Constraints
I had to take the aerial with me on our flight so I was really limited to wire antennas. I obtained a 10m 'roach pole' collapsible fibre glass mast as a support, as Namibia is almost devoid of the portable amateur's friend, the tree. Thanks to SOTA Beams for the Maxi-pole. This made it through Namibian custom eventually, although there was a delay as it was in a gun bag I had borrowed and ended up in the Police room along with some South-African hunters and a friendly Namibian officer. Most airlines allow a 'sports bag' in addition to your regular baggage allowance.
EZNEC modelling
I decided to model possible antennas, with this pole as a support, using Roy Lewallen's excellent EZNEC software. All the designs were modelled using a real, Sommerfield-Norton, ground, with fairly low conductivity. I supposed that dry desert soils would be poor conductors. My first design was:-
Quarter Wave Vertical
I initially assumed that, wanting a good signal at a low angle to contact the U.K., a vertical antenna would be my best choice. Above is the vertical plane radiation pattern from a simple quarter-wave vertical with 4 elevated and angled radial counterpoises. The radiation pattern looks good but the actual strength was disappointing - the maximum gain was only 0.4dBi - so I decided to try other options:-
Inverted V
The top one metre section of the pole is too thin to support a wire dipole, so this aerial is modelled with the centre at 9 m AGL. The gain curve shown is in the optimum direction at right angles to the wire. It is not as low angle as the vertical but the maximum gain is much greater at 6.38 dBi.
The Designs Compared
Comparing the two designs the choice is obvious. The inverted V has better gain at almost every elevation angle in the best direction. It is much worse in the line of the wire but this might not matter too much as most contacts would be in the north to north-west direction. If I was careful about the direction it was erected in, it should work well.
Construction
The centre piece of the dipole was made from 1/8" Perspex™. I think you can see that the wire was held by inserting it through a hole and looping it over the body to rest in two notches. This method seems to hold the very slippery ETFE wire very well. A 50Ω BNC plug was added. I constructed a simple 1:1 balun for the aerial centre, using RG178 and a 2" dust iron core, and then cut a resonant 20 metre dipole from 19/0.2 ETFE insulated silver-plated copper wire, supplied by Henry Weslake at a bargain price of 10p/metre - RS charge 70p!
The mast was guyed with 2mm Mammut kernmantle cord, supplied by Taunton Leisure, which was also used to pull out the inverted V. I made a guying plate from scrap aluminium. This slid over the pole down to the 5 metre level.
The cords and aerial were wound on small pieces of thin plywood to prevent tangling in transport.
Erection and use
Erection was fairly simple and could be managed by one person quite easily. I did take tent pegs, but found that large stones were more effective almost everywhere. It took around 20 minutes to put up and less than 5 to dismantle.
It did require quite a large space, around 20 m in diameter with the antenna support cords extending to 25 m from the mast base. This was not a problem on most Namibian campsites, where pitches are often 100 m apart!
Here is the antenna erected in the western Kalahari.You can just make out the guy lines and the antenna. The coax feed, RG58, was taped to the pole to prevent it pulling sideways on the thin upper section and bending it.
