ANTENNA FUNDAMENTALS
An antenna is an electric circuit of a special kind. In the ordinary type of circuit the dimension of coils,capacitors and connections usually are small compared with the wavelength that corresponds to the frequency in use.when this is the case most of the electromagnetic energy escapes by radiation in the form of electromagnetic waves. when the circuit is intentionally designed so that the major portion of the energy is radiated,we have an antenna.
Usually,the antenna is a straight section of conductor,or a combination of such conductor.Very frequently the conductor is a wire,although rods and tubing also are used .in this chapter we shall use the term “WIRE”to mean any type of conductor having across section that is small compared with its length.
The strength of the electromagnetic field radiated from a section of wire carrying radio-frequency current depends on the length of the wire and the amount of current flowing.( it would also be true to say that the field strength depend on the voltage across the section of wire, but it is generally more convenient to mean sure current. The electromagnet field, consist of both magnetic and electric energy, but it is generally more convenient to mean sure current. The electromagnetic filed consist of both magnetic and electric energy, with the total energy equally divided between the two one cannot exist without the other in a electromagnetic waves, and the voltage in an antenna is just as much a mean sure of the field intensity as the current ). other things being equal,the field strength will be directly proportional to the current .It is therefore desirable to make the current as large as possible,considering the power available. In any circuit that contains both resistance and reactance , the largest current will flow ( for given a mount of power ) when the reactance is “ Tuned out “-- in other words, when the circuit in made resonant at the operating frequency. So it is with the common type of antenna ; the current in it will be largest, and the radiation therefore greatest,when the antenna is resonant.
In an ordinary circuit the inductance is usually concentrated in a coil, the capacitance in a capacitor , and the resistance is principally concentrated in resistors, although some may be distributed around the circuit wiring and coil conductor. Such circuits are said to have lumped constant. In an antenna, on the other hand, the inductance, capacitance,and resistance are distributed along the wire. Such a circuit is said to have distributed constant. Circuits with distributed constant are so frequently straight-line conductor that they are customarily called linear circuits.
