From the material presented here, it is clear that the theory underlying the SNF approach is complex and involved to implement. However, it is also very elegant and provides one with many measurement options and powerful capabilities. The numerical implementation of the theory can be efficiently deployed through the use of the fast Fourier transform (FFT) enabling transforms of even electrically large antennas to be accomplished in a matter of a few seconds on a modern powerful digital computer. With the advent of commercially available SNF test systems, the user can exploit these techniques, largely unimpeded by the burden of the theory or the implementation thereof. The material presented here highlighted some of the fundamental concepts and limitations the user needs to be aware of in order to use these test systems with confidence.

/pdf/spherical-near-field-antenna-measurements-5bm37osur0.pdf

Spherical near-field antenna measurements

The calibration of the Soil Moisture and Ocean Salinity (SMOS) payload instrument, known as Microwave Imaging Radiometer by Aperture Synthesis (MIRAS), is based on characterization measurements which are performed initially on-ground prior to launch and, subsequently, in-flight. A good calibration is a prerequisite to ensure the quality of the geophysical data. The calibration scheme encompasses both the spaceborne instrument and the ground data processing. Once the system has been calibrated, the instrument performance can be verified, and the higher level geophysical variables, soil moisture and ocean salinity, can be validated. In this paper, the overall calibration approach is presented, focusing on the main aspects relevant to the SMOS instrument design and mission requirements. The distributed instrument, comprising 72 receivers, leads to a distributed internal calibration approach supported by specific external calibration measurements. The relationship between the calibration data and the routine ground processing is summarized, demonstrating the inherent link between them. Finally, the approach to the in-flight commissioning activities is discussed.

http://ieeexplore.ieee.org/iel5/36/4456805/04447655.pdf

SMOS Calibration

A complete antenna pattern characterization procedure for spherical near-field antenna measurements employing a high-order probe and a full probe correction is described. The procedure allows an (almost) arbitrary antenna to be used as a probe. Different measurement steps of the procedure and the associated data processing are described in detail, and comparison to the existing procedure employing a first-order probe is made. The procedure is validated through measurements.

/pdf/theory-and-practice-of-the-fft-matrix-inversion-technique-6yt1ifb4sw.pdf

Theory and Practice of the FFT/Matrix Inversion Technique for Probe-Corrected Spherical Near-Field Antenna Measurements With High-Order Probes

An iterative probe correction technique is presented to correct for the errors caused by the application of the traditional first-order probe correction for nonideal first-order probes in spherical near-field antenna measurements. The technique may be applied, with certain restrictions, for more general probes as well.

/pdf/iterative-probe-correction-technique-for-spherical-near-47scgu0j5y.pdf

Iterative probe correction technique for spherical near-field antenna measurements

[1] In this paper, an odd-order probe for spherical near-field antenna measurements is defined. A probe correction technique for odd-order probes is then formulated and tested by computer simulations. The probe correction for odd-order probes is important, since a wide range of realistic antennas belongs to this class. To the authors' knowledge, the proposed technique is the first practical high-order probe correction technique that has been formulated in detail, has been tested, and has been shown to work.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2004RS003063

Odd-order probe correction technique for spherical near-field antenna measurements

An investigation is performed to study the error of the far-field pattern determined from a spherical near-field antenna measurement in the case where a first-order ( 1) probe correction scheme is applied to the near-field signal measured by a higher-order probe. INTRODUCTION A spherical near-field antenna measurement with probe correction is a well-established technique for the determination of the far-field pattern of an antenna Hansen[1]. For so-called first-order probes, that are probes with the spherical mode index 1 and thus a simple sinusoidal azimuthal pattern, the probe correction is particularly efficient, and such probes are employed extensively at the DTU-ESA Spherical Near-Field Antenna Test Facility [2]. However, in some cases it may be advantageous to employ so-called higher-order probes for which the spherical mode content is non-zero for 1. As well, it may be difficult to manufacture probes with insignificant mode content for 1 . The purpose of the present investigation is to demonstrate, by a series of simulated measurements, the errors of the far-field pattern that result from the use of the first-order ( 1) probe correction scheme in the case where the near field is measured with a higher-order probe. In particular, this effect is investigated for off-axis measurements where the antenna under test (AUT) is not centered on the axis of the probe. The level of the errors will demonstrate the necessity of developing an efficient higher-order probe correction technique. THEORY It is beneficial to make use of the spherical wave expansion when a spherical near-field measurement confuguration is used to sample the radiated field of an antenna under test (AUT). The radiated field is then expressed as a finite series of spherical vector wave functions: 2 (3) 1 1 ( , , ) ( , , ), N n smn smn s n m n E r k Q F r (1) where k is the wave number, is the intrinsic impedance, N is the truncation number for the significant spherical wave modes, smn Q are the mode coefficients, and (3) ( , , ) smn F r are the spherical vector wave functions Hansen[1], with r, and being the coordinates of the standard spherical coordinate system. The determination of the far field by a probe-corrected spherical near-field measurement is based on solving the Q coefficients of the radiated field of the AUT from the measured signal using the probe-corrected transmission formula Hansen[1]: 2 N n i i 1 1 ( , , , ) ( ) ( ) m n smn m s n s n m n w A Q e d e P kA , (2) where w is the signal received by the probe, A is the measurement distance, is the probe rotation angle and ( ) im n i m e d e and are the rotation coefficients and the probe response constants, respectively. It is noted that the transmission formula (2) includes a summation over all the the -modes of the probe. ( ) s n P kA SIMULATED MEASUREMENTS The AUT model and the general probe model are presented in Fig. 1 and in Fig. 2 (a), respectively. The AUT is a set of four Huygens sources. Four different probes were generated from the general probe model. For each of the four probes the complex excitations of the electric Herzian dipoles 1, 2, 3, 4, 5 and 6 were , 1, , i i, and i , respectively, where = 1, 0.3, 0.1 and 0.03 for the probes 1, 2, 3 and 4, respectively. The normalized power spectra of the azimuthal spherical modes of the four probes, corresponding to the different values, are shown in Fig. 2 (b). It is seen that all the power is on odd-valued -modes, and the power of each of the probes on modes 7 is insignificant for most practical purposes. Importantly, the power

Errors of first-order probe correction for higher-order probes in spherical near-field antenna measurements

High-Accuracy Calibration of the SMOS Radiometer Antenna Patterns at the DTU-ESA Spherical Near-Field Antenna Test Facility

Aspects of Antenna Pattern Characterization of an L-Band Space Radiometer

In this paper, an electrical performance verifi- cation methodology for large reflector antennas is pro- posed. The verification methodology was developed for the BIOMASS P-band (435 MHz) synthetic aperture radar (SAR), but can be applied to other large deployable or fixed reflector antennas for which the verification of the entire antenna or payload is impossible. The two-step methodology is based on accurate measurement of the feed structure characteristics, such as complex radiation pattern and radiation efficiency, with an appropriate measurement technique, and then accurate calculation of the radiation pattern and gain of the entire antenna including support and satellite structure with an appropriate computational soft- ware. A preliminary investigation of the proposed meth- odology was carried out by performing extensive simulations of different verification approaches. The experimental validation of the methodology included measurements of the prototype BIOMASS feed in several structural configurations with spherical, cylindrical, and planar near-field techniques. The measured characteristics for the feed structure were then used in the calculation of the radiation pattern and gain of the entire reflector antenna. The main emphasis of the work was on the assessment of the achievable pattern and gain uncertainty for the entire antenna and its compliance with the BIO- MASS SAR requirements.

/pdf/electrical-performance-verification-methodology-for-large-4oyn3jbd6k.pdf

Electrical performance verification methodology for large reflector antennas: based on the P-band SAR payload of the ESA BIOMASS candidate mission

The typical antenna measurement system setup working above 20 GHz makes use of frequency multipliers and harmonic mixers, usually working in standard waveguide bands, and thus several parts need to be procured and interchanged to cover several frequency bands. In this paper, we investigate an alternative solution which makes use of a standard wideband VNA without external frequency conversion units. The operational capability of the Planar Near-Field (PNF) Antenna Measurement Facility at the Technical University of Denmark was recently extended to 60 GHz employing an Agilent E8361A VNA (up to 67 GHz). The upgrade involved procurement of very few additional components: two cables operational up to 65 GHz and an openended waveguide probe for tests in U-band (40-60 GHz). The first tests have shown good performance of the PNF setup: 50-60 dB dynamic range and small thermal drift in magnitude and phase, 0.06 dB and 6 degrees peak-to-peak deviations over 4 hours. A PNF measurement of a 25 dBi Standard Gain Horn was carried out and the results were compared to those from the DTU-ESA Spherical Near-Field Facility with a good agreement in the validity region. Uncertainty investigations regarding cable flexing effects at 60 GHz have shown that these introduce an uncertainty of about 0.02 dB (1 sigma) around the main beam region indicating a very good performance of the PNF setup.

/pdf/60-ghz-antenna-measurement-setup-using-a-vna-without-7004gby1um.pdf

Jeppe Majlund Nielsen

Papers

Errors of first-order probe correction for higher-order probes in spherical near-field antenna measurements

High-Accuracy Calibration of the SMOS Radiometer Antenna Patterns at the DTU-ESA Spherical Near-Field Antenna Test Facility

Aspects of Antenna Pattern Characterization of an L-Band Space Radiometer

Electrical performance verification methodology for large reflector antennas: based on the P-band SAR payload of the ESA BIOMASS candidate mission

60 GHz antenna measurement setup using a VNA without external frequency conversion