# Active Radar Homing with single antenna

From what I remember there are a few A2A and S2A missiles which have their own onboard radar system but I don't think they have multiple radar antennas for phased Array use.

I'm curious how radar tracking could be implemented without a phased array system?

• You could physically turn the antenna. Or multiple directional antennas pointed in different directions. May 2, 2022 at 18:37
• I don't know anything about missiles, but you could have a dish with a quadrant detector. As long as your wavelength is somewhat smaller than the detector you can test how far off axis the signal is by looking at the relative intensity in each quadrant. May 2, 2022 at 19:46
• No one has yet mentioned it, but astigmatism can also be productively used here.
– jonk
May 3, 2022 at 4:53

## 3 Answers

First of, note that target tracking was done with radar systems from day one, long before ESAs came on the scene.

One way is to use monopulse tracking. In order to determine angle of arrival (AoA) of the returned signal in 2 axis, X & Y (or Az and El) your antenna needs to provide 3 outputs:

1. A SUM output, which is the normal output of an antenna.

2. A Delta-EL output, which is the difference between the upper and lower halves of the antenna, and

3. A Delta-AZ output, which is the difference between the left and right halves of the antenna.

The SUM output and the Delta-EL output are then processed in a complex (using amplitude and phase) fashion to determine the AoA of the signal in the vertical (Y) direction. Similarly, the SUM output and the Delta-Az output are processed to determine the AoA of the signal in the horizonal (X) direction.

At radar frequencies, phased array antennas don't have to look like multiple, separate antennas. If you take off the radome, you'll probably find a printed circuit board with multiple elements and a feed and switching network. Also note that in a "lock-on, homing" application a radar antenna doesn't have to be continuously steerable. It may simply have fixed beams that are slightly offset toward 12-, 3-, 6-, and 9-o'clock positions, and by comparing returns, it determines which way to steer until the returns are equal.

• That multiple beam concept is similar to what was done by lobing, or rotating the feed horn of the reflector. The feedhorn was positioned so that it is slightly off center of boresite. The feedhorn was then rotated, or spun. If the return signal was centered on the boresite of the antenna, you would see a constant amplitude signal. If it wasn't, there would be an amplitude modulation of the signal. May 3, 2022 at 0:01
• IIRC some early AA stuff simply spun the whole missile to sweep a slightly angled beam, and rather then going to boresight tried to maintain target bearing as a constant angle, which given closing range guarantees a hit eventually. It is not an optimum strategy but it is simple and given a sufficient speed and manoeuvrability advantage... May 3, 2022 at 9:34

It depends how complex you want to go. A "typical" tracking Radar will implement a Twist Cassegrain antenna, with a monoupulse feed/comparator. It creates 3 channels; Sum, Difference Azimuth and Differeance elevation. Amplitutde comparsion is then conducted to centre the target on bore sight. This is typically very complex with high resolution ADCs required. A simpler solution would to implement two high gain (for instant Yagi) antennas, which can be pivioted in Azimuth and Elevation, and work out the math to have each return signal similar in strength. This is explained in simple terms, it's obviously a lot harder to implement in reality.