The reflectors must be fitted with standard waveguide flange-type connectors. The face of these flanges should be machined flat and smooth and should not contain protrusions or indentations. Considerable care must be exercised in keeping the mating connector flange surfaces smooth and clean. Accurate alignment of the interior surfaces of the joining waveguides at the flange junction also is very important. The back of the flange which makes contact with the connecting bolts should be nominally flat and free of soft materials including paint. The connecting holes of the flange should be symmetrically and accurately aligned to the rectangular waveguide opening. These precautions must be observed when using a nonreflecting waveguide port in a precision measurement system. Reference [1] W. J. Anson, A guide to the use of the modified reflectometer technique of VSWR measurement, J. Res. NBS, Vol. 65C, No. 4, Oct.-Dec. 1961, 217-223. The measurement technique utilized in reflection measurements is described in this paper. coaxial attenuators are normally measured in a system having a charistic impedance of 50 ohms. Because measurement limits of uncertainty egraded by any deviation from this characteristic impedance, the types lowable connectors are limited. Connectors having a known plane of ence, such as the sexless precision connectors or Type N connectors ng Mil C 39012, are acceptable. Limits of uncertainty also depend the VSWR of the individual attenuator, quality of the connector, and agnitude of the attenuation [1]. al easurements are made by the substitution method, which requires that onnectors used be asexual or that the attenuator have a male connector e port and a female connector at the other. If an adapter is required mply with the foregoing, it must be supplied with the attenuator. The nation will be calibrated as one unit. See Part 1, Section 200.103 (c) and (d), also Section 200.109 (d). 1. Insertion Loss. The 1959 IRE standard [2] gives two definitions for insertion loss, one in which system mismatch is not specified, the other in which the system is nonreflecting. The definitions contradict each other since the insertion loss of an attenuator will be different for each case. The measurement procedure for both definitions is to open the system, insert the attenuator, and note the relative power absorbed by the load (or detecting device) before and after insertion. The insertion loss in decibels is computed from these two values. Either definition is entirely adequate for a single, unique system, but if the loss (or gain) measurement is to be transferred from one laboratory to another, more must be specified about system conditions. system (TG = L = 0). (c) 2. Attenuation. This is defined as the insertion loss in a nonreflecting In actual measurement these initial conditions cannot be achieved because of imperfections in connectors or adapters and the improbability in reducing system reflections to zero. Since attenuation cannot be measured exactly, the more practical term "standard attenuation" is coming into more general use. 3. Standard Attenuation. This is defined as the insertion loss of a linear two-port device in a nonreflecting system which is initially connected together at the insertion point by a standard (d) connector pair or waveguide joint. The nonreflecting condition is obtained in the standard waveguide sections to which the standard connectors or waveguide joints are attached. The standard attenuation is the ratio expressed in decibels of the power absorbed by the load before and after insertion of the two-port device being calibrated. References [1] R. W. Beatty, Microwave attenuation measurements and standards, NBS Monograph 97, April 1967. [2] IRE standards on antennas and waveguides: [3] [4] Definitions of terms, Proc. IRE, Vol. 47, No. 4, 1959, 568-582; Standards 59 IRE 2.S1. R. W. Beatty, Effects of connectors and adapters on accurate attenuation measurements at microwave frequencies, IEEE Trans. Instr., Vol. 13, Dec. 1964, 272-284. R. W. Beatty, Insertion loss concepts, Proc. IEEE, Vol. 52, (c) (d) TG Where IG and TL are defined as the reflection coefficients of the generator and Ĩoad respectively. Standard connector pair or waveguide joint. A "standard connector" is one which is made precisely to standard specifications for the particular type of connector under consideration. Standard connector pairs usually have low but measurable loss and reflections [1,3,4]. riable attenuators are normally calibrated in a system having a stic impedance of 50 ohms. Because calibration limits of uncerdegraded by any deviation from this characteristic impedance, the lowable connectors are limited. Connectors having a known plane ce, such as, the sexless precision connectors or Type N connectors 1 C 39012, (b) are acceptable. Since only change in insertion loss 1, both ports may have identical connectors. Limits of uncertainty upon VSWR of the attenuator, the quality of the connectors, and de of attenuation [1]. etting of the variable attenuator, or some other setting if specified the reference during calibration. Variable attenuators must have lity of setting of 0.1 dB or better; incremental (step) attenuators a repeatability of 0.01 dB or better. art 1, Section 200.103 (c) and (d), also Section 200.109 (d). 1. Insertion Loss. The 1959 IRE standard [2] gives two definitions of insertion loss, one in which system mismatch is not specified, the other in which the system is nonreflecting. The definitions contradict each other since the insertion loss of an attenuator will be different for each case. The measurement procedure for both definitions is to open the system, insert the attenuator, and note the relative power absorbed by the load (or detecting device) before and after insertion. The insertion loss in decibels is computed from these two values. If the attenuator is variable and remains in the system, the initial and final powers absorbed by the load for two settings are used. This determination is more properly called "Change in Insertion Loss." Either definition is entirely adequate for a single, unique system, but if the loss (or gain) measurement is to be transferred from one laboratory to another, more must be specified about system conditions. = = 0). (c) 2. Attenuation. This is defined as the insertion loss in a nonreflecting system (TG IL In actual measurement these initial conditions cannot be achieved because of imperfections in connectors or adapters and the improbability in reducing system reflections to zero. Since attenuation cannot be measured exactly, the more practical term "standard attenuation" is coming into more general use. 3. Standard Attenuation. This is defined as the insertion loss of a linear two-port device in a nonreflecting system which is initially connected together at the insertion point by a standard connector pair or waveguide The nonreflecting condition is obtained in the standard waveguide sections to which the standard connectors or waveguide joints are attached. The standard attenuation is the ratio expressed in decibels of the powers absorbed by the load before and after insertion of the two-port device being calibrated. 4. Incremental Attenuation. Incremental attenuation is the change in attenuation of a variable attenuator between a reference setting (usually zero) and any other setting. The same restraints on system conditions apply as for attenuation and standard attenuation. The term "differential attenuation" is sometimes applied to this case and usually refers to two non-zero settings. References [1] R. W. Beatty, Microwave attenuation measurements and standards, NBS Monograph 97, April 1967. [2] IRE standards on antennas and waveguides: Definitions of terms, Proc. IRE, Vol. 47, No. 4, 1959, 568-582, Standards 59 IRE 2.S1. [3] R. W. Beatty, Effects of connectors and adapters on accurate attenuation measurements at microwave frequencies, IEEE Trans. Instr., Vol. 13, Dec. 1964, 272-284. [4] R. W. Beatty, Insertion loss concepts, Proc. IEEE, Vol. 52, No. 6, June 1964, 663-671. (c) Where IG and I are defined as the reflection coefficients of the generator and foad respectively. (d) Standard connector pair or waveguide joint. A "standard connector" is one which is made precisely to standard specifications for the particular type of connector under consideration. Standard connector pairs usually have low but measurable loss and reflections [1,3,4]. |