Coaxial lines

Particle Contamination. In assembling large, high voltage equipment such as coaxial lines, contamination by metal particles may occur which may decrease the dielectric strength under various conditions by 5 to 10-fold (44—45). Metal needles are the worst contaminants and electrostatic traps or adhesive areas have been designed to cope with them (46). 

Hippel 1954b). The values of e and e" were derived from microwave theory by placing a sample of material against the end of a short-circuited transmission line, such as a waveguide or a coaxial line. This technique has applicability to high and low loss materials and in the present day has found applicability for the measurement of powders, grains and pulses (Nelson 1972,1991). 

A method which circumvents many of the disadvantages of the transmission line and cavity perturbation technique was pioneered by Stuchley and Stuchley (1980). This technique calculates the dielectric parameters from the microwave characteristics of the reflected signal at the end of an open-ended coaxial line inserted into a sample to be measured. This technique has been commercialized by Hewlett Packard with their development of a user-friendly software package (Hewlett Packard 1991) to be used with their network analyzer (Hewlett Packard 1985). This technique is outstanding because of its simplicity of automated execution as well as the fact that it allows measurements to be made over the entire frequency spectrum from 0.3 MHz to 20 GHz. 

Some care must be exercised with this technique, as errors are introduced at very low frequencies and at very high frequencies, as well as for low values of dielectric constant and loss factor. The technique is valid for the frequencies of 915 and 2,450 MHz, for materials with loss factors greater than 1. The temperature range of the probe is limited to approximately 60°C. However, new probe development is nearing completion. Interpretation for lower loss materials such as fats and oils must be treated with caution. Typical open-ended probes utilize 3.5 mm (0.138 in) diameter coaxial line. For the measurement of solid samples, probes with flat flanges may be utilized (Hewlett Packard 1991). 

Stuchley M. and Stuchley S. 1980. Coaxial line reflection methods for measuring dielectric properties of biological substances at radio and microwave frequencies — a review. IEEE Transactions on Instrumentation and Measurement. 29(3) 176-183. 

In the overlapping range 1500-15000MHz the results were obtained using a coaxial line cell terminated by a movable short circuiting plunger( 7). 

The sample termination section or cell can be so simple as the end of a coaxial line inner conductor in contact with liquid filling a volume bound by the outer conductor or placed between the end and a grounded plane terminating the outer conductor. These and other arrangements have, again from transmission line theory, an admittance y. (ito) related to... 

Distributed circuit methods use coaxial lines, waveguides and resonant cavities at microwave frequencies. The circuits are designed for measuring an attenuation factor and a phase factor, from which sample dielectric properties can be calculated. The sample may form the dielectric medium between the two conductors of a coaxial line (Scaife et al, 1971), or an open coaxial line is brought into contact with the sample surface (Roussy and Pearce, 1995). Fagan et al, (2004) used an open coaxial line method to demonstrate that the moisture and salt contents of processed cheese could be predicted by measuring dielectric properties over a range of frequencies. 

The admittance of the sample cell terminated by the coaxial line is then given by... 

Figure 5. Simplified drawings of sample cells, (a) Open coaxial line cell (b) lumped capacitance cell (c) open-ended coaxial cell. (Reproduced with permission from Ref. 113. Copyright 2000, Marcel Dekker, Inc.)...

For a coaxial line of negligible resistance, filled with dielectric, the characteristic impedance is given by... 

Figure 16 Typical waveform observed for the reflection of a fast-rise voltage pulse from a thin electric sample enclosed in a coaxial line...

The gas displaced from the tank will flow up the outer coaxial line to the dispenser for liquefaction or disposal. Flow of liquid hydrogen will continue until the automobile tank is full. 

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