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Controversies in HF contests Packet clusters A packet cluster system allows radio operators or shortwave listeners to " spot " transmitting stations by submitting their call sign and frequency of operation to a computer system which redistributes the information to all other connected users of the system .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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