BALUNs and LINE ISOLATORs
To keep in mind.
• When you directly feed a symmetrical antenna with a coaxial cable, there is a good chance that you experience problems with common-mode currents.
• Common-mode currents (sheath currents) flow on the outside of the outer shielding of the coaxial cable.
• During transmission these unwanted currents cause interference (RF ingress and feedback) anywhere along the path of the coaxial cable; during reception they couple noise into the wanted signal path.
• Furthermore, the radiation pattern of the antenna is disturbed. The Front-to-Back ratio can be considerable reduced. It behaves like you would superimpose the pattern of a random omnidirectional antenna to your directional pattern.
• A voltage type balun in the feedpoint only solves the problem when the antenna is perfectly symmetrical. (Which is seldom the case).
• A current balun always works; it acts according to the “Current Forcing” principle.
• A current balun can be considered “adequately working”, as soon as common-mode choke values reach 1000 Ohms. (More is better). For power levels in excess of 1000 watts, the CMC impedance value definitely needs to be higher.
• Coiled coax cable current baluns are much more narrow-banded than ferrite core based baluns, and are much more subject to detuning by environmental effects.
• A single ferrite sleeve, or even a few of them, have no effect on the HF frequencies. (Only on VHF/UHF).
• Only from about 10 sleeves on, some choking or balancing effect is noticeable, roughly above 14 MHz. Ferrite bead or sleeve loaded coax solutions are surpassed. Often the choking impedance obtained is too low to be effective on HF. (Apply on the highest HF bands and 50 MHz only).
• A large ferrite torroid with 10 windings of 5 / 6 mm coaxial cable reaches the same choke impedance as 10² (or 100 ! ) single sleeves of the same permeability. This would equal some 2 meters or more of coaxial cable stuffed with sleeves. (Heavy and large).
• Common-mode type current baluns or line isolators can be considered as RX noise reducing when their value rises above roughly 2000 – 3000 Ohms. (Also here: more = better)
• The imaginary part of the common-mode impedance of a current balun can be cancelled (to some extent) by a complementary common-mode impedance of the coaxial cable connected. This depends on the cable length and some environmental variables). For that reason it is very important that the resistive part of the impedance is as large as possible.
• When a current balun at the antenna feedpoint alone is not sufficient to remove all common-mode current effects, a line isolator can be added.
• This device is preferably installed at the base of the tower, or at the entrance of the house, near the ground and with 1 side connected to a good clean RF earth.
• Antennas (e.g. verticals) with an un-symmetrical feedpoint can also benefit from an UNUN line isolator. Here the line isolator prevents the coax from acting as a radial, thus potentially feeding RF back to the radio room.
• Other (non coaxial) cables installed near the antenna (rotator or remote antenna switch control cables) can pick up common mode currents and feed them back to the shack as well. These cables should be choked by ferrites too.
• Iron powder cores are not suitable for construction of CMC based baluns . Ferrite is.
• Not all ferrite is equal. We can distinguish ferrite materials for the lower HF bands, for the general HF spectrum or for the VHF/UHF spectrum.
Beware of counterfeit ferrite materials ! !