Testing a balun in isolation

Question

Consider a microwave RF source going to an unbalanced coaxial cable, then using a 1:1 balun to feed a dipole/Yagi style antenna. How can I test this 1:1 balun in isolation, independent of the type of balun used?

The following diagram shows a proposed test mechanism:

Case 1: Measure the RF power outputted from the coaxial cable, suppose this is x dBm.

Case 2: Apply a balun to turn coaxial cable into 2 halves of a balanced line. Use the coaxial ground to create 2 short transmission lines and feed these into a combiner. If the balun is working as designed, summing the balanced line signals should cancel them out, and y dBm will be at the noise floor. Otherwise I should be able to measure how close to balanced the line is by comparing how low y dBm is relative to x dBm.

At microwave frequencies I expect I will need to use phase-matched cables (or at least the same length of cable for UHF and below) for the 2 paths following the balun.

Does this testing mechanism make sense? Is there a better way to test the function of a generic 1:1 balun?

Answer 1

Your proposed setup only measures the insertion loss. If that's what you are interested in measuring, you could do what you proposed or use two copies of the same balun connected back-to-back and split the total insertion loss. You could do the latter using another balun of known characteristics.

However, when we talk about performance of a balun, we are more interested in the balanced end being actually balanced. If it's a voltage balun, instead of a combiner, you can measure the power at two ports and look at the imbalance. In lower frequencies like VHF and HF, it is also common to use a tapped load (R1+R2 series with the intermediate point grounded where R1 + R2 = 50) and measure the return loss at various tap points. A 1:1 current balun (Guanella transformer) should give fairly invariant measurements regardless of the tap, as long as the magnetizing impedance of the winding is very high. At a microwave frequency, a 1:1 Guanella transformer is typically implemented with electrically 1/4-wavelength differential striplines or coplanar waveguides.