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27-7 Connection of noise bridge
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524 Measurements and adjustment techniques A perfectly resonant antenna will have a reactance reading of zero ohms, and a resistance of 50 to 75 Real antennas might have some reactance (the less the better), and a resistance that is somewhat different from 50 to 75 Impedance-matching methods can be used to transform the actual resistive component to the 50- or 75- characteristic impedance of the transmission line The results to expect: 1 If the resistance is close to zero, then suspect that there is a short circuit on the transmission line Suspect an open circuit if the resistance is close to 200 2 A reactance reading on the XL side of zero indicates that the antenna is too long, while a reading on the XC side of zero indicates an antenna that is too short An antenna that is too long or too short should be adjusted to the correct length To determine the correct length, we must find the actual resonant frequency Fr To do this, reset the reactance control to zero, and then slowly tune the receiver in the proper direction downband for too long and upband for too short until the null is found On a high-Q antenna, the null is easy to miss if you tune too fast Don t be surprised if that null is out of band by quite a bit The percentage of change is given by dividing the expected resonant frequency Fexp by the actual resonant frequency Fr, and multiply by 100: Change = Fexp 100% Fr [276]
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Connect the antenna, noise bridge, and receiver in the same manner as above Set the receiver to the expected resonant frequency (ie, approximately 468/F for half-wavelength types and 234/F for quarter-wavelength types) Set the resistance control to 50 or 75 , as appropriate for the normal antenna impedance and the transmission line impedance Set the reactance control to zero Turn the bridge on and listen for the noise signal Slowly rock the reactance control back and forth to find on which side of zero the null appears Once the direction of the null is determined, set the reactance control to zero, and then tune the receiver toward the null direction (downband if null is on XL side and upband if it is on the XC side of zero) A less-than-ideal antenna will not have exactly 50- or 75- impedance, so some adjustment of R and C to find the deepest null is in order You will be surprised how far off some dipoles and other forms of antennas can be if they are not in free space, (ie, if they are close to the earth s surface)
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We can operate antennas on frequencies other than their resonant frequency if we know the impedance: XC = X = or XL = X = 2340 159,155 68 + C [278] 55 2340 68 C [277]
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The RF noise bridge 525 Now, plug X calculated from Eq 277 or 278 into Xf = X/F where F is the desired frequency in MHz
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The noise bridge can be used for a variety of jobs We can find the values of capacitors and inductors, determine the characteristics of series- and parallel-tuned resonant circuits, and adjust transmission lines Transmission line measurements Some antennas and non-noise measurements require antenna feedlines that are either a quarter-wavelength or half-wavelength at some specific frequency In other cases, a piece of coaxial cable of specified length is required for other purposes: for instance, the dummy load used to service depth sounders is nothing but a long piece of shorted coax that returns the echo at a time interval that corresponds to a specific depth We can use the bridge to find these lengths as follows: 1 Connect a short circuit across the unknown terminals and adjust R and X for the best null at the frequency of interest (Note: both will be near zero) 2 Remove the short circuit 3 Connect the length of transmission line to the unknown terminal it should be longer than the expected length 4 For quarter-wavelength lines, shorten the line until the null is very close to the desired frequency For half-wavelength lines, do the same thing, except that the line must be shorted at the far end for each trial length The velocity factor of a transmission line (usually designated by the letter V in equations) is a decimal fraction that tells us how fast the radio wave propagates along the line relative to the speed of light in free space For example, foam dielectric coaxial cable is said to have a velocity factor of V = 080 This number means that the signals in the line travel at a speed 080 (or 80 percent) of the speed of light Because all radio wavelength formulas are based on the velocity of light, you need the V value to calculate the physical length needed to equal any given electrical length For example, a half-wavelength piece of coax has a physical length of (492 V)/FMHz feet Unfortunately, the real value of V is often a bit different from the published value You can use the noise bridge to find the actual value of V for any sample of coaxial cable as follows: 1 Select a convenient length of the coax more than 12 ft in length and install a PL-259 RF connector (or other connector compatible with your instrument) on one end, and short-circuit the other end 2 Accurately measure the physical length of the coax in feet; convert the remainder inches to a decimal fraction of a foot by dividing by 12 (eg, 32 ft 8 in= 3267 ft because 8 in/12 in = 067) Alternatively, cut off the cable to the nearest foot and reconnect the short circuit 3 Set the bridge resistance and reactance controls to zero 4 Adjust the monitor receiver for deepest null Use the null frequency to find the velocity factor V = FL/492, where V is the velocity factor (a decimal fraction); F is the frequency in megahertz; and L is the cable length in feet
526 Measurements and adjustment techniques Tuned circuit measurements An inductor/capacitor (LC) tuned tank circuit is the circuit equivalent of a resonant antenna, so there is some similarity between the two measurements You can measure resonant frequency with the noise bridge to within 20 percent (or better if care is taken) This accuracy might seem poor, but it is better than you can usually get with low-cost signal generators, dip meters, absorption wavemeters, and the like A series tuned circuit exhibits a low impedance at the resonant frequency and a high impedance at all other frequencies Start the measurement by connecting the series tuned circuit under test across the unknown terminals of the dip meter Set the resistance control to a low resistance value, close to zero ohms Set the reactance control at midscale (zero mark) Next, tune the receiver to the expected null frequency, and then tune for the null Make sure that the null is at its deepest point by rocking the R and X controls for best null At this point, the receiver frequency is the resonant frequency of the tank circuit A parallel resonant circuit exhibits a high impedance at resonance and a low impedance at all other frequencies The measurement is made in exactly the same manner as for the series resonant circuits, except that the connection is different Figure 27-8 shows a two-turn link coupling that is needed to inject the noise signal into the parallel resonant tank circuit If the inductor is the toroidal type, then the link must go through the hole in the doughnut-shaped core and then connect to the unknown terminals on the bridge After this, proceed exactly as you would for the series tuned tank measurement Capacitance and inductance measurements The bridge requires a 100-pF silver mica test capacitor and a 47- H test inductor, which are used to measure inductance and capacitance, respectively The idea is to use the test components to form a series-tuned resonant circuit with an unknown component If you find a resonant frequency, then you can calculate the unknown value In both cases, the series tuned circuit is connected across the unknown terminals of the dip meter, and the series tuned procedure is followed
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