polarization angle can be adjusted for maximum received signal and 

 locked in that position. If the antenna will be used with more 

 than one satellite, it will be convenient to make the polarization 

 as well as the antenna position remotely controlled. 



PREAMPLIFIER 



Considerable thought should be given to the preamplifier as it will 

 play an important role in determining the error rate of the system 

 and the size of the antenna. 



Many types of amplifiers are discussed in detail in the material 

 that follows. For purposes of this discussion, the important 

 parameter is noise temperature, with a low noise temperature 

 being more desirable. 



To maximize performance of the system, it is standard practice to 

 mount the preamplifier on the antenna, making the connection between 

 the preamplifier and antenna as short as possible. For a similar 

 reason, the downconverter is usually located as close to the 

 preamplifier as practical. Several manufacturers supply a pre- 

 amplifier and downconverter in one package for direct mounting 

 at the antenna's terminals. 



DOWNCONVERTER 



The purpose of the downconverter is to translate the satellite signal 

 from the vicinity of 1694.5 MHz down to a more convenient frequency. 

 If the user is independently assembling a station using a surplus 

 VHF WEFAX or APT receiver^, the downconverter output frequency is 

 usually in the vicinity of 137 MHz. Where the user is buying a 

 complete package receiver, this frequency may be in the neighborhood 

 of 70 MHz. Other conversion frequencies are also used. 



RECEIVING SYSTEM 



Although all the components discussed above can be considered 

 part of the receiving system, the term "receiver" is generally 

 reserved for that part of the receiving system that contains the 

 power c-upplies and control circuits and provides most of the 

 amplification. Special units may be acquired or available equipment 

 adapted for this application. 



In addition to the usual functions provided by the receiving system, 

 it is desirable that the receiver lock onto and track a pilot carrier. 

 This carrier is transmitted from the Wallops CDA station and is 

 returned on a nominal frequency of 1694.450 MHz. The receiver should 

 track the deviation from this pilot carrier frequency (+ 20 kHz) to 

 compensate for drift in the satellite translation frequency and 

 control internal receiver drift as well. 



^1 Nagle, John J., "A Method of Converting the SMS/GOES WEFAX 

 Frequency (1691 MHz) to the Existing APT/WEFAX Frequency 

 (137 MHz)." Technical Memorandum NESS 54, NOAA/NESS Office 

 of System Engineering, April, 1974. 



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