There seems to be a myth among many newly licensed radio amateurs that an antenna works better if it costs a lot of money. This may have originated from magazine reviews which appear to be written to keep the advertising revenue up rather than serving the needs of the reader. The antenna shown here costs around one tenth the price of a commercial vertical, yet it will perform as well as (and in many cases better than) a trapped vertical antenna. This antenna is based on a 10 Metre long fibreglass fishing pole - these can be obtained from G3CWI by mail order for around £25 each delivered. From rallies they cost slightly less. It is important to use fibreglass poles, not carbon fibre. The pole can be used without cross pieces on a single band. In the case of the 7 MHz band the pole is slightly short for a straight wire but is long enough provided the wire is wound in a spiral on the pole. My fishing pole measures 32' 5" long, the calculated wire length for a quarter wave on 7 MHz is nearly one foot longer... These poles will collapse inside the sections unless each joint is secured with PVC tape, for more permanent installations glue could be used. The antenna can be made dual or triple band for the 7/10/14 MHz bands by the addition of parallel wires. The one shown here is a triple band version. To minimise interaction between the wires it is necessary to space the wires well apart. Spacers are easily and cheaply made from 16mm wide oval electrical conduit which can be obtained for a nominal sum from DIY stores such as B & Q. By itself the plastic conduit is quite weak, to strengthen the point where the spacers are fixed to the pole it is necessary to add wooden dowels. These only need to be around 120 mm in length by 12 mm or so in diameter, and can either be used to join two lengths of conduit "back to back" or pushed into the conduit from one end. I used 480mm lengths of conduit joined "back to back" plus the single 480mm length for a top support. To lessen the visual impact of the antenna, I covered the white conduit with ordinary black PVC insulating tape. The wire lengths are calculated from the formula L = 234/F, where F is the frequency in MHz and L is the wire length in feet. These lengths work out to around 33' 3", 23' 2" and 16' 7" for the 7, 10 and 14 MHz bands. The lengths for 7 and 10 MHz were more or less correct, however probably due to interaction between the wires the 14 MHz wire needed lengthening by around 4" for minimum SWR. Wire size is not critical, but it is probably better to avoid the thinnest "hookup" wire. Note, ground conductivity/loss and elevated/buried radials make a significant impact on both the performance and tuning of a ground mounted vertical. In the case of buried radials the vertical may resonate significantly lower in frequency than expected. The pole can be fixed to a square section wooden post with cable ties (Ty-Raps) and will self support. Beware than most of the DIY grade cable ties are not strong enough to use for this purpose. For "belt and braces" security guy lines can be added. These need not be elaborate, polypropylene garden twine secured to plastic tent pegs is good enough in all but the strongest of wind. Like all quarter wave verticals this one needs radials, the more the better. I currently have 16 cut into the lawn which return to an earth rod set into an old plant pot. The plastic junction box and RG58 coax shown below was only used for testing. For actual use RG213 coaxial cable is used. The SWR is very good on all three bands with a minimum around 1.2:1 on all bands. Results are much as you would expect from a ground mounted vertical. On 7 MHz the vertical is generally 2 or 3 S-units better than my 100' long doublet (apex at 30 feet above ground) for stations beyond Europe and often stronger into Europe as well. On 10 MHz the difference is less but the above generally applies. On 14 MHz the antenna is worse by 2 or 3 S-units on signals out to 2000 miles compared to the doublet, however beyond 3000 miles the vertical is better than the doublet.. DX worked in early March 2006, running 60 Watts of CW, include: 7 MHz 4X4, 3B8, 5R8, 5H1, 9M2, 9M6, FS, A45, VR2, VY2, VK, ZL, BV4, J88 and SU © D Johnson, G4AON 2006 |
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