Like the human liver that remove toxic from the body, microwave filter plays an important role in removing interference in any wireless communications. But besides than removing interference, microwave filter is also a key component in creating interference. When is that needed? you would say. It is needed when there is a need for blocking out a channel from usage, and naturally this aggressive action can only be found in military usage. Today lets talk about microwave filters in electronic warfare. But before that, a bit of recap on some theories about filters.
Generally filter characteristics can be categorized into 4 categories, namely, Butterworth (Maximally flat), Chebyshev, Generalized Chebyshev and Elliptic. When I say filter characteristic, it refers to unique mathematical function and polynomial which best describe the desired filter response. Some of the filter characteristics can be found in Fig. 1. In short, it is purely mathematical function developed by well-known mathematicians. But the question is how can we realize these mathematical function in hardware such that it performs the main filtering function for any wireless system? As you may be aware, there are various wireless application such as military radar, mobile communication, satellite communication and etc and each of them will have different form factors, spectrum utilization, power, channel, size and linearity requirements.
Modern Warfare, Modern Solution
Much of the modern filter theories presented above were discovered in 1940s during World War II where many synthesis and design approaches were introduced. Some of these advanced filters were demonstrated using hollow waveguide cavities or coaxial lines. World War II was the first war in history where electronics play a huge part in the progress of the war. Alan Turing invented the Turing Machine as a way to decrpyt the German naval enigma machine. The Turing Machine would go on to be the foundation of our modern computers. Similarly, the advancement in filtering technology was mainly pushed by military needs as the war progress. For example, the radar was invented as a response to the need for an early response system against the Blitz during the Battle of Britain. The radar system installed allowed the Royal Air Force to use radio triangulation technique as a mean of direction finding the incoming threat during the course of the battle, allowing the RAF to plot an intercept vector before the threat arrive. The RAF would go on to win the Battle of Britain.
Electronic Counter Measure and Jamming
So moving into the 21st century, where our lives are entwined with the electronics that we created, wars that are fought also demands a dominance in the area of the frequency spectrum. One interesting application of microwave filters in war is the use of tunable filter in military radar Electronic Counter Measure (ECM) and Jamming. The principle is simple, cut off all means of communication of the enemy in a war zone. In today’s world, just imagine what it would be when you are disconnected from the world without any working communication devices. You are basically alone, helpless and susceptible to danger! Similarly, disrupting the means of communication of the opposing fighting force would mean a complete breakdown in military cohesion. In this age of information, the ability to disrupt/maintain communication would mean the difference between victory and defeat.
So how does a tunable filter plays a role in this? A tunable filter will be able to perform a broadband frequency sweep in order to search for the intruder’s spectrum used in communication. Once the channel has been detected, an extremely high power signal can be generated through power amplifier to overwhelm the whole channel, causing the signal to be covered up by the high power signal, in another word, the signal is jammed. As a result, the intruder will not be able to communicate and unable to perform any means of military coordination. However on the defense, if the opposing force has the technological capability, the intruder may also use a tunable filter to perform a similar search for an available spectrum for establishing a new channel for communication. On top of that, it can also be used to counter-jam the aggressor’s communication channel and devices.
So now, warfare does not only happen on land, sea and air, but also transcends into the frequency spectrum, making filter technology an utmost importance in electronic warfare application. The advancement of such a simple device may have a great impact which can become one of the deciding factor for a victory.
There is a lot more filter applications to be demonstrated and discussed besides usage in military. Stay tuned for more interesting reads like this one.