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Spatial Distribution of Thermal Radiation at Microwave Frequencies


George, Nicholas A. (1959) Spatial Distribution of Thermal Radiation at Microwave Frequencies. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/CERS-Y395.


Theoretical and experimental radiation patterns are given in spectral form for the thermal radiation from thin slots or heated wires having dimensions of the order of the comparison wavelength. Maxwell's equations and noise theory form the basis of the analyses in which three independent methods are used to predict a spatial distribution which exhibits interference minima and maxima. In the first, the wave equation is solved for a noise-excited transmission line which is suddenly short- and open-circuited at alternate ends. By a study of the trapped noise currents, it is found that the radiation pattern has an interference structure which is smoothed as the loss is increased. Secondly, a formula is derived for the radiation pattern of a heated wire by a computation of its absorption in an isothermal enclosure and by an application of the principle of detailed balancing. Finally, the pattern of a long thin slot is computed directly using the Lpontovich-Rytov distributed source generalization of Nyquist's noise formula.

Fraunhofer pattern measurements are taken for a thin slot excited by a gaseous discharge at 10,100 ± 200°K. The pattern measuring apparatus is a Dicke radiometer having the following characteristics: frequency 9200 mc/s, bandwidth to the detector 16 mc/s, modulation frequency 1000 c/s, and residual noise level 0.3 rms°K.

The theory and the experiment demonstrate an interference phenomenon even though the source excitation is spatially extended and uncorrelated in time and space. The patterns are not even approximately Lambertian, e.g., a thin slot of 9.5 pi radians length exhibits a pattern having nine relative maxima in 180° with the maximum emission at 63° from the normal.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:(Electrical Engineering and Physics)
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Electrical Engineering
Minor Option:Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Papas, Charles Herach
Thesis Committee:
  • Unknown, Unknown
Defense Date:1 January 1959
Additional Information:Author's name listed in 1959 Caltech commencement program and thesis file as: Nick George.
Record Number:CaltechETD:etd-02022006-142926
Persistent URL:
Related URLs:
URLURL TypeDescription ItemCaltech Antenna Laboratory Technical Report 18
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:460
Deposited By: Imported from ETD-db
Deposited On:06 Feb 2006
Last Modified:18 Oct 2023 19:12

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