Monday, January 20, 2014

HF Antennas impedance matching.

HF Antennas impedance matching.

Antenna BALUNs and UNIONs
Generally a balun consists of two wires (primary and secondary) and a toroid core: it converts the electrical energy of the primary wire into a magnetic field. Depending on how the secondary wire is done, the magnetic field is converted back to an electric field..

Balun
A Balun is a device that joins a balanced line (one that has two conductors, with equal currents in opposite directions, such as a twisted pair cable) to an unbalanced line (one that has just one conductor and a ground, such as a coaxial cable). A balun is a type of transformer: it's used to convert an unbalanced signal to a balanced one or vice versa. Baluns isolate a transmission line and provide a balanced output.

In a Balun, one pair of terminals is balanced, that is, the currents are equal in magnitude and opposite in phase. The other pair of terminals is unbalanced; one side is connected to electrical ground and the other carries the signal.

The Baluns (transformer tipe) also translate impediment  differences. (450Ohm to 50m )

Some impedance matching guide lined.

  1. A (1 to 1) BALUN is mostly used with center taped Dipoles. (50 Ohm Antenna to 50 Ohm Coaxial cable)
  2. A (2 to 1) BALUN is mostly used with HF quads and loop Antennas (100 Ohm Antenna to 50 Ohm Coaxial cable)
  3. A (4 to 1) BALUN is mostly used with Off center dipoles, Windom and Sky wire loops 80m and 160m band  (200 Ohm Antenna to 50 Ohm Coaxial cable)
  4. A (6 to 1) BALUN is mostly used with Off center dipoles that is installed higher than about 25m. (300 Ohm Antenna to 50 Ohm Coaxial cable) you could also use 300 ohm robin cable and the mach it to 50 ohm.
  5. A (9  to 1) BALUN is mostly used with end fed long wire antennas.  (450 Ohm Antenna to 50 Ohm Coaxial cable) you could also use 450 ohm ladder line and the mach it to 50 ohm.

50 Ohm coaxial cable vs 450 Ohm or 600 Ohm ladder or window line?


Coaxial cable 50,75 or 92Ohm


Ladder feed line

The primary advantages of coax with respect to ladder line are
  1. Most transceivers are equipped with 50 ohm coax connectors, whereas using ladder line requires a Balun or balanced tuner
  2. Coax cable is not affected by nearby metal objects, unlike ladder line
  3. The impedance of coax doesn't change when it rains or snows, unlike ladder line
The primary advantages of ladder line with respect to coax are

  1. Lower loss at frequencies of 28 MHz and up words
  2. Easier to make connections
  3. Can drive a balanced antenna (e.g. a dipole) without a Balun