Trevor Lloyd Wadley, (1920 – 21 May 1981) was a South African electrical engineer, best known for his development of the Wadley Loop circuit for greater stability in communications receivers and the Tellurometer, a land surveying device.

The invention that made Trevor Wadley most famous was the Tellurometer (literally “earth measurer”), an accurate distance-measuring tool that used radio waves.

When the first accurate survey was done in South Africa by the second director of the Royal Observatory in Cape Town, Sir Thomas Maclear, he used a Gunter’s Chain, an interlocking chain, for measuring distances. Until the 50s, theodolites (surveying instruments with a rotating telescope for measuring horizontal and vertical angles) were used to measure distances between beacons.

In 1954 Colonel Harry Baumann, director of the Trigonometric Survey of South Africa, asked the CSIR to invent a portable instrument that could measure distances within an accuracy of 1:100 000 using the latest radio technology.

This task was handed to Wadley and, within six months, in consultation with Jules Fejer, an expert on radio wave propagation, he found a solution to the problem. His Tellurometer measures the travel time of a radio wave by comparing the phase of the transmitted signal with that received from a second instrument.

By using microwaves, he produced an instrument that could be operated in broad daylight and could measure distances to an accuracy of 3:1 000 000 (exceeding Baumann’s requirement) over a range of 3km to 50km. The Tellurometer was also light and portable, and could be carried in a rucksack and used in very rugged terrain.

Wadley’s prototype operated at 3GHz with a wave length of 10cm but later models (MRA3, 1956) operated at 10GHz and 3cm and were more accurate.

Wadley demonstrated his instrument in South Africa, England and elsewhere, to great acclaim. 

Land surveying was revolutionised by Wadley’s invention, which was exported worldwide and earned over R300 million (in 60s terms) in foreign earnings for South Africa. By 1958, hundreds of Tellurometers were in use in more than 60 countries.

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