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Balance vs unbalanced antenna
- Thread starter Se7en
- Start date
In a most general sense:
Basically (in most but not all cases) a balanced antenna is one where the elements are fed and are of equal length on both sides such as a dipole antenna. Balanced antennas can be fed with open wire transmission line or with coax with a Balun. One of the exceptions is the OCF dipole. It is one of those antennas that has the balance feed point shifted away from the center. A dipole has a nominal impedance value of 73 ohm (it can be fed with 75 ohm balanced line, directly with 75 ohm/50 ohm coax or with cox and a 1:1 Balun), but when the fed point is shifted away from the middle the impedance value starts going up. The 14% point is somewhere between 300 and 200 ohms, which is why you will see some use a 4:1 Balun or a 6:1 Balun.
Whereas an unbalanced antenna has one element being fed such as a ground plane, or a coaxial dipole. A HF vertical antenna using radials is definitely an unbalanced antenna and can be fed directly with coax or through a 1:1 Unun. Many multi-band vertical antennas will be fed with a 4:1 Unun to help the antenna tuner find a low impedance point over a wide frequency range.
If you go through the ARRL Antenna Handbooks, or most any other antenna handbooks you will generally find the same results/conclusions.
Basically (in most but not all cases) a balanced antenna is one where the elements are fed and are of equal length on both sides such as a dipole antenna. Balanced antennas can be fed with open wire transmission line or with coax with a Balun. One of the exceptions is the OCF dipole. It is one of those antennas that has the balance feed point shifted away from the center. A dipole has a nominal impedance value of 73 ohm (it can be fed with 75 ohm balanced line, directly with 75 ohm/50 ohm coax or with cox and a 1:1 Balun), but when the fed point is shifted away from the middle the impedance value starts going up. The 14% point is somewhere between 300 and 200 ohms, which is why you will see some use a 4:1 Balun or a 6:1 Balun.
Whereas an unbalanced antenna has one element being fed such as a ground plane, or a coaxial dipole. A HF vertical antenna using radials is definitely an unbalanced antenna and can be fed directly with coax or through a 1:1 Unun. Many multi-band vertical antennas will be fed with a 4:1 Unun to help the antenna tuner find a low impedance point over a wide frequency range.
If you go through the ARRL Antenna Handbooks, or most any other antenna handbooks you will generally find the same results/conclusions.
NQR
I am including any baluns/ununs as part of the antenna SYSTEM.
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the current known as the common mode current. It creates an external field from an apparently symmetrical balanced line (cf Figure 1c), creating losses and reducing the noise immunity of the balanced line.
I think of a balanced antenna as having the same impedance for both sides of the circuit. So both the hot & ground side are connected to the same kind of thing. A center fed dipole or a full wave loop are balanced antennas. An off-center fed dipole or a long wire are unbalanced antennas.
Balanced feed-line is twin-lead, or ladder/window line. Coax is un-balanced feed line. The problem with coax is that RF can get on the outside of the coax shield thus to RF the coax can look like three different wires, the center wire, the inside of the shield & the outside of the shield. We do NOT want RF to get on the outside of the coax shield since this is how RF gets back into the shack.
Balanced feed-line is twin-lead, or ladder/window line. Coax is un-balanced feed line. The problem with coax is that RF can get on the outside of the coax shield thus to RF the coax can look like three different wires, the center wire, the inside of the shield & the outside of the shield. We do NOT want RF to get on the outside of the coax shield since this is how RF gets back into the shack.
Basically a balanced antenna is any antenna is not ground referenced meaning they do not depend of a ground connection to return RF currents back to the feedpoint. Dipoles, quad loops, delta loops etc are all balanced antennas while monopoles, groundplanes, mobile antennas etc are unbalanced. Even single longwires are unbalanced as they require a connection to ground in order to function properly.
Wrong. A gamma fed yagi is unbalanced and none of the conditions you mention apply. What you describe is more like RFI problems which can occur with any antenna type.
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