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Cambridge Underground 1971 pp 22-23

THE HARDWARE SCENE

As a follow-up to his Simple Emergency Light reported in Descent 13, Steve Smith, Cave Engineer Extraordinary, was rumoured to be working on yet another top-secret device. A raid on the safe in the Engineering Department yielded a number of blueprints; long study suggested that he was working on a scheme for transmitting V.H.F. messages directly down electron ladders on long pitches, replacing the expensive and easily snagged telephone cable. A perfectionist by temperament, he had abandoned the project on discovery that the speed of light is in fact 3 x 1010 cm. per sec., not 3 x l09.

The scheme was based on the phenomenon that wave-propagation is unaffected by shorting at voltage nodes (which occur every half-wavelength). Given an electron ladder with reasonably regular rung-spacing, and unrusted C-link fastenings, a simple transmitter can be clipped across both sidewires at any point and, using a wavelength of four rungs, a receiver can similarly be clipped across at the bottom. Police high-band portable radios are easily adapted for the purpose. At this point, lesser intellects in the club took up the thread spun by the inventor. Detailed calculations - in abbreviated form at the end of this article - brought them to two conclusions. Firstly, transmission at V.H.F. frequencies requires a rung-spacing of forty inches and as yet the club has no ladder of this type. Secondly, with the widespread 10 inch spacing used by the club, wavelengths suitable to the ladder are in the Ultra-High Frequency range.

Work is now concentrating on the second line of development. Though the pioneer, Smith, is sceptical, a device is almost complete which will allow members on subsequent descents of Black Shiver to fend off boredom by watching B.B.C.2 in colour. Possibly of more value is our work on DEWDROP (Distant Early Warning Device for Rockfalls On Pitches), an adaption of the system for radar to protect cavers from falling boulders on climbs such as the entrance shaft of Reyfad; details will be released later.

As to the first result of the calculations, a committee chaired by our President is praying for "a meaningful dialogue at the Highest Level" to see if the speed of light can be reduced by a factor of ten. There is also a project under way at the Biology labs. to make the greater rung-spacing viable by selective breeding of cavers, but proctorial disapproval over the matter of stud-fees makes this very much a long term venture.

Technical Data

(i) With l0 inch rung spacing the wavelength is 2 x 10 / 39 metres.

Assuming suitable Voltage Standing Wave Ratio, this gives a propagation frequency of ~300 x 39 / 20 = 585 Megahertz, roughly the frequency of V.H.F. Television.

(ii) For the DEWDROP radar system, a frequency of 1170 MHz was chosen, which requires rung spacing of half the standard l0 inches. As the club had no such ladder, experiments were made with a phased structure of two ladders fastened together with sellotape, but results were not encouraglng.

(iii) The Characteristic Impedance (Z0) of ladder made with 5/16" 1663 mild-steel C-links, 10 cwt. 7/14 constr. 30 WP aero cable, and alloy HT 30 WP 16 SWG " O.D. rungs may be expressed as sqrt(L/C), where L = the inductance per unit length, and C, the capacity per unit length. =

[we're going to need a graphic here, Webmaster]

= 550 ohms - a reasonable figure for most designs of interface systems

ANDREW NICHOLS (C.U.C.C.)


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An After Dinner Story