This is a very common- symbol used in school-textbooks as a symbol for antenna.
This website (http://www.chegg.com/homework-help/questions-and-answers/figure-shows-arrangement-generating-traveling-electromagnetic-wave-shortwave-radio-region--q1017746) mentioned this as the symbol of Transmission line.
However, I did-not found any information about this symbol in google and wikipedia. As well, outside school-textbooks I yet not seen this symbol in any professional circuit diagram. What is this symbol? and in which sign-convention it is/was used?