2 Wavelength ,2 Meter Bi-Square Beam , 5dbd gain
I've built 3 of these antennas, they are inexpensive(under $10.00) and work very well.
The first one took a couple of hours to build. The others took about 1 hour.
Their radiation pattern is similar to a figure 8 with max. signal through the loop,
but they may be used as a" near-omnidirectional " antenna. I have used both 14 and 12
gauge insulated wire, 10 gauge will also work. Stranded wire is preferred over solid
because it is less prone to breaking. Insulated or uninsulated wire? Using insulated wire
results in dielectric heating and a loss of approx. . 0.3 db(not much) but will last much longer.
It's up to you.
THE MATH: loop size = 1970/FMHZ or 1970/146= 13.493 feet (13 ft. 6 inches)
length of each side = 13.493/4 = 3.373 feet
distance from center of the frame to wire = 3.373 X .707 = 2.385 feet
**** I cut the loop 14 ft 6 inches to allow extra length for fastening to the frame ****
Fold the wire in half and cut it into 2 equal lengths.
Remove 3 inches of insulation from both ends of each of the 2 equal lengths.
The remaining insulated portions are the 13 ft. 6 in.(6ft 9in X 2 each) needed for the antenna,
while the 3 inch stripped sections at each end are used to secure the wire to the frame.
The wire maybe fastened to the frame in many different ways, the idea is to loop the ends
around or through your fastener, using the shortest possible loop, bending the wire back
on itself and wrapping the ends tightly. The ends can then be soldered and later sealed.
BALUN
Since this is a balanced antenna and coax is an unbalance feed line a balun is needed for the transition.
Using the balun reduced pattern distortion and false/erratic VSWR readings.
Cut a 22 inch length of coax. Remove the outer insulation completely. By pushing the shield wire toward
the center it will expand like the old finger puzzles. Remove the shield wire. This will become the sleeve.
On the coax cable that is to be used to feed the antenna, measure back 22 inches from the end which
is to be connected to the antenna. Remove approx.1 inch of the outer insulation between the 22 and 23
inch marks.
Slide the 22 inch shield wire, removed earlier, over this coax and down to the bottom of the exposed
area. This connection between the shield and sleeve may be soldered or simply secured by wrapping with
multiple turns of small gauge wire. The exposed shield wire (sleeve) should be covered completely by heat
shrink tubing or wrapping with electrical tape. It may be water proofed by covering with silicone compound.
This will leave 1.5 to 2.5 inches of coax exposed at the feed end of the cable. Strip the insulation back
about 1 inch from the end making sure that the sleeve you installed does not touch the shield at this point.
Separate the coax shield (not the sleeve) from the inner conductor. Twist it together into a separate wire lead.
Remove about 1/2 in. of the insulation from the inner conductor. Your BALUN is now ready to be connected
to this antenna or any other antenna that requires a 1 to 1 balun for 2 meter operation.
(does well on 440mhz also)
FEED SYSTEM
*RULE OF THUMB: A resonant antenna, made up of an odd multiple of 1/4 wavelengths per leg
is usually( "I" ) current feed. 1,3,5,7,9 quarters
A resonant antenna, made up of an even multiple of 1/4 wavelengths per leg is
usually ("E" ) voltage feed. 2,4,6,8,10 quarters.
* Referring to center feed antennas *
Each leg of this antenna is one ( 4/4 ) wavelength long and so ("E") voltage fed.
I used 1/4 wavelength of 450 ohm ladder line as the matching section to match it to 50ohm coax.
246/146 = 1.685ft X .98(VF) = 1.65 ft or 1 ft 8 in. or 20 inches.
NOTE:( VF) is the Velocity Factor of the transmission line.
One end of the ladder line is soldered to the feed point. I started by removing 4 inches of insulation
from the other end. Then by removing more insulation as necessary and attaching and
sliding the 50ohm coax toward the feed point, a point of lowest VSWR can be found. The ladder
line and coax cable with balun should be brought away from the feed point back toward the center of
the antenna and secured with plastic ties and/or electrical tape. Solder the coax to the ladder line at the
point of lowest VSWR and clip off and discard the unused portion of the ladder line.
( my lowest SWR was achieved with 13 inches of ladder line remaining)
The maximum VSWR from 144 to 148 MHz did not exceed 1.5:1 on the antenna(s) that I built.
If ladder line is not available, 20 inches may be added to each of the two sections of wire. This
extra length is than folded back along the arm of the frame maintaining equal distance between
them as much as possible. The 3/4 inch width of the arm provides good separation, one wire on
top the other on the lower side. If you use this method you must insure that a good solder joint
is made where the wire is wrapped at the screw. A poor connection at this point could result in
the wrap acting as an inductor that decouples the antenna from the matching section. Also
insure that the remaining wire used in the matching section is well insulated. This will help keep
the impedance more constant during wet and dry conditions.
CHEAP FRAME
I mounted my antenna on a wooden frame. While many people are skeptical of using wood in antenna
construction, it does hold up well if sealed with a product such as "KILTS PRIMER" and painted.
Most houses are framed from wood and many are over one hundred years old. I had a 8 foot long, 2
Meter beam made from a 1X4 pine board that I took down after 6 years of use inorder to mount a 10
and 6 Meter Quad Loops antenna ( hope to add one of these antennas inside the 6 Meter loop soon.)
The 1X4 was still in good condition.
If we look back at the calculations, we see that it is 2.385 ft. from the center of the frame to the wire.
To determine the span we multiple 2.385 by 2 = 4.77 ft. or approx. 4 ft. 9 in.. In this case a 5 ft. arm
would be left with 1.5 in. to the tip from the wire attachment points. I cut 2 each 3/4X3/4in. 5 ft.
long square sticks from a 1X board. I purchased a 2X2 in. by 8 ft. pine stick at the local
"Home Improvement Store". One of the 3/4X3/4X5 is attached to the 2X2 vertically, rising 2 ft. 6 in.
above the top. The other 3/4X3/4X5 is centered on the top of the 2X2 horizontally and both are
secured to the 2X2(and each other) with(1 5/8 in.) drywall screws. Prime and paint after assemble.
The antenna wire is also attached using the drywall screws. Measure from the top of the 2X2 up
the vertical 3/4X3/4 2 ft. 4.5 in. (1/2 of 4ft. 9in.). Install a drywall screw angling in from the top at
this point. Repeat this moving down from the top on the 2X2. These 2 screws provide the top and bottom
anchors for the antenna wire. At 2ft 4.5in. from the center along each horizontal arm install 2 screws, one on the
top and one on the bottom of the arm, slanting in from the outside. Make sure that these screws do not touch
each other inside the wood.(this will produce the "DUHHHH" effect, a major headache and dysfunctional antenna).
All that is left is to install the antenna wire(taut but not tight) by wrapping the bare ends around the screws
using the smallest possible loop on the side arms and pulling it over the top or bottom screw. Solder the ends
of the wire. Solder the ladder line to the loop keeping it in position with electrical tape until tuning is complete.
Attach and slide the coax with balun up the ladder line(removing insulation from the ladder line as needed)
until the lowest VSWR is achieved at the center of the band(146MHZ). Solder all the electrical connections,
secure the matching and feed wires and antenna wire if you like with plastic ties,
seal all connections with silicone compound.
Now go aim it at a repeater where you received reports of being scratchy and get an opinion or two.
You now have 4ea. 1/2 wave antennas transmitting and receiving instead of a single 1/2 wave as with
antennas like the J-pole.( no disrespect to the J-pole, Thanks again Robert)
If the wire is rotated 90 degrees(separation at the top and feed at the bottom) it will be horizontally
polarized.
Replace the 2X2 with a 6 in. square plywood center for an emergency indoor antenna with 5dbd gain
and a low radiation angle due to the stacking effect inherent with this design.
NOTE: Multiplying the loop (13.493 ft.) by 1.04 gives the length of a reflector that when combined
with this driven element at .15 wavelength spacing and retuning, will result in a 2 element beam
having approx. 9.5 dbd gain. Open the reflector loop on one side only. (works, i'm using one)