V.F. Mikhailov – Dr.Sc. (Eng.), Professor, Saint-Petersburg State University of Aerospace Instruments
On-board antennas of returned spacecraft on the descent trajectory are exposed to high-temperature heating and plasma, and for some descent trajectories, the temperature can exceed 5000 K. Under these conditions, due to a temperature change, the dielectric loss tangent of the antenna’s thermal protection sharply increases and, as a result, the antenna efficiency decreases significantly, which may result in loss of radio connection. An analytical calculation of the efficiency of onboard antennas for high-temperature heating conditions is not possible due to the lack of initial data on the temperature dependences of the electrical characteristics of thermal protection. The use of experimental studies requires the creation of a high-temperature heating adequate to flight temperature conditions. For this purpose, only plasma heating is suitable. Traditional experimental methods during the operation of the studied antenna or to transmit or receive harmonic oscillations due to the presence of heating plasma in the measurement zone do not give reliable data.
It is proposed to use plasma not only as a heating medium, but also as a source of high-frequency radiation. In this case, in addition to the antenna under study, an external channel antenna is used to measure the noise characteristics of the plasma. This antenna is an ellipsoid of revolution, in the far focus of which is the studied antenna, heated by plasma. The temperature regime of heating should correspond to flight. To ensure this condition, the plasma heat flux density is regulated. The plasma characteristics are controlled by a calorimeter and a spectrograph. The studied antenna and the antenna of the external channel work on a radiometric
receiver that measures the power of noise radiation. Since the noise power is proportional to the noise temperature of the radiation source, all the relations obtained in this work are recorded for the noise temperature. Research is carried out in several stages. Moreover, at the first stage, the standard onboard antenna is replaced by a radiator without heat protection made of copper. This allows the antenna to be exposed to the plasma for a long time without destruction. For such an emitter it is possible to measure the noise radiation of the plasma without the influence of the noise radiation of the heated thermal protection.
An expression is obtained for calculating the efficiency of onboard antennas for heating conditions adequate to flight, according to the results of radiometric measurements. During measurement, the radio brightness temperatures of both the studied antenna and the external channel antenna are recorded. Moreover, the measurements are performed both in the presence of a heating plasma, and without it. The results of experimental studies are presented. The calculation of the efficiency of the onboard antenna was
carried out according to the obtained analytical ratio. We studied airborne antennas with thermal protection from Al2O3 and SiO2.
The results obtained show that the considered experimental research method determines the efficiency of onboard antennas for conditions adequate to flight.
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