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Note: Noise temperatures are in kelvin and frequencies in GHz.

Note: Noise temperatures are in kelvin and frequencies in GHz.

primarily by Arthur B. Crawford and Henry W. Anderson. The 2.39-GHz measured radiation patterns and gains were found to agree well with theory (Crawford, Hogg and Hunt 1961); both azimuth and elevation pointing were available. The dual-channel TWM, provided by the Derek Scovil group at the Murray Hill Laboratory, received waves of both clockwise and counterclockwise circular polarization, with a noise temperature of 7K. Ohm carried out an exhaustive study of the uncertainties in the measured and estimated noise contributions. The overall system noise temperature (at zenith) was 21 K.

The third low-noise system operated at 4.17 GHz as a sensitive receiver for the Crawford Hill station of the Telstar Project (Jakes 1963). The antenna was the same as for the 2.39-GHz Echo experiment (Figure 4.2), but gain, beamwidths, and pointing characteristics were checked at the new shorter wavelength and found satisfactory when compared with theory. The TWM, with a 5-K noise temperature, was provided by the Scovil group to amplify both senses of circular polarization.

The "first-ever" live TV from Europe was obtained via Telstar with this receiver. The overall zenith system noise temperature was just less than 17K. W. C. Jakes Jr. was project engineer. The main US ground station was at Andover, Maine.

Table 4.1 shows that the zenith atmospheric sky noise, measured by tipping the antenna beam in elevation, is within 0.25 K of the theoretical computation in the microwave band; the theoretical values are computed for average summer conditions of temperature and humidity (USA).

The table also shows that in no case does the estimated system temperature exceed the measured system temperature. Of course, none of the estimated system temperatures contain any contribution from the CMBR.

However, it is amusing that, in the case of system 1, the estimated is the same as measured; this indicates sizable uncertainties, probably in both quantities.

Many of the uncertainties in the noise contributions from the microwave circuitry can be avoided by switching between the antenna and a precisely calibrated cold load located near the antenna input per se as shown by Penzias and Wilson (1965a), at 4.08GHz. With this improved measuring system they were able to deduce 3.5 K excess to the expected antenna temperature; this excess is interpreted as the CMBR.

The very low-loss switch used in these measurements is a treasure in microwave radiometry. It is made of a gently squeezed section of waveguide of circular cross section. Penzias and Wilson quote only 0.027 dB of loss for their switch; to my knowledge, it was first mentioned by George C. Southworth in his book on microwave technology and was first used in radiometry by Douglas H. Ring at K-band.

However, measuring the contribution of the lower hemisphere (back lobes) to the antenna temperature is quite another matter (for some estimated values, see Table 4.1). Ideally, one would measure the antenna radiation pattern over the lower hemisphere, measure the ground etc. radiation over that same hemisphere, and integrate the product of those two over the hemisphere. This radiation is comprised of both emission from the ground surface and reflection of sky temperature by that surface (Hogg 1968). Penzias and Wilson calculated a net contribution of just less than 1K for the antenna per se. Apparently further research on antenna design, measurements and siting is called for.

As implied in the historical introduction, some equipment, designed and built for practical (economic) application, can impact scientific studies, provided the quality is good. An example of this is the fruitful use of electromagnetic and electronic equipments, designed for microwave satellite communications development, in pursuing the microwave cosmic background radiation.

Recently, the importance of science and engineering innovation to the USA has been emphasized in a proclamation by the President of the US, backed by a report issued by a panel supported by the National Academies, and chaired by Norman Augustine (2007). That there is fruitful feedback between the two is well exemplified by the exercise we have just discussed. The cosmolo-gists and astronomers who carry on such research and innovation are to be commended.

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