ANTENTOP- 01- 2019, # 023

Spreading and Underground Antennas

Because of this the spreading and underground antennas were excellent for field espionage operation from suburbs of large cities, and for stationary operation from places located at remote distances from large cities and direction finding facilities.

After WWII in the World were developed underground and spreading antennas for stationary underground radio sites operating in a broad frequency range - from 9 kHz up to 50 MHz. Spreading antennas as well were used for field military communications as reserve antennas. The information on usage of underground and spreading antennas during World War II and modern times can be found in the internet, see for example, Reference [1].

The theory for both underground vertical and underground horizontal antennas was developed in different countries. However publications on the thematic were classified. In the USSR only one book on the subject was open for public and only after 20 years when the book was published. The book is named Spreading and Underground Antennas, Reference [2].

Design of Spreading and Underground Antennas

 Figure 1 shows a typical an old spreading antenna with the Diagram of Directivity. The maximum direction of the spreading antenna is in line with the element of the antenna. Originally such antenna was invented by downed by shell vertical antenna of field radio set.

How the antenna does work? As is known from the theory of antennas, a horizontal wire antenna, installed close to the ground, during operation establishes in the ground a mirror image, see Figure 1. The currents flowing in an antenna and the mirror image are mutually cancelled at low altitudes foiling the antenna ability to radiate waves horizontally polarized. So a spreading antenna should poorly radiate horizontally polarized waves.

It is known that vertically polarized radio waves are not absorbed in soil to such a degree as horizontally polarized radio waves. Therefore, with the spreading antenna there is some radiation of vertically polarized radio waves present. If soil under antenna has good conductive, the antenna will be less effective compare to antenna placed above the soil with poor conductivity.

The spreading antenna as well may be considered as a kind of feedline being created by wire in the insulation and soil, as lossy second wire of the feedline. Figure 2 shows a spreading antenna. Termination shown on the end of the antenna is the theoretical load that shows losses on radiation wire and losses of the electromagnetic energy in the soil.

Figure 3 shows an underground wire antenna. Underground antenna usually is wire covered with plastic or aerial isolation by thickness H and installed inside of the ground.

Figure 1

Old Spreading Antenna with DD

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