Radcom Feb 2019 “cable balun” – a deeper look

This article is a continuing discussion of Radcom Feb 2019 “cable balun”.

The article Baluns in antenna systems explores some different dipole and feed line configurations and the effectiveness of common mode chokes at various locations on the feed line.

Models 1, 2 and 3 particularly show the effect of a quarter wave vertical common mode conductor grounded and isolated, and a half wave vertical common mode conductor grounded.

These illustrate that those common mode conductors can be viewed to some extent as a ‘single wire’ transmission line, and the impedance presented at the dipole feed point is low or high in keeping with simple transmission line analysis of a shorted or open line of quarter or half wave length.

The question then arises with the Radcom “cable balun”, does it behave similarly, to what extent does the folding of the conductor affect its quarter wave resonance.

NEC models

One way to explore this is to construct an NEC model of the structure and a reflection of itself.

Three quarter wavelength a side folded

Above is the serpentine structure of three quarter wavelength folded, and below it, a reflection of itself. The whole structure is fed in the middle and the impedance vs frequency charted.

Above, the impedance vs freq, both in cartesian and polar form are plotted and show a clear self resonance around 7.15MHz.

Three quarter wavelength a side straight

For comparison, a plain dipole of the same total wire length is modelled.

Above, the plain dipole of same total wire length.

Above, the impedance vs freq, both in cartesian and polar form are plotted and show a clear self resonance around 7.15MHz.

There is a little difference in the resistance plots as might be expected because it is mainly radiation resistance which is affected by the folding of the conductor. The reactance plots are very similar, and importantly, the resonant frequencies are almost equal.

To all intents and purposes, they are both radiating transmission lines of 3λ/4.

Cable balun

So there should be little surprise when the 3λ/4 “cable balun” is extended by a further λ/4 to make a total of λ and connected to ground, that a low impedance to common mode current is presented at the feed point.

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.

It does exactly what is predicted from a very simple analysis. If you follow the thinking, you can see that it is quite naive to think the “cable balun” alone effectively reduces common mode current in general.