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BOBTAILING OVER THE BAY John Michael Sadler - KD8JZP North Branch, Michigan The Thumb Area of Michigan
boasts a mixture of farms, woods, lakes, rivers, hills, and valleys. I
live in northern Lapeer County, on the side of one of those hills. Looking
to the north, I can see for miles and reaching 2 meter repeaters at 40+
miles is easy enough with my copper J-Pole mounted at 20 ft, but I have
yet to get across Saginaw Bay.
And, facing south from my back deck, I see the land rise above my house and it continues for several hundred yards and further into the state land. Needless to say, communicating with radios and repeaters located in the Detroit area is a challenge. Repeaters as close as 25 miles proved unreachable. Determined as I am, I began reading and
researching 2 meter directional antennas, hoping to find something that
might provide the performance necessary to connect with hams downstate.
" The 2 Meter 2 Element Bobtail Beam is a high gain antenna yielding about 10dB compared to a single 1/4 wave vertical antenna or about 13 dBi + in free space according to computer models and much research on the web and various antenna book references. According to research, the Bobtail antenna was invented by Woodrow Smith, W6BCX around 1948."Well, after realizing I already had most of the necessary materials just taking up room in my barn, I decided to give it a shot. Referring to the pictures in this
article will convince most people that the frame was easy to build,
using 3/4 inch PVC, while keeping dimensions as close as possible
(see related article).
Side elements spacing is 40 1/4 inches from center. Front to back measures 12 1/8. For elements, I used 1/4 inch O.D. copper tubing normally used for fuel oil furnace feedlines. The tubing comes in rolls and wasn't straight, but rolling it between a flat surface and a smooth piece of plywood while applying some bodyweight does a surprizingly good job. 1/4 inch brass ferrels were slipped onto and soldered to the elements to maintain proper positioning in 1/4 inch holes drilled in the corner and center PVC fittings. Grooves were filed in the bottom of the copper elements to provide for a better soldering connection of the #12 THHN wiring. Driven front element lengths were
trimmed to about 20 3/8 inches. The rear reflectors were cut at 21 5/8
inches. An RF balun was formed from 6 turns of the feedline RG-8X coax in
a 5 inch diameter and secured with ty-wraps and Scotch electrical
tape.
The coax center conductor was soldered
directly to the bottom of the center (driven) element, and the shield is
connected to the 2 front corner elements via the THHN. See photo
above.
The rear reflectors are connected electrically together, but isolated from the front elements. Coax connections were sealed with E-6000 silicone adhesive sealer. Using these methods and dimensions, the rig tuned in at 1.2:1 SWR at 146.5 mhz., and weighs in at a scant 5 1/2 lbs. Initial testing of the Bobtail had some real surprizes... I raised it on a 12 ft tripod on my deck, and upon power-up, I immediately noticed a marked improvement in reception on repeater frequencies that previously were quite noisy at best. I switched to a previously inaudible repeater 30 miles due south and received a clean signal... even BEFORE I pointed the Bobtail toward south.
Transmitting was equally improved. I am now cleanly reaching repeaters in Detroit. I've made contact with Lansing and even Jackson, some 75 miles away.To the north, I'm reaching the other side of Saginaw Bay... all from a 12 ft. tripod. This antenna is SWEET ! I've aquired a 40 ft. tower recently and will be erecting it when the weather lends itself better to setting a concrete foundation. I intend to fly the J-pole for local repeaters and nets. But it will be just under the rotating Bobtail that should carry me easily across the bay and over the hump. By the way... this antenna
is GREAT ! Mail to mike48723 (at symbol
here) yahoo.com (no spaces) http://www.hamuniverse.com/2meterbobtailbeam.html
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