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Globally, fine particulate matter (PM2.5) air pollution is a leading contributor to death, disease, and environmental degradation. Satellite-based measurements of aerosol optical depth (AOD) are used to estimate PM2.5 concentrations across the world, but the relationship between satellite-estimated AOD and ground-level PM2.5 is uncertain. Sun photometers measure AOD from the Earth's surface and are often used to improve satellite data; however, reference-grade photometers and PM2.5 monitors are expensive and rarely co-located. This work presents the development and validation of the aerosol mass and optical depth (AMOD) sampler, an inexpensive and compact device that simultaneously measures PM2.5 mass and AOD. The AMOD utilizes a low-cost light-scattering sensor in combination with a gravimetric filter measurement to quantify ground-level PM2.5. Aerosol optical depth is measured using optically filtered photodiodes at four discrete wavelengths. Field validation studies revealed agreement within 10 % for AOD values measured between co-located AMOD and AErosol RObotics NETwork (AERONET) monitors and for PM2.5 mass measured between co-located AMOD and EPA Federal Equivalent Method (FEM) monitors. These results demonstrate that the AMOD can quantify AOD and PM2.5 accurately at a fraction of the cost of existing reference monitors.

The equations for low amplitude space‐charge waves in an electron stream containing heavy positive ions are considered. At frequencies a little below the ion plasma frequency there should be a wave which increases in amplitude as it travels (negative attenuation). Ion motion tends to limit the rate of increase. In a beam filling a conducting cylinder and confined by a uniform magnetic field there should be an increasing wave at frequencies near the ion cyclotron frequency. Such waves might explain observed fluctuations in long electron beams.

While orbital radio relays probably could not compete with microwave radio relay for communication over land, they might be useful in transoceanic communication. Three sorts of repeaters appear to be consistent with microwave art: (a) 100-ft reflecting spheres at an altitude of around 2200 miles; (b) a 100-ft oriented plane mirror in a 24-hr orbit, at an altitude of 22,000 miles; (c) an active repeater in a 24-hr orbit. Cases (a) and (b) require at 10cm wavelength 250-ft-diam antennas and 100-kw and 50kw power, respectively; in case (c), using 250-ft antennas on the ground and 10-ft antennas on the repeater, only 100 watts on the ground and 0.03 watt on the repeater would be required; in this case one should probably use smaller antennas on the ground. In cases (b) and (c) the problem of maintaining the correct orientation and position of the repeater is critical; perturbations by the moon and sun might cause the satellite to rock or wander prohibitively.

Many microwave tubes make use of a long electron beam. The radio frequency excitation on such a beam can be expressed in terms of two space-charge waves, one of which has negative energy and negative power flow. The electron beam may pass through resonators, through lossy surroundings, through slow-wave circuits. In this paper the low-level operation of klystrons, resistive-wall amplifiers, easitrons, space-charge-wave amplifiers, traveling-wave tubes and double-stream amplifiers is explained in terms of waves on electron beams and on circuits. Noise is discussed in terms of such waves.

Many ideas important to microwave tubes, including the interaction of electrons with waves, occurred very early. An orderly understanding came much later. The RF diode equation (1933-1944) gave quantitative results concerning microwave tubes rather than insight. The klystron and the concept of velocity modulation and phase focusing (1938-1939) led to a clear picture of important phenomena as well as to powerful microwave devices. Insight and performance were extended by the traveling-wave tube and the wave analysis of its behavior (1946-1947). This led to explorations in terms of the behavior of coupled circuits (1951, 1954) and negative-energy waves (1951). Backward-wave oscillators provided further understanding and a new tool (1952-1953). This background of understanding which grew up concerning microwave tubes led to analysis of and improvement in noise behavior (1950-1959). Finally, the invention of the parametric amplifier led to very-low-noise microwave tubes (1959-1961). Crossed-field tubes have always been highly effective but difficult to analyze and design. Nonetheless, useful high-power amplifiers and oscillators have been built.

Propagation on a. helix as a function of frequency is characterized by ranges in which there is a slow wave which can be explained as very nearly a transverse electromagnetic wave traveling along the wire, and ranges in which the helix does not propagate; these are associated with "forbidden regions" of propagation constant pointed out by Sensiper. Each mode of propagation consists of an infinite number of spatial Fourier components called spatial harmonics. The complicated behavior with frequency of the modes of a helix is explained as an effect of the coupling through spatial harmonics of the slow wave which travels with about the velocity of light along the wire to fast free-space or waveguide waves which travel with about the velocity of light along the axis. The qualitative effects of such coupling agree with a tape-helix analysis of helixes in free space and helixes surrounded by a conducting tube. The case of the bifflar helix is also treated.

In electron beam devices the thermal velocities of the electrons leaving the cathode limit the current density which can be attained with given cathode current density, cathode temperature, potential, and angular spread of arriving electrons. By using Liouville's theorem, limiting expressions are obtained which are valid for any point focus or line focus concentrating system using steady electric and magnetic fields. Current densities in perfectly focused beams are also evaluated. Curves are presented which relate the nearness of approach to the maximum attainable current density with the fraction of cathode current necessarily lost in the concentrating system. Use of the expressions obtained is illustrated by applying them to cathode ray tubes and electron multipliers.

In Chapter VI we have expressed the properties of a circuit in terms of its normal modes of propagation rather than its physical dimensions. In this chapter we shall use this representation in justifying the circuit equation of Chapter II and in adding to it a term to take into account the local fields produced by a-c space charge. Then, a combined circuit and ballistical equation will be obtained, which will be used in the following chapters in deducing various properties of traveling-wave tubes.

The voltage and current modulation of ideal cylindrical electron beams in Brillouin flow, as well as beams in zero magnetic field, are studied by means of Laplace transform. With a large-diameter beam of this class, suddenly accelerated from a temperature-limited cathode and without transverse velocities, the minimum noise figure of an amplifier is found to be smaller than it would be for a narrow, essentially one-dimensional (filament or sheet) beam, or for a confined-flow beam with the same diameter, longitudinal velocity and direct current. Certain space-charge wave solutions obtained in field analyses of beams from shielded diodes, which have never been detected experimentally, are found to be nonexistent in the sense that no phenomenon taking place in a vacuum tube excites them.

Traveling-wave tubes will have gain even if the r-f field at the mean position of the electron stream is purely transverse. The addition of a longitudinal magnetic focusing field reduces the gain due to transverse fields and increases the electron velocity for optimum gain.

This is a review paper which discusses in simple terms high-power klystrons, double-tuned circuits for reflex klystrons, noise in traveling-wave tubes and its reduction by velocity-jump or space-charge-wave deamplification, periodic magnetic focusing of electron beams, and the backward-wave oscillator.

The existence of artificial earth satellites and of very low-noise maser amplifiers makes microwave links using spherical satellites as passive reflectors seem an interesting alternative to cable or tropospheric scatter for broad-band transatlantic communication. A satellite in a polar orbit at a height of 3000 miles would be mutually visible from Newfoundland and the Hebrides for 22.0 per cent of the time and would be over 7.25° above the horizon at each point for 17.7 per cent of the time. Out of 24 such satellites, some would be mutually visible over 7.25° above the horizon 99 per cent of the time. With 100-foot diameter spheres, 150-foot diameter antennas, and a noise temperature of 20°K, 85 kw at 2000 mc or 9.5 kw at 6000 mc, could provide a 5-mc base band with a 40-db signal-to-noise ratio. The same system of satellites could be used to provide further communication at other frequencies or over other paths

In a simple electron gun consisting of a cathode and two apertured planes held at different potentials the apertures act as electron lenses. When the gun is immersed in a uniform axial magnetic field the aperture spacings and potentials can be chosen so that the emerging electrons have no radial velocities.

The subject of this talk is “Physical limitations in electron ballistics”. It is pleasant to have a chance to talk about such physical limitations, because there is so little we can do about them. And, although these limitations are apt to be discouraging, a knowledge of them is very valuable, for it keeps us from spending time trying, like the inventors of perpetual motion machines, to do the impossible.

It is proposed that if Imax and Imin are the maximum and minimum noise currents in a noise standing wave on an electron stream, then the product of their magnitudes cannot be smaller than |Imax||Imin|=αeI0B(ωωq)(kTceV0). Various assumptions lead to a factor α ranging between 0.8 and 1. I0 is beam current, B is band width, ω is frequency, ωq is effective plasma frequency, Tc is cathode temperature, and V0 is the voltage specifying the velocity of the electrons.

By using a new theorem concerning noise in electron streams together with work by D. A. Watkins, an expression for the minimum noise figure of a traveling‐wave tube with a uniform helix is deduced. For reasonable values of tube parameters this minimum lies around 6 db.

It is conjectured that the design of the cochlea is influenced by two conflicting requirements: (1) the cochlea should act as a precise frequency analyzer and (2) waves propagating along the basilar membrane should be transmitted without reflections. Accurate frequency analysis is possible only if the mechanical properties of the cochlea change rapidly with distance along the basilar membrane. Reflections of waves traveling on the basilar membrane will be negligible, however, only if these same mechanical properties change slowly. A compromise between these two requirements is possible if a loss constant δ related to the sharpness of response of the basilar membrane to a pure tone is related to the number N of wavelengths of the wave on the basilar membrane [N/(δ)1/2?1]. Furthermore, if sizable changes in the displacement occur only over distances larger than the width of a hair cell, then δ must be larger than the ratio of the width w of a hair cell to the distance d along the basilar membrane over which...

Most of the profitable work on information theory has been on codes for correcting errors in digital transmission, and various practical and effective codes are now available. Considerable attention has been given to encoding for efficient transmission over analog channels. A bound on the information rate imposed by quantum effects has been found. Schemes have been devised for reducing the redundancy of signals from various sources, such as text, facsimile, and TV, but these have not found practical use.