OPERATING INSTRUCTIONS TYPE 874-LBA/-LBB SLOTTED LINES GENERAL RADIO COMPANY
GENERAL RADIO COMPANY WEST CONCORD, MASSACHUSETTS 617 369-4400 5 ALE 5 NEW 01781 617 OFFICES ENGINEERING ENGLAND* 22 Baker Avenue West Concord, Massachusetts 01781 Telephone 617 646-0550 METROPOLITAN NEW YORK* Broad Avenue at Linden Ridgefield, New Jersey 07657 Telephone N.Y. 212 964-2722 N.J.
INSTRUCTION MANUAL TYPE 874-LBA/-LBB SLOTTED LINES Form 0874-0215-1 April, 1966 Copyright 1962 by General Radio Company West Concord, Massachusetts, USA GENERAL WEST R A D I 0 CONCORD, COMPANY MASSACHUSETTS, USA
TABLE OF CONTENTS Section 1. GENERAL DESCRIPTION. 1 Section 2. 2.1 2.2 2.3 THEORY • Characteristic Impedance and Velocity of Propagation. Traveling and Standing Waves. Line Impedance 1 1 2 3 Section 3. 3.1 3.2 3.3 DESCRIPTION. Slotted Line Design Generator Detector. 10 10 12 13 Section 4. 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 OPERATION Connections and Adjustments . Detector Tuning . Probe Penetration Adjustment. Measurement of Wavelength .
NOTE: All instructions for the Type 874-LBA apply equally to the Type 874-LBB except that the maximum operating frequency of the Type 874-LBB extends to 8.5 Gc. Figure 1. The Type 874-LBA Slotted Line and the Type 874-W50, 50-ohm Termination Unit are shown connected for the measurement of the VSWR of Type 874-QN Adaptors. These adaptors are used to connect components fitted with type N Connectors to devices fitted with Type 874 Connectors. SPECIFICATIONS Characteristic Impedance: 50 ohms, ±0.5%.
GENERAL DESCRIPTION- THEORY SECTION 1 GENERAL DESCRIPTION One of the important basic measuring instruments used at ultra-high frequencies is the slotted line. With it, the standing-wave pattern of the electric field in a coaxial transmission line of known characteristic impedance can be accurately determined. From aknowledge of the standing-wave pattern several characteristics of the circuit connected to the load end of the slotted line can be obtained.
~ TYPE 874-LBA SLOTTED LINE ~~~~~~~-----------------unit length, C, is defined as the characteristic impedance, Z 0 , of the line. (1) This is an approximation which is valid when line losses are low. It gives satisfactory results for most practical applications at high frequencies. In the next paragraph, transmission-line behavior will be discussed in terms of electromagnetic waves traveling along the line. The waves travel with a velocity, v , which depends on L and C in the following manner: 2.
THEORY generator is called the reflected wave. The combination of these two traveling waves produces a stationary interference pattern which is called a standing wave, as shown in Figure 2b. The maximum amplitude of the standing wave occurs when the incident and reflected waves are in phase or when they are an integral multiple of 360° out of phase. The minimum amplitude occurs when the two waves are 180', or an odd integral multiple thereof, out of phase.
~--TY_P_E_8_7_4-_LB_A__S_L_O_T_T_E_D__ L_IN_E________________________________________ ratio of the maximum to minimum voltages, which is called the voltage standing-wave ratio, VSWR, is VSWR = Emax = E min 1 + 1 lr I lr I (Sa) The standing-wave ratio is frequently expressed in decibels. VSWR in db = 20 log 10 0 E max Emin phase with each other.
THEORY le At a voltage minimum, the two voltage waves are opposing and the two current waves are aiding. Again the effective impedance is a pure resistance and is equal to the characteristic impedance of the line divided by the VSWR.
~--T_Y_P_E_8_74_-L_B_A__ S_L_O_T_T_E_D_L_I_N_E_______________________________________ If the line loss cannot be neglected, the equations are: Zx Yx = Zo = Zp Z 0 tanh?Y.i Zo Zp tanhcr.i yp Y tanh.:rl Yo YP tanhcrl Yo (13a) minimum located with the load connected, e will be negative. The points corresponding to half-wavelength distances from the load can be determined by shortcircuiting the line at the load and noting the positions of the voltage minima on the line.
THEORY electrical distance away (for instance, at the load) is desired, the problem can be solved using the Smith Chart in the following manner: First, locate the point on the chart corresponding to the known impedance, as shown in Figure 6. (For example, assume that Zp = 20 + j25 ohms.) Then, draw a line from the center of the chart through Zo to the outside edge of the chart.
~ TYPE 874-LBA SLOTTED LINE ~-------------------------VSWR as a ratio, ~~~· or in db on the appropriate scale. (In the example of Figure 6, the VSWR is 3.2 or 10.1 db.) 2.3.4.2 Calculation of Impedance at the Load from theVSWRand Position of a Voltage Minimum. In impedance measurements in which the voltage standingwave pattern is measured, the impedance at a voltage minimum is a pure resistance having a rnagni- z tude of~.
THEORY The example plotted on the chart in Figure 7 shows the procedure for determining the load impedance when the VSWR is 5 to 1, and the electrical distance between the load or a half-wavelength point and a voltage minimum is 0.14 wavelength. The unknown impedance, read from the chart, is 23 - j55 ohms. The Smith Chart can also be used when the line between the load and the measuring point is not lossless. The procedure for correcting for loss is outlined in Paragraph 4.6.3.
~--T_YP_E__ 87_4-_L_BA__S_L_O_T_T_E_D__ L_IN_E______________________________________ sion line. The impedances at points 1 and 2, a quarterwavelength apart, are related by the equation Zo2 zl = (16a) z2 The example plotted on the chart is the same as that used for the impedance example of Figure 7. or zl = 2. Zo Y2 (16h) 2.3.4.4 Admittance Measurements Using the Smith Chart.
DESCRIPTION INNER CONDUCTOR PROBE ADJUSTING SCREW FELT WASHER (0814-o072~) Figure 9a. Cross-sectional views of the carriage on the Type 874-LDA Slotted Line, showing the crystal mount and the adjustable probe. RING NUT (0.874-6126) . SLOT IN COAXIAL LINE PROBE FLAT SPRING PROBE CARRIAGE CARRIAGE Figure 9b. Rear view of the drive mechanism, showing the arrangement of the nylon cord. CARRIAGE Figure 9c. Installation of the nylon cord, viewed from the top of carriage.
~ TYPE 874-LBA SLOTTED LINE ~----------------------~- 874-D20.L TUNING STUB TYPE 1360-A MICROWAVE OSCILLATOR Figure 10. Use of a modulated source for measurements with the Type 874-LBA Slotted Line. The built-in crystal detector and a standing-wave indicator are used to detect the voltage induced in the probe. The probe is tuned by means of the adjustable stub shown. STANDING WAVE INDICATOR 874-L8A SLOTTED 874-G ATTENUATOR NE 74-D20L TUNING STUB UNIT OSCILLATOR 25- OR 50-~a METER Figure 11.
DESCRIPTION wave modulation, as well as a regulated power supply, for the unit oscillators in slotted -line use. 3.3 DETECTOR As mentioned previously, either the built-in crystal detector4 or an external receiver can be· used as a detector. 3.3.1 CRYSTALRECTIFIERAND STANDING-WAVE METER. The most commonly used and the most generally satisfactory detector is the built-in crystal rectifier' with one of the several commercially available VSWR indicators.
~--T_YP_E_8_7_4-_LB_A__S_L_O_T_T_E_D_L_I_N_E______________________________________ SECTION 4 OPERATION 4.1 CONNECTIONS AND ADJUSTMENTS. 4.1.1 GENERAL. In use, the slotted Une is fed from an oscillator which is connected to one end of the line. The circuit to be measured is connected to the other end. If a Type DNT detector is to be used and the Type 87 4MRL Mixer Rectifier (Paragraph 3. 3. 2.
OPERATION 3 em Physical 3.2cm Electrical 50-0HM ~==j~~b=~COAXIAL LINE tFZ=P-g~~§P==~ POl NT AT WHICH IMPEDANCE IS MEASURED 874-WN OR WO (b) POSITION OF EFFECTIVE SHORT OR OPEN CIRCUIT 874-WN3 OR W03 POLYSTYRENE BEAD Figure 13. Methods of Short- and Open-Circuiting. (a) Use of Type 874-WN3 Short-Circuit Termination Unit or Type 874W03 Open-Circuit Termination Unit to make a short circuit or opencircuit when measuring point is located 3 em. from face of bead, as in upper figure.
~--T_Y_PE__ 87_4_-L_B_A_S_L_O_T_T_E_D__L_I_N_E______________________________________ 4.2.2 HETERODYNE DETECTOR. When the DNT Detector is used, care must be taken to tune the local oscillator to beatwith the desired signal and not with one of its harmonics. Harmonics of the oscillator signal can beat with harmonics of the signal picked up from the slotted line and produce an output at the intermediate frequency if the local oscillator is tuned to a wrong frequency.
OPERATION The probe coupling or the oscillator output should be adjusted until the output from the detector is in a satisfactory range. If the crystal detector is used, this means the maximum output to be measured should not correspond to an input beyond the squarelaw range if the square -law characteristic is to be depended upon (refer to Paragraph 3. 3.1), and the probe coupling should not be large enough to affect the measurements appreciably.
~ TYPE 874-LBA SLOTTED LINE ~~~~~~~------------------ Figure 14. Method of improving the accuracy of the determination of the position of a voltage minimum on the line when the VSWR is low. 0 line case, the spacing, A, between points on the line at which the rf voltage is V2 times the voltage at the minimum, is measured, as shown in Figure 15. The VSWR is related to the spacing, A, and the wavelength, )\, by the expression ~ ~ VSWR ~ ~ f=! ..J > ..J l.
OPERATION The electrical distance between the unknown and the minimum found on the line can be determined A.S outlined in Paragraph 4.5.2. At very high standing-wave ratios, the losses in the slotted line and in any connecting line or cable used can have an appreciable effect on the measurements. To keep this error as low as possible, the voltage minimum nearest the load should be measured. A correction for the loss in the line can be made as outlined in Paragraph 4.6.3. 4.6.
~ TYPE 874-LBA SLOTTED LINE ~-----------IMPEDANCE COORDINATES- 50·0HM CHARACTERISTK IMPEDANCE Figure 16b. Example of the use of the Smith Chart for line length corrections when the line has an appreciable amount of loss. (See Paragraph 4.6.3.) Min.= 0 .12A. toward "-... ~· ! Attenuation = 0. True VsWR ofload=l3db or ., L ~ 4 .5tol Measured VSWR of lood / ....
OPERATION When VSWR is greater than 10 tan e, the following approximation is valid: The impedance of an unknown connected to the slotted line by a line or cable having an appreciable loss can be calculated from the slotted -line me asurements by use of the Smith Chart exactly as outlined in Paragraph 2.3.4.
~--T_Y_P_E_8_74_-_LB_A__ S_L_O_T_T_E_D__ L_IN_E_______________________________________ as a Type 874-Gl0,10-DB Pad, between the oscillator and the slotted line. If the resultant decrease in input cannot be tolerated, the oscillator tuning can be adjusted to compensate for the frequency shift. The oscillator frequency can be checked on a receiver or a heterodyne frequency meter. Signal generators, in general, are loosely coupled, and the frequency shift is usually small. length is short.
OPERATION (4) If theVSWR is less than lO,measure the relative output from the detector at several minima and maxima. Actually, only one minimum and one maximum need be measured, but because of the variations in probe coupling along the line, greater accuracy can be obtained if several minima and maxima are averaged or if the probe coupling is calibrated, as outlined in Paragraph 5. 2. If the VSWR is greater than 10, use the width-of-minimum method, outlined in Paragraph 4.6.2, to determine the VSWR. 4.7.
~~T~YP~E~8~7~4-~LB~A~S~L~O~T~T~E~D_L~I~N~E-------------------------------------(For greater accuracy, the variation in probe coupling can be calibrated, and corrections made, as outlined in Paragraph 5.2.) The effective position of the measured minimum with respect to the load is then measured by short-circuiting the line at the antenna by means of a Type 874-WN3 or -WN. The approximate position of the minimum is found. The gain is then increased to improve the resolving power.
OPERATION IMPEDANCE COORDINATES- 50-0HM CHARACTERISTIC IMPEDANCE Figure 17. Example of the use of the Smith Chart to calculate antenna impedances. (See Paragraphs 4.7. 3.1 and 4. 7.3.2.) 10, the approximate form can be used. Also, 8 is negative, since it lies on the load side of the shortcircuit minimum. If the cable is long enough to have appreciable loss, corrections can be made as outlined in Paragraph 4.6.3. Therefore, = = = 2 VSWRcos 8 9.4 ohms = 50 4. 7.
~ TYPE 874-LBA SLOTTED LINE ~-------------------------vantageous to measure the impedance at the front face of the polystyrene bead in the unknown connector. (Refer to Paragraph 4.1.3.) In order to determine the impedance at this point, the electrical distance from the insulator in the connector and the position of a voltage minimum on the slotted line must be found. Figure 18. Approximate equivalent circuit of connecting lead reactances encountered when components are measured.
OPERATION If the admittance of the unknown is desired, rather than the impedance, the admittance, Ym, appearing across the end of the line, is calculated from the VSWR and from t he position of a voltage minimum, as outlined in Paragraph 2.3.4.4. The following equations should be used to correct for the lead reactances: Gx Gm = (36) D I Bm Bx = (1- =~j dotted capacitor of Figure 18. The ratio of the two capacitances can be estimated from the physical configuration of the circuit.
__S_L_O_T_T_E_D__L_IN_E________________________________________ ~--TY_P_E_8_7_4-_LB_A the ungrounded terminal, and the outer conductor is extended in the form of a disk for a ground plane. The line can be short-circuited at the. terminal by means of a very low inductance disk (supplied) or the mount can be disconnected and replaced by a Type 874-WN3 Short-Circuit Termination Unit.
MISCELLANEOUS SECTION 5 MISCELLANEOUS 5.1 OPERATION AT FREQUENCIES BELOW 300 Me. Since the probe travel is only 50 em, it will not always be possible to measure both a voltage minimum and maximum on the line at frequencies below 300 Me, as the range of travel of the probe is onehalf wavelength at 300 Me. At frequencies below 150 Me, where the line is less than a quarter of a wavelength, it will never be possible to measure both a voltage maximum and minimum on the line directly.
~ __T_Y_P_E_8_74_-L_B_A__ S_L_O_T_T_E_D_L_I_N_E_______________________________________ SECTION 6 SERVICE AND 6.1 GENERAL. The two-year warranty given with every General Radio instrument attests the quality of materials and workmanship in our products. When difficulties do occur, our service engineers will assist in any way possible.
SERVICE AND MAINTENANCE POLYSTYRENE BEAD END CASTING 6.5 CENTERING OF PROBE IN SLOT . (0874-0700) OUTER CONDUCTOR TEFLON INSULATOR (0874-0603) (0874- 7230) SPRING FINGERS SLOT FOR PROBE INNER CONDUCTOR (0874-6119) CENTER CONDUCTOR (0874-6121) RETAINING RINGS OUTER CONDUCTOR (0874 -OSlO) (0874- 6123) To check the centering of the probe shield, hold the line up to a light and sight from one end along the slot. Move the carriage along the line and observe the centering of the probe shield.
TYPE 87 4 COAXIAL COMPONENTS TYPE 874 CABLE CONNECTORS CONNECTOR TYPE ~ ::t: 0 0 .,., 874- A2 RG-BA/U RG-98/U RG-lOA/U RG-87A/U RG-11'6/U RG-156/U RG-165/U RG-!66/U RG-213/U RG-214/U RG-215/U RG- 225/U CABLE -CA CABLE LOCKING PANEL FLANGED -CLA -PBA -CLBA -CBA -PBBA TYPE 874 ADAPTORS PANEL LOCKING -PLA -PLBA PANEL LOCKING RECESSED -PRLBA "' w a._ >- I- w ..J "'u< ~ ::t: 0 ~ i: 0 z w ..J "'u< -..J a._ a._ < ~ ::t: 0 0.,., 0 .,.,0 i: 0 z ~ ::t: 0 0 .,.
