Many hams when changing to work on the air symmetric antennas - dipoles, squares, etc., - nourishing them unbalanced coaxial cable in these conditions, the cable is not fully manifests its screening properties: during transmission, it radiates energy, and in receive mode receives signals (and noise) from all directions. In order to eliminate this effect, apply balun devices that connect between the cable and the antenna. English is a device called a BALUN from words "ВALanced-UNba lanced out", i.e. symmetric - asymmetric. There are many types of balun One of them, is quite simple and effective described V. Galneryus (UP2NV):
"...The design of the balun transformer is extremely simple. The main feeder is connected segment, the same coaxial cable length of 1 - 1.5 m (see figure), and bi-part feeder is wound on large size ferrite ring. If one ring is bi-part feeder does not fit, can be folded together several rings of the same size. It is possible to use ferrite rings all brands of magnetically soft ferrites with permeability 400.. 2000".
As reported UP2NV, testing of such balun device showed that it works well in the frequency range from 2 to 30 MHz. Note that this device does not change resistance is a transformer with a 1:1 ratio), i.e. it can be connected to your existing symmetric antennas, without changing anything in their design.
Author: W. Galneryus (UP2NV); Publication: www.cxem.net