# Monopulse Antennas

A monopulse antenna is one method of realizing a tracking radar. The word "monopulse" implies that with a single pulse, the antenna can gather angle information, as opposed to spewing out multiple narrow-beam pulses in different directions and looking for the maximum return.

Here's a block diagram of a monopulse antenna, including the comparator network. The blue squares represent 180-degree hybrid couplers (such as rat-races). You can follow the simple arithmetic as the analog signals are added and subtracted to form the four receiver channels. According to convention, elevation angle is Θ (theta).  What happens when the target is not along the boresight of the radar? Let's look at the geometry of target with respect to angle, for a simple monopulse where only two antenna elements (horn perhaps) are used. The target is some distance L from the upper quadrants of the monopulse antenna. It is slightly farther away (ΔL) from the lower quadrants, which varies as the sine of the angle and the distance separating the antennas. Let's look at the sum and difference outputs versus elevation angle Θ from boresight. In this case we will space the antennas one wavelength apart, and did the math using a combination of Eagleware Genesys (to model the transfer function of the comparator network) and Excel (to model the horn). Here we have assumed an omni-directional antenna (no gain pattern). Now we "overlay" the horn antenna pattern to see the pattern you'd observe when you test this puppy in an anechoic chamber: The null depth is a measure of how well the signals cancel each other. In our example we have achieved 38 dB (the difference between sum and delta in dB). In practice, you will be happy to get 20-30 dB...

Cool stuff!

Author : Unknown Editor