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494 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s<br />

and to noise figure by<br />

–1 ⎛ NF ⎞<br />

T = KT log ⎜ ⎟<br />

⎝ 10 ⎠<br />

– 1<br />

(21.9)<br />

e 0<br />

Noise temperature is often specified for receivers and amplifiers in combination<br />

with, or in lieu of, the noise figure. Applied to antennas, the noise temperature concept<br />

relates the amount of thermal noise generated to the resistive loss components of the<br />

antenna feedpoint impedance.<br />

The antenna/receiver system will be afflicted by three different noise sources external<br />

to the receiver:<br />

• The thermal noise temperature (T R ) of the resistive loss portion of the feedpoint<br />

impedance<br />

• A sky noise temperature (T SKY ) that depends on where the antenna main lobe is<br />

pointed<br />

• A ground noise temperature (T GND ) that consists of components reflected from the<br />

sky as well as components caused by internal thermal agitation of the ground<br />

Figure 21.1 Contributors to antenna noise temperature.

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