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Propagation predictions
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Propagation predictions for the VLF through low VHF bands are published each month in several magazines Ham radio operators often use those in QST magazine (Fig 2-39), while SWLs and others tend to prefer those published in magazines such as Monitoring Times These charts relate the time of day in Universal Coordinated Time (UTC, formerly, GMT) and the frequency for transmission to different parts of the world
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Fading
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Skip communications are not without problems One phenomenon is fading (ie, a variation in signal strength as perceived at the receiver site) This problem can sometimes be overcome by using one of several diversity reception systems Three forms of diversity technique are used: frequency diversity, spatial diversity, and polarity diversity In the frequency diversity system (Fig 2-40), the transmitter will send out two or more frequencies simultaneously with the same modulating information Because the two frequencies will fade differentially, one will always be strong The spatial diversity system (Fig 2-41) assumes that a single transmitter frequency is used At the receiving site, two or more receiving antennas are used, spaced one-half wavelength apart The theory is that the signal will fade at one antenna while it increases at the other A three-antenna system is often used Three separate, but identical, receivers, often tuned by the same master local oscillator, are connected to the three antennas Audio mixing, based on the strongest signal, keeps the audio output constant while the radio-frequency (RF) signal fades Polarity diversity reception (Fig 2-42) uses both vertical and horizontal polarization antennas to receive the signal As in the space diversity system, the outputs of the vertical and horizontal receivers are combined to produce a constant level output Another form of fading, selective fading, derives from the fact that fading is a function of frequency The carrier and upper and lower sidebands of an AM signal have slightly different frequencies, so they arrive out of phase with each other Although this type of fading is lessened by using single-sideband (SSB) transmission, that does not help AM users In those systems, some people use a filtering system that eliminates the carrier and one sideband; it then reconstitutes the AM signal with a product detector SSB receivers with stable local and product detector oscillators, and a sharp intermediate-frequency (IF) bandpass filter, can be used to reduce the effects of differential fading of AM signals because of the phasing of the lower sideband (LSB), upper sideband (USB), and carrier components Carefully tune the receiver to only one sideband of the signal, and note when the heterodyne beatnote disappears The correct point is characterized by the fact that you can then switch among USB, LSB, and continuous-wave (CW) modes without changing the received signal output
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56 Radio-wave propagation
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2-39 Propagation prediction charts appear in magazines such as QST every month
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(Courtesy of the American Radio Relay League)
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XMTR 1 F1
RCVR 1
AF 1
XMTR 2 F2
F2 F2 F1 F1
RCVR 2
Composite audio out
2-40 Frequency diversity reduces fading
Antenna 1
/2 RCVR 1 Antenna 2 RCVR 2 /2
Composite audio output
Antenna 3
RCVR 3
Master local oscillator
2-41 Spatial diversity
58 Radio-wave propagation
Horizontal antenna
RCVR 1
H audio
Composite audio output
Vertical antenna
RCVR 2
V audio
2-42 Polarization diversity
CHAPTER
Transmission lines
TRANSMISSION LINES AND WAVEGUIDES ARE CONDUITS FOR TRANSPORTING RF SIGNALS
between elements of a system For example, transmission lines are used between an exciter output and transmitter input, and between the transmitter input and its output, and between the transmitter output and the antenna Although often erroneously characterized as a length of shielded wire, transmission lines are actually complex networks containing the equivalent of all the three basic electrical components: resistance, capacitance, and inductance Because of this fact, transmission lines must be analyzed in terms of an RLC network
Parallel and coaxial lines
This chapter will consider several types of transmission lines Both step-function and sine-wave ac responses will be studied Because the subject is both conceptual and analytical, both analogy and mathematical approaches to the theory of transmission lines will be used Figure 3-1 shows several basic types of transmission line Perhaps the oldest and simplest form is the parallel line shown in Figs 3-1A through 3-1D Figure 3-1A shows an end view of the parallel conductor transmission line The two conductors, of diameter d, are separated by a dielectric (which might be air) by a spacing S These designations will be used in calculations later Figure 3-1B shows a type of parallel line called twin lead This is the old-fashioned television antenna transmission line It consists of a pair of parallel conductors separated by a plastic dielectric TV-type twin lead has a characteristic impedance of 300 , while certain radio transmitting-antenna twin lead has an impedance of 450 Another form of twin lead is open line, shown in Fig 3-1C In this case, the wire conductors are separated by an air dielectric, with support provided by stiff (usually ceramic) insulators A tie wire (only one shown) is used to fasten each insulator end to the main conductor Some users of open line prefer the form of insulator or supporter shown in Fig 3-1D This
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