engleski

Active Double Negative Transmission Line with Negative Group Velocity

The basic element of any passive microwave component is a transmission line. The phase shift introduced by any kind of transmission line is dependent both on physical length of the line and on the frequency of applied signal. This is the fundamental reason why the design of broadband passive components such as matching networks, couplers or hybrid junctions has been challenging engineering issue for many years. The increase of the either physical length or the frequency in ordinary transmission line (LC line) causes decrease of the introduced (negative) phase shift. In recently introduced [1] passive backward-wave line (either the line made out of meta-material with negative permeability and negative permittivity or CL line), the increase of frequency also causes the decrease of introduced (positive) phase shift. On contrary, the increase of the physical length causes the increase of introduced phase shift due to backward wave propagation [2]. It would be desirable to manufacture the transmission line whose introduced phase shift decreases with frequency (the line with negative group velocity). Cascading such a line with an ordinary transmission line would, in principle, allow possibility of designing frequency independent components with fixed, predetermined phase shift. It can be shown, that due to energy conservation, the negative group velocity is possible only in vicinity of frequency where particular passive structure exhibits high loss. This property significantly limits practical applications. But, if one allows usage of active components (or active materials), it is possible to achieve losless transmission line with negative group velocity. Here, we propose the usage of negative inductance and negative capacitance as the basic elements of active double negative LC transmission line. These non-conventional elements can be achieved by negative converters with operational amplifiers. At the present state of the art, this approach is feasible only at low microwave frequencies, at which the operational amplifiers are commercially available (up to few GHz), but we envisage that this basic principle can be extended to distributed MMIC amplifiers, as well. This would enable manufacturing of the integrated active negative group velocity line. The dispersion characteristic of active double negative transmission line was studied analytically and it was shown that combination of such a line with conventional line can yield fixed phase shift in extremely broad band. Possible applications for broad-band matching networks have been discussed. The theoretical analysis was verified by measurements on the experimental active double negative transmission line in low frequency range of 300 kHz. The measurement results show increase of the introduced phase shift with frequency, i.e. the negative group velocity, as it was predicted theoretically.

backward wave ; meta-material ; negative group velocity

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