Practical_Antenna_Handbook_0071639586
470 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s Coaxial cable Loop Loop Possible location for loop H-lines Waveguide H-field Figure 20.18 Loop (inductive) coupling. Inductive, or loop coupling, is shown in Fig. 20.18. A small loop of wire (or other conductor) is placed such that the number of magnetic flux lines it cuts is maximized. This form of coupling is popular on microwave receiver antennas as a way of making a waveguide-to-coaxial cable transition. In some cases, the loop is formed by the pigtail lead of a detector diode that, when combined with a local oscillator, downconverts the microwave signal to an intermediate frequency (IF) in the 30- to 300- MHz region. Aperture, or slot, coupling is shown in Fig. 20.19. This type of coupling is used to couple two sections of waveguide, as on an antenna feed system. Slots can be designed to couple electric, magnetic, or electromagnetic fields. In Fig. 20.19, slot A is placed at a point where the E-field peaks, so it allows electrical field coupling. Similarly, slot B is at a point where the H-field peaks, so it allows magnetic field coupling. Finally, we see slot C, which allows electromagnetic field coupling. Slots can also be characterized according to whether they are radiating or nonradiating. A nonradiating slot is cut at a point that does not interrupt the flow of currents in the waveguide walls. The radiating slot, on the other hand, does interrupt currents flowing in the walls. A radiating slot is the basis for several forms of antenna, which are discussed at the end of this chapter. 0111057 FIG 18-18 A C B Figure 20.19 Slot coupling.
C h a p t e r 2 0 : M i c r o w a v e W a v e g u i d e s a n d A n t e n n a s 471 Microwave Antennas Antennas are used in communications and radar systems over a phenomenally wide range of radio frequencies. In both theory and practice, antennas are used until operating frequencies reach infrared and visible light, at which point optical techniques take over. Microwaves are the transition region between ordinary “radio waves” and “optical waves”, so (as might be expected) microwave technology makes use of techniques from both worlds. For example, both dipoles and parabolic reflectors are used in microwave systems. The purpose of an antenna is to act as a transducer, converting signals propagating on the two conductors of a conventional transmission line or “bouncing off the walls” inside a waveguide to an electromagnetic wave propagating in free space. In the process, the antenna also acts as an impedance matcher between the waveguide or transmission line impedance and the impedance of free space. Antennas can be used equally well for both receiving and transmitting signals because they obey the law of reciprocity. That is, the same antenna can be used to receive and transmit with equal success. Although there might be practical or mechanical reasons to prefer specific antennas for one or the other mode, electrically they are the same. In the transmit mode, the antenna must radiate electromagnetic energy. For this job, the important property is gain G. In the receive mode, the job of the antenna is to gather energy from impinging electromagnetic waves in free space. The important property for receiving antennas is the effective aperture A e , which is a function of the antenna’s physical area. Reciprocity suggests that large gain goes hand in hand with a large effective aperture. Effective aperture is defined as the area of the impinging radio wavefront that contains the same power as is delivered to a matched resistive load across the feedpoint terminals. The Isotropic “Antenna” Antenna definitions and specifications can become useless unless a means is provided for putting everything on a common footing. Although a variety of systems exist for describing antenna behavior, the most common system compares a specific antenna with a theoretical construct, called the isotropic radiator, which we first encountered in Chap. 3. Since an isotropic radiator is a spherical point source that radiates equally well in all directions, the directivity of the isotropic antenna is unity (1) by definition, and all other antenna gains are measured against this standard. From spherical geometry, we can calculate isotropic power density at any distance R from the point source: P P = 4πr d 2 (20.19) where P d = power density, in watts per square meter P = power in watts input to the isotropic radiator r = radius in meters at which point power density is measured Example 20.5 Calculate the power density at a distance of 1 km (1000 m) from a 1000W isotropic source.
- Page 439 and 440: C h a p t e r 1 8 : a n t e n n a s
- Page 441 and 442: C h a p t e r 1 8 : a n t e n n a s
- Page 443 and 444: C h a p t e r 1 8 : a n t e n n a s
- Page 445 and 446: CHAPTER 19 VHF and UHF Antennas The
- Page 447 and 448: C h a p t e r 1 9 : V H F a n d U H
- Page 449 and 450: C h a p t e r 1 9 : V H F a n d U H
- Page 451 and 452: C h a p t e r 1 9 : V H F a n d U H
- Page 453 and 454: C h a p t e r 1 9 : V H F a n d U H
- Page 455 and 456: C h a p t e r 1 9 : V H F a n d U H
- Page 457 and 458: C h a p t e r 1 9 : V H F a n d U H
- Page 459 and 460: C h a p t e r 1 9 : V H F a n d U H
- Page 461 and 462: C h a p t e r 1 9 : V H F a n d U H
- Page 463 and 464: C h a p t e r 1 9 : V H F a n d U H
- Page 465 and 466: C h a p t e r 1 9 : V H F a n d U H
- Page 467 and 468: CHAPTER 20 Microwave Waveguides and
- Page 469 and 470: C h a p t e r 2 0 : M i c r o w a v
- Page 471 and 472: C h a p t e r 2 0 : M i c r o w a v
- Page 473 and 474: C h a p t e r 2 0 : M i c r o w a v
- Page 475 and 476: C h a p t e r 2 0 : M i c r o w a v
- Page 477 and 478: C h a p t e r 2 0 : M i c r o w a v
- Page 479 and 480: C h a p t e r 2 0 : M i c r o w a v
- Page 481 and 482: λ o = c / f 8 3× 10 m / s = 9 C h
- Page 483 and 484: C h a p t e r 2 0 : M i c r o w a v
- Page 485 and 486: C h a p t e r 2 0 : M i c r o w a v
- Page 487 and 488: C h a p t e r 2 0 : M i c r o w a v
- Page 489: C h a p t e r 2 0 : M i c r o w a v
- Page 493 and 494: C h a p t e r 2 0 : M i c r o w a v
- Page 495 and 496: C h a p t e r 2 0 : M i c r o w a v
- Page 497 and 498: C h a p t e r 2 0 : M i c r o w a v
- Page 499 and 500: C h a p t e r 2 0 : M i c r o w a v
- Page 501 and 502: C h a p t e r 2 0 : M i c r o w a v
- Page 503 and 504: C h a p t e r 2 0 : M i c r o w a v
- Page 505 and 506: C h a p t e r 2 0 : M i c r o w a v
- Page 507 and 508: C h a p t e r 2 0 : M i c r o w a v
- Page 509 and 510: C h a p t e r 2 0 : M i c r o w a v
- Page 511 and 512: CHAPTER 21 Antenna Noise Temperatur
- Page 513 and 514: C h a p t e r 2 1 : A n t e n n a N
- Page 515 and 516: C h a p t e r 2 1 : A n t e n n a N
- Page 517 and 518: CHAPTER 22 Radio Astronomy Antennas
- Page 519 and 520: C h a p t e r 2 2 : R a d i o A s t
- Page 521 and 522: C h a p t e r 2 2 : R a d i o A s t
- Page 523 and 524: C h a p t e r 2 2 : R a d i o A s t
- Page 525 and 526: C h a p t e r 2 2 : R a d i o A s t
- Page 527 and 528: C h a p t e r 2 2 : R a d i o A s t
- Page 529 and 530: Figure 22.9 Interferometer pattern.
- Page 531 and 532: CHAPTER 23 Radio Direction-Finding
- Page 533 and 534: C h a p t e r 2 3 : R a d i o D i r
- Page 535 and 536: C h a p t e r 2 3 : R a d i o D i r
- Page 537 and 538: C h a p t e r 2 3 : R a d i o D i r
- Page 539 and 540: C h a p t e r 2 3 : R a d i o D i r
C h a p t e r 2 0 : M i c r o w a v e W a v e g u i d e s a n d A n t e n n a s 471<br />
Microwave <strong>Antenna</strong>s<br />
<strong>Antenna</strong>s are used in communications and radar systems over a phenomenally wide<br />
range of radio frequencies. In both theory and practice, antennas are used until operating<br />
frequencies reach infrared and visible light, at which point optical techniques take<br />
over. Microwaves are the transition region between ordinary “radio waves” and “optical<br />
waves”, so (as might be expected) microwave technology makes use of techniques<br />
from both worlds. For example, both dipoles and parabolic reflectors are used in microwave<br />
systems.<br />
The purpose of an antenna is to act as a transducer, converting signals propagating<br />
on the two conductors of a conventional transmission line or “bouncing off the walls”<br />
inside a waveguide to an electromagnetic wave propagating in free space. In the process,<br />
the antenna also acts as an impedance matcher between the waveguide or transmission<br />
line impedance and the impedance of free space.<br />
<strong>Antenna</strong>s can be used equally well for both receiving and transmitting signals because<br />
they obey the law of reciprocity. That is, the same antenna can be used to receive<br />
and transmit with equal success. Although there might be practical or mechanical reasons<br />
to prefer specific antennas for one or the other mode, electrically they are the same.<br />
In the transmit mode, the antenna must radiate electromagnetic energy. For this job,<br />
the important property is gain G. In the receive mode, the job of the antenna is to gather<br />
energy from impinging electromagnetic waves in free space. The important property<br />
for receiving antennas is the effective aperture A e , which is a function of the antenna’s<br />
physical area. Reciprocity suggests that large gain goes hand in hand with a large effective<br />
aperture. Effective aperture is defined as the area of the impinging radio wavefront<br />
that contains the same power as is delivered to a matched resistive load across the feedpoint<br />
terminals.<br />
The Isotropic “<strong>Antenna</strong>”<br />
<strong>Antenna</strong> definitions and specifications can become useless unless a means is provided<br />
for putting everything on a common footing. Although a variety of systems exist for<br />
describing antenna behavior, the most common system compares a specific antenna<br />
with a theoretical construct, called the isotropic radiator, which we first encountered in<br />
Chap. 3.<br />
Since an isotropic radiator is a spherical point source that radiates equally well in all<br />
directions, the directivity of the isotropic antenna is unity (1) by definition, and all other<br />
antenna gains are measured against this standard. From spherical geometry, we can<br />
calculate isotropic power density at any distance R from the point source:<br />
P<br />
P<br />
=<br />
4πr<br />
d 2<br />
(20.19)<br />
where P d = power density, in watts per square meter<br />
P = power in watts input to the isotropic radiator<br />
r = radius in meters at which point power density is measured<br />
Example 20.5 Calculate the power density at a distance of 1 km (1000 m) from a 1000W<br />
isotropic source.