Microwave Power Measurement Research Articles

An oscillatory torque-operated wattmeter for the 8 mm waveband

An instrument is described which enables absolute measurements of microwave power to be made in the 8mm waveband. The measurements are absolute, in the sense that the calibration of the instrument depends only on measurements of mass, length and time. Microwave power levels of a milliwatt and less have been measured by this method, and the accuracy of the instrument is of the order of a few per cent.

A proposed method of microwave power measurement

The paper describes the basic principles of a new method of microwave power measurement. According to the proposed method, the microwave power is converted into heat by a power-dissipating element placed in a waveguide. Around this element a closed air duct is formed with vertical upward and downward branches. The air heated by the dissipative element causes a deviation of a small vane placed in the duct and suspended on a horizontally stretched torsion filament, and the angular displacement of a mirror mounted on this vane is observed through an autocollimator.

A proposed new method of measuring microwave power and impedance using Hall effect in a semiconductor

A proposed new method of measuring microwave power and impedance using Hall effect in a semiconductor

A survey of microwave power-measurement techniques employed at the National Bureau of Standards

The bolometric technique of power measurement is an important part of the microwave art. The paper describes certain refinements and extensions of this basic method which have been developed at the Boulder Laboratories of the National Bureau of Standards and which provide the basis for a microwave power-calibration service.The attendant problems may be divided into three categories: (i) measurement of the substituted or bolometric bias power, (ii) evaluation of the d.c. r.f. substitution error, and (iii) determination of the bolometer mount efficiency.In the first area, the Laboratory has developed precise and automatic d.c. instrumentation which permits an accuracy of 0.1% to be realized on a routine basis. In the second and third areas, microcalorimetric techniques enable a determination of the total microwave power dissipated within the bolometer mount; this, when compared with a simultaneous bolometric measurement, determines the combined effect of the substitution error and mount efficiency.Another interesting tool for evaluating bolometer-mount efficiency is provided by the Kerns impedance method. The implementation of this technique has always proved a real challenge, but recent applications of modified reflectometer techniques to the problem have resulted in improved accuracy. Consistent agreement, within a half per cent, with microcalorimetric determinations has been realized at X-band frequencies.Because a bolometer completely absorbs the power being measured, the problem of comparing or calibrating a bolometer mount in terms of a second mount is inherently more difficult than that of comparing two voltmeters or ammeters where simultaneous observations of the same quantity are possible. This problem has, in fact, been a major limitation to the accuracy so far achieved in the art.Two methods of dealing with this problem have been developed which employ directional couplers and related techniques.

The design and performance of transverse-film bolometers in rectangular waveguides

Equations are derived which determine the design of film-type bolometers used for the measurement of microwave power. The results are discussed in relation to the concept of impedance in rectangular waveguides.Measurements on an improved tunable-type instrument at a wavelength of 3cm indicate that power levels between 1 and 100mW can be measured in terms of a d.c. calibration with an error of not more than ±2%.

Resistive-Film Calorimeters for Microwave Power Measurement (Correspondence)

Two papers published recently in these Transactions have described calorimetric techniques for the measurement of microwave power at the milliwatt level which are free from the limitations inherent in existing methods using resistance-type milliwatt-meters.

Recent Developments in the Field of Microwave Power Measurements at the National Bureau of Standards

Recent Developments in the Field of Microwave Power Measurements at the National Bureau of Standards

The Hall Effect and Its Application to Microwave Power Measurement

Hall effect and radiation pressure, as different manifestations of the same basic phenomenon, have both been successfully applied to the measurement of microwave power, using semiconductors for the purpose. Attention is called to the part played by displacement current at these frequencies and the conclusion is drawn that such currents are much more significant in relation to radiation pressure than to Hall effect. A wattmeter depending for its operation on the Hall effect is described and it is shown to give accurate measurements whatever the nature of the load.

Broad-Band Calorimeters for the Measurement of Low and Medium Level Microwave Power II.Construction and Performance

The construction and performance of a series of rugged, broad-band twin-Joule calorimeters, using dry loads, are described.These calorimeters operate over the frequency range of 0 to 75,000mc. The over-all measurement error, computed as the rms value of the maximum individual errors from known independent sources, is shown to lie between 1 and 21/2 per cent for power levels between 1 and 100mw. Power measuring techniques are discussed and a method using the heating and cooling cycle of the calorimeter is described in detail. Power comparison measurements between the calorimeters and several bolometer mounts illustrate the increasing inefficiency of bolometer mounts with increasing frequency.

Broad-Band Calorimeters for the Measurement of Low and Medium Level Microwave Power I. Analysis and Design

Design considerations for a group of broad-band calorimetric power meters, capable of accurately measuring low (zero to one milliwatt) and medium (zero to 100 milliwatts) power levels over a frequency range from to 75,000 mc, are presented. The power meters are of the nonadiabatic, twin, dry-load type and utilize the substitution of dc power. The conflicting requirements imposed upon the design by the need to realize broad-band performance, adequate sensitivity, reasonably short response time, negligibly small rf-dc equivalence error, freedom from zero drift and from other types of error are discussed. An analysis is given of the known sources of error which enables the accuracy of the individual instruments to be reliably estimated.

The Hall effect and its application to power measurement at 10 Gc/s

The paper describes experiments on the measurement of microwave power at 10 Gc/s employing the Hall effect produced in single crystals of n- and p-type germanium when erected on the axis of a hollow metal rectangular waveguide carrying the power. So far as is known, this is the first recorded observation of the Hall effect at this frequency, and it has been shown possible to apply the effect, with the help of a suitable phase-adjustment device, to the design of a wattmeter. For this purpose the Hall output from a crystal was calibrated against the power measured independently, and the relationship was found to be practically linear.From these power measurements and consideration of the conditions of operation of the crystal, the Hall coefficients for the n- and p-types of germanium used were deduced and shown to be of the same order of magnitude as the d.c. values.An investigation of the impedance of the crystal circuit led to the conclusion that the contribution to the Hall effect from the displacement current in the crystal was very small compared with that of the conduction current.The success of this application depends largely upon the development of suitable crystal units, and towards that end new techniques were introduced.

Hall effect and its counterpart, radiation pressure, in microwave power measurement

Hall effect and radiation pressure are shown to be different manifestations of the same basic phenomenon, and their relationship is deduced for a plane wave transmitted through a slab of material of given electrical characteristics. In particular, the contributions to these effects of both the conduction and displacement components of current in the material are examined, and it is shown that the latter can be a significant supplementary factor at ultra-high frequencies. Both Hall effect and radiation pressure have been used for microwave power measurement, and some aspects of these applications are discussed in the light of the present analysis. To permit a better assessment of the Hall-effect-wattmeter problem, numerical calculations have been made at 4000 Mc/s for n-type germanium and silicon showing that a high-resistivity material is necessary to achieve a satisfactory performance with a transmission type of instrument. Similar computations for polythene predict that a measurable Hall effect arising from displacement current in this material should exist, and it is suggested that observations based on that effect might offer a new approach in further exploration of the properties of dielectrics.

An instrument for the absolute measurement of low-level microwave power in the 3 cm band

The paper describes an instrument termed the `vibration wattmeter? which consists of a cavity in which is suspended a thin metal rod mounted on a fine glass spindle with a quartz fibre suspension. The rod is free to execute torsional oscillations in the horizontal plane. The oscillating system receives periodic impulses from the interaction between the metal rod and the electromagnetic field within the cavity, and by the automatic switching of the microwave source a steady amplitude of mechanical oscillations can be maintained. The amplitude of these oscillations is measured by means of a small mirror attached to the spindle and a lamp-and-scale arrangement. The relationship between the power delivered to the cavity and the amplitude of the mechanical oscillations can be found theoretically, and with knowledge of the specific couple of the quartz fibre suspension and of the damping factor of the mechanical system, measurements of power can be made. The instrument measures microwave power with an error estimated as not exceeding ±2%. The measurement is absolute, since the calibration depends only on measurements of mass, length and time.

Microwave Power Measurements Employing Electron Beam Techniques

A new electron beam technique for measuring microwave power flow, either cw or pulse, in waveguides is described. This technique consists of accelerating an electron beam transversely through an evacuated section of waveguide carrying power in the TE10 mode. The transit time of the electrons is adjusted to a value which gives maximum interaction of the field in the guide with the electrons, i.e., electrons gain maximum energy. The energy gained by the electrons is measured by means of a dc stopping potential which can be related to the field. Power is then calculated from the Poynting vector. The instrument takes the form of a sealed-off vacuum tube having a short section of waveguide as part of the tube. The ends of the waveguide may be sealed using known window techniques. The theory for the ideal case is presented and then means of correcting for the various perturbations present in an actual tube are discussed. The theory and the corrections are verified by experiment. The technique appears to have definite value for monitoring or measuring high- and medium-level cw or pulse-power flow. Theoretically the device is self-calibrating and therefore might make a good primary standard. Its suitability as a primary standard is under further investigation.

A resonant-cavity torque-operated wattmeter for microwave power

A sensitive method of microwave power measurement is described which makes use of the mechanical force exerted by the electromagnetic field on a small vane in a resonant cavity. It is shown that the force on the vane is a simple function of the Q-factor of the cavity, the power absorbed in it and the perturbation of its resonant frequency caused by the vane. The results of a comparison between an experimental wattmeter based on this principle and a water calorimeter are given, and the requirements of a practical instrument are discussed.

Measurement of microwave power

Measurement of microwave power

The Operation of Bolometers Under Pulsed Power Conditions

The dynamic behavior of Wollaston (thin platinum) wire bolometers under pulsed power conditions is examined and shown to be responsible for certain errors in the measurement of microwave power. These errors, in order of importance, arise from the nonlinearity of bridge null arm current as a function of bolometer resistance, from the nonlinear cooling of the bolometer in the interval between pulses, from the time variation of VSWR of the bolometer and from the time variation of bias power in the bolometer. The total error is found to be proportional to the bolometer resistance excursion during a pulse; the resistance excursion, in turn, is proportional to the pulse energy. All of these errors, with the exception of that due to nonlinear cooling, are amenable to calculation. The latter error was evaluated by subtracting the sum of the computed errors from the experimentally determined total error. The total error is about 14 to 15 per cent per 100 ohms resistance excursion for two widely used bolometer types under typical conditions. More than half of the above error may be eliminated by suitable design of the bridge circuitry.

A Microwave Microcalorimeter

This paper gives a brief account of a calorimeter developed at the National Bureau of Standards for the measurement of microwave power with relatively high accuracy (better than 1 percent) and at low (milliwatt) power levels. The calorimeter is identifiable as an electrically (dc) calibrated, aneroid micro-calorimeter of the Joule twin type. A bolometer mount assembly serves as a wave-guide termination for the absorption of microwave power and constitutes the calorimetric body whose temperature rise is observed. This arrangement enables the use of a novel indirect measurement technique and tends to insure calorimetric equivalence of dc and rf heating. The equivalence error is investigated with the aid of heat flow analysis and auxiliary experiments, and an upper bound for the error is assigned.

Mismatch Errors in Microwave Power Measurements

Expressions are derived for error due to mismatch when a UHF or microwave power meter is calibrated by comparison with a standard power meter. Three different methods are considered: (a) alternate connection to a stable power source, (b) the use of a microwave junction which simultaneously supplies power to the uncalibrated power meter and the standard power meter in a known ratio, (c) alternate connection to a microwave junction. The relative merits of the methods are discussed. Expressions are derived for error due to mismatch when using a calibrated power meter in the following situations: (a) direct connection of power meter to power source, (b) reduction of power into the power meter by means of an attenuator, (c) reduction of power into the power meter by means of a directional coupler.

A torque-operated wattmeter for 3-cm microwaves

The wattmeter depends for its action on the torque exerted by the electromagnetic field in a waveguide on a vane suspended inside the guide by means of a fine quartz fibre. The deflection of the vane can be measured by means of a small mirror attached to it and a lamp-and-scale arrangement. The specific couple of the quartz fibre being known, the torque can be calculated. The relationship between power and torque can be determined by an auxiliary experiment in which nodal positions of certain standing-wave patterns are determined. The theory of this method of calibration is based on the theorem of adiabatic invariance of action and has been explained in a previous paper. The intrument will measure microwave power with an error not exceeding about ± 1½%. The measurement is absolute, depending only on measurements of mass, length and time, and the amount of power absorbed is insignificant.