Basic Concepts and Terminology

In this post I enlist some basic concepts and terminology involved in the domain of microstrip antennas. A proper comprehension of these concepts is imperative before any further deliberation on microstrip antennas.

Stripline and Microstrip

The terms stripline and microstrip are often encountered in the literature, in connection with both transmission lines and antennas. A stripline or triplet device is a combination of three parallel conducting layers separated by two thin dielectric substrates, the center conductor of which is analogous to the center conductor of coaxial transmission line. If the center conductor couples to a resonant slot cut orthogonally in upper conductor, the device is said to be a stripline radiator.

On the other hand, microstrip consists of two parallel conducting layers separated by a single thin dielectric substrate. The lower conductor functions as a ground plane and the upper conductor may be a simple resonant circular or rectangular patch, or a resonant dipole, or a monolithically printed array of patches or dipoles and the associated feed network.

Single Microstrip Line

A basic microstrip transmission lines consists of a conductive strip and a ground lane separated by a dielectric as shown in the figure below.

Fig: Single microstrip line

The figure above shows the general structure of microstrip line. The conduction strip, which is the microstrip line, has a width W, a thickness t. It is placed on the top of dielectric substrate, which has a relative dielectric constant €r, with a thickness of h, and a ground plane below on the substrste.

The single transmission line forms the basics of microstrip and therefore a basic knowledge of its working imperative. As mention earlier, two parameters, the effective dielectric constant €r and the characteristic impedance describe transmission characteristics.

Microstrip Antenna

A microstrip antenna in its basic form consists of a metallic radiating patch on one side of dielectric substrate, which has a ground plane on other side. The radiating patch can be considered as extension of microstrip line.

Fig: Rectangular Microstrip patch antenna

Assuming a voltage input at feedline, when operating in transmission mode, current is excited on feedline to the patch of a vertical electric field between the patch and the ground plane. So therefore the patch element resonates at certain wavelength and this result in radiation.

An Introduction to Microstrip Antennas

Microstrip Antennas started out as an extension of microstrip circuit elements, a modification of microstrip structures into radiating elements. But over the years due to the compact and thin profile structure of the microstrip antennas which are amenable to conformal configurations, and numerous other advantages as well, microstrip antennas have been a subject of extensive research and  extensive commercial use.

A Little bit of history:

Deschamps was the first to give the concept of microstrip radiators way back in 1953. But as is the case with most scientific breakthroughs of that time, the first patented documentation of microstrip antennas is in the name of Gutton and Baissinot of France in 1955. The first ones to make practical antennas were Howell and Munson.

Robert (Bob) Eugene Munson is regarded as the father of practical microwave patch antennas. Although the patch antenna was first theorized by G. A. Deschamps in 1953, but it was not put to use for many years (by Munson), first on a datalink for Sidewinder missile, then on Sprintmissile's semi-active seeker.

So was it really Munson's genius or was he just in the right place at the right time??

The 1960s and 1970s witnessed 

• the move to higher frequencies and solid state devices
  
• the move away from chassis and terminal wiring to printed circuits
  
• a demanding need for conformal arrays (the Sprint missile array was on the curved surface of the nose cone).
  

At this point of time. the height of the Cold War, Munson was a defense worker at Ball Aerospace. So Munson had just the right needs and facilities at his disposal for development of microstrip antennas, which the likes of Deschamps, Gutton and Baissinot were not able to take advantage of. These situational advantages apart it must have taken all of Munson's genius to make such outstanding progress in this field. Munson's name appears on many U.S patents, 29 at last count just on antennas. Today he's retired and living on a 160 acre vegetable farm outside Boulder. Financial benifits apart Munson's reputation in the field of microwave engineering exceeds all his contemporaries. The above information on Munson was obtained from the Microwave hall of fame, whereas a simple Google search on Deschamps hardly yielded any results.