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New for October 2024. This page was split off of our monopulse comparator network page, then we added a physical example and a circuit model. Enjoy!
The monopulse comparator network (also known as an "arithmetic network") is what processes the four quadrants of a monopulse antenna into the SUM, Delta AZ, Delta EL, and Delta Q signals. Sure, you can do this subtraction in the digital domain, but it is not as cool and it costs you some DC power to process it. What would you rather listen to, a classic analog recording of Roy Orbison's four-octave voice singing "Only the Lonely", or some auto-tuned digital crap?
A monopulse antenna has four feeds, which can be horns or other radiators. We'll use the convention of the Cartesian coordinate system, where the quadrants are labeled starting in the upper right, then go counter-clockwise, in this case, we labeled the quadrants A, B, C and D.
Below is a schematic representation of a monopulse using 90 degree hybrid couplers, in this case, branchline couplers. The trick is to add a 90 degree section to one of the inputs of the coupler, converting it to a 180 degree coupler (click here to learn more). The layout has an advantage over the ratrace comparator in that none of the signals are "trapped" inside the network. The disadvantage is the bandwidth of this approach sucks!
In this comparator, all of the blue lines are quarterwavelength. The lengths of the four skinny black lines labeled "(L1)" don't matter much, but they all must be the same. Again, use care when you lay this out, errors of a few mils can mess up the response.
Physical representation
Below is the branchline coupler monopulse network realized on alumina. It matches the previous schematic exactly. We added some stars so you can see where the extra 90-degree lines were added to convert the branchline 90 degree couplers into 180 degree couplers
Below is the same image without the annoying stars. We know people are going to cut and paste it into their own presentaions. Feel free to use it! We marked it so you won't forget where it came from!
Simuation in Microwave Office
Here is a model of the branchline monopulse. Go to our download area and get a copy of its Microwave Office file.
We replaced the ABCD nomenclature with Q1, Q2, Q3 and Q4, just to confuse everyone.
Here are the quadrant port matches. We usually don't plot return loss with the Y axis beyond -50 dB, but we want you to see the overall shapes. Weird thing is, two of the quadrants are different from the other two. We don't offer you an explanation.
Here are the port matches at AZ, SUM and EL ports. They land exactly on top of the quadrant matches. Here, AZ and Q have more bandwidth than SUM and EL.
Now let's look at the phases of the four quadrants out the AZ port. Here, you can see Q1 and Q4 are in-phase, but 180 degrees from Q2 and Q3. This indicates that the paired signals will subtract from each other.
Now let's look at the phases of the four quadrants out the EL port. Here, you can see Q1 and Q2 are in-phase, but 180 degrees from Q3 and Q4. This indicates that the paired signals will subtract from each other.
Now let's look at the phases out the SUM port. All quadrants are in-phase. Therefore, they will add up.
Now we need a way to drive the four quadrants equally. We used four ideal amplifiers with 0 dB gain, driven from Port 1. That port is 12.5 ohms impedance because it needs to drive four fifty-ohm ports in parallel.
Finally, here is the monopulse overall response. The SUM port collects all of signal, at least at center frequency, when the incoming signal is aligned with boresight. AZ and EL show deep nulls, which is what we want. By steering the antenna until signal is minimizes at AZ and EL, you know you are pointed at the target!
Go to our download area and get a copy of the Microwave Office file that produced thse plots.