How to Build a 1:1 Balun for an HF
Inverted V Antenna
A 1:1 balun (balanced-to-unbalanced transformer) is
an essential component when connecting a coaxial cable (unbalanced) to an HF
inverted V antenna (balanced). It minimizes RF current on the coaxial shield,
reducing interference and ensuring efficient antenna performance. Here's a
step-by-step guide to constructing a 1:1 balun suitable for HF bands.
Materials Required
- Toroidal
Core: A ferrite core with a material suited for HF, such as an
FT-240-43.
- Enamel-Coated
Copper Wire: Two 18-gauge wires (approx. 3-4 meters each).
- Plastic
Enclosure: Weatherproof box to house the balun.
- SO-239
Connector: For coaxial cable connection.
- Binding
Posts or Eyelets: For connecting antenna legs.
- Heat-Shrink
Tubing: To insulate wires.
- Cable
Ties: For organizing wires.
- Epoxy
or Hot Glue: To secure the components.
- Screwdriver
and Soldering Kit: For assembly.
Steps to Build the Balun
- Prepare
the Core
- Clean
the toroidal core with a dry cloth.
- Cut
two equal lengths of enamel-coated wire, each about 3 meters long. These
wires will form the bifilar winding.
- Wind
the Toroid
- Twist
the two wires together for better manageability (optional).
- Wrap
the twisted wires around the core uniformly, forming 10-12 turns. Ensure
the turns are tight and evenly spaced to minimize inductive loss.
- Leave
about 10 cm of wire at each end for connections.
- Strip
and Solder the Wires
- Strip
the enamel coating from the ends of the wires using sandpaper or a wire
stripper.
- Test
the continuity with a multimeter to ensure good electrical contact.
- Connect
the Wires to the SO-239 Connector
- Solder
one end of the wire pair to the SO-239 connector. One wire goes to the
center pin (unbalanced), and the other goes to the connector body
(ground).
- Ensure
the connections are firm and insulated with heat-shrink tubing.
- Create
the Balanced Output
- At
the other end of the wire pair, attach binding posts or eyelets. These
will serve as connection points for the two arms of the inverted V
antenna.
- Encapsulation
and Mounting
- Place
the toroid and connections inside the plastic enclosure. Secure them with
epoxy or hot glue to prevent movement.
- Drill
holes in the enclosure for the SO-239 connector, binding posts, and
mounting bolts.
- Seal
the enclosure with screws and weatherproof it with silicone sealant to
protect against moisture.
- Test
the Balun
- Connect
the balun to a dummy load and use an antenna analyzer or SWR meter to
ensure it functions correctly.
- Check
for minimal SWR across the desired HF bands and adjust as needed.
Installation Tips
- Mount
the balun at the feed point of the inverted V antenna, ideally at the
apex.
- Ensure
the antenna arms are symmetrically deployed for balanced operation.
- Use a good-quality coaxial cable to connect the balun to your transceiver, and include a choke near the feed point to further reduce RF on the shield.
Advantages of a DIY 1:1 Balun
- Cost-Effective:
Building your own balun is significantly cheaper than purchasing a
commercial unit.
- Customization:
You can tailor the balun to your specific HF band requirements.
- Learning
Experience: Constructing a balun helps deepen your understanding of RF
principles.
With this 1:1 balun, your HF inverted V antenna will achieve
optimal performance, ensuring clear and efficient communication on the bands.
Enjoy building and experimenting!
Dr. Prudhvi Raju Kakani - VU2IKY
How to build 1: 1 balun
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