BASIC PARAMETERS

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 e*r*

· The bandwidth is
directly proportional to the width* W*.

Radiation Efficiency

Radiation efficiency is the ratio of power radiated into space, to the total input power.

The radiation efficiency is less than 100% due to

• conductor loss

• dielectric loss

• surface-wave power**
**

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

Directivity

• 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 *ε*r
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: