A balanced feed line is a transmission line in which the two conductors carry equal currents in opposite directions, and neither conductor is connected to ground. The electromagnetic fields are balanced around the pair of conductors.
Common examples include:
- Open-wire line (two parallel wires separated by spacers)
- Twin-lead (flat ribbon cable with two conductors)
- Ladder line (similar to open-wire but with insulating webbing)
In radio astronomy, balanced feed lines are often used with dipole antennas, Yagi antennas, and some low-frequency receiving arrays because they have very low loss and are less affected by nearby objects when installed correctly.
Why is it called “balanced”?
If one conductor carries +1 amp, the other carries −1 amp. The currents are equal and opposite:
As a result:
- Radiation from the feed line largely cancels.
- Less unwanted pickup of local electrical noise.
- The feed line itself contributes less to the antenna pattern.
Balanced vs. coaxial cable
| Balanced line | Coaxial cable |
|---|---|
| Two equal conductors | Centre conductor plus shield |
| Neither side grounded | Shield usually grounded |
| Very low loss | Higher loss, especially at higher frequencies |
| Can radiate if imbalance occurs | Well shielded from external signals |
| Often 300–600 Ω | Usually 50 Ω or 75 Ω |
Coaxial cable is unbalanced, because the outer shield and inner conductor have different roles.
In radio astronomy
For low-frequency arrays (such as those used for solar, Jupiter, or galactic observations), balanced feed lines can be advantageous because:
- Losses are extremely low.
- Large dipole arrays naturally have balanced outputs.
- They can provide good common-mode noise rejection.
However, most modern radio astronomy receivers, SDRs, LNAs, and spectrum analysers use 50 Ω coaxial inputs, so a balun (BALanced-to-UNbalanced transformer) is often placed between the antenna and the receiver.
For example, if you use a simple half-wave dipole for observing the 1420 MHz hydrogen line, the dipole is a balanced antenna but your SDRplay receiver input is unbalanced 50 Ω coax, so a balun at the feed point is generally recommended.
Your antenna project
From your earlier description of replacing the dish with an antenna consisting of two horizontal wires joined in the middle, supported by poles at each end, with a single line dropping from the centre to the receiver, that arrangement resembles a centre-fed balanced wire antenna. If the two horizontal wires are symmetrical and fed at the centre, the antenna itself is balanced. Whether the feed line is balanced depends on what cable runs down to the computer:
- Twin-lead, ladder line, or open-wire → balanced feed.
- Coaxial cable → unbalanced feed (usually with a balun at the feed point).