Radiometric method accurately calibrates the principal line sections of an antenna, including the section which contains the antenna-feed component /mode generator and polarizer/. Precise knowledge of the absolute antenna temperature is not required

Otoshi, T. Y.

Stelzried, C. T.

English

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August 1970	 Brief 70-10344 
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Radiometric Evaluation of Antenna-Feed Component Losses 
Simplified Antenna Network 
Low noise antenna networks are frequently used 
in communications systems which measure atmos-
pheric and cosmic background noise at microwave 
frequencies. Most of these applications require pre-
cise knowledge of the antenna line-loss between the 
antenna aperture and the receiver input port. 
The determination of this portion of line-loss per-
mits a precise calculation of the antenna noise-tem-
perature component (operating noise-temperature) 
which is due to the external environment. High cali-
bration accuracy is required since an uncertainty of 
0.01 db in the dissipative line-loss corresponds to 
about 0.67°K uncertainty of antenna noise-tempera-
ture. 
Although most sections of an antenna line can 
usually be evaluated by direct insertion-loss measure-
ments or by theoretical calculations, the section which
contains the antenna-feed component (mode genera-
tor and polarizer) is difficult to calibrate because more 
than one waveguide mode is propagated and the field 
is circularly polarized. 
A radiometric method has been developed to 
calibrate accurately the principal line sections of an 
antenna shown in the figure. The first line section 
is the feed horn having a loss factor, L 1 , that is easily 
calculated; the second line section contains the wave-
guide components having the unknown loss factor, 
L 2 ; the third line section contains single-mode wave-
guide components with a loss factor, L3, that can be 
calibrated by direct insertion-loss measurements. The 
radiometric method consists of measuring the operat-
ing noise-temperature with the normal feed compo-
nents installed; these components are then replaced 
with a dominant-mode waveguide section having 
(continued overleaf) 
Now
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a known loss factor. The operating noise-temperature 	 Note: 
is measured again, and with the addition of several
	 Requests for further information may be directed to: 
approximations based on theoretical considerations,
	 Technology Utilization Officer 
the loss factor can be calculated	 NASA Pasadena Office 
One of the main advantages of this radiometric
	 4800 Oak Grove Drive 
method is that precise knowledge of the absolute
	 Pasadena California 91103 
antenna temperature is not required Error analysis
	 Reference TSP70 10344 
calculations indicate that a fairly large error in the
	 Patent status: 
antenna noise-temperature, defined at the antenna
	 inquiries about obtaining rights for the commercial 
aperture, has a negligible effect on the calibration
	 use of this invention may be made to NASA, Code 
accuracy. 'A combination of the individual probable
	 GP, Washington, D.C. 20546. 
errors, assuming they are uncorrelated, results in a
	 Source: C. T. Stelzried and T. Y. Otoshi of 
total probable error for the line-loss of approximately
	 Jet Propulsion Laboratory 
0.0016 db. The largest source of error in the calibra-
	 under contract to 
tion originates from mismatches between individual
	 NASA Pasadena Office 
components.	
.	 (NPO-1 1238)


Radiometric evaluation of antenna-feed component losses

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