Balun Measurements

Why can't you find the specifications for the baluns on the market? The only specifications you'll see are the ratio, the power rating and a bandwidth that is more hoped for than reality. The power rating is also often ambiguous. We publish the specification for our baluns. We are proud of the products we make, and we're happy to give you the specifications that will help you select the correct balun for your project. Here are some measurements and characteristics necessary to generate a balun's specifications.

Winding inductance affects a balun's low frequency response, output balance, and isolation factor.

It is commonly accepted that winding reactance to load impedance = 5:1. At the RADIO WORKS, a 10:1 ratio is required for precision baluns.

Coefficient of coupling

To keep losses low and bandwidth wide, a very high coefficient of coupling is required. Current-type baluns have no tertiary windings and coupling can be as high as 100%.

To measure the coefficient of coupling:

  1. Voltage balun's tertiary winding must be disconnected.
  2. Measure inductance of the main winding
  3. Measure with tertiary winding open-circuited
  4. Measure with tertiary winding short-circuited.
  5. K2 = 1- Lsc/Loc
  6. K must be at least .98. The larger this number the better the tolerance to mismatch and the wider the bandwidth.

Core Saturation

A balun must be able to operate without saturating while high power is applied and the load is moderately mismatched. Core saturation can cause signal distortion, RFI, TVI, and balun failure.

The measurement technique for this specification requires applying increasingly higher power to a balun until the core saturates which is indicated by a change in the transmitted carrier as monitored on an oscilloscope.

Output balance

Balanced drive to the antenna is one reason for using a balun in the first place. Output balance should be excellent over a moderate range of mismatch.

To measure output balance, a signal generator is connected to the balun's input.

  1. Use a center-tapped matched load. The signal generator is set to the measurement frequency.
  2. Measure voltage from each output to ground. Use RF voltmeter or scope.
  3. A dual trace scope also will show any phase difference between the two outputs.

Alternate technique

  1. Place a center-tapped load resistor at output of the balun.
  2. Measure input impedance with suitable bridge with center-tap on resistor load grounded.
  3. Remove ground on load resistor- note change in input impedance. The smaller the change the better. In a well designed balun, this change will be less than 1%.

4. This test can be done with balun in case with all windings connected.

There are several other measurements that must be made. All of them require good lab equipment or high power transmitters. If you have access to the equipment, there are several books and articles that outline the various procedures. Balun design is interesting, but you soon learn that building a good balun is more than just winding a few turns of wire around any available core.

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