2 Meter (2 Element)
Bobtail Beam Project
Updated May, 2010
(See 70cm Bobtail feedback at
end of 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 our research, the Bobtail
antenna was invented by Woodrow Smith, W6BCX around 1948. We have
compiled data from various sources to present this project in condensed
form so you won't have to wade into all that theory, graphs, tables, etc
when all you want to do it to get a project up and running on the air in
as minimal time as possible and start having some more fun on the 2 meter
band.
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!
In general,
the SWR is very near 1:1 when tuned properly and according to research is
wide banded.
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!
The drawings below are shown using PVC pipe as
the support for the antenna however, you can use most any non-conductive
element support in a frame such as wood. A non-conductive mast may help to
keep the pattern as designed.
Description, details, formulas and lengths
(Refer to Drawing)
Element
lengths:
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.
Driven
Section each = 20.4
inches (20 13/32 inches)
Reflector Sections
each = 21.6 inches
(21 39/64 inches)
Note:
These lengths should be very close to
final lengths.
Some length adjustment may be needed for
lowest SWR.
Formulas: Note -
These formulas seem to be very close. Adjust as needed.
Front
Elements~~~ 248 / freqmhz = length in feet (multiply by 12 =
inches)
Reflector Elements~~~ 263 / freqmhz = length in feet ) multiply
by 12 = inches)
Spacing = .15 X one wavelength at 146mhz (one wave
aprox 80.32inches)
Example for 147.00mhz:
Front elements------248 /
147 = 1.68 feet X 12 = 20.24 inches
Reflector elements-----263 / 147
=1.78 feet X 12 = 21.46 inches
11808 / 147 = 1 wavelength = 80.32
inches (both front and rear equal)
.15 wavelength spacing = 80.32 X .15
=12.04 inches
As stated
earlier, these formulas are not exact but should be
close.
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.
As an example for 10 meters, 28.400mhz, the width
of it would be about 34 feet plus; with over 9 feet reflector
lengths. Also remember, the bobtail beam is 1 wavelength wide at your
design frequency and requires that you fashion 2 supports for the vertical
elements and some way to keep them
separated!
Description:
The project is shown here using a combination of
ideas but is shown using PVC pipe as a support for the vertical
elements.
The 2 meter Bobtail beam driven section is basically
a stand alone center 1/4 wave vertical with 2 more 1/4 wave verticals the
same length as the stand alone section on each side that are connected to
each other independent of the center vertical which acts as the driven
element in a 2 element (2 section) beam.
The outside elements
are 1/2 wavelength from the center element on both the driven section and
the reflector, yielding an antenna about 81 inches wide at 2
meters.
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.
Spacing between driven and reflector section is
about 12 inches.
(not critical)
The 2 Element Bobtail Beam is
very broadbanded and usually is easily tuned for best match to 50 ohm
coax.
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
diameters of all elements should be the same.
1/8 inch welding rods
make good lightweight elements if you can get them long enough or you may
use aluminum tubing, small diameter copper tubing, etc or stiff
wire of small size for better stability.
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.
Mount it as close to the center vertical radiator as
possible.
Editors note:
There are
numerous ways to build this antenna using PVC, wood, fishing poles and
other non-conductive materials for the
support and aluminum tubing, welding
rods, old TV antenna elements, large diameter stiff wire, brazing rods and
other materials for the verticals. Use your
imagination!
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.
One
other note of interest is the fact that the orientation of the beam
towards the station of interest is at 90 degrees to the parallel supports
and not off the end as in standard beams or yagi's. See drawing for
reference.
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!
Feedback from
Builders!
70cm version- Photo courtesy of Rich,
K2RRF
"I had very good luck on a 70cm model using 5" front to
back spacing and 12" side to side spacing the driven elements were 6.5
inches and the reflectors were 7". The only reason I used 5" front to back
and not 4" was it was the closest I could get the pvc t's together but it
worked out.Thank you for sharing this antenna project with us and I hope
this helps someone who wants to build a 70cm model." 73 K2RRF Rich in
Trenton NJ
See other versions below by other
builders:
2 METER BOBTAIL BEAM
ROVER
2 METER BOBTAIL BEAM (Attic
Version)
2 METER BOBTAIL BEAM
KD8JZP
Builders and
experimenters.....Any input you may have with your experimentation
or results with this antenna is welcomed....
6 meters, 440
anyone?
email your experimentation to n4ujw at
hamuniverse.com...
Subject: bobtail
beam..........NOT (None)....... please.
Your
experimentation could lead to your project being published
here!
73
