The last breakthrough in the continuing miniaturization of electronics systems is made possible by the integration of circuit function and radiating elements into single module. Traditionally, electronic circuits and the antenna system have been designed by different groups of designers using different types of designs tools, working independently on either side of a well-defined interface, very often with very little interaction. This approach leads to separately packaged circuits and antenna subsystems, connected by cables and waveguides. Nowadays, the integration of circuits and antennas into single modules has been made possible by the common technological features of RF and microwave circuit and printed microstrip antennas.
Active Integrated Antennas (AIAs) in which the active devices are integrated directly into antenna structure have been developed by the interest in the potential for sensor applications and spatial power combiner applications.
With the advantages of no feed line losses especially at the high frequency, low power consumption, size and weight reduction, reliability enhancement, AIAs are getting much attention in the today’s wireless communication system. In AIAs the active nonlinear device can generate high level of harmonic radiations. Electromagnetic interference which is caused by harmonic radiations degrades the system performance. To overcome this problem, a band stop filter between active device and antenna can be used. But this technique increases the total size, insertion loss, and fabrication cost. A better way to solve this problem is to make a harmonic suppression antenna which can suppress harmonic radiations.
In this thesis, first of all, the theory of microwave two-port oscillators, active integrated antenna oscillator, and harmonic suppression antenna, varactor diode are introduced.
Secondly, a harmonic suppression antenna operating at the frequency of 4 GHz is proposed. The antenna used a T-shaped microstrip-line-fed slot in order to achieve wide bandwidth. Both the conventional T-shaped microstrip-line-fed slot antenna and the harmonic suppression antenna have -10 dB bandwidth about 2 GHz, which is 50 % of the operating frequency of 4GHz. Equivalent circuits of the both the proposed antennas are introduced and the simulation results are compared with the CST simulation results. Radiation patterns, gains, and size of both the proposed antennas at the fundamental frequency are almost the same. The measured results of return losses and radiation patterns at harmonic frequencies show that this proposed antenna is excellent at harmonic radiation suppression. Those characteristics make this antenna attractive to integrated active antenna applications.
Thirdly, an active integrated harmonic suppression antenna which belongs to oscillator type is presented. To achieve wide bandwidth, a folded slot with open ended microstrip line is employed. The operating frequency is 5.8 GHz. Return losses of the antenna at fundamental and harmonic frequencies are measured and compared with those of the reference folded slot antenna. In order to measure and compare radiation patterns at the same frequency, both antennas are integrated with the same varactortuned active circuit to make oscillator type AIAs.
Finally, an inclined square wide slot harmonic suppression antenna with the operating frequency of 4 GHz is discussed. Harmonic suppression is achieved by using a defected ground structure (DGS), a narrow microstrip-line-fed, and a small grounded inclined square patch. Return losses and radiation patterns of the antenna at the fundamental and harmonic frequencies are measured and compared with those of the conventional inclined square wide slot antenna.
