Editing Chain Home (section)

=== "Less unpromising" ===
[[File:Arnold Frederic Wilkins.jpg|thumb|upright=0.8|Arnold Wilkins carried out most of the theoretical and practical work that proved radar could work.]]
Watt wrote back to the committee saying the death ray was extremely unlikely, but added:

{{quote|Attention is being turned to the still difficult, but less unpromising, problem of radio detection and numerical considerations on the method of detection by reflected radio waves will be submitted when required.{{sfn|Watson|2009|p=45}}}}

The letter was discussed at the first official meeting of the Tizard Committee on 28 January 1935. The utility of the concept was evident to all attending, but the question remained whether it was actually possible. [[Albert Rowe (physicist)|Albert Rowe]] and Wimperis both checked the maths and it appeared to be correct. They immediately wrote back asking for a more detailed consideration. Watt and Wilkins followed up with a 14 February secret memo entitled ''Detection and Location of Aircraft by Radio Means''.<ref>{{cite tech report |url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a110586.pdf |title=New Eye for the Navy: The Origin of Radar at the Naval Research Laboratory |first=David |last=Allison |date=29 September 1981 |publisher= Naval Research Laboratory |page=143}}</ref> In the new memo, Watson-Watt and Wilkins first considered various natural emanations from the aircraft – light, heat and radio waves from the engine ignition system – and demonstrated that these were too easy for the enemy to mask to a level that would be undetectable at reasonable ranges. They concluded that radio waves from their own transmitter would be needed.{{sfn|Watson|2009|p=45}}

Wilkins gave specific calculations for the expected reflectivity of an aircraft. The received signal would be only 10<sup>−19</sup> times as strong as the transmitted one, but such sensitivity was considered to be within the state of the art.{{sfn|Bowen|1998|p=9}} To reach this goal, a further improvement in receiver sensitivity of two times was assumed. Their ionospheric systems broadcast only about 1&nbsp;kW,{{sfn|Bowen|1998|p=9}} but commercial shortwave systems were available with 15 amp transmitters (about 10&nbsp;kW) that they calculated would produce a signal detectable at about {{convert|10|miles|km}}. They went on to suggest that the output power could be increased as much as ten times if the system operated in pulses instead of continuously, and that such a system would have the advantage of allowing range to the targets to be determined by measuring the time delay between transmission and reception on an [[oscilloscope]].{{sfn|Watson|2009|p=45}} The rest of the required performance would be made up by increasing the [[Gain (electronics)|gain]] of the antennas by making them very tall, focusing the signal vertically.{{sfn|Bowen|1998|p=10}} The memo concluded with an outline for a complete station using these techniques. The design was almost identical to the CH stations that went into service.{{sfn|Watson|2009|p=45}}