Name: A Source Book in Astronomy and Astrophysics, 1900-1975 - A Measurement of Excess Antenna Temperature at 4080 Mhz
Author: Arno A. Penzias

A Source Book in Astronomy and Astrophysics, 1900-1975

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Author: Arno A. Penzias | Date: 1965

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One of the most important twentieth-century discoveries bearing on cosmology came about quite serendipitously. When testing the microwave-receiving system intended to be used for measurements of the high-latitude continuum radiation of our galaxy, scientists at the Bell Telephone Laboratories found a few degrees of unexpected noise temperature. Because the excess noise showed no sidereal, solar, or directional variation, they first supposed that the noise arose in the receiving antenna itself. Here Arno Penzias and Robert W. Wilson conclude that there remains an external noise contribution with an antenna temperature of at the wavelength of 7.3 cm.In a companion letter in the same issue of the Astrophysical Journal, Robert Dicke and his colleagues explained the excess 3° radiation as the residual temperature of the primeval explosion that initiated the expansion of the universe. It was not surprising that Dicke was one of the first to realize the importance of this radiation, since nearly twenty years earlier he had used a sensitive microwave radiometer to derive an upper limit of 20° for the residual temperature of the "radiation from cosmic matter." Nevertheless, the idea was not altogether new, for George Gamow, Ralph Alpher, and Robert Herman had already shown that a residual temperature of 5° results from the expansion of the 10 billion degree primeval fireball (see selection 131).If the cosmic noise discovered by Penzias and Wilson is the relic fireball radiation, it should have a blackbody spectrum. Because of the interactions of matter and radiation, the primeval radiation will very rapidly reach thermodynamic equilibrium, and the resulting thermal spectrum will be preserved for all time as the universe expands and cools. While P. G. Roll and D. T. Wilkinson confirmed the initial measurements, G. B. Field and J. L. Hitchcock verified the expected thermal spectrum by examining the excitation of interstellar molecules at the shorter wavelength of 2.5 mm. Balloon measurements were subsequently used to confirm the thermal spectrum at submillimeter wavelengths (see figure 132.1). The microwave background radiation has also provided additional evidence for the isotropy of the universe as a whole. Following the pioneering observations of E. K. Conklin and R. N. Bracewell, and of Wilkinson and R. B. Partridge, the background radiation has been found to be isotropic to 0.1% on all angular scales greater than 1 min of arc. Most recently, an anisotropy of a few millidegrees has been detected and attributed to the motion of the earth relative to the radiation at a velocity of 390 km sec-1. When this component is removed, the radiation is isotropic to 1 part in 3,000.

A Measurement of Excess Antenna Temperature at 4080 MHz

Arno A. Penzias and Robert W. Wilson

(Astrophysical Journal 142, 419–421 [1965])

MEASUREMENTS of the effective zenith noise temperature of the 20-foot horn-reflector antenna (Crawford, Hogg, and Hunt 1961) at the Crawford Hill Laboratory, Holmdel, New Jersey, at 4,080 Mc/s have yielded a value about 3.5° K higher than expected. This excess temperature is, within the limits of our observations, isotropic, unpolarized, and free from seasonal variations (July, 1964-April, 1965). A possible explanation for the observed excess noise temperature is the one given by Dicke, Peebles, Roll, and Wilkinson (1965) in a companion letter in this issue.

The total antenna temperature measured at the zenith is 6.7° K of which 2.3° K is due to atmospheric absorption. The calculated contribution due to ohmic losses in the antenna and back-lobe response is 0.9° K.

The radiometer used in this investigation has been described elsewhere (Penzias and Wilson 1965). It employs a traveling-wave maser, a low-loss (0.027-db) comparison switch, and a liquid helium-cooled reference termination (Penzias 1965). Measurements were made by switching manually between the antenna input and the reference termination. The antenna, reference termination, and radiometer were well matched so that a round-trip return loss of more than 55 db existed throughout the measurement; thus errors in the measurement of the effective temperature due to impedance mismatch can be neglected. The estimated error in the measured value of the total antenna temperature is 0.3° K and comes largely from uncertainty in the absolute calibration of the reference termination.

The contribution to the antenna temperature due to atmospheric absorption was obtained by recording the variation in antenna temperature with elevation angle and employing the secant law. The result,

is in good agreement with published values (Hogg 1959; DeGrasse, Hogg, Ohm, and Scovil 1959; Ohm 1961).

The contribution to the antenna temperature from ohmic losses is computed to be

In this calculation we have divided the antenna into three parts: (1) two nonuniform tapers approximately 1 m in total length which transform between the

round output waveguide and the 6-inch-square antenna throat opening; (2) a double-choke rotary joint located between these two tapers; (3) the antenna itself. Care was taken to clean and align joints between these parts so that they would not significantly increase the loss in the structure. Appropriate tests were made for leakage and loss in the rotary joint with negative results.

The possibility of losses in the antenna horn due to imperfections in its seams was eliminated by means of a taping test. Taping all the seams in the section near the throat and most of the others with aluminum tape caused no observable change in antenna temperature.

The backlobe response to ground radiation is taken to be less than 0.1° K for two reasons: (1) Measurements of the response of the antenna to a small transmitter located on the ground in its vicinity indicate that the average back-lobe level is more than 30 db below isotropic response. The horn-reflector antenna was pointed to the zenith for these measurements, and complete rotations in azimuth were made with the transmitter in each of ten locations using horizontal and vertical transmitted polarization from each position. (2) Measurements on smaller horn-reflector antennas at these laboratories, using pulsed measuring sets on flat antenna ranges, have consistently shown a back-lobe level of 30 db below isotropic response. Our larger antenna would be expected to have an even lower back-lobe level.

From a combination of the above, we compute the remaining unaccounted-for antenna temperature to be

at 4,080 Mc/s. In connection with this result it should be noted that DeGrasse et al. (1959) and Ohm (1961) give total system temperatures at 5,650 Mc/s and 2,390 Mc/s, respectively. From these it is possible to infer upper limits to the background temperatures at these frequencies. These limits are, in both cases, of the same general magnitude as our value.

Note added in proof. The highest frequency at which the background temperature of the sky had been measured previously was 404 Mc/s (Pauliny-Toth and Shakeshaft 1962), where a minimum temperature of 16° K was observed. Combining this value with our result, we find that the average spectrum of the background radiation over this frequency range can be no steeper than

This clearly eliminates the possibility that the radiation we observe is due to radio sources of types known to exist, since in this event, the spectrum would have to be very much steeper.

Crawford, A. B., Hogg, D. C., and Hunt, L. E. 1961, Bell System Tech. J. 40, 1095.

DeGrasse, R. W., Hogg, D. C., Ohm, E. A., and Scovil, H. E. D. 1959, "Ultra-low Noise Receiving System for Satellite or Space Communication," Proceedings of the National Electronics Conference 15, 370.

Dicke, R. H., Peebles, P. J. E., Roll, P. G., and Wilkinson, D. T. 1965, Ap. J. 142, 414.

Hogg, D. C. 1959, J. Appl. Phys. 30, 1417.

Ohm, E. A. 1961, Bell System Tech. J. 40, 1065.

Pauliny-Toth, I. I. K., and Shakeshaft, J. R. 1962, M.N. 124, 61.

Penzias, A. A. 1965, Rev. Sci. Instr. 36, 68.

Penzias, A. A., and Wilson, R. W. 1965, Ap. J. 142, 1149.

APPENDED FIGURE

Fig. 132.1 Comparison of observations of the relict cosmic blackbody radiation with the theoretical monochromatic brightness in erg cm-2 sec-1 sterad-1 Hz-1 as a function of wavelength, according to the Planck spectral distribution function for a 2.8°K blackbody. The maximum brightness occurs at 0.18 cm. The data point recorded by Penzias and Wilson is denoted by an arrow and the data points obtained by other radio astronomers by open circles. The data near the peak of the radiation curve were taken by balloon or rocket-borne equipment, and separate observations are denoted by a sequence of crosses and a sequence of dots. (From R. A. Alpher and R. Herman, Proceedings of the American Philosophical Society 119, 325 [1975]).

1. R. H. Dicke, P. J. E. Peebles, P. G. Roll, and D. T. Wilkinson, Astrophysical Journal 142, 414 (1965).

2. R. H. Dicke, R. Beringer, R. L. Kyhl, and A. B. Vane, Physical Review 70, 340 (1946).

3. P. G. Roll and D. T. Wilkinson, Physical Review Letters 16, 405 (1966); T. F. Howell and J. R. Shakeshaft, Nature 210, 1318 (1966); G. B. Field and J. L. Hitchcock, Physical Review Letters 16, 817 (1966).

4. D. P. Woody, J. C. Mather, N. S. Nishioka, and P. L. Richards, Physical Review Letters 34, 1036 (1975).

5. E. K. Conklin and R. N. Bracewell, Physical Review Letters 18, 614 (1967); D. T. Wilkinson and R. B. Partridge, Nature 215, 719 (1967).

6. G. F. Smoot, M. V. Gorenstein, and R. A. Muller, Physical Review Letters 39, 898 (1977).

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Chicago: Arno A. Penzias, "A Measurement of Excess Antenna Temperature at 4080 Mhz," A Source Book in Astronomy and Astrophysics, 1900-1975 in A Source Book in Astronomy and Astrophysics, 1900-1975, ed. Kenneth R. Lang and Owen Gingerich (Cambridge: Harvard University Press, 1979), 873–876. Original Sources, accessed July 18, 2025, http://www.originalsources.com/Document.aspx?DocID=962LE8IZYKF2FTW.

MLA: Penzias, Arno A. "A Measurement of Excess Antenna Temperature at 4080 Mhz." A Source Book in Astronomy and Astrophysics, 1900-1975, Vol. 142, in A Source Book in Astronomy and Astrophysics, 1900-1975, edited by Kenneth R. Lang and Owen Gingerich, Cambridge, Harvard University Press, 1979, pp. 873–876. Original Sources. 18 Jul. 2025. http://www.originalsources.com/Document.aspx?DocID=962LE8IZYKF2FTW.

Harvard: Penzias, AA, 'A Measurement of Excess Antenna Temperature at 4080 Mhz' in A Source Book in Astronomy and Astrophysics, 1900-1975. cited in 1979, A Source Book in Astronomy and Astrophysics, 1900-1975, ed. , Harvard University Press, Cambridge, pp.873–876. Original Sources, retrieved 18 July 2025, from http://www.originalsources.com/Document.aspx?DocID=962LE8IZYKF2FTW.