If you were to make an analogy between a radar and the human body, the antenna on the radar would be the eyes – it sees what the radar detects. There are many different antenna types that exist today and these technologies are continually growing and becoming more complex. The technologies range from the dipole antenna (2 wires), to patch antennas and, most recently, metamaterial antennas that could, in the far future, provide a method for cloaking. Cloaking, in the context of metamaterial cloaking and electromagnetic (EM) radiation, is when the EM field propagates around the object with the metamaterial properties, or in other words it makes the object seem invisible. Aren’t antennas cool?
PRECO Electronics has been in the heavy-duty radar industry for almost 20 years and we utilize many types of antennas in our radars, including the patch antenna, the stacked patch and cavity backed slot antenna. However, the industry is now primarily using the patch array antenna. Let’s take a look at how antennas work to provide us with the information we need to create the most effective object detection radar possible.
Before going into details about antenna design, it’s good to get an understanding of how the patch antenna radiates. There are two main methods for radiation when either the H-fields, the same magnetic fields that make a compass needle point north, or Electric Fields (E-fields) add in phase. The patch antenna radiates by fringing E-fields that add in-phase. See, that wasn’t so bad. The E- fields usually radiate in the same direction as the feed coming into the patch (Figure 1). In a typical patch antenna with one feed line it will be linearly polarized.
Figure 1. Patch antenna emphasizing the fringing fields that create radiation
After you design the patch, the next thing to do is stack them in a way that causes the array to radiate in a particular direction. For heavy-duty equipment radars we want the area of detection to be wide on the azimuth plane (looking left and right) and narrow on the elevation plane (looking up and down). To do this it’s best to have one element in the azimuth direction and multiple elements in the elevation direction. To determine how many elements you need in the elevation direction you can use the following equation:
Equation 1. HPBW of an antenna array
Depending how narrow you want the beam, increasing N (number of elements) will make the beam width narrower.
Now that we know how the antenna radiates and how many patches we need for our design, the next step is to electrically tie them together in an array (also known as antenna array feed) and weight them (having a varied magnitude per antenna). This weighting can be seen in Figure 2, where the element with the most red (left side of the image) has a higher magnitude and the elements on the end with more green have a lower magnitude (elements on the right side of image). Notice how the red is apparent at the edge of the patch antenna showing how the edge fringing fields give rise to radiation.
Figure 2. E-Field of antenna array (half of the array)
For a better visual display of what the radiation pattern looks like, Figure 3 and Figure 4 both show a 3D radiation pattern overlaid on the antenna array (Figure 3) and of just the 3D polar plot of the radiation pattern (Figure 4). The darker red corresponds to high antenna gain, while the green corresponds to lower antenna gain.
|Figure 3. 3D Polar Plot Directivity and Antenna Array||Figure 4. 3D Polar Plot Directivity|
For the most part, that’s how the patch array works and performs aiming for high gain and efficiency, with minimal sidelobes. Each array is going to look different depending on the application, for heavy-duty applications this is what more arrays will look like. The ‘Antenna Array’ section is technical and condensed, if you would like a more in-depth blog about it say so in the comments with the code word: “more sidelobes!” and a picture of Will Ferrell.
You remember the elementary playground come-back, “I’m rubber you’re glue, whatever you say bounces off me and sticks on you.” Well it turns out you can say the same thing to a radar and it would actually be true, “I’m human you’re radar, whatever you radiate at me will bounce and be picked up by you.”
Two antennas met on a roof, fell in love and got married. The ceremony wasn't much, but the reception was excellent.