Radcom Feb 2019 “cable balun”

A correspondent asked for an explanation of a novel balun described in Radcom Feb 2019 by K3MT.

Above is a diagram of the so-called “cable balun”.

To evaluate it, I have inserted it into one of the NEC models used for the article Baluns in antenna systems, the model used for Model 3 which I will repeat here for convenience.

Model 3

The key characteristics of Model 3 are:

  • half wave dipole for the 40m band;
  • 20m height (λ/2) above average ground;
  • feed point is 5% off centre to expose the model to some practical outcomes of less than perfect symmetry;
  • feed line is coax, and it runs from a physically small transmitter located at ground height directly up to the feed point;
  • lower end of the feed line and transmitter are effectively grounded; and
  • no discrete balun or common mode choke.

Above is the current distribution diagram from the model, the dark green curve is the magnitude of current. Note the common mode current on the vertical feed line, its magnitude relative to that on the dipole elements is relevant.

Cable balun

The model was changed to insert the cable balun, all other details remain the same.

Above is the current distribution diagram from the model, the dark green curve is the magnitude of current. Note the common mode current on the vertical feed line, its magnitude relative to that on the dipole elements is relevant.

The common mode current on the lower half of the feed line can easily be seen to be very similar to the case of Model 3. It is a little harder to evaluate the common mode component on the upper half of the feed line as it is the sum of the complex currents in each of the three parallel conductors, but it will resolve to a common mode current very similar to that of Model 3.

The cable balun has not significantly reduced the common mode current in this scenario. If there are scenarios in which the cable balun reduces common mode current significantly, it is for reasons other than simply the dimensions and geometry set out in the article.

Conclusions

The so-called “Cable Balun” is presented without any evidence that it effectively reduces common mode current generally, or in any specific scenarios, nor is a dredible detailed explanation given of its mode of operation.

It did not significantly reduced common mode current in a NEC model scenario of a practical dipole that should demonstrate its effectiveness.

Further discussion at Radcom Feb 2019 “cable balun” – a deeper look.