Some standard content:
Standard of the Ministry of Electronics Industry of the People's Republic of China Antenna Test Method
Impedance Measurement
This standard applies to the impedance measurement of antennas. 1 Measurement of input impedance
1.1 Influence of input impedance
SJ2534.1485
The input impedance of the antenna at a specified end pair (or port) affects the interaction between the antenna and its associated circuits. Antenna impedance is an important factor when considering power conversion, noise and stability of active circuit components. It is usually the impedance of the antenna that limits the useful bandwidth of the antenna.
1.2 Several optimal impedance relationships
The optimal impedance relationship between the antenna and its associated circuit depends on the application. In some receiving situations, in order to obtain the minimum noise figure, the antenna impedance should be lower than the matching impedance. In some transmitting situations, in order to obtain near-maximum power efficiency, it may be required that the antenna impedance is greater than the equivalent output impedance of the source at the antenna terminals. However, in most applications, the matching condition is ideal. That is, the antenna and the circuit are conjugate matched, and the antenna obtains the maximum power at this time. 1.3 Mismatch loss
When the conjugate matching condition is not met, part of the effective power will be lost, as shown in the following formula: Piast
PavailablewwW.bzxz.Net
Where: Pavailable
Pliost
effective power;
lost effective power;
input impedance of an antenna;
input impedance of the circuit at the antenna end;
the conjugate complex number of the circuit input impedance.
This formula is useful in measuring the power gain of a mismatched system. Zant -Zcet
Zant +Z*cet
Most antennas are connected to electronic circuits via transmission lines, and the required matching or mismatching can be adjusted at either end of the transmission line. However, in practice, it is usually more advantageous to achieve matching closer to the antenna port, which generally minimizes the loss of the transmission line and the peak voltage on the line, while maximizing the useful bandwidth of the system. When the antenna is not ideally matched to the transmission line, a reflected wave is generated on the transmission line; the ratio of reflected power to transmitted power is:
Prerl= JZant -Zo
Zant +Zo
-characteristic impedance of the transmission line.
Where Z
This ratio is related to the voltage reflection coefficient and the voltage standing wave ratio (VSWR) by the standard transmission line formula: Pren
VSWR-1
VSWR+1
·(3)
If the characteristic impedance of the transmission line is a pure real number and the electronic circuit and the transmission line are ideally matched, the useful power loss caused by the reflection at the antenna port can be obtained from this ratio.
1.4. Special issues in measuring antenna input impedance Issued by the Ministry of Electronics Industry of the People's Republic of China on January 5, 1985 1
Implementation on July 1, 1986
SJ2534.14—85
The input impedance is measured at the antenna. The measurement method is the same as that of ordinary impedance measurement for a single port. However, there is an inherent special problem in radiating mechanisms, namely that the input impedance varies with the external environment of the antenna. For this reason, when making impedance measurements, the antenna should be set up in a location that simulates its operating environment. Usually this requirement is easy to approach for narrow-beam antennas because the beam of such antennas can be pointed away from reflective obstacles. However, this may be more difficult for omnidirectional antennas because the surrounding structures affect the input impedance.
1.5 Measurement method
Impedance or complex reflection coefficient measurements can be made using impedance bridges or slotted lines at frequencies where these techniques can be applied. Due to the increasing adoption of wideband, swept frequency network analyzer systems can be used for many applications. To measure the impedance or the scattering matrix of the entire network. Network analyzers are suitable for computer-controlled data collection, storage and display, and are also suitable for analog display using XY recorders and optical displays. In the microwave frequency band, the measuring instrument is characterized by the use of various overlapping diagrams to link the trajectory of the reflection coefficient with the impedance coordinates to automatically sweep the complex reflection coefficient and transmission coefficient. Usually two sets of coordinate systems are used, one set contains the resistance and reactance components of the impedance, and the other set contains the magnitude and phase of the impedance. The impedance measured in this way is the normalized impedance relative to the reference transmission line impedance. This form is very convenient at microwave frequencies where the transmission line is universal. 2 Mutual impedance measurement in array antennas
2.1 Overview
In array antennas, there are usually interactions between array elements. This interaction seriously affects the performance of the antenna. 2.1.1 Measures of interaction
2.1.1.1 Mutual impedance
An important measure of interaction is the mutual impedance between antenna array elements. The mutual impedance between any two elements is defined as follows: m
where: 1, the current at the reference point of the nth excitation element (4)
V the voltage generated at the reference point of the mth element when all elements except the excitation element are open circuited at their respective reference points. 2.1.1.2 Cross-coupling coefficient
The mutual coupling between two elements can also be described by the incident wave, reflected wave and coupled wave measured on the transmission line connected to each element. This coupling is described by the cross-coupling coefficient of the scattering matrix: S
where: a. The incident wave of the excitation element n at the reference point of the transmission line; bm
bm the wave received in the transmission line of the mth element when all array elements except the excitation element are connected to matching loads. 2.1.1.3 Factors affecting mutual coupling
(5)
The measure of mutual impedance or mutual coupling depends on the type of element, the size and spacing of the elements in wavelengths, the geometric arrangement of the elements and their surroundings. In general, the magnitude and phase angle of mutual impedance or mutual coupling decrease with increasing spacing. 2.1.2 Selection of reference points
The reference terminals (or ports) at which the currents and voltages are determined may be chosen as most convenient. But in any case they should be specified. In tower radiators of broadcast array antennas, the reference terminal is usually chosen at the bottom of the tower, even though the towers may be of different heights. The influence of the bottom insulator can usually be deducted from the measured value. In dipole arrays used for frequencies such as high or very high frequencies, the reference terminal is generally chosen at the point of maximum current in the radiating part of the element. In this case the current distribution in the dipole itself can be measured using a ring probe mounted in the dipole slot, or it can be measured along the surface of the dipole. In microwave arrays, the reference point is often selected at the waveguide port connected to the unit, or on the array surface of the waveguide unit. When the unit feeding transmission line is uncertain or unequal in length, the symmetrical probe method is used to ensure consistent measurement of the reference surface. 2.1.3 Self-impedance and complex reflection coefficient
For a given antenna array, the impedance matrix or scattering matrix can be composed of a set of mutual impedance values Zm or a set of scattering coefficients. These matrices 2
SJ2534.14—85
include self-impedance B or complex reflection coefficient Sm. These two types of matrices are related to each other and can be transformed from one type to another according to matrix transformation. The above-mentioned self-impedance term or reflection coefficient term is the characteristic of each unit independently excited in an array environment under specific load conditions. It is different from the active impedance or active reflection coefficient when the entire array is excited. In the latter case, the value measured at the port is affected by the mutual coupling of the entire array. 2.1.4 Active Impedance and Active Reflection Coefficient
The active impedance or active reflection coefficient is defined as the input impedance or reflection coefficient of a cell when all other cells are excited according to the actual operating conditions of the array.
Generally speaking, the active input impedance or active reflection coefficient will change with changes in the excitation because changes in the relative phase or relative amplitude of the excitation produce different terminal resultant voltages. Such changes occur, for example, when the directivity pattern of a broadcast tower array is being adjusted or when the main lobe of a large planar array is being scanned. If the impedance or scattering matrix is known, the active impedance or reflection coefficient of all cells in the array can be calculated under all scanning conditions. 2.1.5 The role of the partially filled scattering matrix
For microwave arrays, the scattering matrix is convenient for both measurement and analysis. By transforming the excitation throughout the array according to a specified set of array incident wave excitations, the performance of the active array can be determined using a partially filled scattering matrix consisting only of the external coupling coefficients of the given excited cells. The advantage of this method is that it does not require the inversion of a large impedance matrix or the determination of a complete scattering matrix. 2.2 Measurement methods
The measurement methods of interaction vary depending on the operating frequency band and array type, and are usually divided into the following categories. 2.2.1 Method 1
In antenna arrays in the medium frequency range, mutual impedance is usually determined directly from voltage and current measurements. For example, in broadcast antenna arrays composed of electrically small tower elements, the mutual impedance of such an array must be known to determine the branch circuit and coupling circuit requirements, which will provide the appropriate current amplitude and phase relationship within the array elements to produce the required directional characteristics. Such antenna arrays usually use tower elements arranged in an irregular geometry. The elements are of different heights, and the current amplitudes are different and the phase relationship is also irregular, in order to produce the radiation pattern that best matches the required coverage area. In antenna arrays containing only a few radiating elements, the non-repeating relative position of each element in the array can lead to significant differences in mutual impedance between different pairs of elements. At very low frequencies, antenna arrays are generally not used, however, when they are used, the mutual impedance is determined by direct measurement of current and voltage as in the medium frequency case. 2.2.2 Method 2
In the higher RF range, when current and voltage are not easy to measure, the mutual impedance between array elements can be derived from the input impedance measured at their reference point under appropriate conditions. Terminal conditions of two or more coupled elements are required, such as short-circuited and open-circuited terminals, in which case the self-impedance of the element can be measured. Such measurements are based on strict analogies between the currents and voltages between the terminals in a radiating element system and the currents and voltages in the corresponding circuit network. Such measurements are usually made using impedance bridges, admittance meters, or vector voltmeters. 2.2.3 Method 3
UHF and microwave antennas consist of a large number of identical, regularly arranged radiating elements, with the mutual impedance between all pairs of elements having the same relative position being approximately constant, except for some pairs of elements near the edge of the array. In this case, measurements are sometimes made on a finite-size array. The array has the same element design and arrangement as the prototype array. Measuring the mutual impedance or mutual coupling between the elements near the center of the finite array allows the determination of a local impedance or scattering matrix that is approximately equal to that of the prototype array. However, caution should be exercised in interpreting the results in terms of various aspects of array performance, such as the active impedance or the gain and shape of the element pattern as a function of beam scanning.
When the mutual impedance value of a finite array is used for a prototype array, the above quantity will have a significant error due to the neglect of the effect of the missing elements in the finite array.
To overcome this shortcoming, waveguide technology can be used to simulate the effect of the elements in an infinite array. This technology is based on the mirror theory and results in the pointing of a finite number of discrete beams. 3
Additional Notes:
SJ2534.14-85
This standard was proposed by the Standardization Research Institute of the Ministry of Electronics Industry. This standard was drafted by the 39th Institute of the Ministry of Electronics Industry. The main drafters of this standard are Ke Shuren, Zhang Yifeng, and Wang Shuhui. 4
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