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CHAPTER
Measurements and adjustment techniques
THIS CHAPTER EXAMINES SOME OF THE INSTRUMENTS AND TECHNIQUES FOR TESTING
antenna systems, whether brand new installations, or in troubleshooting situations on older antenna The basic radio system is shown in Fig 27-1 We have several elements in the system: transmitter, low-pass filter, impedance-matching unit, a coaxial relay if the receiver is separate from the transmitter, and the antenna Connecting these elements are lengths of transmission line In most modern radio systems below the microwaves, the transmission line is coaxial cable The low-pass filter and matching unit may be considered optional by some, but some designers regard them as essentially standard equipment, especially for HF and low-band VHF systems The low-pass filter has the job of removing harmonics from the output signal that could interfere with other radio systems or radio and TV broadcast reception It will pass only those frequencies below a certain cutoff point The impedance matching unit is used to tune out impedance mismatches that cause the transmitter to see an excessive VSWR These units are common in VLF, MW, and HF communications stations, as well as AM broadcasting stations The matching unit also provides additional attenuation of the harmonics, so it makes the output even cleaner than is possible with the low-pass filter alone Another reason to use the matching unit is to allow the radio transmitter to put out the maximum allowable RF power Modern solid-state final amplifiers are not tolerant of VSWR In addition, these units use fixed-tuned low-pass filters for each band rather than the wide-range pi networks common on vacuum tube transmitters Solid-state transmitters usually include a feedback automatic load control (ALC) that reduces output power when a high VSWR is sensed The VSWR cut-in knee begins around 15:1 and completely shuts off the transmitter when the VSWR gets high (typically above 25:1 or 3:1) The external T/R relay is not used on most modern systems because the receiver and transmitter are typically housed inside of the same box The coaxial relay was used in days past, when
Copyright 2001 - the McGraw-Hill Companies
516 Measurements and adjustment techniques
Antenna
Receiver
T/R relay
Low-pass filter
Impedance matching unit
Transmitter
27-1 Basic communications radio station setup
a separate receiver needed to use the same antenna In addition, many modern solid-state equipments use PIN diode T/R switching
Transmission lines
The transmission line is not merely a wire that carries RF power to the antenna It is actually a complex circuit that simulates an infinite LC network There is a characteristic impedance Zo, also called surge impedance, which describes each transmission line This impedance is the square root of the ratio of the capacitance and inductance per unit of length When a load having a resistive-only impedance equal to the surge impedance of the transmission line is connected, then we will see maximum transfer of power between the line and the antenna We cannot deal extensively with transmission-line theory here, and refer the reader instead to previous chapters We must, however, have at least some idea of what the circuit looks like Figure 27-2 shows a model of a transmission line in which Zo is the surge impedance of the line, R1 is the load impedance of the antenna, and R2 is the output impedance of the transmitter In a properly designed system all three impedances either will be equal (Zo = R1 = R2) or a matching network will make them equal We must consider the electrical situation along the transmission line in order to understand the readings that we see on our instruments Figure 27-3 shows several possible situations These graphs are of the RF voltage along the line, with voltage on the vertical axis and transmission line length (expressed in wavelengths of the RF signal) along the horizontal axis When the system is matched (Zo = R1), the voltage
Transmission line
Calculating standing wave ratio 517
R1 In
R2 Out
L L C Inductance per unit length Capacitance per unit length
27-2 Equivalent circuit of transmission line
is the same everywhere along the line (Fig 27-3A) This line is said to be flat But when Zo and R2 are not equal, then the voltage varies along the line with wavelength In mismatched systems, not all of the power is radiated by the antenna, but rather is reflected back to the transmitter The forward and reflected waves combine algebraically at each point along the line to form standing waves (Fig 27-3B) We can plot the voltage maxima (Vmax) and minima (Vmin) Keep this graph in mind for a few minutes because you will refer again to it when we deal with VSWR Two special situations occur in transmission line and antenna systems that yield similar results The entire forward power is reflected back to the transmitter (none radiated) if the load (ie, antenna) end of the transmission line is either open or shorted The voltage plot for an open transmission line (R2 is infinite) is shown in Fig 27-3C, and that for the shorted line is shown in Fig 27-3D Note that they are very similar to each other except for where the minima (Vmin = 0) occur The minima are offset from each other by 90 (ie, quarter-wavelength)
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