Practical_Antenna_Handbook_0071639586
Share Embed Flag
Download ePaper

Practical_Antenna_Handbook_0071639586

SV1FKA
24.07.2018 Views

C h a p t e r 3 : A n t e n n a B a s i c s 99 resistance of the conductor gradually diminishes their amplitude, they continue to travel so long as the wire goes on “forever”. A real antenna, however, has finite length. Therefore, the traveling waves are interrupted when they reach the end of the conductor. To simplify our visualization of conditions on the line, assume that the transmitter is turned on just long enough to allow one cycle of RF (solid line) to drive the line (Fig. 3.8A). Because it is an alternating, or changing, current, it has a changing magnetic field associated with it. At the end of the conductor, the current path is broken, thus causing the magnetic field to collapse. That, in turn, creates an induced voltage at the end of the conductor that causes a new current to flow back toward the source, as in Fig. 3.8B. The current traveling from the transmitter toward the end is called the incident, or forward, current, and the returning current is called the reflected current. Similar terminology is used for the voltages on the line. An observer standing at a point midway along one side of the dipole would thus see a single cycle of RF current and voltage followed sometime later by another cycle. The direction (or polarity) of the second current, the polarity of the second voltage compared to the original pulse of energy, and the time interval between them will depend on the length of the dipole with respect to the wavelength of f 0 and the observer’s exact Incident wave Direction of travel Reflected wave Direction of travel Standing wave (stationary) Figure 3.8 Standing wave formation on an antenna. (A) Incident (forward) traveling wave. (B) Reflected traveling wave. (C) Resultant standing wave.

100 P a r t I I : F u n d a m e n t a l s position on the line. One example is shown in Fig. 3.8C, but it is only one of an infinite number of possible scenarios. If now we let the transmitter send a continuous flow of sinusoidal energy of frequency f 0 to the dipole, a continuous flow of forward and reflected voltages and currents will result. (Of course, since the energy is RF energy, the instantaneous direction of the currents and the polarity of the voltages will be reversing many thousands or millions or billions of times each second.) Because they share a single conductor, the two waveforms must pass each other as they travel in opposite directions along the conductor. To our observer standing at the same point on the dipole, however, all that is evident to him at that point is a single voltage or current that is the vector sum of both voltages or both currents, respectively! When, for example, the two voltages at that point are in phase with each other they reinforce, and the resultant seen by the observer is at its maximum value; when they are out of phase they cancel, and the resultant is at a minimum. The exact value of the maximum voltage at that point, and the phase of the resultant relative to the incident wave, depend on the exact length of the line in wavelengths or fractions thereof. The truly amazing part, however, is that for a conductor of any finite length, such as our dipole antenna, the points at which the maxima and minima of the resultant voltage or current occur (Fig. 3.8C) are stationary! In other words, for a continuous drive signal of fixed f 0 the maxima and minima stand still even though both the incident and the reflected waves are constantly traveling out and back along the wire. Thus, the resultant is referred to as a standing wave of current or voltage. The development of the standing wave on an antenna by actual addition of the traveling waves is illustrated in Fig. 3.9. In this sequence of “snapshots” of either currents or voltages taken at five different times on one side of a dipole of arbitrary length, the incident or forward waveform is represented by a thin solid line and the reflected waveform by a broken line. The resultant, or standing, wave along the antenna is represented by a heavy solid line. At the instant pictured in A, the forward and reflected waveforms perfectly coincide. (That is why the broken line is not visible.) The result is a standing wave having twice the amplitude of either traveling wave. In B, taken when the source voltage has progressed through one quarter (or 90 degrees) of a single cycle of f 0 , the waveforms have traveled a little farther in opposite directions, and the amplitude of the resultant decreases, but the points of maximum and minimum standing current or voltage do not move. When the traveling waves have moved to a position of 180 degrees phase difference, as shown in C, the resultant is zero along the entire length of the antenna. (The heavy black line representing the resultant is thus drawn along the centerline of the antenna itself.) The continuing movement of the traveling waves, shown in D another 90 degrees later, builds up a resultant in a direction opposite to that in A. Finally, the in-phase condition of the traveling waves results in a standing wave in E equal in amplitude, but 180 degrees out of phase with, the standing wave in A. If a ruler or other straightedge is now laid over Fig. 3.9 from top to bottom so as to intersect all five curves at the same distance from the generator at the left-hand side of the drawing, as shown by the vertical dotted line in the figure, it will be clear that neither the maxima nor the minima of the resultant waveform move to the left or the right over the full 360 degrees of each cycle of RF. In other words, as the traveling waves move past each other, the standing wave changes only its amplitude and phase—never its position!

C h a p t e r 3 : A n t e n n a B a s i c s 99<br />

resistance of the conductor gradually diminishes their amplitude, they continue to<br />

travel so long as the wire goes on “forever”.<br />

A real antenna, however, has finite length. Therefore, the traveling waves are interrupted<br />

when they reach the end of the conductor. To simplify our visualization of conditions<br />

on the line, assume that the transmitter is turned on just long enough to allow<br />

one cycle of RF (solid line) to drive the line (Fig. 3.8A). Because it is an alternating, or<br />

changing, current, it has a changing magnetic field associated with it. At the end of the<br />

conductor, the current path is broken, thus causing the magnetic field to collapse. That,<br />

in turn, creates an induced voltage at the end of the conductor that causes a new current<br />

to flow back toward the source, as in Fig. 3.8B. The current traveling from the transmitter<br />

toward the end is called the incident, or forward, current, and the returning current is<br />

called the reflected current. Similar terminology is used for the voltages on the line.<br />

An observer standing at a point midway along one side of the dipole would thus<br />

see a single cycle of RF current and voltage followed sometime later by another cycle.<br />

The direction (or polarity) of the second current, the polarity of the second voltage compared<br />

to the original pulse of energy, and the time interval between them will depend<br />

on the length of the dipole with respect to the wavelength of f 0 and the observer’s exact<br />

Incident<br />

wave<br />

Direction of travel<br />

Reflected<br />

wave<br />

Direction of travel<br />

Standing wave (stationary)<br />

Figure 3.8 Standing wave formation on an antenna. (A) Incident (forward) traveling wave. (B) Reflected<br />

traveling wave. (C) Resultant standing wave.

logo

products

Resources

Company

Terms of service
Privacy policy
Cookie policy
Cookie settings
Imprint
Change language
Made with love in Switzerland
© 2024 Yumpu.com all rights reserved

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!