The Evolution of the G3BDQ Grounded Antenna part 4 – by John Heys

Part 4. ‘Capacitive Top Loading’ Mark V (Fig. 4)

Being, for various reasons unable to raise the height of the vertical wire at the far end of the antenna, and so make that vertical section more efficient, I decided to try some form of top loading. A conventional ‘Top Hat’ of radial spokes at the top of the vertical wire was impossible, so I resorted to the well known parallel wires with spreaders that has been used by generations of ships. The effective capacitance to ground of two spaced horizontal wires is determined by two factors, their length and their spacing.

g3bdq-vertical-grounded-antenna-p4The longer and further apart they are the greater becomes the top loading capacitance. If the spacing is increased beyond a certain amount the increases in capacitance increase very slowly; a question of diminishing returns. I had successfully used this kind of capacity loading some years earlier. Point ‘ T ‘ on figure 4 connects to two wires, one the main wire running back to the shack, and the other wire which is terminated by an insulator close to point ‘T2’ .The wires are spaced 2 metres apart. Ideally a spacing of four metres would give added capacitance, and was the favoured spacing on battleships and other large vessels. This spacing would be difficult to achieve on an amateur antenna installation.

The spreaders used on my antenna were each made up from a pair of 1 metre long garden canes, centre joined by a 6 inch (150mm) length of aluminium tubing. The canes must be weatherproofed. I used a couple of coats of ‘Yacht Varnish. The length of the loading wires is 25 feet ( 8m ) but can be sized longer or shorter than this and depends upon the available garden space.

Adding this extra capacity resulted in stronger signals both in and out showing that its construction had been worthwhile.

John Heys G3BDQ from Vital Spark September 2010.

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