Editing Chain Home (section)

==Description==

===Mechanical layout===
[[File:The 360ft transmitter towers at Bawdsey Chain Home radar station, Suffolk, May 1945. CH15337.jpg|thumb|right|Three of the four transmitter towers of the Bawdsey CH station as seen in 1945. The antennas proper are just visible at the extreme right. These towers, as all of Chain Home, were built by [[J. L. Eve Construction]].]]

Chain Home radar installations were normally composed of two sites. One compound contained the transmitter towers with associated structures, and a second compound, normally within a few hundred metres distance, contained the receiver masts and receiver equipment block where the operators (principally WAAF, [[Women's Auxiliary Air Force]]) worked.{{sfn|Neale|1985|p=74}} The CH system was, by modern terminology, a "[[bistatic radar]]", although modern examples normally have their transmitters and receivers far more widely separated.

The transmitter antenna consisted of four steel towers {{convert|360|feet|m}} tall, set out in a line about {{convert|180|feet|m}} apart. Three large platforms were stationed on the tower, at 50, 200 and 350 feet off the ground. A 600 ohm transmission cable was suspended from the top platform to the ground on either side of the platform (only on the inside of the end towers). Between these vertical feed cables were the antennas proper, eight half-wave dipoles strung between the vertical cables and spaced ½ of a wavelength apart. They were fed from alternating sides so the entire array of cables was in-phase, given their ½ wavelength spacing. Located behind each dipole was a passive reflector wire, spaced 0.18 wavelength back.{{sfn|Neale|1985|p=74}}

The resulting ''[[curtain array]]'' antenna produced a [[Polarisation (waves)|horizontally polarised]] signal that was directed strongly forward along the perpendicular to the line of the towers. This direction was known as the ''line of shoot'', and was generally aimed out over the water. The broadcast pattern covered an area of about 100 degrees in a roughly fan-shaped area, with a smaller [[side lobe]] to the rear, courtesy of the reflectors, and much smaller ones to the sides. When the signal reflected off the ground it underwent a ½ wavelength phase-change, which caused it to interfere with the direct signal. The result was a series of vertically-stacked lobes about 5 degrees wide from 1 degree off the ground to the vertical. The system was later expanded by adding another set of four additional antennas closer to the ground, wired in a similar fashion.{{sfn|Neale|1985|p=74}}

The receiver consisted of an [[Adcock array]] consisting of four {{convert|240|foot|m}} tall wooden towers arranged at the corners of a square. Each tower had three sets (originally two) of receiver antennas, one at 45, 95 and 215 feet off the ground. The mean height of the transmitter stack was 215 feet,{{sfn|Neale|1985|p=74}} which is why the topmost antenna was positioned at the same altitude in order to produce a reception pattern that was identical to the transmission. A set of motor-driven mechanical switches allowed the operator to select which antenna was active. The output of the selected antenna on each of the four towers was sent to a single [[radiogoniometer]] system (not Watt's own huff-duff solution). By connecting the antennas together in X-Y pairs the horizontal bearing could be measured, while connecting together the upper and lower antennas allowed the same goniometer to be used to measure the vertical angle.{{sfn|Neale|1985|pp=74-75}}

Two physical layout plans were used, either 'East Coast'<ref>{{Cite web |url=https://artuk.org/discover/artworks/a-type-ch-chain-home-radar-station-on-the-east-coast-7381 |title=A 'Type CH' (Chain Home) Radar Station on the East Coast &#124; Art UK |access-date=12 February 2018}}</ref> or 'West Coast'.<ref>{{Cite web |url=https://artuk.org/discover/artworks/a-type-ch-chain-home-radar-station-on-the-west-coast-7544 |title=A 'Type CH' (Chain Home) Radar Station on the West Coast &#124; Art UK |access-date=12 February 2018}}</ref> West Coast sites replaced the steel lattice towers with simpler guy-stayed masts, although they retained the same wooden towers for reception. East Coast sites had transmitter and receiver blocks protected with earth mounds and blast walls, along with separate reserve transmitter and receivers in small bunkers with attached 120&nbsp;ft aerial masts. These reserves were in close proximity to the respective transmitter/receiver sites, often in a neighbouring field. West Coast sites relied on site dispersal for protection, duplicating the entire transmitter and receiver buildings.

===Transmitter details===
[[File:J M Briscoe24 07 200713 07 40IMG2106 CH TX.JPG|thumb|right|Chain Home transmitter, [[RAF Air Defence Radar Museum]] (2007)]]
[[File:Chain Home valve, London Science Museum.jpg|thumb|Chain Home transmitting valve, Science Museum, London. The valve was capable of being dismantled and consequently had to be continuously vacuum pumped while operating. This was done via the piping to the left.|alt=]]

Operation began with the Type T.3026 transmitter sending a pulse of radio energy into the transmission antennas from a hut beside the towers. Each station had two T.3026's, one active and one standby. The signal filled space in front of the antenna, flooding the entire area. Due to the transmission effects of the multiple stacked antennas, the signal was most strong directly along the line of shoot, and dwindled on either side. An area about 50&nbsp;degrees to either side of the line was filled with enough energy to make detection practical.{{sfn|Neale|1985|p=74}}

The Type T.3026 transmitter was provided by Metropolitan-Vickers, based on a design used for a BBC transmitter at [[Rugby, Warwickshire|Rugby]].{{sfn|Neale|1985|p=78}} A unique feature of the design was the "demountable" [[Valve (electronics)|valves]], which could be opened for service, and had to be connected to an [[Diffusion pump#Oil diffusion pumps|oil diffusion vacuum pump]] for continual evacuation while in use. The valves were able to operate at one of four selected frequencies between 20 and 55&nbsp;MHz, and switched from one to another in 15 seconds. To produce the short pulses of signal, the transmitter consisted of [[Hartley oscillator]]s feeding a pair of tetrode amplifier valves. The tetrodes were switched on and off by a pair of mercury vapour [[thyratron]]s connected to a timing circuit, the output of which biased the control and screen grids of the tetrode positively while a bias signal kept it normally turned off.{{sfn|Neale|1985|pp=78-79}}

Stations were arranged so their fan-shaped broadcast patterns slightly overlapped to cover gaps between the stations. However, it was found that the timers controlling the broadcasts could drift and the broadcasts from one station would begin to be seen at others, a phenomenon known as "running rabbits".{{sfn|Neale|1985|p=74}} To avoid this, power from the [[National Grid (Great Britain)|National Grid]] was used to provide a convenient phase-locked 50&nbsp;Hz signal that was available across the entire nation. Each CH station was equipped with a phase-shifting transformer that triggered it at a different point on the grid waveform. The output of the transformer was fed to a [[Phase-shift oscillator|Dippy oscillator]] that produced sharp pulses at 25&nbsp;Hz, phase-locked to the output from the transformer. The locking was "soft", so short-term variations in the phase or frequency of the grid were filtered out.{{sfn|Neale|1985|p=80}}

During times of strong ionospheric reflection, especially at night, it was possible that the receiver would see reflections from the ground after one reflection. To address this problem, the system was later provided with a second pulse repetition frequency at 12.5&nbsp;pps, which meant that a reflection would have to be from further than {{convert|6000|miles|km}} away before it would be seen during the next reception period.{{sfn|Neale|1985|p=74}}

===Receiver details===
In addition to triggering the broadcast signal, the output of the transmitter trigger signal was also sent to the receiver hut. Here it fed the input to a [[time base generator]] that drove the X-axis deflection plates of the CRT display. This caused the electron beam in the tube to start moving left-to-right at the instant that the transmission was completed. Due to the slow decay of the pulse, some of the transmitted signal was received on the display. This signal was so powerful it overwhelmed any reflected signal from targets, which meant that objects closer than about {{convert|5|miles|km}} could not be seen on the display. To reduce this period even to this point required the receiver to be hand-tuned, selecting the decoupling capacitors and impedance of the power supplies.{{sfn|Neale|1985|p=79}}

The receiver system, built by [[A.C. Cossor]] to a TRE design, was a multiple-stage [[superheterodyne]]. The signal from the selected antennas on the receiver towers was fed through the radiogoniometer and then into a three-stage amplifier, with each stage housed in a metal screen box to avoid interference between the stages. Each stage used a [[Amplifier#Class B|Class B amplifier]] arrangement of EF8s, special low noise, "aligned-grid" pentodes.{{efn|Introduced in 1938, the EF8 was not technically a pentode as it had 4 grids making it a hexode. However, the purpose of the fourth grid and the alignment of the remaining grids was to reduce the partition noise from which pentodes generally suffer. Since the device exhibited pentode characteristics, all literature generally describes it as a 'pentode.<ref>{{Cite web |title=EF8 Low-noise variable-MU R.F. amplifier pentode |website=electron Tube Data sheets |url=https://frank.pocnet.net/sheets/046/e/EF8.pdf}}</ref> It is not clear whether the device was specifically developed for the chain home system.}} The output of the initial amplifier was then sent to the [[intermediate frequency]] mixer, which extracted a user-selectable amount of the signal, 500, 200 or 50&nbsp;kHz as selected by a switch on the console. The first setting allowed most of the signal through, and was used under most circumstances. The other settings were available to block out interference, but did so by also blocking some of the signal which reduced the overall sensitivity of the system.{{sfn|Neale|1985|p=79}}

The output of the mixer was sent to the Y-axis deflection plates in a specially designed high-quality CRT.{{sfn|Neale|1985|pp=79-80}} For reasons not well explained in the literature, this was arranged to deflect the beam downward with increasing signal.{{efn|The image of the operator console on this page appears to offer the solution; the line is not being drawn across the ''top'' of the display, but the middle, where it is the widest and thus provides the greatest resolution. The tube is then placed in a box with the upper section covered, so the line on the middle of the CRT appears at the top of the resulting opening. Of course this could also be operated upward.}} When combined with the X-axis signal from the time base generator, echoes received from distant objects caused the display to produce ''blips'' along the display. By measuring the centre point of the blip against a mechanical scale along the top of the display, the range to the target could be determined. This measurement was later aided by the addition of the ''calibrator unit'' or ''strobe'', which caused additional sharp blips to be drawn every {{convert|10|miles|km}} along the display.{{sfn|Neale|1985|p=81}} The markers were fed from the same electronic signals as the time base, so it was always properly calibrated.

===Distance and bearing measurement===
[[File:Chain Home screen shot -NEDAD.2013.047.058A.jpg|thumb|right|Chain Home display showing several target ''blips'' between 15 and 30 miles distant from the station. The marker at the top of the screen was used to send the range to the fruit machine.]]
[[File:WAAF radar operator Denise Miley plotting aircraft on a cathode ray tube in the Receiver Room at Bawdsey 'Chain Home' station, May 1945. CH15332.jpg|thumb|right|The operator display of the CH system was a complex affair. The large knob on the left is the goniometer control with the ''sense'' button that made the antenna more directional.]]

Determining the location in space of a given blip was a complex multi-step process. First the operator would select a set of receiver antennas using the motorized switch, feeding signals to the receiver system. The antennas were connected together in pairs, forming two directional antennas, sensitive primarily along the X and Y axes respectively, Y being the line of shoot. The operator would then "swing the gonio", or "hunt", back and forth until the selected blip reached its minimum deflection on this display (or maximum, at 90 degrees off). The operator would measure the distance against the scale, and then tell the plotter the range and bearing of the selected target. The operator would then select a different blip on the display and repeat the process. For targets at different altitudes, the operator might have to try different antennas to maximize the signal.{{sfn|Neale|1985|p=75}}

On the receipt of a set of [[polar coordinates]] from the radar operator, the plotter's task was to convert these to X and Y locations on a map. They were provided with large maps of their operational area printed on lightweight paper so they could be stored for future reference. A rotating straightedge with the centrepoint at the radar's location on the map was fixed on top, so when the operator called an angle the plotter would rotate the straightedge to that angle, look along it to pick off the range, and plot a point. The range called from the operator is the line-of-sight range, or ''[[slant range]]'', not the over-ground distance from the station. To calculate the actual location over the ground, the altitude also had to be measured (see below) and then calculated using simple [[trigonometry]]. A variety of calculators and aids were used to help in this calculation step.

As the plotter worked, the targets would be updated over time, causing a series of marks, or ''plots'', to appear that indicated the targets' direction of motion, or ''track''. ''Track-tellers'' standing around the map would then relay this information via telephone to the filter room at [[RAF Bentley Priory]], where a dedicated telephone operator relayed that information to plotters on a much larger map. In this way the reports from multiple stations were re-created into a single overall view.<ref>{{cite web |url=http://www.raf.mod.uk/history/fightercontrolsystem.cfm |title=The RAF Fighter Control System |publisher=RAF |date=6 December 2012 |access-date=10 February 2013 |archive-url=https://web.archive.org/web/20130118225830/http://www.raf.mod.uk/history/fightercontrolsystem.cfm |archive-date=18 January 2013 |url-status=dead}}</ref>

Due to differences in reception patterns between stations, as well as differences in received signals from different directions even at a single station, the reported locations varied from the target's real location by a varying amount. The same target as reported from two different stations could appear in very different locations on the filter room's plot. It was the job of the filter room to recognize these were actually the same plot, and re-combine them into a single track. From then on each track was identified by a number, which would be used for all future communications. When first reported the tracks were given an "X" prefix, and then "H" for Hostile or "F" for friendly once identified.{{sfn|Neale|1985|p=81}}{{efn|Other codes may have been used as well, this is not intended to be an exhaustive list.}} This data was then sent down the telephone network to the Group and Section headquarters where the plots were again re-created for local control over the fighters.

The data also went sideways to other defence units such as [[Royal Navy]], Army anti-aircraft gun sites, and RAF [[barrage balloon]] operations. There was also comprehensive liaison with the civil authorities, principally [[Air Raid Precautions]].

===Altitude measurement===
[[File:Royal Air Force Radar, 1939-145. CH15331.jpg|thumb|right|Plotting and reporting tracks was a manpower intensive operation. This image shows the receiver station at RAF Bawdsey, the home of CH development. It is commanded by Flight Officer Wright, on the phone. The radar operator is just visible in the background, just right of centre. She communicated with the plotter, in the foreground wearing headphones, via intercom so the readings could be made out even under attack.]]

Due to the arrangement of the receiver antennas, the sensitive area had a number of [[side lobe]]s that allowed reception at multiple vertical angles. Typically the operator would use the upper set of antennas at {{convert|215|feet|m|abbr=on}}, which had the clearest view of the horizon. Due to the half-wave interference from the ground, the main lobe from this antenna was directed at about 2.5 degrees above the horizontal, with its sensitive region extending from about 1 to 3 degrees. At the ground the gain was zero, which allowed aircraft to escape detection by flying at low altitudes. The second lobe extended from about 6 to 12 degrees, and so on. This left a distinct gap in the reception pattern centred at about 5.2 degrees.

This reception pattern provided CH with a relatively accurate way to estimate the altitude of the target. To do this, the motorized switch in the receiver hut was used to disconnect the four receiver masts and instead select the two vertically displaced antennas on one mast. When connected to the radiogoniometer, the output on the display was now effected by the relative signal strength of the two lobes, rather than the relative strengths in X and Y in the horizontal plane. The operator ''swung'' the radiogoniometer looking for the peak or minimum reception, as before, and noted the angle.

The number reported by the operator was the line-of-sight range to the target, or ''[[slant range]]'', which included components of both the horizontal distance and altitude. To convert this to the real range on the ground, the plotter used basic [[trigonometry]] on a [[right angle triangle]]; the slant range was the [[hypotenuse]] and the open angle was the measurement from the radiogoniometer. The base and opposite sides could then be calculated, revealing the distance and altitude. An important correction was the curvature of the Earth, which became significant at the ranges CH worked at. Once calculated, this allowed the range to be properly plotted, revealing the grid square for the target, which was then reported up the chain.

When the target was first detected at long range, the signal typically did not have enough of a return in the second lobe to perform height finding. This only became possible as the aircraft approached the station. Eventually this problem would recur as the target centred itself in the second lobe, and so forth. Additionally, it was not possible to determine the difference between a signal being compared between the first and second or second and third lobe, which caused some ambiguity at short ranges. However, as the altitude was likely determined long before this, this tended not to be a problem in practice.

This pattern left a set of distinct angles where reception in both lobes was very low. To address this, a second set of receiver antennas was installed at {{convert|45|feet|m}}. When the lower antennas were used, the pattern was shifted upward, providing strong reception in the "gaps", at the cost of diminished long-range reception due to the higher angles.

===Raid assessment===

Another critical function of the CH operators was to estimate the number and type of aircraft in a raid. A gross level of the overall size could be determined by the strength of the return. But a much more accurate determination could be made by observing the "beat" rate of the composite echoes, the way they grew and diminished over time as they entered into different sections of the antenna reception pattern. To aid this, the operator could reduce the pulse length to 6 microseconds (from 20) with a push-button. This improved the range resolution, spreading the blip out on the display at the cost of lower returned energy.{{sfn|Neale|1985|p=76}}

Raid assessment was largely an acquired skill and continued to improve with operator experience. In measured tests, experimenters found that acquired skill was so great that experienced operators could often pick out targets with returns less than the current [[signal-to-noise ratio]]. How this was accomplished was a great mystery at the time–the operators were spotting blips in static that were larger than the signal. It is currently believed this is a form of [[stochastic resonance]].{{sfn|Neale|1985|p=76}}

===Fruit machine===
[[File:Royal Air Force Radar, 1939-1945. CH15178.jpg|thumb|right|The fruit machine greatly simplified measurement and calculation, driving the plotter directly]]

Operating a CH station was a manpower-intensive situation, with an operator in the transmitter hut, an operator and assistant in the receiver hut, and as many as six assistants in the receiver hut operating the plotters, calculators and telephone systems. In order to provide 24-hour service, multiple crews were needed, along with a number of service and support personnel. This was then multiplied by the reporting hierarchy, which required similar numbers of WAAFs at each level of the Dowding system hierarchy.

Plotting the angle of the target was a simple process of taking the gonio reading and setting a rotating straightedge to that value. The problem was determining where along that straightedge the target lay; the radar measured the [[slant range]] straight-line distance to the target, not the distance over the ground. That distance was affected by the target's altitude, which had to be determined by taking the somewhat time-consuming altitude measurements. Additionally, that altitude was affected by the range, due to the curvature of the Earth, as well as any imperfections in the local environment, which caused the lobes to have different measurements depending on the target angle.{{sfn|Neale|1985|p=81}}

As no small part of the manpower required was dedicated to calculation and plotting, a great reduction could be made by using as much automation as possible. This started with the use of various mechanical aids; these were eventually replaced by the ''fruit machine'', an [[electromechanical]] [[analogue computer]] of some complexity.{{sfn|Neale|1985|p=81}} It replicated all of these devices and tables in electrical form. An electrical repeater, or [[synchro]], was added to the gonio dial. To measure the range, a new dial was added that moved a mechanical marker to a selected blip on the display. When a particular target was properly selected, the operator pushed a button to activate the fruit machine, which then read these inputs. In addition to the inputs, the fruit machine also had a series of local corrections for both angle and altitude, as measured by calibration flights and stored in the machine in telephone [[Stepping switch|uniselectors]]. These corrections were automatically added to the calculation, eliminating the time-consuming lookup of these numbers from tables. The output was the altitude, which then allowed the plotters to determine the proper over-ground distance to the target.{{sfn|Neale|1985|p=76}}

Later versions of the fruit machine were upgraded to directly output the position of the aircraft with no manual operation. Using the same buttons to send settings to the machine, the operator simply triggered the system and the outputs were used to drive a [[T-square]]-like indicator on the chart, allowing the operator to read the calculated location directly. This reduced the number of people needed at the station and allowed the station to be reorganized into a much more compact form. No longer did the operator call readings out to the plotters; now they sat directly beside the plotting table so they could see if the results looked right, while the tellers could see the plot and call it into the area plotting room. A further upgrade allowed the data to be sent to the local plotting room automatically over the phone lines, further reducing the required manpower.{{sfn|Neale|1985|p=81}}