The value of an aerial is dependent upon its height above the surface of the earth. The greater its height the wider will be the field of force or strain set up in its neighborhood and consequently more powerful electric waves will be developed. Proximity to all large conductors, such as smokestacks, telephone lines, etc., is always avoided because these obstacles would absorb appreciable amounts of the energy sent out from the station and also shield it somewhat from the incoming waves. Aerials are usually constructed of conductors made up of a number of wires stranded together. High frequency currents only travel near the surface of conductors and stranded wires consequently offer less resistance because they possess more surface than a solid conductor of equal cross section.
The aerial is always carefully insulated by means of special high tension insulators, made of insulating composition molded into a corrugated bar having iron rings embedded in each end to which the wires may be fastened.
Aerials take many different forms, but may be classified into two general groups called the vertical aerials and flat top aerials.
Vertical aerials compose the grid, fan, cage and umbrella forms.
Flat top aerials are known as the T, inverted U, L and V types, according to their shape.
The Pyramid Aerial is only employed in ultra-powerful stations and is becoming an obsolete form.
The Fan Aerial is a good type of especial value in crowded quarters.
The Grid Aerial is probably the best form of vertical aerial, but is gradually giving way to those of the flat top class.
The Cage Aerial is rarely used nowadays and may be considered obsolete.
The Umbrella Aerial is a very good type now being employed in many high power stations. A metallic pole or mast insulated at the base used to support the wires, so that it is part of the aerial itself.