Model 1115d - Isolation/ Choking 1:1 Balun, 1.5-54 MHz, 5kW

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  • Model 1115d - Isolation/ Choking 1:1 Balun, 1.5-54 MHz, 5kW
  • Model 1115d - Isolation/ Choking 1:1 Balun, 1.5-54 MHz, 5kW
  • Model 1115d - Isolation/ Choking 1:1 Balun, 1.5-54 MHz, 5kW
  • Model 1115d - Isolation/ Choking 1:1 Balun, 1.5-54 MHz, 5kW
  • Model 1115d - Isolation/ Choking 1:1 Balun, 1.5-54 MHz, 5kW
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Building on the original 1:1 design by Dr. Jerry Sevick, this balun has two (2) low permeability cores in a stacked configuration to maximize choking impedance on HF bands from 7- 54MHz. By utilizing Mil-Spec coax, the SWR, transformation and power handling are excellent while providing a choking impedance that exceeds 10,000 ohms. Even on 80m the choking impedance is over 2,000 ohms.  If you have RFI primarily from the lower to middle frequencies or operate more on the low bands (160m-40m) you should consider the model 1116d for enhanced suppression and isolation on these bands.

N connectors and mounting options are available in "Accessories & Connectors" section of the website

Sometimes called a choke balun or common mode choke, this 1:1 ratio current balun is the best for feedline isolation and choking. It has the widest operating frequency range, lowest core stress and provides the best overall balance of any balun for given cost, size, and weight. Please click on the graph shown below to see the complete results.

model-1115d-choking-thumb.jpg  model-1115d-thumb.jpg
Click to Enlarge
Tests performed with AIM-4170c Network Analyzer and precision 50 ohm non inductive load.


Regardless of the antenna you're using, an isolation balun can provide numerous advantages

    • Prevents unwanted RFI by eliminating feedline common mode currents and radiation
    • All power goes to the antenna, improving efficiency
    • Reduces noise or EMI picked-up by your coax shield
    • Power is balanced between driven elements of antenna
    • Can help overcome a less than optimal ground


It must be pointed out that a 1:1 balun should never be used on the second harmonic of a half-wave center-fed dipole fed with coax (like an 80 meter dipole being used on 40 meters). The impedance can be as great as 5,000 ohms creating very high voltages which can bring about voltage breakdown and/or excessive heating. This exception ONLY applies to Coax Fed HALF WAVE CENTER FED DIPOLES WHEN USING A 1:1 BALUN AT THE FEEDPOINT.  This caution does not apply to fan dipoles or any dipole that utilize loading coils to create resonance on the bands in use.



 Core Material  Custom mix low permeability ferrite by Fair Rite Products for maximum bandwidth, lowest loss and highest efficiency.
  Large 2.4 inch cores are coated and sealed for long term durability.
 Winding Type   Stacked dual core, coaxial wound 1:1 current balun provides larger effective core area.
 Winding Material  Mil Spec 50 ohm coax rated 19kW @ 1MHz, 9kW @ 10 Mhz and 4kW @50MHz. 
 Silver flashed braid and center conductor with solid Teflon dielectric.
 Power Rating  1 to 35 MHz - 5kW, 10kW intermittent. 35 to 54 MHz - 5kW.  
 All ratings at resonance. High SWR will reduce power handling ability.
 Useable Frequency  1 to 54 MHz
 Insertion Loss Less than .1db
 Connectors  SO-239 connectors are gold center conductor with Teflon insulation.
 Studs and eyebolts are 1/4 inch.
 Alternate connectors and Mounting Options are available in the Accessories section.
 Hardware  All Stainless Steel
  Enclosure Type
  NEMA rated 4x marine grade junction box for outdoor installations.
  Cover utilizes integral neoprene gasket for weatherproof integrity.
 Dimensions  4x4x2 inches for main body of unit
 Dimensional Drawing of Standard Enclosure
 Additional Info  Very high efficiency. 
 Will not heat up or saturate at like many of the "less expensive" designs.

Installation Notes


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Warranty Information

Noise Reduction

It is a seldom appreciated fact that feedlines which are not adequately decoupled can act as efficient vertical antennas that degrade an otherwise excellent radiation pattern. The addition of a quality feedline isolation ("choke") balun can significantly reduce feedline radiation and dramatically decrease RFI and TVI. It does this by providing the antenna with balanced current at the feedpoint and by effectively preventing the coax shield from acting as part of the antenna.

While the most common advice is to improve the station's RF ground, the root of the problem is in the poor isolation of the feedline from antenna currents. If your goal is to reduce feedline radiation and improve reception by reducing noise, feedline isolation baluns are an excellent choice. Adding an additonal isolation balun at the point where the feedline exits the near field area of the antenna will substantially reduce unwanted feedline radiation and reception of EMI without the need for improved station grounding. 

If your antenna SWR is already low and you wish to reduce feedline radiation and improve reception, a feedline isolation balun is recommended. Adding one at the base of a vertical antenna will substantially reduce unwanted feedline radiation (RFI) by preventing your antenna from using the your coax feedline as a radial. This can also reduce the need for improved station grounding.

Benefits to Vertical Antennas

When quarter-wave antennas are constructed over a near perfect radial system, they have a feedpoint impedance of about 36 ohms. When they are constructed over a less than optimal radial-system there is a loss introduced into the feed system that adds to the 36-ohm figure. This improves the SWR but there is a loss in the efficiency of the antenna, signals transmitted and received have a higher take-off angle and often there is current introduced onto the feedline. 

With my SteppIR vertical I use two of these baluns. One at the base of the antenna and another midway along the feedline after the near field of the antenna. You can also install the balun where it enters your shack if you would rather not break the feedline run. The balun balances the RF currents at the antenna, is terrific for keeping RF out of your operating environment and greatly reduces the noise inherent in a vertical by decoupling the feedline. 

With a ground-mounted quarter-wave vertical, regardless of the radial situation, but especially with poor radial systems, there is the need for a feedline isolation balun to keep common mode currents off the feedline. 

Benefits to Yagis

Yagi antennas especially, benefit from improved balanced drive and superior feedline isolation, but even simple dipoles benefit from properly selected and installed Feedline Isolation baluns. In addition, receiver noise may also be reduced by eliminating stray EMI picked up by the coax shield.

Another very useful application for a feedline isolator is installing them in series with a yagi antenna’s normal feed system. The proper location is between the antenna’s matching device/system and the feedline. Doing this will not affect the antenna and prevents the feedline from acting as part of the antenna. In beam installations, using a feedline isolator in series with the antenna’s feed system can substantially improve the antenna’s front-to-back and front-to-side ratios.

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