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Noise Temperature of ESSCO Metal Space Frame Radomes

It is often the case that engineers will incorrectly assume that the total radome transmission loss (losses due to the dielectric membranes plus losses due to the metal space frame) should be treated as a resistive attenuator at 290 °K much like that which is done for atmospheric loss. This is not the case however, because the radome has both resistive and scattering components that add to make up the total transmission loss. 

The resistive components of the radome membranes are very small, which are attributed only to the actual resistive losses of the electrically thin, 40 GHz and below, dielectric membrane.  Generally this  is less than 0.1 dB for most membrane types. ESSCOLAM membranes, used in microwave applications of 20 GHz to 30 GHz, have dielectric "resistive" losses that are between 0.01 and 0.04 dB. The remaining membrane loss as it relates to total transmission loss is typically 0.1 to 0.5 dB  and is caused by the membranes frequency dependency reflection coefficient. The resistive component increases only slightly at higher frequencies and is only responsible for ~1° Kelvin of the "radome noise temperature". The largest part of the membrane transmission loss occurs because of the small reflection coefficient of the radome membrane - even where it is electrically thin -- and this reflection coefficient reflects some of the incident radiation away from the antenna system. This reflected power is incident signal power that is lost by reflection from the outside surface of the radome and does not add thermal noise to the system temperature. This can be considered as a reduction in main beam gain.

The dielectric membrane will also scatter a small additional amount of ground noise back at the antenna that will be collimated into the feed system. This again has a very little effect since the vast majority of this scattered energy hits the antenna system at angles that do not result in collimating that energy onto the subreflector. This is why the spherical shape of the radome and the size of the radome relative to the antenna diameter are carefully selected by ESSCO's engineers to minimize this radiation from being collected in the feed system. ESSCO can also supply higher performance radome membranes such as GORE-TEX® that have much lower resistive losses and reflection coefficients for specialty applications, such as radio telescopes, demanding the highest level of performance and maximum G/T for antenna systems operating in the millimeter wave bands.   

The other portion of the overall radome transmission loss is caused by physical blockage from the metal space frame members. This is only a fraction of the incoming plane wave radiation field that would otherwise be collected in the antenna aperture. Physical blockage of the antenna aperture plane by the metal space frame reduces the effective gain of the antenna system by that amount, but it is important to understand that it does not add noise temperature back into the system, as would be the case for a resistive loss of the same magnitude. In many cases, the main beam antenna gain lost due to the metal space frame aperture blockage is completely offset by improved antenna main beam gain for antennas operating inside of the radome where the harmful effects of wind deformation to the antenna aperture have been eliminated. The added aperture blockage from the metal space frame does however scatter some amount of sky and ground noise into the aperture plane and the fraction of this scattered energy that is actually collected into the antenna feed system is then seen as added noise due to the radome metal space frame. The metal space frame blockage is virtually constant with frequency and the scattered ground noise from it into the antenna system is a small and nearly constant amount.

ESSCO has developed industry leading and accepted proprietary codes, which can accurately predict the scattered noise that is added into the system due to the radome membrane and radome frame as a function of frequency and antenna look angle. Specific sky and ground noise properties can be assumed to account for typical operations in both warm and cold climates and at both sea level and high altitude sites.

In summary, the main beam gain "G" of the antenna system is directly reduced by the total transmission loss of the radome but the noise temperature of the antenna system is only increased by the sum of the resistive loss of the radome membrane (usually ~ 1 °K) plus the actual level of ground and sky noise scattered back into the antenna system by the metal space frame and the small reflection coefficient of the membrane. This total radome added noise temperature component is in the range of 7 °K to 12 °K for typical ESSCO metal space frame radomes using cost effective membranes in the microwave and 20/30 GHz bands. Achievement of this level of noise temperature requires that the radome has been correctly sized; where the center of the antenna aperture is located as near as possible to the center of the radome. ESSCO can also optimize the radome geometry for any specific customer application so as to minimize the effect of the radome on G/T. 

If desired in any specific application, ESSCO can utilize higher performance ultra low loss membrane materials that will further reduce the transmission loss and noise temperature of the metal space frame radome for more demanding millimeter wave systems and radio-astronomy applications. In those specialized applications using ultra low loss membranes, radome noise temperatures of less than ~4 °K with lower overall transmission losses in the range of 0.6 dB to 0.7 dB can be achieved in the microwave and low millimeter wave bands.