Low-loss Materials for Application in Millimetre

and Submillimetre Waves Ranges

 

V.V. Meriakri

 

1 Introduction

 

Low-loss materials are needed for millimetre (MM) and submillimetre (SMM) guiding systems (dielectric and microstrip waveguides, quasi-optical lens waveguides) and their components (phase shifters, directional couplers and power deviders, prisms, polarizers etc.), for antenna systems (lenses, antenna covers, frequency selective screens), as well as for transmission windows for vacuum tube generators (e.g. gyrotrons) [1-6].

Information concerning materials complex refractive indices n^*= n- ik is also of a great interest for MM and SMM introscopy, materials and media testing (first of all for aquametry), for propagation problems including indoor propagation, for some industrial and medical applications [1,3,7-9].

Here we present properties of the different low-loss materials and substances (liquid and solid-state dielectrics, ferroelectrics, semiconductors, ferrites, composite, building, nature and common use materials and media in MM and SMM waves ranges, mainly at the wavelengths l=3.0¸0.6 mm. There are many papers [1,2,4,5,10,11] with results of investigation of low-loss materials in MM and SMM ranges (let us consider as low-loss materials these with loss tangent tand=2n/n²-k² less than 10^-2).

However in many cases it is difficult to compare the results of the different authors because the results of measurements on nominally the same material gave considerable differences between assigned values of n and k. There are clearly at least two possible explanations for this. First, that different specimens of the same material do not necessary have the same n and k due to technology peculiarity and some other reasons. The second possible explanations is that some or all the measurements made, even by the same technique, were susceptible to systematic errors that were not considered in the publications.

The National Physical Laboratory (UK) has carried out a complex experiment on intercomparison of measurements techniques of 13 research groups from different countries using the same specimens at frequencies 30¸900 GHz (l=10¸0.33 mm) [12]. For investigation share were used following measurements methods: dispersive Fourier transform spectroscopy, open Fabry-Perot resonators, optically pumped laser spectroscopy, free space power transmission and reflection measurements, four-port and six-port reflectometers.

The intercomparison of the results of low-loss materials (polyethylene, quartz, rexolite, macor, beryllia) properties measurements has shown that the disagreement in measurements of real n and imaginary k parts of n^* by different techniques and authors amounts to (0.3¸10)% for n and an order of magnitude for k. Therefor here we present mainly the results of measurement investigation in MM and SMM ranges made in our Laboratory in Institute of Radioengineering and Electronics Russian Academy of Sciences using quasi-optical lens beam waveguide spectroscopy methods [5,13,14]. This version of MM and SMM spectroscopy has some advantages in frequency range 50¸500 GHz.

2.   Measurement method and circuits.

For wavelengths l longer than approximately 6¸8 mm effective waveguide and resonator technique of low-loss materials properties measurement is elaborated [15,16]. On the other hand for wavelengths shorter than 0.5 mm very good Fourier transform and laser spectroscopy methods are available [1,2,16].

            However, there are some difficulties in carrying out material investigation in the wavelength interval from 5¸4 mm to 0.6¸0.5 mm. The reason is that the waveguide technique is ineffective due to an decrease of the waveguide dimensions, gaps between waveguide and sample walls [17], 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 [13,14]. 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 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 [14], whereas in conventional free space measurements these values may be in this case more than 10^-1 [18].

            The methods of determination n^*, complex permittivity e^*=e₁-ie₂, complex permeability m^*=m₁-im₂ are based mainly on measuring the dependencies of the transmission t=½t½e^ijt and reflection r=½r½e^ijr coefficients modules and phases on frequency, specimen thickness l, polarization of the wave, etc.

            The typical block diagram of a quasi-optical measuring set up for wavelengths l=4¸0.6 mm is shown in fig 1 [14].

Equipment fig. 1 allows to measure n, k; complex permittivity e^*=e₁ - ie₂, t, r in very wide intervals n from 1.05 to 20,  from approximately 1 to 10^-6,  from 10^-5 to 1, and  from »1 to 10^-4.

The method of measuring  and , where l -sample thickness, f-frequency, T - temperature appears to be the most universal. In the case of liquids it was used tray of variable thickness with a readout accuracy Dl = ±1 mm or if absorption is low measurement chamber had short-circuiting plunger making it possible to vary the liquid layer thickness.

To determine the k of low loss solid materials with n from 1.3 to 1.6 the  value is measured in immersion liquids featuring practically the same refractive indices and certain absorption.

In interferometer (see III, fig. 1)  or  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.

 

.

                                                                                          Fig. 1

 

Here I - resonator for low loss material properties measurement, II -transmission measuring circuit,
III -Michelson or Max - Zender interferometer, IV - reflectometer. 1 - BWO, 2 - magnet, 3 - horn,
4 - modulator,   - 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.

 

Measurements of the  tensor components in ferrites were performed with the aid of an interferometer with circularly polarized waves which are the natural waves of the longitudinally magnetized material. The diagonal, () and non-diagonal () components of  are related to the  of right and left circularly polarized waves. The most accurate method of determination  is by measuring  by reversing the direction of the permanent longitudinally magnetic field. Alternately,  was determined by measuring the Faraday rotation angle q of a linearly polarized wave: .

Of special interest are the measurements birefringence and dichroism of crystals ,  (i, j=x, y, z and i¹j). At small , and  when , ,  and  cannot be determined individually with adequate accuracy rotating a sample between polarizers and frequency sweeping were used. There are  two  adjacent  frequencies n_1,2 =1/l_1,2 corresponding to a circular polarization waves at the crystal output. These frequencies and the angle of axis i rotation relative to the electrical field vector of the incident wave, and reflection coefficients ,  allow us to find , and .

            The experimental set up for very low-loss specimens is based on a hemispherical Fabry-Perot resonators (see I, Fig. 1). The quality factor of these resonators is more then 50000.

 

3. Some comments concerning low-loss materials

Low-loss materials are non-polar. Their absorption features are of four types [19,21].

1.   Sharp single-phonon absorption due to lattice vibrations in the crystalline regions. Usually the frequencies of such vibrations have been observed below l £ 0.5 mm.

2.   A multi-phonon absorption continuum arising from the vibrational density of states in the crystalline regions and extending into the MM and microwave region.

3.   A continuum absorption arising from vibration in the amorphous region. This absorption may extend into MM and microwave region too.

4.   An absorption due to impurities and degradation products.

Practically at MM and SMM wavelengths the dielectric losses in non-polar materials both lattice losses and losses due to impurities, charge carriers plays a large role.

At a fixed temperature the theoretical limit of the lattice losses in a solid state dielectrics in the MM wavelength range is determinated by a multi-phonon mechanism primarily by two-phonon losses in corresponding ideal crystal. The crystals with low coefficient of heat expansion and high lattice heat conduction, high Debye frequency and sound velocity, high crystal symmetry must have very low losses. The crystals with diamond structure (diamond, silicon) satisfy these criteria to the highest degree. Crystals with Zinc blende and wurtzite structure, sapphire, polyethylene, and some others belong to them too.

Practically losses depend also on material processing, material purity and homogeneity.

 

4. Material properties

Non-polar liquids. Non-polar liquids are of interest as low-loss substances for MM region applications (immersion and cooling liquids, substances under test in aquametry, etc.)

The use of non-reflecting lens beam wavequide measurement techniques improves the accuracy of determination n and k of low-loss liquids because of lesser errors associated with wave beam defocusing effects and standing waves in the measuring channel [14,22]. Some n and tand values for the most transparent non-polar liquids are listed in Table 1.

 

Table 1 (T=18°¸20°C)

liquid	n ± 0.3%	tand ´ 103±10%	l, mm	note
cyklohexane	1.424	0.50	0.63
octane	1.396	0.74	0.63
decane	1.407	0.83	0.63
nonane	1.405	0.93	0.63
carbon tetrachloride	1.490	3.5	1.0	[23]
cysdecalin	1.474	4.2	1.0	[23]
transdecalin	1.461	0.6	1.0	[23]
benzene	1.510	5.2	1.0	[23]
pentane	1.380	1.9	1.2
toluene	1.510	5.4	1.2
1,4 dioxan	1.343	5.0	1.0
fluorinert cooling liquid FC-43	1.380	1.3	1.0	[24]
CS2	1.343	3.0	1.0	[22]
crude oil	1.470-1.570	0.8-1.4	2.0

 

Table 1 presents characteristics of chemical pure liquids (with the exception of crude oil). Practically all these liquids have tand ~10^-4¸10^-3 in centimetre wavelength region. At MM waves fast increase of not only absorption a (dB/cm) but also increase of tand is observed. This increase is connected with so named induced dipoles in non-polar liquids [23]. Additional absorption in practically used non-polar liquids arises also due to their humidity. Water is a very lossy liquid at MM and SMM wavelengths.

Table 2 presents e₁, e₂ and a for liquid water [8].

 

Table 2

l, mm	T, grad C	e1	e2	a, dB/mm
2	20 30	6.28 6.86	8.81 10.5	41.1 46.0
4	20 30	9.10 10.9	16.0 18.9	29.5 31.9
8	20 30	18.1 23.5	28.0 30.8	18.8 18.9

 

Therefore even very small water content in non-polar liquids evokes additional losses, e.g. at l=2 mm the dependence a(W), where W is the concentration of water by volume, for crude oil is about 2 dB/cm per 1% water [8]. Table 1 gives for dry crude oil a(0)» 0.2 dB/cm.

Low-loss solid state dielectrics. These dielectrics (polymers, ceramics, crystals, composite materials) have in MM and SMM regions n from 1.05 to 10 and tand <10^-2. Some results of polymers investigations are summarized in Table 3.The measurements of n and tand was carried out at approximately 20°C on polymers the highest commercially purity.

 

            Table 3

material	n ± 0.5%	tand ´ 103±10%	l, mm	note
polytetrafluorethylene (PTFE),unsintered	1.25-1.44 1.42	0.23-0.26 0.05	0.63	density 1.3-2.2 g/cm3
PTFE, sintered, teflon	1.43	0.7	1.3
polyethylene	1.52	0.6	0.63
polypropylene	1.51	0.6	0.63
TPX	1.45	0.7	1.0
Rexolite	1.59	1.0	2.2
Duroid	1.48	1.0	3.0
polystyrene	1.59	2.9	1.0
teflon-4MB	1.42	1.2	0.63

 

Unsintered PTFE features the lowest losses. Investigations of PTFE physical and chemical properties [14] indicate that unsintered PTFE has a higher crystallinity than sintered PTFE (teflon), approximately 95% and 60% respectively). The observed difference in losses can be ascribed to the absorption in the polymer’s amorphous phase analogous to the absorption in non-polar liquids.

The refractive index of PTFE is related to the material density d as

n = 1+ 0.196 d,

where d is in g/cm³, 1.15< n< 1.3.

The value of d can be varied either by changing the pressure at which the PTFE powder is moulded, or else by making the PTFE porous.

It should be stressed that for accurate measurements of n and a for materials with n = 1.3¸1.5 we have used low-loss immersion liquids featuring practically identical refractive indeces (see Table 1).

The dependence of a on frequency n = 1/l = 2¸50 cm^-1 for low-loss polymers is in a good accordance with low [2]

a_Nepers= 8.10^-4n + 1.4.10^-4n ².

Tables 4 and 5 present the characteristics of the most interesting low-loss materials with n >1.6: ceramics and glasses (Table 4) and crystals (Table 5).

It should be stressed that values of n in Table 4 belong to the concrete specimens under test and these values may change to material density and technology of preparation.

 

Table 4 (T=18°¸20°C)

material	n ± 0.5%	tand ´ 103±10%	l, mm	note
BN	1.73	1.0	o.95	d=1.45 g/cm3
BN	2.07	0.64	1.22	d=1.9 g/cm3
SiO2	1.92	2.0. 0.67	0.65 0.26
MgF2	2.16	0.9 0.6	1.0 1.2
BeO	2.63	1.2	1.0
AlN	2.88	4.0 7.0	3.0 1.4
Al2O3	3.10	0.26 1.3	2.18 0.95
BaTiO3+TiO2 (50%)	6.1	1.1	0.95
SrTiO3	15.1	35	1.0
TiO2	9.4	10	1.4
MgAl2O4	3.14 2.90	1.5 0.6	1.0 2.5	[16]
La7/12Na1/4TiO3	9.3	12	2.2
ZnS	2.89	1.9	3.0
ZnSe	3.02	2.2	3.0
Na2O 6Al2O3	3.6	2.0	2.3
SiO2 (fused)	1.95	1.4	0.85

 

In frequency range 100¸400 GHz tand for materials presented in Table 4 increases depending on f as f^g (g = 0.5¸1.0).

Table 5 shows the characteristics of the low-loss crystals.

The results presented in Table 5 show that the lowest losses among solid state materials at room temperature have high resistive silicon and artificial (made using the method of chemical vapor deposition, CVD, [31,32]) diamond. This result is in accordance with theoretical predictions [20,21].

The  best  results  were  achieved  n  the  gold-doped silicon (tand = 3´10^-6  at l» 2 mm). These losses are due to free carriers of charges, because according to [28, 31] the lower limit of loss tangent value due to intrinsic lattice loss in silicon is equal to about 3´10^-8.. For Ge this value is equal 2´10^-7. The same estimation of the lower limit of lattice loss tangent values have been obtained for the diamond, GaAs, GaP, and InP respectively: tand »10^-9, 10^-4, 2.5´10^-4 [31]. In GaAs, GaP and InP the experimental values of tand are practically equal to the theoretical predictions for lattice loss. The observed losses in diamond can be explained by the inclusions of non-diamond phases containing amorphous carbon and nanographite.

 

Table 5 (T=18°¸20°C)

material	n ± 0.5%	tand ´ 103±10%	l, mm	note
SiO2	ne=2.14 no=2.10	0.55 0.56 0.4	2.18 2.18 0.66
Al2O3	ne=3.40 no=3.07	0.15 0.25 0.60	2.14 2.14 0.61	[26] [26] [27]
GGG	3.51	1.3 0.7	1.1 2.18	[30]
LiNbO3	ne=6.7   no=5.1	6.0 2.0 4.7 1.5	1.0 2.2 1.0 2.2
LiTaO3	ne=6.30 no=6.45	10 7.0	1.2 1.2
TGS	ny=2.31 nx=2.91 nz=2.74	8.4 15 620	1.1 0.9 2.0
Ge (r =400 Om.cm)	3.15	3.3	2.0
GaAs(r=108Om.cm)	3.61	0.2	2.24
InP (r =107Om.cm)	3.55	0.2	2.16	[28]
GaP (r=108Om.cm)	3.34	0.1	2.16	[28]
Si (r=25 kOm.cm)	3.42	0.08	1.4	[27]
Si (r=40 kOm.cm)	3.42	0.025	2.0
Si (r=150 kOm.cm)	3.42	0.003	2.0	[29]
diamond, CVD	2.40	0.05	2.1	[32]
diamond, CVD	2.39	0.008	1.95	[31]

 

The dependence of tand for diamond and Si on frequency can be approximated by f ^-1 law due to conductivity of these crystals, whereas tand for InP, GaAs, GaP practically invariable in frequency range 100¸500 GHz.

In Table 5 also some low-loss anisotropic crystals properties are presented.

These crystals are of interest for MM and SMM polarizers and polarization convertors [33]. So TGS crystal is a very good polarizer for short MM and SMM region due to very large dichroism in this material.

The photoconductivity in low-loss silicon and germanium allows to create MM-wave optically controlled attenuators and modulators, e.g. for dielectric wavequides [34].

            Low-loss ferrites. Ferrites are very important materials for non-reciprocal devices. Here we present some results of investigation of over 50 types of polycrystal ferrites made by our group [35]. Table 6 shows characteristics of the ferrites with the lowest losses among each type of ferittes.

 

Table 6 (T=18°¸20°C)

material	n ± 0.3%	tand ´ 103±10%	l, mm	Saturation magnetization, Gauss
YIG ferrites	9.68-3.82	0.75 1.4 2.5	2.16 0.85 0.6	<2200
LiZn ferrites	3.85-4.0	1.2	2.16	<4800
NiZn ferrites	3.54-3.95	1.0 1.5 3.5	2.16 1.10 0.60	<5200

 

            The best ferrites have the Faraday rotation angle q about 8¸11 grad/mm for YIG ferrites and 10¸16 grad/mm for NiZn ferrites.

            Low-loss composite materials. Now there are many low-loss materials for microwave and MM wave region are available. These materials usually based on PTFE or polyethylene as a matrix and Al₂O₃, MgO, TiO₂ as a admixture. Table 7 shows the characteristics of composite material based on PTFE at frequency 480 GHz depending on concentration of admixture by weight, W, %.

 

Table 7

admixture	n±0.5%		tand´103±20%
	W=5%	W=10%	W=5%	W=10%
MgO	1.45	1.48	3.1	7.6
Al2O3	1.48	1.51	9.1	17.0
TiO2	1.58	1.68	10	23

 

            Here dimensions of admixture powder grains are smaller than 0.1 mm.

            In porous PTEF losses are lower than 0.5 dB/mm up to l=0,6 mm if grains are smaller than 0.25 mm. Composite materials with n>1,7 where obtained by doping polystyrene with high-permittivity powders of TiO₂ [36]. The permittivity of such composite materials is good described by Lichtenekker expression

                        lne*=j_m lne_m+j_ae_a,

where e_m, e_a and j_m, j_a are polystyrene and TiO₂ permittivities and concentrations by volume respectively. Experimentally it was achieved at frequencies near 70 GHz e=4 and tand =1.4´10^-3 for j_a =10% and e_a =11 and tand =1.3´10^-2 for j_a =40%.

            The next practically important composite materials are glass plastics for antenna cowers.

            Some results of investigation of many types of glass cloths and resins used for preparing glass plastics were presented in [37].

            So cloths based on non-alkaline and quartz glass fibers have e =3.6¸6.3 and tand from (0.2¸2.0)´10^-3 at frequencies 30¸35 GHz to (0.3¸4.0)´10^-3 at frequencies 300¸350 GHz.

            Resins used for glass plastics (epoxy and silicon-bounded types) have e= =2.8¸3.1 and tand from (1.2¸2.5)´10^-2 at frequencies 30¸35 GHz to (2.5¸3.5) 10^-2 at frequencies 300¸350 GHz. The permittivity e₁ for each material is practically invariable at frequencies 10¸350 GHz.

            Antenna covers materials based on porous SiO₂ and Al₂O₃ have e =1.15¸3.8 and tand from (1¸5)´10^-3 at frequencies 150¸300 GHz.

            For some applications it is of interest composite dielectrics with dispersion n connected not with material properties but with dimensions of insertions into composite material matrix. For example the artificial dielectric [38] consisting of teflon  with cylindrical periodical holes (hole diameter d=2.5 mm, grating period 5 mm) has in the interval of l 2.5 mm >l >4mm n increasing from 1.36 to 1.4 with two resonant regions where n_min=1.31 and n_max=1.39 and tand »3´10^-3 instead of tand <10^-3 at the other frequencies.

            Natural, building and common use materials. These materials are of great interest for communications and traffic applications as well as for instruments for non-destructive test of materials, manufactured articles and environment [38, 30].

            Table 8 shows building and natural materials and substances properties.

 

            Table 8  (l=2 mm, T=20°C)

material	n ± 1%	tand ´ 102±10%	r, g/cm3	note
brick (red)	1.78	3.5	1.5
brick (silic.)	1.82	4.2	1.8
concrete	2.40	5.5	1.7
ashalt	1.50	8.0	1.3
sand	1.55	2.5	1.8
soil	1.60 1.60	3.8 2.5	1.5 1.5	180 -390
snow	1.12	0.65 2.6	0.23 0.23	-360 -10
pine tree wood	1.4	3.4/2.0	0.5
glass, window	1.45	5.0
organic glass	1.60	1.5
marble	1.50	1.0
ebonite	1.67	1.1		l= 0.6 mm
cardboard	1.80	6.0
cautchuck	1.66	30
glues	1.57-1.72	1.0-2.0		l= 1.7 mm
phenolone	1.8	3.9		l= 1.7 mm
polymyde	1.66	1.8		l= 1.7 mm
veneer	1.5	10		l= 7.6 mm
plaster	1.7	0.7

 

            Here less value of tand for pine tree wood corresponds to electrical field perpendicular to wood fibers. Water content in wood materials is less than 8%.

            In Table 9 clothes materials are presented. Here ½t½² and ½r½² are power transmission and reflection coefficients, n_eff - effective refractive index for materials

                        n_eff =largt/2pd,  where d - material thickness.

 

Table 9  (l=1.6 mm)

material	½t½2, %	½r ½2, %	neff- 1	l, mm
cloths for tents	82-94	£ 1.3	0.2- 0.3	0.3 - 0.5
cloths for coat, wool	77- 84	£ 3.0	0.1- 0.2	2- 4
cloths for suits, wool	85- 98	£ 1.0	0.2- 0.3	0.5- 1.0
silk	89- 93	£ 1.0	0.28- 0.35	0.15- 0.25
leather, natural	79- 85	£ 5.0	0.22- 0.28	0.9- 1.5
leather, artificial	75- 89	£ 5.0	0.22- 0.28	0.7- 0.8
fur, artificial	75- 89	£ 3.0	0.04- 0.07	4.0- 12
astrakhan	71	£ 1.0	0.14	35
cloths for shirts	92- 95	£ 5.0	0.18- 0.23	0.2- 0.3

 

Frequency dependence of n for materials in Tables 8 and 9 is weak but tand and ½t½² vary considerably as frequency changes. For example tand of brick at l=6¸7 mm is only 10^-2, tand of concrete is 6´10^-3. Transparency of clothes decreases as frequency increases from 98¸75% at l=1.6 mm to 76¸30% at l=0.5 mm.

All materials in Table 9 have strong dependence of frequency on moisture for practically dry materials (see Table 9). ½t½²=98¸75% in temperature interval T=5°¸20°C, for moisture W=28% ½t½²=80¸25%.

 

5.   Conclusion

This short review shows modern situation with low-loss materials for applications in MM and SMM wavelengths region. This situation is continuously changed: material properties becomes better, new, first of all artificial, materials are created, new information concerning frequency, temperature dependence of n and tand is appeared as well as information how these values  are changed under different external effects..

 

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18. V.V. Meriakri, I.P. Nikitin, Iris Effects in Quasu-Optical Measurements of Dielectrics, in book Quasi-Optical technology of MM and SMM Waves ranges Kharkov, Ukraine, pp. 55-58, 1989, (in Russian).

19. G.J. Davis, J.Haigh, Submillimetre Spectra of Pure High and Low Density Polyethylene, Infrared Physics, vol. 14, pp. 183-188, 1974.

20. B. M. Garin, A.V. Galdetskii, Two Phonon Absorption of Far Infrared Radiation in Polymer Crystals, Opt. Spectroscopy (USSR), vol. 50, pp. 540-543, 1981.

21. B.M. Garin, One-Phonone Dielectric Losses by Exitation of Sound, Sov. Phys. Solid State, vol. 32, No 11, pp. 1917-1920, 1990.

22. V.N. Apletalin, B.M. Garin, V.V. Meriakri, Dielectric Properties of Liquids in the Submillimeter Band, Radio Engin. and Electronic Physics, (USSR), vol. 28, No 1, pp. 1-15, 1983.

23. G.J. Davis, M. Evans, Use of Genelized Lagevin Theory to Describe Far Infrared Absorptions in Non-Polar Liquids, Jour. Chem. Soc., Faraday Trans., 2, vol. 72, pp. 1194-1205, 1976.

24. M.N. Afsar, K.J. Button, Millimeter and Submillimeter Wave Measurements of Complex Optical and Dielectric Parameters of Materials, Intern. Journ. Infrared Millimeter Waves, vol. 2, pp. 1029-1044, 1981.

25. R.J. Cook et. al., Comparison of Cavity and Open-Resonator Measurements of Permittivity and Loss Angle at 35 GHz, IEEE Trans. Instrum. Measur., vol. IM-23, No 4, pp. 438-442, 1974.

26. Yu.A. Dryagin, V.V. Parshin, A Method to Measure Dielectric Parameters in 5-0.5 MM Wavelength Band, Intern. Journ. Infrared and Millimeter Waves, vol. 13, No 7, pp. 1023-1032, 1992.

27. B.M. Garin, A.N. Kopnin, V.V. Meriakri et. al., About the Dielectrics with Minimum Losses in MM and SMM wavelength band, 1 Ukrainian Symp. Prysics and Techniques of MM and SMM Radio Waves, Kharkov, Digest, Part 1, pp. 86-87, 1991, (in Russian).

28. B.M.Garin et al., Extremely Low Loss Materials at the Near Millimeter Wavelengths Range, 21 Intern. Conf. On Infrared and Millimeter Waves, Digest, Berlin, Germany, paper CT 15, 1996.

29. R. Heidinger, J. Molla, V.V. Parshin, Step to Intrinsic Absorption in Doped Silicon, Ibid, paper AW 8.

30. V.V. Meriakri, Material Properties in the Millimeter Range, Proc. of the 3^rd Intern. Kharkov Symp., Physics and Engineering of MM and SMM Waves, Ukraine, pp. 121-123, 1998.

31. B.M. Garin, A.N. Kopnin, M.P. Parkhomenko et. al., Losses in Diamond at Millimeter Range, Ibid., pp. 278-280.

32. R. Heidinger, Dielectric Property Measurements on CVD Diamond Grades for Advanced Gyrotron Windows, Proc. of the 19^th Int. Conf. On Infrared and MM Waves, Sendai, Japan, pp. 277-278, 1994.

33. V.V. Meriakri, E.F. Ushatkin, Polaroids for SMM Wave Range Using Dixroic Crystals, Pribory i Technika Experimenta, No 5, pp. 170-171, 1976, (in Russian).

34. B. Murmuzhev, V. Meriakri, Millimeter Wave Low-Power Electronic and Optoelectronic Devices Based on Dielectric, Ferrite, and Semiconductor Wavequedes, Proc. of SPIE Intern. Conf. on Smart Structures and Materials 1996, San Diego, USA, vol. 2722, pp. 270-272, 1996.

35. S.G. Abarenkova, G.A. Kraftmacher, V.V. Meriakri et. al., Polycrystal Ferrites Properties at SMM Waves, Elektronnaya Technika, Materials, No. 3, pp. 34-38, 1977 (in Russian).

36. V.V. Meriakri, I.P. Nikitin, E.E. Chigrai, Dielectric Properties of Materials at the Near Millimeter Wavelengths Range, Radiotechnika, No. 6, pp. 88-95, 1995, (in Russian).

37. E.E. Chigrai, V.V. Meriakri, Millimeter Wave Characteristics of Glass Plastics for Antenna Covers, Proc. of the 3^rd Intern. Conf. on Antenna and Techniques, Sevastopol, Ukraine, pp. 403-404, 1999.

38. V.V. Meriakri, E.E. Chigrai, Properties of Materials for Practical Use at the MM and SMM Wavelengths, Colchester, UK, Digest, pp. 68-69, 1993.