# `Microstrip Antenna - Transient Fields (the Movie)`

 Previous Antennas List Antenna Theory .com Patch Antenna Menu Bandwidth & Fringing Fields Feeding Methods

On this page, I will present a movie showing the fields under a microstrip antenna. In this numerical experiment, a short pulse will be launched from the end of a microstrip, which will travel towards the patch antenna. Some of the pulse will radiate away, and some of the power will be reflected back down the microstrip line. This type of simulation gives a little bit better idea of what is going on with a patch antenna, specifically when short pulses (short waveforms, or brief applied voltages) are incident upon a microstrip antenna.

Specifically, consider a patch antenna that is mounted on a ground plane, with a dielectric with permittivity equal to 2.2. The thickness of this dielectric is 0.795 mm (millimeters). The patch antenna will be 1.25 centimeters wide and 1.56 centimeters long (you should be able to tell what frequency this antenna will radiate well at - if not, see intro to patch antennas page). The microstrip antenna will feed the patch offset from the center, as shown in Figure 1.

Figure 1. Offset feed for Patch Antenna.

The transient pulse will be of the form given by exp(-(t-T0)/T )^2, where T0 is the time delay and will be 45 pS (picoseconds, 10^-12), and T is a parameter that controls the rate of rise and fall, which is 15 pS. This function is plotted in Figure 2.

Figure 2. Incident (transient) pulse fed to a Patch Antenna.

In the following video, we will view the z-directed electric field, immediately below the patch antenna. Note that the surface of the patch is normal to the z-axis. We can clearly see the incident pulse propagate down the microstrip line, be disturbed by the microstrip antenna, then some of the fields are reflected, some radiate away, and some stay resonant below the patch and eventually radiate away or reflect back down the microstrip line.

This video shows a Gaussian pulse travel down the microstrip. Some energy is reflected back. Incidentally, taking the Fourier transform of the incident pulse and the returned signal and taking the ratio would give S11 (return loss) as a function of frequency for this antenna.

If you would like to see how this numerical electromagnetics simulation was developed, see the patch antenna numerical example page.