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2 Meter (2 Element) Bobtail Beam Project The 2 Meter 2 Element Bobtail
Beam is a high gain antenna 10db gain compared to a simple 1/4 wave vertical from a 6 element antenna is well worth the time, effort and fun in constructing this antenna! The 2 meter Bobtail beam is patterned after the Bobtail Curtain which is well known for it's very narrow pattern and high gain down on the HF bands. Just do a search on Google or any good search engine to learn more about the Bobtail Curtain from others who have experimented with it. The pattern of the standard Bobtail antenna is very narrow. The Bobtail beam is even more narrow off the front (driven section) with only minor backside radiation giving you a front to back ratio of around 17 to 19 degrees and a feed point impedance of about 50 ohms! These numbers may vary slightly with your final building outcome. At 2 meters, the entire antenna is only about .15 wavelength (12
inches)deep and about 81 inches (1 wavelength) wide and is fed
directly with 50 ohm coax. The separation between the front and rear
elements may be changed to suit your building requirements and the .15
wavelength (12 inches) is a good starting point and may have to be widened
by experimentation. At some point, all of the lengths and separation
between the front and back elements may start to interact with each other
and change the feed point impedance. Experiment! Depending on your construction materials, this beam can be made
very lightweight and can be turned with any small TV type rotor or by hand
if needed!
Formulas below are a combination of several formulas found in doing research on the Bobtail and may need tweaking but are a good starting point. Your lengths may be somewhat different as with any antenna project. The results from the foumulas below may
be long which should help in tuning for lowest SWR and good match for
direct coax connection. Yes....you should be able to
"scale" this antenna for just about any frequency desired depending on the
final size of the antenna and your support structure using the formulas
above! Going up in frequency is most likely your best bet as this antenna
will get very large as you go down in frequency and supporting it may be a
problem depending on your location. Description: The rear elements (the reflector section) are about 6% longer than the vertical elements on the front side (station side) of the beam and ALL 3 are connected together. All 3 front elements are the same length and all 3 reflector elements are the same length. Don't get confused here. The reflector lengths are all the same length and about 6% longer than the front elements. All front elements are the same length and about 6% shorter than the reflectors. As with most beams, the front section elements are shorter than the reflector elements. Both front and rear sections are 1 full wavelength wide from side to side. The center (driven) vertical (on the front section) is fed directly, using 50 ohm coax as an air wound choke and the shield of the end of the air wound choke coax is connected to a 1/2 wavelength wire or other conductor on each side to the outside verticals on the driven section. Center conductor from end of choke to the center driven element. See drawing. There is no
electrical connection between the driven or the reflector
section. Construction lengths are not
critical except that all of the front side (driven section elements) are
exactly the same length (20.4 inches in the example above) and the rear
(reflector) elements are all exactly the same length (21.6 inches). The
Bobtail Beam is fed with a simple air choke consisting of 6 or
7 turns of feedline coax wrapped around a suitable form such as PVC
Pipe. One end to feed point on the Bobtail Beam and the other end to your
rig. Editors note: It is suggested that the front section and the
rear section be insulated from each other...in other words....separated by
a non-conductive boom support structure. Experimentation with various methods of construction should give
you a high gain beam for 2 meters loaded with lots of fun for an
inexpensive price compared to a commercial beam of comparable gain!
Remember, the computer models show about 10dB gain compared to a 1/4 wave
vertical! Even if you can't get 10db over a 1/4 wave vertical, this project should improve your signal greatly! Builders and
experimenters.....Any input you may have with your experimentation
or results with this antenna is welcomed....
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