CN103630864B - A kind of calibration steps for free space material electromagnetic parameter test system - Google Patents

Abstract

The present invention provides a kind of calibration steps for free space material electromagnetic parameter test system, the calibration steps using vector network analyzer and the formula pre-seted, it is achieved the calibration to free space material testing system two-port network scattering parameter matrix.Use such scheme, can more effectively carry out free space material testing system calibration, error term transmission/reflection model is separated by setting up, need not use precise clamp that dual-mode antenna is moved, only simple alignment part need to be utilized to combine time domain gate technique, complete the calibration of free space material electromagnetic parameter test system, it is to avoid dependence to high precision measurement fixture.

Description

A kind of calibration steps for free space material electromagnetic parameter test system

Technical field

The invention belongs to the collimation technique field of electromagnetic parameter test system, in particular a kind of for free space The calibration steps of material electromagnetic parameter test system.

Background technology

Along with the high-tech areas such as developing rapidly of microwave technology, Aeronautics and Astronautics, mechanics of communication and information technology are to sky Line, the requirement of microwave device improve the most therewith so that microwave/millimeter wave material serves more and more important in these areas Effect.The electromagnetic parameter of material is its fundamental characteristics, and the performance of various microwave/millimeter wave devices and equipment is the most up to standard and material The electromagnetic parameter of material has important relationship, therefore first has to determine the electromagnetic property of used material in device R&D process, also It is accomplished by material therefor is carried out dependence test.

The most conventional material electromagnetic parameter test method has Transmission line method, free-space Method, Resonant-cavity Method and single spy Head bounce technique, wherein free-space Method is to utilize dual-mode antenna to launch microwave/millimeter wave signal to irradiate test sample, measures it anti- Penetrating transmission parameter, inverting obtains material electromagnetic parameter.Material preparation is required low by free space Material Testing Technology, it is only necessary to system The planarizing material of certain area is had, it is not necessary to carry out accurate cutting processing, be suitable for nondestructive testing for meeting certain thickness, and can Conveniently carry out high/low temperature test, be more suitable for the millimeter wave test that difficulty of processing is higher.

As it is shown in figure 1, the main test instrunment of free space material testing system is vector network analyzer 11, two-port is divided Do not connect two antennas 13 and 14 (conventional point focusing antenna), between antenna, place tabular detected materials, vow net can by GPIB or LAN bus is controlled carry out data acquisition and carry out electromagnetic parameter inverting by main control computer 12.

Carry out material electromagnetic parameter test and to obtain the S parameter of transmission/reflex system, it is necessary to whole before testing System system is calibrated, and i.e. carries out two-port network calibration for detected materials, with eliminate vector network analyzer internal and The error of dual-mode antenna, obtains the microwave signal true S parameter through the two-port network of measured material composition, and then inverting obtains To material electromagnetic parameter.

According to vector network analyzer principle, set up 12 error models, vow that the calibration of net is through calibrating device Measure, solving system error term, carry out just to obtain real S parameter by error term and measured value when measured piece is measured.Base When coaxial or waveguide transmission/bounce technique carries out testing of materials, use traditional transmission line, therefore have only to utilize pass The SOLT method (i.e. short-circuiting device, open circuit device, matched load, method through) of system just can carry out system calibration, but opens in free space Road device and this kind of calibrating device of matched load are difficult to, so cannot be carried out calibration.

Another kind of conventional method calibration steps is TRL method (i.e. straight-through, reflection, transmission method), may be used on free space In calibration, straight-through measurement is i.e. added without any material, and reflection measurement available standards reflecting plate realizes, and transmission measurement then needs to move Dynamic dual-mode antenna realizes, centered by best transmission line standard the 1/4 of wavelength.Its calibration steps is:

1, two antennas (2 times of focal lengths of focusing anteena spacing seasoning) at a certain distance are aligned placement, vow that net carries out straight-through survey Amount；

2, place short board calibrating device in two antenna centre positions, vow that net carries out two-port reflection measurement respectively；

3, remove short board, by two focusing anteena spacing increase about 1/4 centre wavelengths, vow that net is transmitted measuring.

After completing calibration, vector network analyzer will carry out error term calculating by carrying calibration procedure, then carry out tested Part will be by being calculated true S parameter when measuring.

By analyzing domestic and international list of references and similar techniques, the calibration of free space material testing system uses TRL more Calibration steps.Needing ambulatory transceiver antenna to carry out analogue transmission calibrating device in TRL calibration process, displacement is 1/4 centre wavelength, because of This needs more accurate mechanical clamp to adjust the distance of dual-mode antenna, and frequency is the highest the highest to required precision, and operation is also Increasingly difficult.

Therefore, prior art existing defects, need to improve.

Summary of the invention

The technical problem to be solved is for the deficiencies in the prior art, it is provided that a kind of for free space material The calibration steps of electromagnetic parameter test system.

Technical scheme is as follows:

A kind of calibration steps for free space material electromagnetic parameter test system, wherein, comprises the following steps:

Step 1: arrange and be not connected to dual-mode antenna, vector network analyzer is calibrated；

Step 2: arrange connection dual-mode antenna, places standard reflecting plate in the middle of described dual-mode antenna, utilizes vector network Analyser records the S in two-port network scattering S parameter₁₁And S₂₂, it is designated as respectivelyAnd

Step 3: arrange connection dual-mode antenna, does not place any material in the middle of described dual-mode antenna, utilizes vector network Analyser records the S in S parameter₁₁And S₁₂, it is designated as respectivelyAnd

Step 4: arrange connection dual-mode antenna, does not place any material in the middle of described dual-mode antenna, and launching, antenna is anti- Penetrate position and time domain door is set, utilize vector network analyzer to record the S in S parameter₁₁And S₂₂, it is designated as respectivelyAnd

Step 5: utilize preset formula to calculate the error term of free space error source, E_11A、E_11B、E_22A、E_22B、E_21A、E_12A、 E_12B、E_21B, wherein, E_11A、E_11BIt is respectively two-port directional error, E_22A、E_22BIt is respectively two-port source mismatch error, E_21A、 E_12A、E_12B、E_21BFor transmission and skin tracking error；

Step 6: place detected materials in the middle of described dual-mode antenna, record S parameter and be designated as S_11M、S_21M、S_12M、S_22M；

Step 7: use predetermined formula to calculate S parameter matrix S in detected materials₁₁、S₂₁、S₁₂、S₂₂, complete calibration.

Described calibration steps, wherein, the execution sequence of described step 2, described step 3 and described step 4 is to adjust mutually Change.

Described calibration steps, wherein, in described step 2,

Described calibration steps, wherein, in described step 3, Wherein: e is natural constant, j is imaginary unit, and k is wave number in free space, and d is Standard reflection plate thickness.

Described calibration steps, wherein, in described step 4,

Described calibration steps, wherein, in described step 5,

$E_{22A}=\frac{S_{22M}^P{S}_{11M}^A-e^{-2jkd}{S}_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A}{2e^{-2jkd}(S_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A)}$

$\±\frac{\sqrt{{(S_{22M}^P{S}_{11M}^A-e^{-2jkd}{S}_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A)}^2-4e^{-2jkd}{S}_{11M}^P{S}_{22M}^A(S_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A)}}{2e^{-2jkd}(S_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A)}$

$E_{22B}=\frac{S_{11M}^P{S}_{22M}^A-e^{-2jkd}{S}_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A}{2e^{-2jkd}(S_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A)}$

$\±\frac{\sqrt{{(S_{11M}^P{S}_{22M}^A-e^{-2jkd}{S}_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A)}^2-4e^{-2jkd}{S}_{22M}^P{S}_{11M}^A(S_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A)}}{2e^{-2jkd}(S_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A)};$

Wherein:

$S_{11M}^P=S_{11M}^R-S_{11M}^G,S_{22M}^P=S_{22M}^R-S_{22M}^G,S_{11M}^A=S_{11M}^T-S_{11M}^G,S_{22M}^A=S_{22M}^T-S_{22M}^G;$

Described calibration steps, wherein, in described step 7,

$S_{11}=\frac{\left(\frac{S_{11M}-E_{11A}}{E_{12A}{E}_{21A}}\right)(1+\frac{S_{22M}-E_{11B}}{E_{12B}{E}_{21B}}{E}_{22B})-\left(\frac{S_{21M}}{E_{21A}{E}_{21B}}\right)\left(\frac{S_{12M}}{E_{12A}{E}_{21B}}{E}_{22B}\right)}{S_D};$

$S_{21}=\frac{\left(\frac{S_{21M}}{E_{21A}{E}_{12B}}\right)}{S_D};$

$S_{22}=\frac{\left(\frac{S_{22M}-E_{11B}}{E_{12B}{E}_{21B}}\right)(1+\frac{S_{11M}-E_{11A}}{E_{21A}{E}_{12A}}{E}_{22A})-\left(\frac{S_{12M}}{E_{12A}{E}_{21B}}\right)\left(\frac{S_{21M}}{E_{21A}{E}_{12B}}{E}_{22A}\right)}{S_D};$

In above-mentioned formula, wherein:

$S_D=(1+\frac{S_{11M}-E_{11A}}{E_{12A}{E}_{21A}}{E}_{22A})(1+\frac{S_{22M}-E_{11B}}{E_{21B}{E}_{12B}}{E}_{22B})-\left(\frac{S_{21M}}{E_{21A}{E}_{12B}}\right)\left(\frac{S_{12M}}{E_{12A}{E}_{21B}}\right)E_{22B}{E}_{22A}.$

Use such scheme, it is possible to more effectively carry out free space material testing system calibration, separated by foundation Error term transmission/reflection model, it is not necessary to use precise clamp that dual-mode antenna is moved, only simple alignment part need to be utilized to tie Close time domain gate technique, complete the calibration of free space material electromagnetic parameter test system, it is to avoid to high precision measurement fixture Rely on.

Accompanying drawing explanation

Fig. 1 is free space material testing system configuration figure in prior art.

Fig. 2 is free space material testing system error model of the present invention.

Fig. 3 is free space error term schematic diagram of the present invention.

Fig. 4 is standard reflecting plate free space instrumentation plan of the present invention.

Fig. 5 is reflection measurement error model of the present invention.

Fig. 6 is straight-through instrumentation plan in the present invention.

Detailed description of the invention

Below in conjunction with the drawings and specific embodiments, the present invention is described in detail.

Embodiment 1

The invention discloses a kind of calibration steps for free space material electromagnetic parameter test system, due to free sky Between calibrating device be difficult to, systematic error is divided into arrow net error source and free space error source, is illustrated in fig. 2 shown below, Fig. 2 vows Net error source 21,22, free space error source 23,24.Only comprised the S in detected materials 25₁₁、S₁₂、S₂₁、S₂₂Parameter Matrix, vows that net error source can carry out full two-port calibration by traditional SOLT method, and after calibration, systematic error is only the most empty Between error source.It is illustrated in fig. 3 shown below.

In Fig. 3, E_11A、E_11BFor directional error, E_22A、E_22BFor source mismatch error, E_21A、E_12A、E_12B、E_21BFor transmission and Skin tracking error, due to passive in free space transmission, so E_21A=E_12A, E_12B=E_21B, therefore system has 6 errors, How to obtain, by measuring of standard component, the key that these 6 error terms are calibrations.

As Fig. 4 shows, adding thickness is the standard reflecting plate of d, the metallic plate of available enough sizes, and its ideally-reflecting is-1, I.e. S₁₁=-1, S₂₁=0, then error model signal flow diagram is converted to as shown in Figure 5.

Derivation signal flow diagram can get the relation of its measured value and error term, formula one:

$S_{11M}^R=E_{11A}-\frac{E_{12A}{E}_{21A}}{1+E_{22A}}$

In like manner can obtain, formula two:

$S_{22M}^R=E_{11B}-\frac{E_{12B}{E}_{21B}}{1+E_{22B}}$

Removing reflecting plate and carry out straight-through measurement, signal is equivalent to by thickness as shown in Figure 6 is the air of d, therefore S₁₁=S₂₂ =0, S₂₁=S₁₂=e^-jkd, wave number during wherein k is free space, then utilizing signal flow diagram to derive can obtain, formula three:

$S_{11M}^T=E_{11A}+\frac{e^{-2jkd}{E}_{12A}{E}_{21A}{E}_{22B}}{1-E_{22A}{E}_{22B}}$

Formula four:

In above-mentioned formula, e is natural constant, and j is imaginary unit, and k is wave number in free space, and d is standard reflection thickness of slab Degree.

E_11ACan be regarded as launching during 1 port output signal the reflection of antenna, its signal may utilize the time domain door merit vowing net Can extract, in the case of i.e. leading directly to, measure S₁₁, and carry out spatial transform, observe time domain waveform, it can be seen that the district that signal is stronger Territory is the reflection launching antenna, and time domain door is added on the S behind the stronger region of signal₁₁It is designated asObtain error term, formula Five:

$S_{11M}^G=E_{11A}$

In like manner, formula six is obtained

$S_{22M}^G=E_{11B}$

Can be tried to achieve 6 errors by above formula six, expression formula is, formula seven:

$E_{11A}=S_{11M}^G$

Formula eight:

Formula nine:

$E_{22A}=\frac{S_{22M}^P{S}_{11M}^A-e^{-2jkd}{S}_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A}{2e^{-2jkd}(S_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A)}$

$\±\frac{\sqrt{{(S_{22M}^P{S}_{11M}^A-e^{-2jkd}{S}_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A)}^2-4e^{-2jkd}{S}_{11M}^P{S}_{22M}^A(S_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A)}}{2e^{-2jkd}(S_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A)}$

Formula ten:

$E_{22B}=\frac{S_{11M}^P{S}_{22M}^A-e^{-2jkd}{S}_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A}{2e^{-2jkd}(S_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A)}$

$\±\frac{\sqrt{{(S_{11M}^P{S}_{22M}^A-e^{-2jkd}{S}_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A)}^2-4e^{-2jkd}{S}_{22M}^P{S}_{11M}^A(S_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A)}}{2e^{-2jkd}(S_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A)}$

Formula 11:

$E_{21A}=E_{12A}=\±\sqrt{(-E_{22A}-1)S_{11M}^P}$

Formula 12:

$E_{21B}=E_{12B}=\±\sqrt{(-E_{22B}-1)S_{22M}^P}$

Wherein

After obtaining error term, the relation between actual value and measured value can be derived by the signal flow diagram shown in Fig. 3 As follows, formula 13:

$S_{11}=\frac{\left(\frac{S_{11M}-E_{11A}}{E_{12A}{E}_{21A}}\right)(1+\frac{S_{22M}-E_{11B}}{E_{12B}{E}_{21B}}{E}_{22B})-\left(\frac{S_{21M}}{E_{21A}{E}_{21B}}\right)\left(\frac{S_{12M}}{E_{12A}{E}_{21B}}{E}_{22B}\right)}{S_D}$

Formula 14:

$s_{21}=\frac{\left(\frac{S_{21M}}{E_{21A}{E}_{12B}}\right)}{S_D}$

Formula 15:

$S_{22}=\frac{\left(\frac{S_{22M}-E_{11B}}{E_{12B}{E}_{21B}}\right)(1+\frac{S_{11M}-E_{11A}}{E_{21A}{E}_{12A}}{E}_{22A})-\left(\frac{S_{12M}}{E_{12A}{E}_{21B}}\right)\left(\frac{S_{21M}}{E_{21A}{E}_{12B}}{E}_{22A}\right)}{S_D}$

Formula 16:

$S_{12}=\frac{\left(\frac{S_{12M}}{E_{12A}{E}_{21B}}\right)}{S_D}$

Wherein, formula 17:

$S_D=(1+\frac{S_{11M}-E_{11A}}{E_{12A}{E}_{21A}}{E}_{22A})(1+\frac{S_{22M}-E_{11B}}{E_{21B}{E}_{12B}}{E}_{22B})-\left(\frac{S_{21M}}{E_{21A}{E}_{12B}}\right)\left(\frac{S_{12M}}{E_{12A}{E}_{21B}}\right)E_{22B}{E}_{22A}$

Real S parameter can be obtained by measurement data according to above formula, i.e. complete calibration.

This method is utilized to carry out the step of free space material testing system as follows:

1) utilize the methods such as tradition SOLT that vector network analyzer is calibrated in the case of being not connected with dual-mode antenna.

2) connect dual-mode antenna, in the middle of two antennas, place standard reflecting plate, as shown in Figure 4, utilize and vow that net records And

3) remove standard reflecting plate, do not place any material between i.e. two antennas, as shown in Figure 6, utilize and vow that net records And

4) do not place between two antennas under any material context, time domain door is set launching antenna-reflected position, utilizes and vow Net recordsAnd

5) formula five-formula 12 formula is utilized to try to achieve the error term shown in Fig. 3, E_11A、E_11B、E_22A、E_22B、E_21A、E_12A、 E_12B、E_21B。

6) place detected materials, record S_11M、S_21M、S_12M、S_22M。

Formula 10 three to formula 17 is utilized to calculate S₁₁、S₂₁、S₁₂、S₂₂, complete calibration.

It should be appreciated that for those of ordinary skills, can be improved according to the above description or be converted, And all these modifications and variations all should belong to the protection domain of claims of the present invention.

Claims (5)

1. the calibration steps for free space material electromagnetic parameter test system, it is characterised in that comprise the following steps:

Step 1: arrange and be not connected to dual-mode antenna, vector network analyzer is calibrated；

Step 2: arrange connection dual-mode antenna, places standard reflecting plate in the middle of described dual-mode antenna, utilizes vector network analysis Instrument records the S in two-port network scattering S parameter₁₁And S₂₂, it is designated as respectivelyAnd

Step 3: arrange connection dual-mode antenna, does not place any material in the middle of described dual-mode antenna, utilizes vector network analysis Instrument records the S in S parameter₁₁And S₂₂, it is designated as respectivelyAnd

Step 4: arrange connection dual-mode antenna, does not place any material in the middle of described dual-mode antenna, is launching antenna-reflected position Install time domain door, utilize vector network analyzer to record the S in S parameter₁₁And S₂₂, it is designated as respectivelyAnd

Step 5: utilize preset formula to calculate the error term of free space error source, E_11A、E_11B、E_22A、E_22B、E_21A、E_12A、E_12B、 E_21B, wherein, E_11A、E_11BIt is respectively two-port directional error, E_22A、E_22BIt is respectively two-port source mismatch error, E_21A、E_12A、 E_12B、E_21BFor transmission and skin tracking error；Described

$\begin{array}{c}{E}_{22A}=\frac{S_{22M}^P{S}_{11M}^A-e^{-2jkd}{S}_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A}{2e^{-2jkd}\left(S_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A\right)}\\ \±\frac{\sqrt{{\left(S_{22M}^P{S}_{11M}^A-e^{-2jkd}{S}_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A\right)}^2-4e^{-2jkd}{S}_{11M}^P{S}_{22M}^A\left(S_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A\right)}}{2e^{-2jkd}\left(S_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A\right)}\end{array}$

$\begin{array}{c}{E}_{22B}=\frac{S_{11M}^P{S}_{22M}^A-e^{-2jkd}{S}_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A}{2e^{-2jkd}\left(S_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A\right)}\\ \±\frac{\sqrt{{\left(S_{11M}^P{S}_{22M}^A-e^{-2jkd}{S}_{11M}^P{S}_{22M}^P-S_{22M}^P{S}_{11M}^A\right)}^2-4e^{-2jkd}{S}_{22M}^P{S}_{11M}^A\left(S_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A\right)}}{2e^{-2jkd}\left(S_{11M}^P{S}_{22M}^P-S_{11M}^P{S}_{22M}^A\right)}\end{array};$

Wherein:

$S_{11M}^P=S_{11M}^R-S_{11M}^G,$ $S_{22M}^P=S_{22M}^R-S_{22M}^G,$ $S_{11M}^A=S_{11M}^T-S_{11M}^G,$ $S_{22M}^A=S_{22M}^T-S_{22M}^G;$

Step 6: place detected materials in the middle of described dual-mode antenna, record S parameter and be designated as S_11M、S_21M、S_12M、S_22M；

Step 7: use predetermined formula to calculate S parameter matrix S in detected materials₁₁、S₂₁、S₁₂、S₂₂, complete calibration；Described

$S_{11}=\frac{\left(\frac{S_{11M}-E_{11A}}{E_{12A}{E}_{21A}}\right)(1+\frac{S_{22M}-E_{11B}}{E_{12B}{E}_{21B}}{E}_{22B})-\left(\frac{S_{21M}}{E_{21A}{E}_{21B}}\right)\left(\frac{S_{12M}}{E_{12A}{E}_{21B}}{E}_{22B}\right)}{S_D};$

$S_{21}=\frac{\left(\frac{S_{21M}}{E_{21A}{E}_{12B}}\right)}{S_D};$

$S_{22}=\frac{\left(\frac{S_{22M}-E_{11B}}{E_{12B}{E}_{21B}}\right)(1+\frac{S_{11M}-E_{11A}}{E_{21A}{E}_{12A}}{E}_{22A})-\left(\frac{S_{12M}}{E_{12A}{E}_{21B}}\right)\left(\frac{S_{21M}}{E_{21A}{E}_{12B}}{E}_{22A}\right)}{S_D};$

$S_{12}=\frac{\left(\frac{S_{12M}}{E_{12A}{E}_{21B}}\right)}{S_D};$

In above-mentioned formula, wherein:

$S_D=(1+\frac{S_{11M}-E_{11A}}{E_{12A}{E}_{21A}}{E}_{22A})(1+\frac{S_{22M}-E_{11B}}{E_{21B}{E}_{12B}}{E}_{22B})-\left(\frac{S_{21M}}{E_{21A}{E}_{12B}}\right)\left(\frac{S_{12M}}{E_{12A}{E}_{21B}}\right)E_{22B}{E}_{22A}.$

2. calibration steps as claimed in claim 1, it is characterised in that holding of described step 2, described step 3 and described step 4 Row order is to exchange mutually.

3. calibration steps as claimed in claim 2, it is characterised in that in described step 2,

$S_{11M}^R=E_{11A}-\frac{E_{12A}{E}_{21A}}{1+E_{22A}};$ $S_{22M}^R=E_{11B}-\frac{E_{12B}{E}_{21B}}{1+E_{22B}}.$

4. calibration steps as claimed in claim 3, it is characterised in that in described step 3,

$S_{11M}^T=E_{11A}+\frac{e^{-2jkd}{E}_{12A}{E}_{21A}{E}_{22B}}{1-E_{22A}{E}_{22B}};$ $S_{22M}^T=E_{11B}+\frac{e^{-2jkd}{E}_{12B}{E}_{21B}{E}_{22A}}{1-E_{22A}{E}_{22B}},$

Wherein: e is natural constant, j is imaginary unit, and k is wave number in free space, and d is standard reflection plate thickness.

5. calibration steps as claimed in claim 4, it is characterised in that in described step 4,