A copy of the original Practical Wireless (PW) article is attached (see bottom of the page). Buy PW monthly!
The Homebase10 is a simple to make wire halo antenna for 10m (28MHz) built using parts available from the local DIY store.The resulting antenna is very effective on 10m despite its small size and light weight.
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.
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. The remainder of each dipole leg is 105cms long added to the ends of the folded dipole section. The folded dipole is made by paralleling up two pieces of the PVC cover multi-strand wire and holding these close together with cable ties. A choke consisting of 6 turns of the coax cable about 5cms diameter close to the feed point helps to keep RF from the outer of the coax.Firstly, assemble the support strut woodwork by taking four pieces of 21 x 12 mm wood 1m long (with the 21mm side vertical) and drill two holes close to one end to line up with metal corner brace holes. The four 50mm corner braces and the drilled aluminium plate are screwed together and to the wooden struts. When screwed together, the four wooden pieces form a cross with the aluminium plate trapped between two of the corner braces and the wooden struts. Coat the wood, the assembled brackets and the nuts & bolts with three layers of outdoor yacht varnish to protect them from the elements. A better alternative may have been to use nylon rods but the rectangular wooden struts allowed a simpler mechanical arrangement.
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
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 VK, 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
20m, matching the antenna successfully with the IC703 auto-ATU, before the 10m
band opened up to allow any QSOs on 10m. Of course, if you strap the coax cable inner and outer together at the base of the feed then the coax feed works as a vertical with the halo being a top capacity hat. This way, bands from 500kHz upwards can be used using an ATU.
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.
An improved version of the design would replace the wooden struts with PVC or nylon rods for reduced weight and increased durability.
DUAL BAND 10M/6M VERSION
The photo shows the arrangement used. Some small adjustment of the 6m lengths may be needed but mine was almost spot on first time.
Being able to cover the SSB/CW/DATA parts of 6m and 10m on a simple but effective antenna is extremely useful.