used for testing between 110 and 200 GHz. This sample serves as proof of validity of the test set-up and assist to indicate accuracy.

-5- ‘Blue diamonds’ results in the long resonator obtained with a 50*50 mm sample. See also Appendix.

-6-‘Black crosses’ results in the long resonator obtained with a 50*50 mm sample. See also Appendix.

-7- ‘Light-blue squares’ for results obtained in the long resonator on Aluminium reference sample, (curves originally in colour).

V.              Sample of the Baffle Material as Proposed for the Planck Satellite

Test results for the baffle material are shown in fig 3 (‘green’squares). Also the results for the CRFP samples (with vacuum deposited Aluminium) are given for two samples (blue and red), to show, that the backside of the baffle material is very good at mm-wave frequencies. The ‘pink’ line is calculated for pure Aluminum.

In fig. 4 the results for the shine side of the sample.

The dot in fig.3 and 4 is for test results for the same sample in a three mirror test facility in Stuttgart University. The three mirror facility has the E-field vector either parallel or perpendicular under an angle of incidence. Results are an average for both situations for incident polarisation.

The comparison is good and the deviating results (high loss for shiny side) are repeated on the facility of the University of Stuttgart (but at one frequency only).

IV.  Conclusion

1.  CFRP Reflector Samples. Small samples of 19*19 mm have been tested in a resonator test set-up to measure the reflection loss. Samples are representative for Planck CFRP reflector (with coating with Aluminium and plasil).

-Reflection loss is only a little bit higher than the loss of pure Aluminium.

– One sample had a yellow color and a higher loss was measured. This has been improved now.

– The results are comparable with results obtained with 50*50 mm samples as reported earlier, except for result of that yellow sample.

–  There is no dependence on orientation of fibre direction (in CFRP). Phenomena reported earlier [4] do not occur for the Planck reflector.

Frequency [GHz]

Fig. 4. Test results for ‘baffle’sample (shiny side) with and without plastic coating

2.   Baffle Samples. Samples of the baffle material have been tested.

– The shiny side (protected by plastic film) shows a higher loss (4 to 5 times) than the backside.

Therefore:Shiny aluminium baffle material presents a 4 to 5 times higher reflection loss and has accordingly an impact on the noise temperature, if (fraction of) spill-over is hitting the baffle, which has such 4 to 5 times higher loss.

The backside of the baffle material (not shiny aluminium) has much lower reflection loss.

The clear conclusion is, that for high accurate results, one should measure the reflection loss.

Results cannot be automatically extrapolated to other frequency bands. The accurate facility as available in Applied Physics Institute in Nizhny Novogorod is of interest, with its (calibrations) corrections to measure under atmospheric conditions.

V.  References

[1]  W. Kasparek, A. Fernandez, F.Hollmann, R. Wacker. Measurement of ohmic losses of metallic reflectors at 140 GHz using a 3-mirror resonator technique. Int. J. of IR&MM Waves. N11, pp. 1965-1707, 2001.

[2]  V. V. Parshin, S. E. Myasnikova, G. Valsecchi, Kees van’t Klooster. "Reflectivity Investigations of Metal Samples in the Frequency Range 110 – 200 GHz". 25th Antenna Workshop on "Satellite Antenna Technology". ESTEC, The Netherlands, pp. 689-696, 2002.

[3]  S. E. Myasnikova, V. V. Parshin, C. van’t Klooster, "Reflectivity of Antenna And Mirrors Reflectors”, Proceedings ICATT2003, Sebastopol.

[4]  C. van’t Klooster, V. V. Parshin, S. E. Myasnikova, ‘Reflectivity of Antenna Reflectors: Measurement at Frequencies between 110 – 200 GHz.’ IEEE APS, OHIO, Columbus 2003.

Источник: Материалы Международной Крымской конференции «СВЧ-техника и телекоммуникационные технологии»

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