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It is often desirable to measure antenna impedance during an operational state. In applications such as experimental studies of ionospheric properties it is desirable to determine the instantaneous impedance. Most applications will involve adaptive tuning such as in antenna filter or multicoupler systems to maintain resonance despite other operations being conducted on the system. Adaptive tuning of HF whip antennas will provide compensation for environmental conditions such as ice load, or proximity to various objects. In cases where the antenna, or its surrounding, is affected by the power level, it is also desirable to measure the impedance over the power range as well as frequency. Two methods of determining impedances will be discussed in this paper. The first method is that of voltage and current probes and the second that of directional couplers for measuring forward and reflected waves.
A method for calibrating an automated network analyzer for antenna impedance measurements through a long interconnecting transmission line is developed. The transmission line is non-precision and of nominal characteristic impedance, loss, and dispersion
This paper examines the development tests performed at Rockwell International in Anaheim, CA on VHF meandering monopole and dipole antennas which are part of the Global Positioning System satellite. The development tests included numerous impedance measurements of individual antennas configured first in their operational positions on a full-scale mockup of the GPS satellite spacecraft and second while mounted on an indoor ground plane. The initial measurements of antennas positioned on the mockup required the mockup to be located in an exceptionally large, obstruction-free environment because of the low operating frequencies (large wavelengths) of the antennas under test, and in our case a suitable environment was an empty parking lot approximately one-half mile away from the necessary test equipment. This situation necessitated frequent transportation of fragile test equipment to and from the test site which was both impractical and time-consuming. To avoid this situation when production units are to be tested later this year, a ten-foot diameter ground plane was constructed in order to perform the antenna parameter measurements indoors, which presents a very reflective environment. To minimize and theoretically eliminate the effects of these reflections on our measurements, the time-domain gating (time filter) feature of the HP 8510 Network Analyzer was utilized at the indoor test site. The gating function removes any time-domain responses outside of the gate span, the span in this case being the radius of the ground plane. When the time-domain response is Fourier-transformed back to the frequency domain, the effect of the unwanted (gated) responses is eliminated in impedance measurements. While the gated, ground plane parameter measurements will not yield the same values as those measured on the mockup, they can be used to establish an impedance, VSWR, or return loss standard from a known “good” antenna against which production antennas can be compared to determine electrical failures.
Measurement of complex permittivity (er) and permeability (µr), both vector quantities of absorptive materials, has gained increasing importance with expanding use of the RF and microwave spectrum, particularly in communications and electromagnetic countermeasure applications. In addition, the network analyzer has seen increasing use in non-destructive measurements to determine the chemical composition of a sample dielectric material. The method described here is suited for the measurement of complex permittivity and permeability of ansorptive materials. These measurements have been made for years using numerous methods. A conventional technique involves a two-step process using a slotted line or network analyzer. First, the sample is backed up by a short circuit and the input impedance is measured. Next, the short circuit is moved ¼ ? from the sample to simulate an open circuit termination (where ? is the incident signal wavelength), and a second measurement is made. The results of these two measurements are used to solve simultaneous equations for er and µr. This procedure is repeated for each frequency of interest. Uncertainties in the measurement include test set-up frequency response, mismatch, and directivity errors, as well as the uncertainty in the physical position of the short circuit.
Paper not available for presentation.
The loop cell is fabricated using two intersecting metal sheets joined at the intersection and forming a 36 deg angle. A section of a loop is mounted between two coaxial panel jacks, one on each sheet at a distance equal to the loop radius from the intersection. A known current through this section of electrically small loop produced calculable E and H fields between the sheets in the plane of the loop. These known fields may be used to determine the antenna factor of small E and H antennas placed in the field if the mutual impedance due to the antenna images in the sheets is negligible and the antenna is not close to the open edges of the cell. Measured and calculated antenna factors agree within ±2 dB between 0.25 MHz and 1000 MHz.
The paper describes a simple but unique antenna test facility suitable for aerospace antenna developments. The total idea can be easily adopted by organizations who wish to carry out antenna measurements with minimum required instrumentation. The facility majorly caters for omni and wide beam antenna measurements, has been set up at ISRO Satellite Centre, Bangalore, India. It has been extensively used for omnidirectional antenna developments in VHF, UHF, L, S, and X-bands for India’s various space programs. Radiation pattern, gain, polarization and impedance measurements can be carried out both in near free space conditions as well as the ground reflection modes. The main feature of the facility is the use of large fiber-glass mounting structures for avoiding reflections and perturbations in radiation patterns due to impressed surface currents, specially in VHF ranges. Field probing is done by the use of a fiber-glass X-Y probe positioner. The facility used Scientific Atlanta 1752 Receiver and 1540 Recorder. Suitable software has been added to the facility for contour plotting of radiation levels, calculation of efficiency isotropy, and polarization properties.