BROAD B.WI) \//CR(H\ \ Vli NOISE SOURCE 617 



theoretical explanation which, when applied to the case in hand, would 

 explain the observed results cjualitatively and cjuantitatively, thereby es- 

 tablishing a new absolute standard noise source for microwave measure- 

 ments. 



'I'he micro\vave noise power from such a discharge tube was measured at 

 3950 mc in cooperation with Mr. C". 1"". Kdwards on his calibrated measuring 

 set on two different occasions, 16 davs apart. The values obtained were 



T 



15.86 (lb and In. 80 db respectively for 10 log ( t^ — 1).'' This places the 



temperature, 7\ in the neighborhood of 11,400 degrees Kelvin. It is believed 

 that the absolute measurements are correct to within ±.25 db or better. 



Having determined the temperature of this noise source, we might ask, 

 "If we should terminate our waveguide in a black body at 11,400 degrees, 

 how much microwave noise power would we get from it?" The black body 

 radiates with three polarizations, only one of which is propagated along the 

 waveguide, and this available power is given by Nyquist:^ 



where /; = 6.61 (10)~^^ joule sec. 



k = 1.381 (10)--3 joule/deg. 



/ = frequency in cycles per sec. 



B = bandwidth in cycles per sec. 



hf 

 At 4000 mc, -■'- is, for T = 290 degrees, 6.6 (10)-" which is so small that the 



denominator of (16) can be replaced by -^. This gives us the familiar 

 expression for thermal noise: 



PxA = kTB watts (17) 



In other words, thermal noise is black body radiation with but one 

 polarization . 



Going one step further we might also ask the question, "If we should 

 examine the radiation from this black body with an ojitical spectroscope, at 

 what wavelength would we lind its maximum radiated energy?" The spec- 

 trosco{)e detects radiation having three polarizations, and Planck's radia- 

 tion law applies. From Wien's displacement law, the wavelength of maxi- 

 mum radiation is given by the relation: 



K,r = 0.289 cm deg. (18) 



"The temperature of the waveguide was 32°C when these values were measured. 

 ^ H. Nyquist, F/iys. Rev., Second Series, Vol. 32, pp. 110-113, July 1928. 



