"Sperrtopf" by DF7DJ

Another solution is the "Sperrtopf". Sorry, I have no correct english term for this principle...

(N. E. Lindenblad, US-Pat. 2.131.108 from 1936), sleeve or bazooka feeding.

The following pictures of Herbert, DF7DJ shows this method in a 2m-Yagi. The outer tube has 10mm inner diameter, the inner conductor has 5,5mm in diameter. The impedance of this lambda/4-Sperrtopf is 37,5Ohm.

One half of the radiator is connected to the outer tube, the other to the inner. At the other end the outer tube is grounded to the boom, the inner tube is connected to the pin of the coax-socket.

This is a well-know solution for commercial dipoles in TV- and BC-antenna-groups and seems to be very good for 2m an 70cm.

Tnx pictures to DF7DJ, here is the link to his homepage: http://www.df7dj.de/html/28_ohm_anpassung.html