Microstrip Antennas In high-performance aircraft, spacecraft, satellite, and missile applications, where size,weight, cost, performance, ease of installation, and aerodynamic profile are constraints, low-profile antennas may be required. Patch antennas are low profile, conformable to planar and nonplanar surfaces, simple and inexpensive to manufacture using modern printed-circuit technology, mechanically robust when mounted on rigid surfaces, compatible with MMIC designs, and when the particular patch shape and mode are selected, they are very versatile in terms of resonant frequency, polarization, pattern, and impedance

Advantages Microstrip dipoles are attractive because they inherently possess a large bandwidth and occupy less space, which makes them attractive for arrays. Linear and circular polarizations can be achieved with either single elements or arrays of microstrip antennas. Arrays of microstrip elements, with single or multiple feeds, may also be used to introduce scanning capabilities and achieve greater directivities.

Disadvantages Microstrip antennas are their low efficiency, low power, high Q (sometimes inexcess of 100), poor polarizationpurity, poor scan performance, spurious feed radiation and very narrow frequency bandwidth,

Diagram

Types

Feeding Methods

microstrip-line feed: It is easy to fabricate, simple to match by controlling the inset position and rather simple to model. However as the substrate thickness increases,surface waves and spurious feed radiation increase, which for practical designs limit the bandwidth (typically 2–5%). coaxial probe feed: It is also easy to fabricate and match, and it has low spurious radiation. However, it also has narrow bandwidth and it is more difficult to model,especially for thick substrates (h > 0.02λ0). Both the microstrip feed line and the probe possess inherent asymmetries which generate higher order modes which produce cross-polarized radiation

Aperture coupled antenna: It has moderate spurious radiation The ground plane between the substrates also isolates the feed from the radiating element and minimizes interference of spurious radiation for pattern formation and polarization purity. Proximity coupling feed: It has the largest bandwidth (as high as 13 percent), is somewhat easy to model and has low spurious radiation. However its fabrication is somewhat more difficult.

Methods of analysis The transmission-line model is the easiest of all, it gives good physical insight, but is less accurate and it is more difficult to model coupling Compared to the transmission-line model, the cavity model is more accurate but at the same time more complex. However, it also gives good physical insight and is rather difficult to model coupling. full-wave models are very accurate, very versatile, and can treat single elements, finite and infinite arrays, stacked elements, arbitrary shaped elements, and coupling. However they are the most complex models and usually give less physical insight.

Physical and effective lengths of rectangular microstrip patch.

Design Equations Effective dielectric constant

Effective length Resonant frequency Considering fringing field

Patch width Patch length

CAD TOOL Computer aided design software Solving complicated problems Requires less computation time, good accuracy, Do not require complex mathematics steps, results closer to measured one. Commercial packages: PCADD 3.0: Window based general purpose antenna analysis and design tool ENSEMBLE 2.0: Implements moment-method solution, capable of handling arbitrarily shaped patches, coupling apertures and feet networks.