The Radar Revolution and its Consequences

World War Two is called the physicists war for a reason, as without the revolutionary advancements made by physicists in somewhat solely academic fields such as Radar, crucial victories may have not taken place. The goal of this article is to not only teach you about the fundamental scientific principles behind Radar but to truly emphasise how WW2 essentially reinvigorated its development. By the end of this article, not only will you be able to show off your new level of scientific knowledge but hopefully will be convinced to submit a late application to UCAS to start a physics degree.

What is Radar and how does it work?

One of the key demonstrators of weaponization is the development and implementation of various radar systems as well as its change in application. RADAR, or RAdio Detection and Ranging, is the use of pulses of radiation in order to detect distant objects. This radiation is commonly in the form of an electromagnetic wave as seen below and has the property of having a constant speed due to the inverse relationship between frequency and wavelength.[1]. The frequency of the wave is the number of oscillations per second and the wavelength is essentially the distance between two equivalent points. 

Therefore, by timing the duration of the pulses journey its possible to determine the distance to the object the wave has bounced off and thus detect its presence, as demonstrated below.

Example of the journey of emitted radiation from a radar transmitter[10]

By dividing this duration by the known speed of electromagnetic radiation we determine the distance the pulse has travelled, by dividing this by two we can then work out the distance to the object as shown using the famous equation below.

Where d is the distance to the object, c is the universal speed of electromagnetic waves, and t is the duration of the pulses journey.

Assuming this foundational knowledge hasn’t induced traumatic memories of being stuck in double-period physics, we can now explore why its so important to learn about through its impact on one of the most famous wars in world history.

The development of Radar during WW2:

Before the war, Radar was primarily used for radio communications as well as tracking changes in weather such as thunderstorms and thus had little application to warfare. Although, after fear of the Germans producing “a death ray” (which thankfully wasn’t possible), research was initiated into how the use of radiation could prove to be an advantage over the axis powers by the famous Tizard Committee.[11]

The fear of a potentially imminent air attack, led to a rapid revolution in radar development, particularly in Great Britain due to its proximity to axis powers. There was thus this massive shift in focus in trying to find a way to hopefully detect incoming enemy objects quickly enough to allow sufficient time for preparation of a defence and evacuation. For example, a coastal defence system known as Chain Home was made operational by the RAF in 1938 to prepare for such an attack and thus was a vast improvement on current measures.[2] Similarly there was a further implementation of a Chain Home Low system to detect low-altitude dive-bombers attempting to avoid the range of the radar systems.[12].

Picture of the chain home system implemented in the UK during World War 2.[14]

The system works similarly to the diagram above by using powerful radio transmitters which reflect off incoming aircraft enabling detection. These radio transmitters were so powerful that “stations in Kent could detect German aircraft flying over France”![15]. This advancement in detection proved to be significantly important in the Battle of Britain as chain home systems were unable to be destroyed enabling the Luftwaffe to be detected.

Despite working effectively in the Battle of Britain, the main disadvantage of this system was its inability to detect enemy forces once they had penetrated the coastline, as radar was only emitted outwards from the UK. This was later solved through the development of an antenna which could rotate mechanically in order to detect objects in all directions producing the classic grid like interface seen frequently in films and tv series’ as shown below.

 

Modern-day grid-like interface produced using radar[17]

This weaponization of radar continued with the development of a more advanced weapon surrounded with an equivalent amount of secrecy as the atomic bomb. This device was known as a proximity fuse capable of being used in anti-aircraft weaponry with an efficacy 10 to 20 times greater than missiles at the time. [3,18] These proximity fuses allow the remote detonation of explosives when near a target as the fired shell acts as a transmitter producing an interference pattern with the approaching object.  

The detonation occurs as the waves reflected off the incoming object have a different frequency compared to the transmitted wave due to a doppler effect. The doppler effect is an important principle within science and can be explained through considering how the pitch of an ambulance’s siren changes as its proximity to us changes as demonstrated below.

As the source of a wave, which in the case of an ambulance is a soundwave, moves away from us the wavelength of the wave increases resulting in the frequency decreasing causing a lower pitch as the peaks of the wave are more spaced out. The extent to which the initial frequency emitted from the source changes is given by the equation below and shows how this change is dependent on the velocity of the source which is important when trying to decide at what frequency should the shell explode.

 

Where fo is the frequency heard by an obersver( or detected by the fired shell),fs is the frequency heard when the emitter is stationary, v is the speed of sound and vs is the velocity of the source. When applying this equation to the proximuty fuse, v is equal to the universal speed of electromagnetic radiation c.
In the case of our analogy the dot is an ambulance.[18]

All of the factors in the above equation had to be carefully considered when producing a fully functioning proximity fuse and a simple mistake could lead to at best a broken weapon, but at worst an explosion which may occur sooner than expected resulting in friendly fire.

Also, when trying to understand the fuse, its important to note that the phase of wave represents the position of a point with respect to the beginning of the wave.

This doppler effect which the shell experiences, results in a difference in phase causing the production of an interference pattern. An interference pattern consists of areas of high intensity and low intensity due to constructive and destructive interference respectively. These areas of high intensity are where the waves have similar phases causing an increase in amplitude, whereas the areas of low intensity are caused by different phases and thus a reduction in amplitude as demonstrated below.  

Effects of constructive interference and destructive interference on the amplitude of the wave.[13]

Due to the decreasing distance caused by the shell approaching the target it cycles between these points of high intensity and low intensity. When the points of high intensity exceed a certain threshold (which occurs when the projectile is a certain distance away from the object) a detonation is triggered by setting off the fuse[3].

This device was key in many crucial victories won by the allies due to the sheer military advantage it provided. For instance, not only did it reduce the number of shells required to destroy an aircraft by 900 but was also key in the winning the Battle of the Bulge in 1945 as well as in the combatting of the Japanese Kamikaze strategy [19].

Implementation of microwave frequencies:

The war caused a desperate need for portable radar systems capable of being attached to warships and aircraft which sparked interest in the development of a new, more accurate, radar system using microwaves. The magnetron was thus developed in 1940 at the University of Birmingham [4] and as described by President Roosevelt was “one of the most important cargos to ever reach US shores” [5] where it was further developed. It was delivered as a part of the famous Tizard Mission in which the magnetron, arguably one of the most significant inventions of the 20th century, was attached to the top of a taxi on its way to Euston station.

Rather than producing radio waves, a magnetron is capable of producing microwave frequencies. Microwaves have a shorter wavelength than radio waves making them more precise allowing the detection of smaller objects. The magnetron itself consists fundamentally of a cathode ( a negatively charged metal) which emits electrons, a ring shaped anode( a positively charged metal) with holes in it and a powerful magnet beneath it. The anode and the cathode produce what is known as an electric field, the electrons emitted from the cathode, since they are negatively charged are accelerated in this field as they try to move towards the positively charged anode

Labelled diagram of magnetron [6]

 However, due to the presence of a magnetic field produced by the strong magnet, their path is significantly curved preventing the electrons from ever reaching the anode. This path is curved as the magnetic force is pointing into the centre of a circle due to it being at 90 degrees to both the magnetic field and the velocity of the particle. Therefore, the magnetic force acts as a centripetal force which means it is “centre seeking” and the radius of the circle is determined by the strength of the magnet used and thus the path of the electron can be controlled. All of these factors controlling the path are contained within the equation below and were carefully considered in the invention of the magnetron to produce the desired frequencies. The video below demonstrates this curved path that a charged particle experiences once it enters a magnetic field where the green arrow represents the magnetic force which the charged particle experiences. 

 

Fundamental equation where R is the radius of the circular path of the electron, m is the mass of the electron,v is the velocity at which the electron moves, B is the strength of the magnetic field and q is the charge of the electron.

For those interested in playing around with this concept which had to be greatly considered in the development of the magnetron please use this inciting and educational link

With this new system, radar was now able to be used in aircraft to detect submerged submarines as well as in “Night-Fighters” for use in low visibility conditions. This was a significant advantage to the allies and thus a massive factor in them eventually winning the war.

 

How we still reap the benefits:

This significant development in radar technologies has paved the way for the study and solving of many present issues. For example, the magnetrons ability to produce microwave frequencies initially intended for detecting objects is capable of being applied to cooking food and thus magnetrons can be found in almost every kitchen on earth. [6]

Furthermore, these developments made in WW2 are now crucial to our current understanding of the earth itself. For instance, radar is frequently used in weather surveillance allowing us to monitor the ever-growing problem of climate change using the same principles in physics exploited during WW2. Meteorologists are capable of detecting precipitation significant distances away , not that we need to be told when it’s raining in Britain. This is made possible as the electromagnetic waves, produced by sources similar to magnetrons, reflect off of these raindrops. 

Moreover, without the significant increase in funding which radar research received, modern-day projects such as the launch of radar altimeters into the orbit of the earth would likely not be as possible. These altimeters allow us to determine the height of the ocean surface by sending out pulses of radiation from orbit which reflect off of the ocean, thus if the height changes the duration of the pulse changes, and therefore the change can be calculated relatively easily using similar methods discussed previously. This further allows us to conduct in-depth analysis of ocean currents, prediction of possible tsunamis, as well as enabling the discovery of planetary waves. [7,8]

After the war, there was also a subsequent evolution of a new area of research known as radar astronomy [9]. Radar astronomy works by reflecting pulses of radiation off distant celestial objects allowing us to determine not only their distance but also the shapes and properties of their surface. This is because the radar pulses will be detected more strongly off a surface which is in a specific direction as its reflected wave will be travelling towards the earth rather than towards outer space. Without this field of research, the surface properties of planets such as Venus and Mars as well as the shape and number of asteroids would be less well known.

Computer modelled image of a local asteroid using radar techniques [16]

 

Overall, WW2 acted as an incubator for innovation in radar technologies due to the significant increase in investment and focus it received due to its ability to aid and eventually end the war. Therefore, without this crucial weaponization that occurred many technologies and key areas of research may not even exist today.

References:

  1. https://scienceworld.wolfram.com/physics/Radar.html#:~:text=Radar%20typically%20consists%20of%20a,radar%20system%20to%20detect%20it.
  2. https://www.iwm.org.uk/history/how-radar-changed-the-second-world-war#:~:text=Radar%20could%20pick%20up%20incoming,masts%20over%20100%20metres%20high.
  3. https://patents.google.com/patent/US3152547A/en
  4. http://histru.bournemouth.ac.uk/Oral_History/Talking_About_Technology/radar_research/the_magnetron.html
  5. https://www.jstor.org/stable/4142066?seq=1
  6. https://www.explainthatstuff.com/how-magnetrons-work.html
  7. https://www.esa.int/Applications/Observing_the_Earth/Radar_altimetry_revolutionises_the_study_of_the_ocean
  8. https://www.eol.ucar.edu/system/files/Radar_handout_FullPage_f.pdf
  9. https://public.nrao.edu/radio-astronomy/the-science-of-radio-astronomy/
  10. https://www.skyradar.com/blog/radar-principle
  11. https://etherwave.wordpress.com/2008/11/12/hump-day-history-the-tizard-committee/
  12. https://www.historylearningsite.co.uk/world-war-two/world-war-two-in-western-europe/battle-of-britain/chain-home/
  13. https://www.youtube.com/watch?v=CAe3lkYNKt8&feature=youtu.be
  14. https://duotechservices.com/chain-home-radar-saved-london-in-battle-britain
  15. https://www.rafmuseum.org.uk/research/online-exhibitions/history-of-the-battle-of-britain/radar-the-battle-winner/
  16. http://spaceref.com/asteroids/radar-observations-shine-new-light-on-near-earth-asteroid.html
  17. https://makersportal.com/blog/2020/3/26/arduino-raspberry-pi-radar
  18. https://www.youtube.com/watch?v=h4OnBYrbCjY&feature=youtu.be
  19. https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6443717

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