W6KOA
the "KITES ON AIR" group
SAN DIEGO, CALIFORNIA USA
For our first dipole antenna designs, commercial
off-the-shelf baluns were used to interface
between the center fed dipole antenna elements
and the coaxial feed line cable. These baluns are
typically used force the current that flows in the
center conductor and braid of the cable to be
equal, thereby minimizing antenna pattern
irregularities as well as minimizing the effects
caused by a portion of the coaxial cable lying on
the ground.  Typically these off-the-shelf baluns
weigh in at almost one pound.

We soon realized that this was equivalent to
requiring the kite to lift an additional 50 feet of
RG-8x coaxial cable or 100 feet of RG-58/U cable
into the air. Therefore, an early task for our kite-
antenna weight reduction activity was to design
and build a light weight ferrite based common
mode choke to replace the heavy commercial
balun. To accomplish this, tiny ferrite beads are
used that are a close slip fit over the coax cable
braid.  A stack of these cores can produce the
same effect as the larger commercial balun but at
a much lower total weight. To go with this light
weight choke design, a 10 inch long, 1 inch
diameter case was designed that weighs only one
ounce.

This new light weight choke can be used at either
to the center of a vertical half wave center fed
antenna design or the 12 foot delta kite which
carries the horizontal dipole. The only
compromise that was made in this light weight
choke is that it cannot be used on the 80 or 160
meter bands – which is no problem for any of the
half wave center fed kite flown antenna dipoles
that we plan to use. The result of this effort was a
choke design that weighed slightly less than three
ounces but still had the equivalent electrical
performance of the stock full size balun.

Our center fed vertical half wave antenna designs
also uses a snap-on ferrite core that is positioned
at ¼ wave down the coaxial cable from the
antenna. Folklore says that this core has the
effect of placing the equivalent of a 100 ohm or
greater resistor (depending on the core used)  in
series with the unbalanced current flowing in the
cables braid, thereby minimizing the coupling
between the feed cable and the lower leg of the
vertical dipole (which hangs down within a couple
feet of the feed line coax.

Snap-on cores can be applied at several places
along the coaxial cable where it runs in parallel
with the lower dipole leg. If this additional weight is
acceptable, the additional "decoupling" effect can
sometimes be beneficial.

Our actual measurements while the kite-antenna
is in the air, show that the addition of one or more
snap-on ferrite core has the effect of lowering the
rectance and stabilizing the SWR while the kite is
flying.
ABOUT BALUNS AND ISOLATING CORES
The photo above show how this component has
evolved as we searched for ways to reduce the
overall weight of our kite-antennas.

The large balun on the left shows the commercial unit
that we used on our first kite-antenna. Even though it
weighed almost one pound, this was not a significant
problem at the time since our first antennas were
carried on a 12 foot delta kite that provided lots of lift.

The choke design in the middle of the photo is our
first attempt at reducing the weight of the balun
function. This choke uses a number of small ferrite
beads that fit tightly around the feed  line coaxial
cable. When combined with a thin wall PVC tube from
our local Home Depot store, the weight came to a little
less than three ounces. This choke design
immediately became the "balun" of choice for both our
horizontal and vertical half wave dipole kite-antennas.

Finally, we built a choke element (pictured on the
right) that uses nothing more than a ferrite core
placed over the coaxial cable at the point where the
cable is attached to the center of the dipole. This
bare bones design works particularly well when
applied to our center fed half wave vertical antenna
designs. Using a single ferrite in this way adds less
than one ounce to the antenna weight.

As a result of using this latest design concept, the
total weight for a complete 20 meter half wave center
fed vertical dipole antenna comes to only three
ounces! When flying this antenna with it's center at 50
feet, and using our lightest weight coaxial cable feed
line, the kite only has to lift one pound of weight.

Our best result so far using this efficient but light
weight kite-antenna is a contact with CN2A in Morocco
(6923 miles) using a 10 watt KX-2 QRP radio.
THE "BALUN" EVOLUTION AT W6KOA
Commercial 1:1balun    3 oz choke  1 oz choke   
Last updated June 6, 2017
THE FINISHED FERRITE BASED COMMON MODE CHOKE IS PICTURED HERE WITH IT'S COMPONENTS
The case for this common mode choke is fabricated from a piece of thin wall PVC tubing that was found in
the plumbing department at our local Home Depot. Two brass machine screws are provided at the top end
of the case for attaching the antenna dipole elements. The interior wiring for both the resistors and coaxial
cable is soldered to these screws thereby eliminating the chance for a loose connection. Close fitting
wooden plugs are epoxied into the tube ends to seal the case. The wooden plugs are additionally secured
to the case at each end using two small sheet metal screws. An eye bolt, held in place with epoxy and a hex
nut, provides an attachment point at the top of the unit. A string of five series connected 2 watt 390 ohm
Allen Bradley carbon resistors are connected between the two brass terminals with a short length of hookup
wire. These resistors are positioned parallel to the cable/ferrite bead assembly inside the case when it is
assembled. The outer covering of the RG-58/U coaxial cable is removed so that the 25 small ferrite beads
can be slid over the braid. Two layers of Scotch tape are first wrapped tightly over the braid in order to
insulate the cores from the copper braid. A small amount of braid below the end of the core stack is exposed
so that the wooden bottom plug can be epoxied to the cable braid. This supports the weight of the coaxial
cable that protrudes beyond the choke assembly (below the choke when the choke is used with a
kite-antenna). The cable end and resistor interconnecting leads are trimmed and dresses as needed so that
they can be attached (soldered) to the two brass screws before final assembly. Sufficient spacing should be
maintained between the components when the unit is assembled. When the internal assembly is completed,
it can be slid into the case from the bottom end and the pre-soldered screws can then be pulled through the
holes at top of the case.