The Homebase 10
A small "weekend project" to build a 10m band wire halo antenna
1.6.08 6

The Homebase 10 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.


SPECIFICATION   

Gain                  Around 0 to -2dBd
Pattern            Near omnidirectional
Polarization     Horizontal 

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 will appear in September 2008 Practical Wireless, so this is just an overview.

POSSIBLE OPTIONS

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 Homebase 10. 

CONSTRUCTION

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.

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.  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)
Timber 12 x21 x 1000mm                    4 off
50mm corner braces                         4 off
M4 nuts 15mm long                          8 off 
M4 bolts                                          8 off (to secure corner braces to struts)
Self-tap screws                                4 off (to support wires at all 4 corners)
Nylon cable ties                                As required
50 ohm coax e.g. RG58                     As required (for feeder and choke)
100 x 200 x 2mm aluminium sheet     As shown (for mast fixing)
Mast clamps                                    2 off (to fix antenna to mast)
Yacht varnish                                   As required
PVC tape or heat-shrink                    As required to waterproof joints.
Polypropylene or nylon cord              As required to tie free ends of dipole together

All parts except the coaxial cable may be obtained from DIY stores. Coax is available from many sources including Maplin.