Colin C. Bantin

Study of compressed log-periodic dipole antennas

The behavior of log-periodic dipole (LPD) antennas, which are compressed along the transmission-line axis, is studied both experimentally and theoretically. Compressed LPD antennas are found to be efficient, of low gain, and frequency independent. With compression, the radiation pattern approaches dipole-like behavior and the bandwidth increases slightly. It is also found that LPD antennas exhibit anomalous frequency-dependent behavior in narrow bands of frequencies ( \approx 1/8 of a log period in width) when a reactive termination is used and when the scaling factor \tau is below about 0.92-0.93.

Sheath waves observed on OEDIPUS A

An important novel feature of the tethered sounding rocket experiment OEDIPUS A (Observations of Electric-field Distributions in the Ionospheric Plasma—A Unique Strategy) was its direct excitation and detection of electromagnetic waves on conductors in space plasmas. We present quantitative evidence about sheath waves excited in the ionosphere by a high-frequency transmitter on one end of the 1-km tether and detected by a synchronized receiver on the other end. An important characteristic of sheath waves is their sequence of sharply defined passbands and stop bands in the frequency range 0.1–5 MHz. The lowest passband is between 0.1 MHz and the plasma frequency near 2 MHz, the bandwidth where existing theory predicts sheath waves. Resonance fringes in this band have been scaled to determine the phase and group refractive indices of sheath waves. These agree reasonably well with the theory, considering the approximations therein. Passbands and stop bands observed in the range between 2 and 5 MHz are not expected on the basis of the current theory. In this range, band limits have clear signatures of the interaction of the tether fields with electrostatic cyclotron waves. Finite wire moment method modeling of the payload shows that in the low-frequency passband, RF coupling along the tether is increased by 20 dB over vacuum conditions. Similarly, isolation is greater than vacuum isolation in the stop bands. Because sheath waves at frequencies up to 2 MHz are guided efficiently along conductors in plasma, they are a significant design issue in the electromagnetic compatibility of avionics at frequencies up to HF on large metal space structures.

Optimization of log-periodic dipole antennas

It is shown that a log-periodic dipole antenna can be optimized by varying the feed-line characteristic impedance while carrying out swept-frequency far-field and impedance measurements. The minimization of narrow-band anomalies in the measured quantities results in a well-defined optimum characteristic impedance.

Precise positioning of RFID tags using a phased array antenna

A means of precisely determining the position of an RFID tag, relative to the tag reader, has been developed and applied to the problem of determining the location of an urban rail train along its guideway. With this scheme the position can, in principle, be determined to centimeter accuracy using a sensing antenna array consisting of two identical elements which measures the abrupt 180 degree phase change that occurs in the differential phase of the sum and difference signals formed by the array as the array passes over a tag. Electromagnetic modelling confirms that a reliable phase detection signal can be derived from an array located under a train with RF tags located on the guideway.

Modeling and optimization of coverage in London Underground Subway Tunnels

A semiautomatic model extraction procedure is used together with an image-based ray tracer to study the radio coverage in London Underground Subway Tunnels. The simulated received signal strength is compared with that obtained via measurements. The prospects of optimizing the location of access points to improve the coverage in the tunnel is also investigated.

Electric fields from RF tag interrogators underneath an urban rail train

RF tag interrogation systems are used for position determination in the automatic control of urban rail trains. Such systems consist of a Tag Interrogator Unit (TIU) which is attached to an antenna mounted underneath the rail car and transmits interrogation pulses to tags installed between the rails. The TIU operates in a complex electromagnetic environment that contains some components which are highly conductive, such as the rail car itself, the wheel bogie sets, the rails and rail spikes into the ground. Other components have lower but still significant conductivities and permittivities different from that of free space, such as the ground, the ballast, and the sleepers (ties). In the design of a positioning system it is important to gain an understanding of the influence of these components on the RF field distribution underneath the rail car, especially in the plane where the tags are located. This paper discusses the use of a computer simulation, using the RF module of the COMSOL Multiphysics finite element software package, to model the situation and to predict the electric field distribution, most importantly in the plane of the tag. The results of the simulation can be used to better understand the response of the TIU and the train position can be better determined.