© WB5CXC 2011
Double T Bar Verticals
Double T Bar Vertical antenna are vertical antenna with cross bars on the top and bottom.  These crossbars provide capacitance loading thus reducing the size of the antenna.  There are several companies that make them commercially, Force 12 and Transworld have reduced sized multi-band Double T or H vertical.  Force 12 also makes full half wave single band Double T Bar verticals.  These commercial antenna are very portable and have been used on DX Expeditions.
The 20M Vertical Double T antenna is designed to use available aluminum antenna tubing.  This tubing can be shipped UPS in 6 foot sections.  The T bars are made from 3/4” tubing and the vertical section are made from 1” tubing.  The T Bars are 6 foot long.  The center sections are 6 foot with a center section.  The center section are made from 2 - 7 1/2” pieces of tubing with a 1” gap for the transmission line.  The middle section are joined together by inserting a piec of 3/4” PVC sch. 40 plastic pipe.  There is a little slop between the PVC and the tubing but is pretty snug.  Use sheet metal screws to secure the tubing to the pipe.
This antenna is designed to be 2 feet off the ground.  You can use a piece of 3/4 PVC Pipe to mount the antenna to a pipe inserted in the ground.  Leave at lease a 2” gap between the antenna and pipe (this is high resistance / high voltage point).  The transmission lead needs to angle away at a 45 degree angle.  This prevents the transmission line from interfering with the antenna. 
Full Size 20M Double T Bar Vertical
Center Section Detail
20M Double Bar T - SWR Curve
20M Double Bar T - Elevation Plot
20M Double Bar T - 3D Plot