Editing Mid-Canada Line (section)

===Impetus===
Construction of the [[Pinetree Line]] had only just started when air planners started to have concerns about its capabilities and siting. By the time it detected a potential attack by [[Jet engine|jet]]-powered aircraft, there would be little time to do anything before the attack reached Canadian or northern U.S. cities.{{sfn|McCamley|2013|p=34}} Additionally, the Pinetree systems used pulsed radars that were fairly easy to jam and were unable to detect targets close to the ground due to [[Clutter_(radar)|"clutter."]] Although expensive in terms of fuel use, it would be possible for Soviet bombers to evade detection by flying lower and plotting a course between the stations.

[[Bennett Lewis]], head of the [[Atomic Energy of Canada Limited|AECL]] [[Chalk River Laboratories]] and former Chief Superintendent of the UK [[Telecommunications Research Establishment]] (TRE) had proposed to the [[Defence Research Board]] (DRB) a system that avoided both of these problems.{{sfn|Whitehead|1995}} Known today as a [[bistatic radar|forward scatter bistatic radar]], it used two antennas, a transmitter and receiver, separated by some distance. The antennas were positioned and aimed so that the signal from the transmitter filled the space above the line between the two stations. An aircraft flying into this region would reflect some signal back towards the receiver, allowing detection at altitudes as great as 65,000&nbsp;ft.{{sfn|McCamley|2013|p=34}}

A major advantage of the system is that it requires much less power to operate effectively. In a conventional radar, the radio signal has to travel to the target and back again. As each leg of the journey is subject to the [[inverse square law]], the resulting [[radar equation]] contains a fourth-power dependence. In contrast, a forward-scatter radar signal always travels about the same total distance, from the transmitter to the receiver, modified only by the altitude of the target. This means it is dependent on the square root of range and not the fourth root, and thus delivers considerably more energy onto the receiver than a conventional radar over the same range. Also, unlike a conventional "monostatic" radar, the transmitter did not have to turn off to allow the receiver to listen for the signal. Since the total amount of energy received at the receiver is a function of both the peak power and the length of the pulse, using a continual signal means the same total energy will be deposited using much lower peak transmitter power. As a result, Lewis' system would require  smaller sites and much less power than conventional radars like those of the Pinetree Line.{{sfn|Willis|Griffiths|2007|p=37}}

The major disadvantage of the system is that it did not indicate the aircraft's location within the beam, unlike a pulsed system where pulse timing can be used to determine range. This means the forward-scatter concept is useful for making a "radar fence" or "trip wire"{{sfn|McCamley|2013|p=34}} that indicates that something is approaching, but not exactly where it is. To help address locating the target to a degree, the proposal was to build two interlinked fences, so that each pair of stations was perhaps {{convert|30|km}} apart, a short enough distance that the radar on an [[interceptor aircraft]] would be able to find the target within that area. Using two overlapping sets also allowed one pair to cover the dead zone directly above the towers of the other.

Lewis' initial concept was to place the transmitters and receivers on [[telephone pole]]s and [[electric power transmission]] towers, which provided both a convenient location as well as the small amount of power needed to run the electronics. In the case of the telephone poles, the lines would also be used to send the data back to the tracking stations. This concept generated a considerable amount of interest, although it was abandoned for reasons that are not entirely clear. Willis and Griffiths speculate it might be the need for 1,000 such radars,{{sfn|Willis|Griffiths|2007|p=36}} but it is also likely that the desired to locate the line further north than the heavily settled areas in southern Canada was likely significant as well. In any event, the simplicity of the concept helped bring it to the attention of air planners.{{sfn|Willis|Griffiths|2007|p=36}}