Voltage or Current Balun for Dipole

Questions & Answers
1

The balun failed on my 40/75 meter dipole antenna. I like to simply swap out the balun and replace with a new balun. Should I select a voltage or current balun for a fan dipole antenna I have cut for 2 bands 40 and 75 meters

2

John, what feedline are you using? Balanced (open line) or unbalanced (coax)? If you are trying to balance out a open wire feedline with high impedance to match a 50 ohm antenna you want a voltage balun. For RF purposes us a current balun.

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3

John,

For your use you probably need a current balun if you are going from twin lead to coax for your antennas. Current baluns should be used wherever possible. Current baluns provide better balance and lower loss. I assume you are not running a linear, so I recommend the 4:1 Intellitron AA-3913 or the LDG RBA- 4:1. I use the LDG on my multiband dipole to transition from the twin lead feed line to the coax going to my radio. As you know, BALUN means “balanced to unbalunced”; generally used to transition from coax (from your radio) to twin lead going to the antenna. This is particularly necessary if you have the twin lead as a “radiator” along with the antenna. If you are using coax only, then a voltage balun may suffice. If you could provide a little more information, we could understand the issue a little better.

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4

I am sorry I did not provide that other very important information :slight_smile: The feed line is about 85 ft 50Ω coax. Thank you

5

John, Thanks for the follow up. Henry has your back! His instructions are above. Enjoy!

6

For a 40/75 meter fan dipole antenna, you should select a 1:1 current balun.

Here is why a current balun is preferred over a voltage balun for this application:

  • Function: A current balun is designed to force equal and opposite currents into the two halves of the balanced dipole antenna, regardless of the load impedance presented at each terminal. This is crucial for maintaining the intended radiation pattern and preventing the feedline (coaxial cable) from becoming part of the radiating antenna system, which causes common mode current (RF in the shack, interference, etc.).
  • Dipole Characteristics: A fan dipole is a balanced antenna, while the coaxial cable used to feed it is an unbalanced line. A balun (balanced-to-unbalanced) is needed to mediate this transition effectively.
  • Performance: Current baluns provide better overall balance and lower loss in real-world, non-ideal conditions. They tolerate load impedance variations better than voltage baluns, which is important for a multiband antenna like a fan dipole where the impedance can change across different frequencies.
  • Voltage Baluns: Voltage baluns work by attempting to force equal voltages at their output terminals. If the antenna load is not perfectly symmetrical (which is common in practical installations due to surroundings, ground, etc.), a voltage balun can lead to unequal currents and potential feedline radiation.

In summary, a 1:1 current balun provides the best performance and ensures the antenna operates as designed on both 40 and 75 meters.

Here is the one I use: Radiowavz B11-ISO 1:1 Choke Balun B11ISO | GigaParts.com

Thanks for giving us the chance to serve you!

7

Thank you Douglas

I will use the current balun with my fan dipole