Active negative group delay circuits and applications

Thatapudi, Thomas (2021). Active negative group delay circuits and applications. University of Birmingham. Ph.D.

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Abstract

The design of a low profile antenna operating over wide bandwidth with enhanced radiation performance is the objective of this work. The methods to improve the operational bandwidth, reduce the size and enhance the radiation properties such as directivity and gain, while minimising the beam squint for an antenna are presented. Two antennas are designed: an electrically small antenna (ESA) integrated with an active circulator for simultaneous signal transmission and reception (duplex operation) operating over a wide bandwidth and a Fabry-Perot cavity antenna having high directivity and gain with reduced beam squint.
A negative group delay (NGD) concept is used in lumped element implementation and in frequency selective surface implementation for achieving the objectives of this work. A detailed study is carried out on the filter based NGD network design. The NGD network impact is clearly demonstrated: in adjusting the impedance characteristics for the design of an ESA; in achieving wideband signal cancellation for the active circulator and in optimising the phase characteristics of a high impedance surface (HIS) for the design of a wideband Fabry-Perot cavity antenna with reduced beam squint.
A low profile ESA is designed using a NGD matching network. The passive NGD network offers wide bandwidth impedance matching for an antenna over the frequency band, far lower than the quarter wavelength resonance frequency associated with the physical length of the antenna. The inevitable transmission loss of the NGD based matching network is compensated using an active circulator. This enables the ESA to be used in duplex mode. An active quasi-circulator operating over a wide bandwidth, working on a signal cancellation principle is designed using NGD networks. The employment of the NGD network achieved signal cancellation over a wide bandwidth. This network also offered extra design flexibility in terms of optimisation and selection of gain blocks. The ESA is integrated with an active circulator of 14% bandwidth operating at 1.8GHz. The average 5dB gains in the transmission and reception paths are used to compensate the losses of the matching network. This novel antenna is suitable for duplex operation over a wide bandwidth.
The prototypes of the ESA and the active circulator are fabricated and tested separately. An ESA operating at 1.08GHz ( =0.98/2) of 50% bandwidth with a physical length of 43mm is designed and tested. This method is more stable and effective than those that are reported in the literature. The performance parameters of the ESA are discussed in detail. A three-port active circulator (clockwise: transmitter, antenna, receiver) operating at 1.5GHz with 200MHz (14%) bandwidth offering 20dB isolation between the transmitter and the receiver is fabricated and tested. Between any pair of ports the average gain in the clockwise direction is 5dB and the isolation in the anti-clockwise direction is greater than 20dB. The noise figure is less than 5dB from the antenna to the receiver. Using the proposed method the bandwidth of the circulator is improved almost 30 times compared to a similar design reported in the literature.
A Fabry-Perot cavity antenna operating at 10GHz is designed with a partially reflective surface (PRS) and a high impedance negative group delay surface. The HIS with NGD characteristics enabled the establishment of the resonance phase condition over a 0.75GHz bandwidth resulting in a wideband cavity antenna with reduced beam squint. The signal absorption along the HIS and the possible loss compensation techniques are also discussed. A lossy design has exhibited only 0.20 per 100MHz beam squint over 1.5GHz bandwidth with degraded directivity and gain. A loss compensated model has exhibited 1.30 per 100MHz beam squint over 0.75GHz with improved directivity and gain of 19dBi and 5.6dBi respectively.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):