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

Please comment :

Using a single feeder cable for both a 20-meter and a 40-meter inverted V antenna can introduce certain challenges

Bharatiya Amateur Radio

Here’s an analysis of the setup and some advice for optimizing performance:

Challenges with a Single-Feeder Cable

Impedance Matching:

Each antenna has its optimal impedance at its resonant frequency (20m or 40m band).
When connected to a single feeder cable, impedance mismatches can occur when transmitting or receiving outside the resonant frequency of either antenna, leading to power loss and reduced efficiency.

Coupling Between Antennas:

Since both antennas are connected to the same feeder, mutual coupling might cause interference. This can distort the radiation pattern and reduce efficiency.

SWR (Standing Wave Ratio):

A single feeder cable might result in a higher SWR on some frequencies, which can strain your transmitter and reduce power output.

Impact on Efficiency

Transmitting Efficiency:

Efficiency is highest when the antenna is resonant. If either antenna is not matched properly due to the shared feeder, transmitting power will be lost as heat in the tuner or mismatched feeder.

Receiving Efficiency:

While receiving, mismatched impedance can lead to signal loss or reduced sensitivity, particularly for weak signals.

Advice for Improvement

Use a Proper Balun:

Install a 1:1 or 4:1 balun at the feed point to improve impedance matching and reduce feedline radiation.

Antenna Switch or Trap Design:

Consider using an antenna switch or designing a trapped antenna system to select the appropriate antenna for the desired frequency band.

Separate Feedlines:

If possible, use separate feeder cables for each antenna. This avoids impedance and coupling issues.

Antenna Tuner:

Employ a good-quality wide-range antenna tuner at the transmitter end to handle mismatches and optimize performance.

Test SWR and Radiation Patterns:

Use an SWR meter or antenna analyzer to measure the SWR on both bands. If issues are observed, adjust the antenna lengths or feeder setup.

Consider Fan Dipole:

Replace the dual inverted V with a fan dipole design. This allows each band to have its resonant dipole element, with a single feeder cable, while minimizing interaction.

Conclusion

While a single feeder cable can work with compromises, separating the feedlines or using traps is generally better for maintaining high transmitting and receiving efficiency. Testing and fine-tuning will help you achieve optimal performance. If you have an SWR analyzer, use it to determine the actual efficiency of your current setup.