Practical_Antenna_Handbook_0071639586
C h a p t e r 3 : A n t e n n a B a s i c s 87 Fields Originally conceived as a way of helping us understand and predict the observed effect of charges and magnets on other objects in the absence of any direct physical connection between the source and the affected objects, fields have acquired a life of their own. This “action at a distance” can be thought of as analogous to the way gravitational forces act. If a voltage is applied between two points (in space, in a circuit, etc.), an electric field is said to exist between those points. Similarly, if a current flows in a wire or other conducting medium, a magnetic field is said to surround the conductor. A common high school physics experiment provides graphic proof of this: As shown in Fig. 3.1, currents flowing in the same direction in two parallel wires will cause the two wires to be attracted to each other, and unrestrained sections of the two wires will actually move toward each other! Reversing the direction of current flow in one of the wires will push the wires apart. The force between the wires is proportional to the product of the magnitudes of the two currents and inversely proportional to the distance between them. Note a very important aspect of magnetic fields: The force is at right angles to the direction of current flow! Whether electric or magnetic, however, all fields originate with electric charges. A motionless electric charge creates a static, or non-time-varying, electric field. A moving electric charge traveling at a constant velocity creates a static magnetic field (which is at right angles to the direction of electron motion). Since a steady electric current in a wire is the result of many electric charges moving with a constant speed past any point in that wire, we can conclude that such a current will result in a steady magnetic field around the wire. However, it is only when we rapidly vary the velocity of electric charges that electromagnetic radiation (and, hence, radio waves) becomes possible. In the same way that you cause your vehicle to go from being stopped at a traffic light to moving smoothly at 30 mph along a city street by accelerating it to the new speed after the light turns green, the only way to cause the velocity of electric charges to vary is by accelerating or decelerating those charges. In short, the possibility that radio waves can exist is the direct result of electric charges undergoing acceleration or deceleration. While there is more – Battery + d I 1 Force I 2 – + than one way to accelerate charged particles, the only method we need to concern ourselves with in this book is the application of rapidly varying voltages and currents to wires or other conducting structures. Two other conditions are necessary for gener- Force I 1 x I 2 d Battery Figure 3.1 The force between current-carrying parallel wires.
88 P a r t I I : F u n d a m e n t a l s ating radio waves at a useful level. The first is that linked time-varying electric and magnetic fields must exist in the same region of space. (This is the thrust of Maxwell’s equations.) The other requirement is that the geometry of the radiator must support and even “encourage” the tendency of these time-varying electromagnetic fields to break away from it and propagate into free space. Developing a familiarity with some of those geometries is one of the purposes of this book. Voltage and the Electric Field As implied by the aluminum foil suggestion earlier, one of the simplest capacitors consists of two parallel metal plates. The capacitance of this geometry is given by C = εA d (3.14) where C = capacitance in farads e (epsilon) = constant called the permittivity of the dielectric A = area of one of the plates d = spacing between plates in same system of units as A; very much less than A ( abbreviated as
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88 P a r t I I : F u n d a m e n t a l s<br />
ating radio waves at a useful level. The first is that linked time-varying electric and magnetic<br />
fields must exist in the same region of space. (This is the thrust of Maxwell’s<br />
equations.) The other requirement is that the geometry of the radiator must support and<br />
even “encourage” the tendency of these time-varying electromagnetic fields to break<br />
away from it and propagate into free space. Developing a familiarity with some of those<br />
geometries is one of the purposes of this book.<br />
Voltage and the Electric Field<br />
As implied by the aluminum foil suggestion earlier, one of the simplest capacitors consists<br />
of two parallel metal plates. The capacitance of this geometry is given by<br />
C = εA<br />
d<br />
(3.14)<br />
where C = capacitance in farads<br />
e (epsilon) = constant called the permittivity of the dielectric<br />
A = area of one of the plates<br />
d = spacing between plates in same system of units as A; very much<br />
less than A ( abbreviated as
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