Hi All, Tom Vales, a member of the list and the TCBA, helped with the following very fine article: HOW EWEN AND PURCELL DISCOVERED THE 21 cm INTERSTELLAR HYDROGEN LINE BY Karl D. Stephan in IEEE Antennas and Propagation, Volume 41, No. 1, February 1999, pages 7 to 16 There is a picture of the 3C22 "lighthouse" tube. Would this work for a HFTTC?