Half Bobtail Antenna
Brian Williams, GW0GHF
Issue 60, February 1999

The popularity of six metre FM will increase because manufacturers are increasingly producing HF transceivers with all-mode 50MHz coverage at decent power levels. And there is room for experiment with aerials; designs which have long been used at HF are producing results at 50MHz: the long wire, G5RV, HF verticals etc are being used. The HB9CV two-element will produce good results but, when mounted vertically, the top part of the mast needs to be made of some non-metallic material.

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The half-bobtail antenna. X-X is the fedpoint for 50-Ohm coax, four inches up from the shorted end of the feeder transformer. The shorting bar 'B' is used to obtain a low VSWR.

A comparable aerial which works well at this QTH, in the loft space, is the half-bobtail. This is a two-element vertical with a broadside vertically polarised gain of about the same as the HB9CV but only needs quarter-wave radiators although it does have a nine-foot top section. This is perfect for lofts because the horizontal section can go right up in the roof ridge. The vertically-polarised gain is about 4dBd, with about 1.5dBd horizontally-polarised radiation from the top section, broadside to the wire. See the diagram for details.

The half-bobtail is rather like an inverted ground-plane and which has two driven elements phased at 180 degrees. The full size half-bobtail has 8dBd gain but it is a very sharp antenna and does not have any horizontal components provided the two halves are electrically balanced. But you need an 18 foot top wire and it is very difficult to attach to the top of a rotator!

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