MILLIMETER WAVE CHARACTERISTICS OF GLASS PLASTICS FOR ANTENNA COVERS

E.E.Chigriai, V.V.Meriakri

Institute of Radioengineering and Electronics, Russian Academy of Sciences

Vvedensky sq. 1, Fryazino, Moscow Reg. 141190, Russia

Phone: (095) 5269266, FAX (095) 7029572, (095) 2038414

E-mail: meriakri@ms.ire.rssi.ru

INTRODUCTION

Our task was permittiyity e = e+ie and loss tangent tand = e/e of low-loss glass plastics and their components used for antenna covers measurements at the frequencies f = 30-350 GHz.

The most interesting frequency interval in these measurements is so named near millimeter band (wavelengths l = 3- 0.8 mm).

For wavelengths l longer than approximately 4¸5 mm standard waveguide and resonator technique of low-loss materials properties measurement can be used. Therefore for glass plastics and their components testing in the frequency range 10-75 GHz we have used standard panoramic network analyzers R-2-61, R-2-65, R-2-68, and R-2-69 with special metal rectangular horns.

On the other hand for wavelengths shorter than approximately 0.6 mm very good Fourier transform and laser spectroscopy methods are available.

However, there are some difficulties in carrying out material investigation in the wavelength interval from 5¸4 mm to 0.6¸0.5 mm (frequencies from 60- 75 to 500- 600 GHz). The reason is that the waveguide technique is ineffective due to a decrease of the waveguide dimensions, gaps between waveguide and sample walls. On the other hand the optical technique is ineffective due to diffraction effect affecting the field structure and not allowing the use of geometrical laws of optics.

The best way for the measurement of material properties is to use quasi-optical lens beam waveguide transmits only the fundamental low-loss mode with enough small cross-sectional dimensions of a wave beam and large losses for higher order modes [1].

In this case the incident on the plane-parallel specimen wave and the wave on the receiving aperture are the same (Gaussian type) and it is possible to estimate the measurement errors due to a thickness l of a plane-parallel specimen, inclination of its boundaries to the beam waveguide axis Y, and cross-sectional dimensions of the specimen a and receiving aperture b. So for beam waveguide consisting of non-reflecting lenses [1] with a » b > 10l, l £ 1 cm, y £ 0.2, n < 5 the magnitude of errors in transmission |t|² and reflection |r|² coefficients are less than 5×10^-3, whereas in conventional free space measurements these values may be in this case more than 10^-1.

Therefore the measurements in the frequency range 75- 350 GHz we carried out with help quasi-optical lens beam wavequide spectrometers, interferometers, and open resonators described in [1].^.

MEASUREMENT METHOD AND SET- UP

The methods of determination complex permittivity e are based mainly on measuring the dependencies of the transmission t=½t½eⁱ^j^t and reflection r=½r½eⁱ^j^r coefficients modules and phases on frequency, specimen thickness l, polarization of the wave.

In Fig.1 set- up for dielectric properties of antenna covers at frequencies 30- 75 GHz with help waveguide panoramic network analyzers is shown. Here 1- rectangular waveguide, 2- bend, 3- horn, 4- support for sample under test, 5- sample, reflecting mirror, 7- directional coupler for reflection coefficient ½t½ measurement.

Fig.1

The typical block diagram of a quasi-optical measuring set- up for wavelengths l=4¸0.6 mm is shown in fig 2 [1].

Here I - resonator for low loss material properties measurement, II -transmission measuring circuit, III - Michelson or Max- Zender interferometer, IV - reflectometer . 1 – backward wave ossilator, 2 -magnet, 3 - horn, 4 -modulator, 5 -lens, 6 - polarizer, 7 - attenuator, 8 - iris, 9 - receiver, 10 - absorber, 11 - mirror, 12 - beam splitter, 13 - amplifier, 14 - synchronous detector, 15 - digital voltmeter, 16 - storage unit, 17 - voltmeter, 18 - light source, 19 - LED, 20 - power supply.

For dielectric characteristics of glass plastics for antenna covers we used only parts II and III of the set up Fig.2. Part II allows us to measure transmission coefficient t. In interferometer (part III) and are measured using primarily the frequency sweeping technique which eliminates spurious interference effects in the sample and beam path and provides unambiguous determination of the interference order.

Equipment Fig.1 and 2 allows to measure complex permittivity e=e₁ + ie₂, t, r in very wide intervals e from 1.05 to 500, from approximately 1 to 10^-6, tand - from 10^-5 to 1, and -from »1 to 10^-4.

Fig.2

Errors in determination l, modules t and r, argt, arg r, Q, and f were : l = 0,01 mm, t and r modules 5%, argt and argr 5-10 degrees, Q -5%, and f =0.2 GHz.

The resulting accuracy of determination of permittivity e and tand was (1- 2)% for e and (20- 30)% for tand.

RESULTS OF MEASUREMENTS

We have investigated many types of glass cloths and resins used for preparing glass plastics [2]. These materials and glass plastics were let us by I.G.Gurtovnik and V.N.Sportsmen.

Cloths based on nonalkaline and quartz glass fibers have e =3.6 - 6.3 and tand from (0.2 –2) 10 at frequencies 30 - 35 GHZ to (0.3 – 4) 10 at frequencies 300 - 350 GHz.

Resins used for glass plastics (epoxy and silicon-bonded types) have e from 2.8 to 3.1 and tand from (1.2 -2.5) 10 at frequencies 30 -35 GHz to (2.5 - 3.5) 10 at frequencies 300 -350 GHz.

Table 1 presents characteristics of the best glass plastics, glass cloths and resins.

Our investigations of more than 50 version of materials testified that e for each sample practically invariable at frequencies 10 - 350 GHz and depends only on technology, tand is slowly grows up (in interval 20 –30%) in frequency range 10 -35 GHz. At higher frequencies tand increases more rapidly.

All these materials were practically isotropic (level of the orthogonal polarization was less than - 40 dB).

The measurements of the dependence of e and tand on water content W in glass plastics show for W = 2-4% the increase of e not more than 0.1, tand becomes twice more.

Table 1.

material

tand´10

l, mm

Plastic based on

TS-8/3-K-TO cloth and epoxy epoxy resin

2.60

2.61

2.62

8.4

1.95

0.87

Plastic based on TS-8/3-TOKK-TO and SPE-25/3 resin

3.23

3.30

3.0

3.26

7.3

4.5

10.7

4.6

0.87

29.9

Plastic based on nonalkaline glass and SPE-25/3

4.30

4,33

4.35

16.

8.7

4.3

0.9

Silica glass

3.77

3.76

0.3

0.6

1.3

2.2

10.0

6.0

2.0

0.82

Nonalkaline glass

6.11

6.32

6.20

7.4

9.8

8.4

5.0

2.0

Resin SPE -25/3

2.88

2.86

9.9

2.0

0.88

Table 2 presents dielectric properties of antenna cover materials based on porous silica and alumina in near millimeter region.

Table 2

no	material	e	tand´10³	l, mm
1	AlO and DS-150 resin	2.07	5	2.0
2	SiO and Cr (0.5%)	3,35	1.2	1.1
3	SiO and TiO (15%)	3.80	1.6	1.25
4	SiO - 107	1.15	4.0	2.0

CONCLUSION

Set-ups for dielectric characteristics of antenna covers measurements at frequencies 30- 350 GHz were elaborated. At frequencies 10- 75- GHz standard panoramic network analyzers were used .for such measurements, at frequencies 75- 350 GHz we used quasi- optical lens beam waveguides.

Permittivty e and loss tangent tand of glass plastics and their components used for antenna covers were measured in this frequency range.. It was shown that permittivity of the cover materials is practically invariable at all this frequencies, and it was not found any anisotropy of e The value of tand increases approximately as f .

REFERENCIES

1. Meriakri V.V., Apletalin V.N., Kopnin A.N. and oths. ”Submillimeter beam wavequide spectroscopy and its application”, in book “Problems of modern radioengineering and electronics”, ed. by Kotelnikov V.A., Nauka, Moscow, 1985, pp. 179-197.

2. Gurtovnik I.G., Sportsmen V.A., “Stekloplastiki dlja radiotekhniki”, Moskwa, Khimia, 1987 (in Russian).