INTRODUCTION What follows is an antenna option combining a very
small footprint, near omni-directionality, low cost, easy sourcing of parts, easy
assembly and easy adjustment. If mounted in the clear, it is capable of very
useful performance just a few dBs down in gain when compared with a large HF
beam. Full details appeared in September 2008 Practical
Wireless, so this is just an overview. Many of us live in small homes with small gardens and our options for effective HF antennas are limited. Although many of us would love to erect a beam for 10m, our neighbours, local councils and wives may not approve. Even a 28MHz two element HB9CV or a Moxon 2-element yagi look huge when mounted over a small semi-detached roof. On 28MHz a half-wave dipole is small but has directionality and nulls unless rotated. Verticals such as CB end fed half-waves or the professionally designed Cushcraft AR-10 can be very effective and are omni-directional, but these can easily pick up switch-mode power supply and PC noise as well as cause TVI through coupling into vertical cables and coaxial down-leads. Many will be familiar with the Cobwebb design from Steve Webb (G3TPW), which is a 14-28MHz horizontal, omni-directional, wire antenna. This works well, but it is very expensive and, in my humble opinion, it looks a bit like a rotary clothes line stuck on a pole up in the sky: my neighbours would not approve! I can make no claims for originality as this antenna takes some of the ideas from various similar concepts such as the VHF halo, the old Cushcraft Squalo, the GM3VLB delta beam and the Cobwebb. The antenna can be assembled for less than £10 and, with reasonable luck, the parts needed will be available from the junk in the back of the shed. All the parts required for this antenna may be bought from the local Homebase, B&Q or similar DIY stores. As it is designed for 28MHz (10m) then the name just had to be the Homebase10.
Next assemble the wire dipole.
Note how the feedpoint attaches to the centre of the folded dipole section. Initially
“tack” the wire onto the corners of the cross. The feed point is attached at
one end of one of the cross members. This helps to provide support as this is
the heaviest part because of the added weight of the coiled coaxial choke. Bring
the coaxial feeder away from the feedpoint back towards the middle of the
antenna along the wooden support strut. The free ends of the wires are pulled together
via a thin piece of insulating nylon or polypropylene cord. Make small loops
in the end of each wire to attach the cord. The losses of the material used to
connect the wire ends together may be checked by putting a length of it in a powered
microwave oven for 60 seconds to see how hot it gets: if it remains cool, the chosen
material should be OK. TESTING AND ADJUSTMENT Connect the 28MHz rig via an SWR bridge to the antenna. Position the antenna in the air clear of other wires and metalwork. This is best done in the garden as some adjustment of the wire length may be needed. Check the SWR at the bottom, middle and top of the 28MHz band. If all is well, the match should be <1.5:1 over about 600kHz of the band. If adjustment is needed, lengthen or shorten the free ends of the wire until the lowest SWR is centred where you want to operate within the band. My version was adjusted to give a low SWR between 28-28.6MHz where most of the SSB, CW and data DX activity is found. Try to position the antenna in the clear when checking resonance each time. Adjustment should not be too critical. Once adjustments have been completed attach the
antenna wire to the corners of the cross in a more permanent fashion ensuring
the soldered connections joining the folded dipole section to the end wires and
the feed point junction to the coax are suitably waterproofed. Joints should be
covered in heat-shrink sleeving or waterproof tape. Use nylon cable ties to
secure the folded dipole wires to each other, the coax choke and the coax
feeder. In my version I added a small extra piece of wood joining the tops of
the wooden strut supporting the feed coax and its opposite part to give this
additional strength. PERFORMANCE Erect the antenna as high as you can and start collecting 28MHz DXCC countries. You now have a small, lightweight but effective, 28MHz DX antenna which should last a few years and give you plenty of fun. If anything should fail you know the whole thing can be rebuilt in a few hours for less than the price of a takeaway. A dual 10m/6m version should be easy to make too. Contacts so far suggest the antenna is working as planned with a near omni-directional radiation pattern. Despite running only 5 or 10W on SSB and CW, reports have been excellent. I've even had 4 QSOs with 599 reports into Europe when running just 50mW. Best DX has included PY, LU, D4 and W. The antenna does not need any matching when used over its intended part of the band, but an auto-ATU such as that in the IC703 helps to optimise the match in other parts of the band. Unexpectedly, the antenna performs pretty well on other higher HF bands too. Indeed I had contacts on 12m and 20m, matching the antenna successfully with the IC703 auto-ATU, before the 10m band opened up to allow any QSOs on 10m. Although only tested at 5-10W
- the most I can run, HI - this antenna should work with full legal power as
long as the losses in the support cord joining the free antenna ends are low. Wire (PVC insulated multi-strand) 10m total approx (including some for prototyping) All parts except the coaxial cable
may be obtained from DIY stores. Coax is available from many sources including
Maplin. |
The antenna consists of two main parts: (a) a wooden X
skeleton section which provides the support struts and (b) a wire dipole folded
into a square “halo” shape. As with the Cobwebb, the centre part of the wire dipole
section is made as a folded dipole (310cms overall length split in the middle
on one side for the feedpoint) which brings the feed point impedance close
to 50 ohms.