TECNICHAL DATA ABOUT BALUN
We can build a balun in various ways:
choke balun (see picture 1) with ratio 1:1 : as we said, the easier way is a coil with 6-8 turns (diameter 20-30 cm for 14-30 mhz) of coaxial cable just below the dipole (or other balanced antenna) feed point; for low band add more turns or use a ferrite (like Amidon) wich has no inflence on I1 and I2.
collins balun (see picture 2) with ratio 1:1: the bandwidth is very wide; we have also a similar one but with ratio 4:1 (see picture 3);
transmission
line balun: a transmission line transformer is composed of two (or more) transmission
lines of X impedance connected between in series at one end and in parallel at the other
one; turning this line as a coil we obtain high impedance for RF;
the balun on picture 4 has a ratio of 1:1;
the balun on picture 5 has a ratio of 4:1;
the naluns on picture 6 have a ratio of 6,25:1
and 9:1;
the line length should be 1/4 lambda at the lower frequency.
We can use a ferrite (as Amidon) to make the size smaller.
bazooka
balun (see picture 7) with ratio 1:1: it's a
simple conductor pipe of 1/4 lambda length placed around the coaxial cable and short-cut
and the low-end; it has a small bandwitdh and usually it's used on VHF;
in VHF it's made with a metallic pipe, in HF it's made with metallic sheet wrapped around
the coaxial cable (es.:carta stagnola) and short-cut with a pin at the low end (short-cut
only the metallic sheet with the cable shield).
VHF balun (see picture 8) with ratio 4:1: it's very common in VHF. The length L should be exactly 1/2 lambda.
All above baluns can be made using ferrite to make the size smaller (see picture 9).
