Tropical Antennas - Getting Some Help From The Ground

In this post we look at little further at the quarter wave antenna and also see how the ground beneath our feet can help us hear the DX.

We will not, however, go into the theory of antennas in this short post but we will make a very efficient DX antenna for the tropical bands.

Quarter wave plus Earth antenna
The best antenna to have is one that is a half a wavelength long. In this case, with the connection to the receiver in the centre, the signal would oscillate along the whole length of wire. This has the effect of funnelling the maximum amount of signal down to our receiver.

Unfortunately a half wavelength of wire is quite long, being around 200 foot on 120 meters so a smaller length is usually easier to fit into suburban back yards.

Luckily for us, we can take advantage of the Earth below our feet to help us make our quarter wavelength of wire into a full half wave.

Again, without going into the details, an electrical image of the antenna is formed in the earth under the wire and this acts as if it were another quarter wave attached to our wire. In effect, this makes our antenna into a full half wavelength one.

Of course, an antenna a full half wavelength long is best but this is still a good second.

What sort of earth do you need?
The soil under the antenna determines the quality of the mirror image quarter wave. If your soil is good quality, thick, rich earth then the image will be good. If the soil, like mine, is poor, sandy soil then the image will not be a strong one.

But, whatever your soil is, the effect can be enhanced by a further piece of wire placed on or in the ground under or near the antenna.

Such a wire is called a radial and need not be connected to the quarter wave although it will work better if it is. It's sole purpose is to strengthen the ground signal and a current will flow in it by induction through the air if it is not connected.

In this case, we have a wire radial that is just a little longer connected to the braid of the coax at the bottom of the vertical section and just run on (or under) the ground around the antenna.

This will make the contribution from the Earth all the stronger and convert our quarter wave into an antenna that is nearly as good as a full half wave yet only, so far as we are concerned, half as long.

In the next post we will improve our antenna even more by the addition of an antenna tuner.