CLIAMS:1. A method for measurement of radiation pattern of a linear array antenna, said method comprising steps of:
placing a probe parallel opposite to said linear array antenna such that center of the probe is dimensionally coinciding with center of a radiating element that is configured in said linear array antenna, wherein said probe is within reactive near field of said linear array antenna and gets proximity coupled to said radiating element;
placing an excitation source at feed of said linear array antenna to induce electric/magnetic field at said probe; and
connecting a Vector Network Analyzer between said probe and said linear array antenna and measuring amplitude and phase of said field that is induced in said probe.
2. The method of claim 1, wherein said method is implemented at bench level and is not location specific and does not require microwave absorbers around antenna.
3. The method of claim 1, wherein distance between said probe and said radiating element is maintained in a range of ~ ? /5 to ? /100.
4. The method of claim 1, wherein an antenna element of said linear array antenna is used as said probe.
5. The method of claim 1, wherein said method provides low power level at said Vector Network Analyzer to minimize nearby reflections in said linear array antenna to ensure stable reading during measurements.
6. The method of claim 1, wherein said method is repeated for one or more radiating element.
7. The method of claim 1, wherein measurement points are based on number of antenna elements that form part of said linear array antenna under test.
8. The method of claim 1, wherein said method provides diagnostic results in respect of performance of each of said radiating elements in said linear array antenna.
9. The method of claim 1, wherein said measured amplitude and phase is plotted using MATLAB program to get radiation pattern of said linear array antenna.
10. The method of claim 1, wherein said measured amplitude and phase is plotted using a standard tool/program to get radiation pattern of said linear array antenna.
,TagSPECI:TECHNICAL FIELD
[0001] The present disclosure generally relates to the field of measurement of radiation pattern of a linear array antenna. More particularly, it pertains to a bench level method that measures radiation pattern in reactive field of the linear array antennas.
BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Antennas when excited with RF source radiate electromagnetic field into free space. In general, the electromagnetic field radiated from an antenna has a reactive electromagnetic field within a predetermined region, and the predetermined region is referred to as a reactive near-field region. Beyond the reactive near-field region the radiated electromagnetic field becomes greater than the reactive electromagnetic field and the region is referred to as a radiating near-field region. The near-field region is also referred to as the Fresnel region, and the electromagnetic field existing in this region is referred to as the Fresnel electromagnetic field.
[0004] The electromagnetic fields radiated from the antenna are transformed almost to plane waves beyond the Fresnel region. The region is referred to as a far-field region or the Fraunhofer region, and electromagnetic field that exists in this region is referred to as a far-field.
[0005] FIG. 1 illustrates a typical distribution of radiation field at different distances from an antenna as is known in the art. As stated and illustrated in FIG. 1 the radiation field around antenna can be divided into three zones depending on the field distribution with respect to the distance R from Antenna under Test (AUT). The three radiating zones with respect to distance R from an antenna having maximum dimension D are defined as under:
Reactive Zone: R= ?/2p or R< 0.62 v(D3 / ?)
Near Field Zone (Frasnel Zone): ?/2p =R=2D2/ ?
Far field Zone (Fraunhofer Zone): R=2D2/ ?

[0006] The field distribution around antenna is defined as radiation pattern. The radiation pattern is a graphical depiction of the relative field strength transmitted from or received by the antenna. Radiation patterns are typically measured at an Outdoor Test Range (Far Field Measurements) or an Indoor Test Range like Compact Range and Near Field Test Range. Any antenna irrespective of its type is required to be tested at one of these test sites.
[0007] Far Field Measurements at an Outdoor Test Range is the first technique of measuring radiation pattern of an antenna. Beyond a particular distance from antenna, known as Raleigh distance, field strength decreases inversely with distance. Measurements in the far field region of an antenna fall within such a zone and absorption of the radiation by probe does not affect the radiation from the antenna. However this requires a minimum distance between the probe and the AUT. The distance between the antennas and probe depends on the wavelength and size of the antenna. A far field region distance R must satisfy following two conditions:

R = D (where, D is the largest physical dimension of the antenna)
R = ?
These two parameters are related as given below.
R=2D2/ ?
Other than minimum distance, the height of the antenna from the ground during far field measurement is also very critical to avoid multi path reflections from the surroundings.
[0008] Antenna for which radiation pattern is to be measured is rotated full 360 deg with respect to the probe at some specified speed depending on the antenna beamwidth. The amplitude and phase are measured using a Vector Network Analyzer at fixed interval during 360 deg rotation. Typically for a beam width of 10 deg, measurements are taken at 0.5 deg interval, thus amplitude and phase is measured for total 720 points. The measured amplitude and phase are then plotted using software. This measurement typically can be done in < 30 minutes. However antenna mounting and other logistics activities may take about a day time(>8hrs) or more.
[0009] The Far filed Radiation Measurement requires High Rise Towers for antenna mounting, RF Source, High Gain Antenna for Electromagnetic Radiation, A Vector Network Analyzer, Positioner, Control unit for positioner, Real Time Data Acquisition Software and Plotting Software for Radiation Patterns. For radiation measurements, the antenna is required to be mounted on the positioner using dedicated/customized mounting fixtures which interfaces antenna to the positioner top. Antenna mounting requires logistic and is a very time consuming activity. Before mounting the antenna on to the positioner, for health check, VSWR (Return Loss) is measured at the antenna ports.
[0010] In array antennas where same radiating element is used in multiple quantities, VSWR measurement is not sufficient to give any indication of non-radiating element in the array. A non-radiating element in the array significantly adversely affects the Side Lobe level which is a very critical parameter for any antenna. This fault can only be detected after radiation measurements, that too by using special methods & procedures in NFTR. It is not possible to detect any fault in Far Field measurements. For fault diagnosis, antenna has to be brought down from the positioner and then after rectification need to be mounted again on positioner for measurements. This consumes significant time and engages personnel.
[0011] In Near Field (or RadiativeNear Field) Radiation Measurements, which is the second technique of radiation measurements of an antenna, absorption of the radiation (by probe) does effect the radiation from the antenna. Typical distance between the Antenna and the probe is 3-10 ?. As the probe effects the radiation from the antenna, theoretical compensations are required for plotting the Radiation Patterns.
[0012] This method requires very precise measurement of radiated field and should be free from multipath reflections. To avoid multipath reflections, an Indoor Chamber with microwave absorbers is required. The test set up requires, a Vector Network Analyzer, Positioner, Control unit for positioner, precise mechanism for Probe movement, Real Time Data Acquisition Software and Plotting Software for radiation patterns.
[0013] Mounting of the antenna is very critical as probe distance from the Antenna (AUT) is mathematically used for plotting the radiation pattern. The amplitude and phase are measured at finite intervals in both Azimuth and Elevation Planes. Typically Planar Measurements are carried out in Radiative Near Field. This requires scanning of the antenna aperture at