How to work 3B7C: Antenna Ideas

In the RadCom article which preceded the D68C expedition, the team recommended, instead, to focus on one band at a time, and to install a resonant sloping dipole, orientated towards D6.

This led to a great deal of positive feedback after the expedition, many operators discovering that such an arrangement worked much better on that specific band than their multiband system, allowing them to work the expedition much more easily.

So the recommendation is very much the same again. A full-size, resonant dipole, cut according to the standard dimensions, out in the clear and, ideally, sloping and orientated towards 3B7C is an excellent starting point for those of you without Yagis or other gain antennas.

Suitable dimensions are given below (you may need to modify them slightly to achieve best SWR, as lengths can be affected by surrounding objects, etc.). One end of the antenna can be suspended from a chimney or gutter, perhaps, and the other end lower down on a fence-post or even near to the ground. Feed with 50 or 75 ohm co-ax, with the centre conductor to the higher half and the outer to the lower half. Seal the feedpoint to keep out moisture, with insulators at either end and you have an excellent single-band antenna.

On the low bands, of course, such an arrangement starts to get prohibitively large, so other approaches have to be tried. 3B7 is a long way from most centres of population, so arrival angles will be low. Even on 40m, the half-wave above ground required to achieve low angle radiation from a horizontal antenna is 20m (67ft), so it is much easier to use some sort of vertical. Actually, on 40m, that then becomes quite easy.

A full-size quarter-wave vertical is just 10m (33ft) high and takes up almost zero ground space! Of course, like any vertical, it will only work properly with an effective ground system, but for practical purposes this means running out as many radials as you can, of whatever length, in whichever directions they can go. Very few of us have the space to put down a textbook radial mat with, for example, 120 quarter-wave radials at exact spacings of 3 degrees!

I have had great success on 40m with an elevated ground-plane consisting of a 10m glass-fibre fishing pole, with the quarter-wave radiator wire running up its length (held with insulating tape), mounted on an insulating support (old fence post, or whatever) just a metre or so high, with two elevated quarter-wave radials (if you use elevated radials, do not have an earth connection in addition). Feed the centre of the co-ax to the vertical radiator and attach the outer to the radials.

Length of half-wave dipole antennas