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The feedline is the link between your radio and your antenna. Its job is simple in concept — carry RF energy from the transmitter to the antenna, and from the antenna to the receiver — but the choice of feedline, how it is installed, and how well it is matched to the rest of the system can have a significant impact on station performance. A poor feedline installation can waste more of your signal than a mediocre antenna.
A transmission line is any conductor system designed to carry electromagnetic energy from one point to another with controlled characteristics. In amateur radio, the term feedline is used interchangeably with transmission line when referring to the cable between the radio and the antenna.
All transmission lines have a characteristic impedance (Z₀) — the impedance the line presents to a signal travelling along it. This impedance is determined by the physical construction of the line (conductor size, spacing, and dielectric material) and is constant along the line's length regardless of how long the line is. The most common characteristic impedances in amateur radio are 50 ohms (coaxial cable) and 300–600 ohms (open-wire and ladder line).
Coaxial cable ("coax") is by far the most widely used feedline in amateur radio. It consists of a central conductor surrounded by a dielectric insulator, which is surrounded by a braided or foil shield conductor, all enclosed in a protective outer jacket.
Coax is popular because it is self-shielding — the outer conductor contains the electromagnetic field inside the cable, so it can be routed near metal objects, run through conduits, and buried without significantly affecting its performance. It is also relatively easy to work with and available with a wide range of standard connectors.
Common coaxial cables for amateur radio:
| Cable type | Z₀ (ohms) | Outer diameter | Loss at 30 MHz (per 30 m / 100 ft) | Loss at 150 MHz (per 30 m / 100 ft) | Typical use |
|---|---|---|---|---|---|
| RG-174 | 50 | 2.8 mm (0.11 in) | ~3.3 dB | ~7.5 dB | Short jumpers only |
| RG-58 | 50 | 5 mm (0.20 in) | ~1.8 dB | ~4.2 dB | Short runs, QRP, receive |
| RG-8X | 50 | 6.1 mm (0.24 in) | ~1.5 dB | ~3.5 dB | Portable operations, moderate runs |
| RG-213 / RG-8 | 50 | 10.3 mm (0.41 in) | ~0.9 dB | ~2.1 dB | General HF/VHF station use |
| LMR-400 | 50 | 10.3 mm (0.41 in) | ~0.6 dB | ~1.4 dB | Lower-loss general purpose |
| LMR-600 | 50 | 14.9 mm (0.59 in) | ~0.4 dB | ~1.0 dB | Long runs, VHF/UHF |
| RG-62 | 93 | 6.1 mm (0.24 in) | ~1.2 dB | ~2.9 dB | Specialised matching |
| RG-11 | 75 | 10.3 mm (0.41 in) | ~0.7 dB | ~1.7 dB | CATV, some antenna matching |
Loss figures are approximate and vary by manufacturer. Always consult the specific cable's datasheet for accurate values.
Key points about coax:
The most important consideration when choosing coax is loss. Loss increases with frequency and cable length, and it increases significantly when the SWR on the line is high. For HF use with reasonable SWR, RG-213 or LMR-400 are excellent choices for typical run lengths. For VHF/UHF, where losses are higher, lower-loss cable like LMR-400 or LMR-600 becomes more important, especially for longer runs.
Thin cables like RG-58 and RG-174 are suitable for short jumper cables and portable use, but should be avoided for permanent station feedlines longer than a few metres, particularly at VHF and above.
Open-wire line (also called parallel wire, window line, or ladder line) consists of two parallel conductors held at a fixed spacing by periodic insulating spacers. It has a much higher characteristic impedance than coax — typically 300 to 600 ohms depending on conductor spacing and diameter.
The great advantage of open-wire line is its extremely low loss, even when operating at high SWR. This makes it an excellent choice for multi-band antenna systems where a single antenna is used on many bands with an antenna tuner. A centre-fed doublet fed with ladder line and a tuner is one of the most versatile and efficient antenna systems possible.
The disadvantage is that open-wire line is not self-shielding. It radiates if it runs near metal objects, if the spacing between conductors varies, or if it is not kept clear of the ground and other structures. It cannot be run through metal conduit, and routing it into the house typically requires more care than coax.
Common open-wire line types:
Hardline is coaxial cable with a solid or corrugated outer conductor (usually copper or aluminium) rather than a braided shield. It offers significantly lower loss than flexible coax of the same diameter but is rigid or semi-rigid and more difficult to install. Hardline is used in commercial installations and by amateurs who need the lowest possible loss for long runs or at VHF/UHF/microwave frequencies. Surplus commercial hardline can sometimes be obtained at reasonable cost.
For maximum power transfer and minimum loss, the feedline's characteristic impedance should match both the radio's output impedance and the antenna's feed point impedance. Most amateur equipment is designed for 50 ohms, so 50-ohm coax is the standard choice.
When the impedances do not match, some of the power is reflected back from the mismatch point, creating standing waves on the feedline. The severity of this mismatch is measured by the Standing Wave Ratio (SWR). A 1:1 SWR means a perfect match; higher values indicate increasing mismatch.
A moderate SWR (up to about 2:1 or 3:1) on coax causes relatively little additional loss at HF with quality cable. But high SWR on coax at VHF/UHF, or on long runs, can result in significant additional power lost as heat in the cable. This is where open-wire line shines — its inherently low loss means that even very high SWR values cause only modest additional loss.
Baluns and ununs are used to transform impedances and to transition between balanced lines (like ladder line and dipoles) and unbalanced lines (like coax).
The feedline must be terminated with connectors to attach to the radio and antenna. Common RF connectors in amateur radio include:
Connector installation quality matters. A poorly installed connector with exposed shield braid, a cold solder joint, or moisture inside is a source of loss and intermittent problems. Take the time to learn proper connector installation for your chosen cable types, and consider using weatherproofing (self-amalgamating tape or heat-shrink with sealant) on any outdoor connections.
Feedline loss is measured in decibels (dB). Every 3 dB of loss means half your power is being lost as heat in the feedline. Here is what that means in practice:
| Feedline loss | Power reaching antenna | Equivalent effect |
|---|---|---|
| 0.5 dB | 89% | Barely noticeable |
| 1 dB | 79% | Minimal impact |
| 2 dB | 63% | Noticeable but acceptable |
| 3 dB | 50% | Half your power is wasted |
| 6 dB | 25% | Three-quarters wasted — serious problem |
| 10 dB | 10% | Most power lost — feedline needs attention |
Loss increases with frequency, cable length, and SWR. The combination of these factors means that a feedline that works fine for HF may be unacceptable for the same length at UHF.
When calculating total system loss, remember that feedline loss affects both transmit and receive. A 3 dB feedline loss costs you 3 dB on transmit (half the power reaches the antenna) and 3 dB on receive (half the received signal reaches the radio) — a total of 6 dB effective loss in the communication link.
Proper feedline installation protects performance and extends the life of your cable:
The best feedline for your station depends on your specific situation:
In most cases, spending money on better feedline gives more performance improvement than spending the same money on a more expensive antenna. The feedline is the part of your station that literally throws power away as heat — minimising that waste is always a good investment.