Antenna vector characterization in the mm- and submm-wave regions.
Development of antenna measurements in the microwave domain has been intense for many years. In the centimeter-wave region, vector analyzers permit good measurements in anechoic chambers. The matching of the antenna can be provided from the measured [S.sub.11] parameter by rotating the antenna illuminated by a far-field source, and the angular antenna pattern can be achieved from an [S.sub.21] measurement.
In the less familiar mm-wave frequency region, the anechoic chamber problems become less difficult to solve, since a small chamber can appear large as compared to the wavelength. The far field conditions are obtained with moderate distances. Consider a feed horn with an aperture A of 15 mm at 100 GHz (wavelength [Lambda] = 3 mm). The antenna pattern of this horn can be obtained by rotating it around its phase center and illuminating it with 100 GHz radiation coming from a similar horn placed at a distance L. The well-known far field condition L[is greater than][2A.sup.2]/[Lambda], gives L[is greater than]1.5 m, which is quite reasonable. The anechoic chamber becomes better, but the standard vector analyzers become less effective and more expensive with increasing frequency. Moreover, they are limited to frequencies below 110 GHz.
Above 110 GHz, antenna scalar measurements have been performed using mm-wave sources, such as a Gunn oscillator, a Gunn followed by a Schottky-diode harmonic generator, a BWO or a carcinotron, and video or bolometric detection. These methods must be adapted to each peculiar frequency requirement. The linearity of the detector's response must be calibrated. Video detectors are small and light. They can be attached directly to the rotating horn, but they may not be sensitive enough. Liquid helium-cooled bolometers are extremely sensitive, but they cannot be moved easily and good mm-wave waveguide rotating joints are difficult to produce.
The ideal detection for antenna characterization must be vector, resulting in phase determinations that are fast and unambiguous. Figure 1 shows the determination of the phase center of a conical horn in its H-plane at 285 GHz using lateral steps of 0.2 mm of axis rotation. Figure 2 shows the determination of the phase center of the same horn using longitudinal steps of 1 mm. The amplitude measurements do not show any appreciable change with geometrical position. The detected phase variations are sensitive. Video or bolometric scalar detections are thus not appropriate. In this paper, a method using a vector network analyzer above 110 GHz to perform vector measurements in the mm- and submm-wave regions is shown.
The basic configuration of the described mm-/submm-wave vector analyzer uses Schottky diode harmonic generators (HG) as millimeter sources. Each of these sources covers a waveguide mm-wave band using frequency multiplication from an 8 to 18 GHz (centimeter) sweeper. As in all vector network analyzers, heterodyne detection is used, since this technique fulfills the requirements of phase information, good sensitivity and good linearity. In a frequency downconversion performed with a heterodyne mixer, where [P.sub.if] is the power of the beat signal and [P.sub.rf] is the power of the incident signal, the linearity can be expressed as
[P.sub.if] (dBm) = [P.sub.rf] (dBm) - L (dB)
where the conversion loss L is constant for a given frequency, assuming one works with a convenient local oscillator power and below the mixer saturation level. Since tunable mm-wave sources are not widely available, a harmonic mixer (HM) fed by a second centimeter sweeper is used instead of a simple mixer as a detector.
The source side (Schottky diode HG fed by the first centimeter sweeper) and the detection side (Schottky diode HM fed by the second centimeter sweeper) are quite symmetrical. The same harmonic order N (ratio of the mm-wave frequency to the cm-wave sweeper frequency) is used on both sides. The second sweeper is maintained at a fixed frequency difference from the first, and their relative phase noise is canceled by the phase loop control. As a result, the relative phase noise of the same successive harmonics of the first and second sweepers are also canceled. This point has important consequences.
Any vector receiver in a vector analyzer has a minimum of two inputs, one for the signal through the DUT and the other for a phase reference signal. In standard mm-wave vector analyzers, this phase reference signal is obtained from a detection chain similar to the detection through the DUT, sampling a signal taken directly from the mm-wave source through a directional coupler. In the vector analyzer, the phase reference comes directly from the reference oscillator that defines the frequency and phase interval between the first and second sweepers.
Based on this principle, mm-wave directional couplers are not necessary for transmission measurements, which widely opens the submm-wave region. A single HM detector can be used and mm-wave ports can be compact, lightweight and inexpensive. Since these mobile mm-wave ports are linked to the analyzer by simple coaxial cables, which can be as long as 5 m each, measuring moving parts is made easy and accurate. At 100 GHz, the typical phase jitter is below 0.1 [degree] (standard deviation for 10 measurements in 1 s), and the phase drift is below a few degrees per hour.
The measurement of the [S.sub.11] return loss of the antenna needs a directional coupler. This calibration method for reflection uses a short, a sliding short and a sliding matched load. Figure 3 shows the measured return losses of feed horns[2,3] from 70 to 110 GHz. Above 110 GHz, due to the lack of directional couplers, no [S.sub.11] parameter was measured. The focus is on the [S.sub.21] parameter measurement used in the angular antenna pattern determination with the simple through calibration procedure.
Dynamic Range and Frequency Coverage
Comparing a harmonic mixer to a simple mixer, Equation 1 is still valid, but the conversion loss L increases with frequency. Below 190 GHz, the dependence of
L (dB) = 22 + 0.15f (GHz) (2)
is observed. The sensitivity of the heterodyne detection can be expressed in terms of signal-to-noise ratio (S/N), or dynamic range (maximum measurable attenuation), obtained by comparing the detected signal [P.sub.if], to the noise level of the receiver [P.sub.n]
S/N (dB) =
[P.sub.rf] (dBm) - L (dB) - [P.sub.n] (dBm) (3)
The vector analyzer's receiver presents a low noise figure of
[P.sub.n] = -150 dBm (4)
This low noise level is observed in the equivalent detection bandwidth of 10 Hz (10 measured points/s). Knowing that the available mm-wave power [P.sub.rf] extracted from the HG can be in the order of 0 dBm up to 60 GHz (from a quadrupler), or -10 dBm up to 110 GHz (from a sextupler), the available dynamic range S/N will be in the order of 120 dB up to 60 GHz, and 100 dB up to 110 GHz. These dynamic range values permit comfortable measurements of antennas. With a typical insertion loss of 20 dB between horns facing each other in far field conditions, side lobes can be observed down to -80 dBc.
In the 110 to 190 GHz interval, using the harmonics 7 to 11 of a WR-6 HG and a power of
[P.sub.rf] (dBm) = 3 - 0.225 f (GHz) (5)
a dynamic range S/N greater than 60 dB at any frequency up to 190 GHz is obtained. Clean measurements are still relatively easy. Figure 4 shows the tests performed on a conical horn-lens antenna at 143 GHz, which were taken twice to demonstrate the repeatability. When using only the horn, a broad peak in amplitude is observed with a phase that is not stationary. When adding a Teflon lens at the horn aperture, the gain on the axis is 10 dB, the angular dB dependence is a parabola around the axis and the phase is stationary, resulting in a Gaussian beam pattern. The phases are shown reversed in sense for clarity.
If a broad dynamic range is needed, the small mm-wave power supplied by the HG can be replaced with a more suitable power from a Gunn oscillator phase-locked to the analyzer. For example, an 80 mW ([P.sub.rf] = 19 dBm) Gunn source at 50 GHz, associated with the analyzer and its harmonic detector, supplies a dynamic range of approximately 140 dB.
For frequencies above 200 GHz, mm-wave Gunn oscillators, phase-locked to the analyzer reference, and followed by Schottky diode harmonic generators and/or harmonic mixers, can also be used as relay sources. This setup involves breaking up the high harmonic number N, the ratio of mm-/submm-wave frequency and the 8 to 18 GHz LO frequency, into a chain of smaller multiplication factors. Compared to the simple use of the centimeter sweepers followed by harmonic generators, the dynamic range is enhanced, but changing the frequency requires mechanical tuning of the Gunn oscillators. Electronically-driven frequency sweeps of the Gunn oscillators are restricted to typically [+ or -]100 MHz. In contrast, jumping from one harmonic to the next is fast and easily achieved by changing the tuning of the vector receiver. A wide frequency interval can be covered on many different harmonics of the Gunn oscillator.
Using a 95 GHz Gunn, followed by a harmonic generator, dynamic ranges on successive harmonics can be observed on a standard WR-6 oversized waveguide harmonic mixer as 110 dB at 190 GHz, 95 dB at 285 GHz, 82 dB at 380 GHz, 70 dB at 475 GHz and 50 dB at 560 GHz. Figure 5 shows the behavior (Gaussian beam, with -20 dBc side lobes) of a scalar horn observed at 285 GHz in the E- and H-planes. The H-plane was offset by -10 dB for clarity.
When using two 95 GHz Gunn oscillators, one as a local oscillator for a frequency multiplier, and the other as a local oscillator of a sensitive harmonic mixer, on successive harmonics, dynamic ranges of 120 dB at 285 GHz, 105 dB at 380 GHz, 92 dB at 475 GHz, 80 dB at 560 GHz, 68 dB at 665 GHz and 45 dB at 760 GHz are obtained. Frequencies up to 1000 GHz can be reached with two Gunns feeding cascaded multiplier/mixers. At the upper frequencies, the electronic phase stability remains good. The mechanical stability must be improved. Figure 6 shows low sidelobes (-30 dBc) on a dual-mode Potter horn at 380 GHz.(5)
The phase variation clearly shows the compensation of two opposite curvatures, one for each mode. The repeatability of two successive traces is less than 3[degrees]. This shift could be due to the mechanical repeatability of the corresponding six microns (0.006 mm).
Fourier Transforms and Time Domain
In reflectometry, the Fourier transform of a response vs. frequency gives the response vs. time. If the propagation speed is known, this time response can be translated in terms of distance. For example, with two small horns looking in the same direction in a sweep from 40 to 41 GHz, Figure 7 shows the signal reflected by the obstacles, which face the wave emitted in free space towards a window. Slow undulations are due to standing waves between obstacles 2.3 m apart, and fast oscillations correspond to standing waves between obstacles 15.4 m apart. The Fourier transform of this signal gives a cross-section in the propagation direction with the respective levels due to the manifold obstacles. It appears that the slow undulations are due to the standing waves between the edge of the table (in A, 1.1 m from the antennas) and the window (in B, 3.4 m from the antennas). The fast oscillations are due to standing waves between the window and the building on the other side of the street (in C, 18.8 m from the antennas). The processing of the Fourier transform data (disregarding the signals coming after a certain delay, hence a certain distance) permits, in a classical manner, one to erase the unwanted reflections. This method permits the quality of the anechoic chamber to be controlled and corrected.
Fourier transform calculation can also show what happens inside waveguide components. Figure 8 shows the positions of the main mismatch contributions for the return losses of the scalar and Potter feed horns.[2,3] The 73 mm scalar corrugated horn presents mismatches close to its input flange, and practically no mismatch at its aperture. The 65 mm Potter horn presents a smaller mismatch close to the input flange, and a main mismatch at 30 mm from the input flange, which is the end of the rectangular/circular transition and the dual mode section.
A new approach to vector network analyzer problems in the mm-and submm-wave regions allows simple, fast and accurate antenna measurements in the frequency interval from 8 to 1000 GHz. This domain is still a frontier for technologies between microwaves and far infrared domains.
1. Millimeter-wave Radio-Astronomy Radiometry Laboratory DEMIRM at Meudon, France, kindly communicated from G. Beaudin and M. Gheudin.
2. Radiometer-Physics-RPG, Germany, from P. Zimmermann.
3. Thomas Keating Ltd., UK, kindly communicated by R. Wylde.
4. Thomson-CSF, France.
5. Potter horn at 380 GHz for extra-terrestrial vapor observations from a stratospherical balloon (PRONAOS project).
6. French patent CNRS-ENS 1989, starting from US patent nr 5 119 035 on June 2nd 1992, European (1993) and Japanese patents.
Philippe Goy was graduated from the Ecole Normale Superieure de Paris, France, where he performed research on mm- and sub-mm-waves. He received his PhD in 1970 in solid-state physics, in the field of cyclotron resonance in metals. Then he worked in atomic physics, studying Rydberg atoms interacting with microwaves. While engaged in this research, he created a new mm-/submm-wave vector network analyzer. Since 1988, he has been a consultant for AB Millimetre Co., where he has worked on the development and production of this analyzer.
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|Date:||Jun 1, 1994|
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