Above is a diagram of the so-called “cable balun”.
My evaluation essentially showed that it was not effective in an example practical scenario where one might want to use a balun, and that of itself, it was not likely to significantly reduce common mode current in most scenarios.
Radcom Mar 2020 published a letter in “The last word” from the author defending the device citing a NEC model of one scenario, curiously though without explanation, a different topology to the diagram above from the original article. Note also that it is a structure in free space with no discussion of how that is relevant to real world antennas near ground.
The author states it is an NEC4 model, and I have run it in NEC-4.2 but I doubt that it will give significantly different results in NEC-2.
I have made two changes to the model:
- remove the RP card as it is supplied by 4NEC2 (no effect on the model); and
- reversed the direction of wire 1 to be consistent with wire 2 for more understandable graphics.
Here is the retyped NEC deck.
CM Typed from Radcom March 2020 pages 97-98. K3MT Letter CM RSGB CABLE BALUN STUDY CM AUTHOR: K3MT@JOKALYMPRESS.COM CM 40 M DIPOLE IN FREE SPACEH CM 20m LENGTH 5MM DIAMETER CM CM THE DIPOLE CM THE BALUN CM THE HALF WAVE FEEDLINE CM NO GROUND - FREE SPACE CM CM VOLTAGE SOURCE 2V AT CENTER OF DIPOLE CM 6700 kHz CM RADIATION PATTERN CE GW 1 48 -10.2 0 10.0 0 0 10.0 0.005 GW 2 52 0 0 10.0 10.5 0 10.0 0.005 GW 3 25 0 0 10.0 0 5.4 10.0 0.005 GW 4 25 0 0 9.5 0 5.4 9.5 0.005 GW 5 5 0 5.4 10.0 0 5.4 9.5 0.005 GW 6 100 0 0 9.5 0 0 -10.0 0.005 GE 0 GN -1 EK EX 0 1 48 0 2.0 0 0 FR 0 0 0 0 6.700 0 EN
Above is a plot of the current distribution of the model given by K3MT to demonstrate the balun performance.
K3MT’s model with longer vertical conductor
To discover whether the common mode current is well controlled by K3MT’s balun, lets extend the vertical conductor (modelling the feed line common mode current path).
Above is a plot of the current distribution of the revised model, it is clear that common mode current on the vertical conductor has increased significantly.
Above is a slice of the antenna pattern which shows significant distortion due to the asymmetry of the antenna system.
K3MT’s model with shorter vertical conductor
If the common mode current is sensitive to vertical conductor length, let’s try a little shorter conductor.
Above is a plot of the current distribution of the revised model, common mode current is significantly lower.
Comparing figures 4 and 6 which show a marked variation in common mode current caused by changing the linear length of conductor from feed point to lowest extremity from 3λ/4 to λ/2 hints that the common mode current may be simply responsive to that length and the nature of termination (ie open end or grounded), and that K3MT’s folded section doesn’t of itself control common mode current.
K3MT’s balun in his demonstration scenario has not isolated the nominal radiator (the flat top dipole) from the influence of the common mode feed line conductor.
The model results are applicable to the structure in free space and not directly extensible to an antenna system near ground, much less a grounded transmitter.