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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.


Map courtesy of Google Maps

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.

I ran across the Bobtail design on Hamuniverse.com and it reminded me of the 6-tower directional system I worked on back in the 80's (WNIC-AM in Detroit). Here is some history behind the Bobtail design taken from the above article:

"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).


Finished 2 Meter Bobtail Beam looking for signals

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.
{It is important that the center conductor be trimmed as short as possible with respect to the braided shield. Trimming it too long does have the same effect as lengthening the element, which will cause the SWR to rise considerably.}


Center rear reflector shown soldered to wire connecting to outer reflectors

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.


What snow? This is fine antenna weather!

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 !
Thanks and enjoy building it...have fun!
John Michael Sadler - KD8JZP
73's

Mail to mike48723 (at symbol here) yahoo.com
Related articles:

http://www.hamuniverse.com/2meterbobtailbeam.html