The first OBARC construction project took place on Saturday March 13th, 2010. There were 10 constructors and 5 facilitators lead by Randy, W2RDM. There were also 5 members at the project offering moral support. The details follow.
Project: Build a J-Pole antenna for 2 meters.
Location: Ocean Acres Community Center. (Same place as meetings.)
Date: Saturday March 13, 2010 at 9:00 AM.
Project Time: Approximately 3 hours.
Cost: $20 or less for all materials.
Randy, W2RDM, is the presenter and project director for the first OBARC construction project. Come at the scheduled time, pay $20 or less and you'll build and subsequently take home a 2 meter unidirectional antenna with 3 db gain. It has a low profile and is easy to install and hide.
The antenna is called a J-pole. The antenna is less frequently called a Zepp antenna (short for Zeppelin) which was first invented by the Germans for use in their lighter-than-air balloons. Trailed behind the airship, it consisted of a single element, one half wave-length long. This was later modified into the J-pole configuration, which became popular with amateur radio operators, as it is effective and relatively simple to build.
The J-pole antenna is an end-fed omni-directional dipole antenna that is matched to the feed line by a quarter wave transmission line stub. Matching to the feed-line is achieved by sliding the connection of the feed line back and forth along the stub until a VSWR as close as possible to 1:1 is obtained. Since this is a half-wave antenna, it will exhibit gain over a quarter-wave ground-plane antenna. The J-pole is somewhat sensitive to surrounding metal objects, and should have at least a quarter wavelength of free space around it.
Under Randy's guidance you'll not only build the antenna, you'll adjust the feed point using an antenna analyzer.
Following are some J-Pole references on the Web
Mike's Tech Blog
The KB9VBR J-Pole Site
Another J-Pole Site
ARRL J-Pole Information