extract from September 2002
Editor: Malcolm Andrew G8NRP
10m HALO PROJECT
by Roger Powell G0AOZ
This aerial is designed to fit into a small attic, loft or roof space, or even into the garden shed. To this end, no waterproofing techniques have been employed in its design, which helps to simplify the construction. There is no reason why the relevant joints and connections could not be weatherproofed, and the aerial mounted outside. Good height above ground is desirable in most aerials, but even during initial tests, this aerial worked successfully at six feet above ground. It is mounted horizontally, provides omni directional polarisation, and is fed with 50 ohm coaxial cable. Dimensions given are for the upper end of the 10m band, i.e. the FM section. To use on the SSB or CW sections of the band, increase the dimensions accordingly, or use a simple matching unit if only occasional excursions into those band areas are envisaged. A simple design for a suitable matching unit is the subject of a separate article, see here.
The aerial utilises 15mm copper water pipe of the type available from plumbers merchants, d.i.y. stores, etc. All four corners and other fixed joints are soldered together using a blow torch. To simplify the build, 90º "Yorkshire" type bends are employed at the corners, and tee pieces are used to attach the gamma match to the main aerial element. To enable the gamma match section at point "A" to be moved along the side of the aerial element, the internal shoulders within the tee piece are filed away until it slides easily on the 15mm tube. Two cuts are made into the "moving" ends of this tee piece and a couple of small jubilee clips enable it to be clamped onto the 15mm aerial element once the best impedance matching point is found. The telescopic sections use 0.5" OD brass tube, which is the best fit that could be obtained without resorting to custom made tube sizes. Again, cuts are made to the copper tubes and small jubilee clips are used to tighten the joints. The aerial can be mounted on wooden slats using conventional plastic water pipe clips, and then affixed to rafters, beams, etc., or any other convenient mounting points. Alternatively, it could be hung from the apex of the roof using polypropylene ropes or cords.
Adjustment and Use
Several movable and telescopic sections have been built into this design to provide a reasonable degree of adjustment, as it is possible that the aerial could to be placed close to objects which may affect its resonance, e.g. water tanks, pipework, etc. The use of a grid dip oscillator or an antenna analyser will be helpful in setting up this aerial, although not essential. I have mounted the halo in the loft space above my garage which puts it at approximately 12ft above ground level. In use the halo produces a quieter background band noise than my unbalanced vertical aerial, and it has been noticed that whilst some signals are weaker than on the vertical, many others are considerably stronger! Prefixes worked to date on both FM and SSB include CE, CX, DL, EA, F, I, HB, LW, OK, OM, PY, SM, SQ, TY7, ZS6, 3Z0 and 4Z5. At the time of writing, there have been no very strong openings, so I would anticipate some good results when propagation allows the band to open up.
A photograph of the completed project and mechanical detail are shown below.