Bandwidth: substrate and patch geometry effects
·The bandwidth is directly proportional to substrate thickness h.
·However, if h is greater than about 0.05 l0 , the probe inductance becomes large enough so that matching is difficult.
·The bandwidth is inversely proportional to er
·The bandwidth is directly proportional to the width W.
Radiation efficiency is the ratio of power radiated into space, to the total input power.
The radiation efficiency is less than 100% due to
Pr –radiated power Pd – power loss due to dielectric
Pt – total power Psw – power loss due to surface wave
Pc – power loss due to conductor
•The directivity is fairly insensitive to the substrate thickness.
•The directivity is higher for lower permittivity, because the patch is larger.
Applications in present-day mobile communication systems usually require smaller antenna size in order to meet the miniaturization requirements of mobile units. Thus, size reduction is becoming major design considerations for practical applications of microstrip antennas. Microstrip antennas are half-wavelength structures and are operated at the fundamental resonant mode TM01 or TM10, with a resonant frequency given by (valid for a rectangular microstrip antenna with a thin microwave substrate) where c is the speed of light, L is the patch length of the rectangular microstrip antenna, and εr is the relative permittivity of the grounded microwave substrate.
Meandering the excited patch surface current paths in the antenna’s radiating patch is also an effective method for achieving a lowered fundamental resonant frequency for the microstrip antenna. For the case of a rectangular radiating patch, the meandering can be achieved by inserting several narrow slits at the patch’s nonradiating edges. The excited patch’s surface currents are effectively meandered, leading to a greatly lengthened current path for a fixed patch linear dimension. This behavior results in a greatly lowered antenna fundamental resonant frequency, and thus a large antenna size reduction at a fixed operating frequency can be obtained. By embedding suitable slots in the radiating patch, compact operation of microstrip antennas can be obtained. some slotted patches suitable for the design of compact microstrip antennas are as follows: