Antenna Feed Systems: Coax, Ladder Line, Open Wire & Feed Point Methods

The feedline is the critical link between your radio and your antenna. Choose the wrong type, install it incorrectly, or ignore common-mode current and you lose power, introduce noise, and distort your antenna pattern. This guide covers every major feed system used in amateur radio — how each works, where each excels, and how to calculate real-world losses.

A feed system has one job: to transfer RF energy from the transmitter output to the antenna feed point — and from the antenna feed point back to the receiver input — with the minimum possible loss and the maximum possible isolation between wanted signal and unwanted common-mode current. Every feedline decision you make is a trade-off between loss, cost, bandwidth, handling of SWR, and physical installation constraints.

The feed system begins at the radio's SO-239 or N-type output connector and ends at the antenna feed point. Everything between those two points — coaxial cable, open-wire line, baluns, ununs, lightning arrestors, connectors, and weatherproofing — is part of the feed system. Each component contributes insertion loss, and each mechanical junction is a potential failure point.

Coaxial cable consists of a central conductor, a dielectric insulating layer, a braided or foil outer conductor (the shield), and a protective outer jacket. The characteristic impedance Z₀ is determined by the ratio of the shield diameter to the centre conductor diameter and the dielectric constant of the insulator. For amateur radio, the two standard impedances are 50 Ω (matches most transceivers and antennas directly) and 75 Ω (CATV standard, widely available as surplus, useful for specific matching applications).

Loss in coaxial cable has two primary mechanisms: conductor loss (I²R heating in the centre conductor and braid) and dielectric loss (molecular heating in the insulator). Conductor loss dominates at lower frequencies; dielectric loss becomes significant above 150 MHz. Both scale with the square root of frequency for conductor loss and linearly with frequency for dielectric loss, which is why the same coax that performs well on 40 m may be inadequate on 70 cm.

Coax connector quality matters enormously

A poorly fitted PL-259 connector introduces 0.1–0.5 dB of loss per connector at HF and substantially more at VHF/UHF. Water ingress at an outdoor connector corrodes the braid and centre conductor, causing the loss to climb over weeks and months — often invisibly until SWR suddenly rises or the feedline fails entirely. Use silver-plated connectors at HF, N-type at VHF and UHF, and weatherproof every outdoor joint with self-amalgamating tape over the connector body followed by a UV-resistant outer wrap.

Open-wire feedline is the oldest form of transmission line in radio — two parallel conductors separated by air or sparse insulating spacers. Its key advantage is extremely low loss even when operated at high SWR, because the dielectric is predominantly air and the conductors are large relative to the skin depth at HF. A 450 Ω window line typically has matched-line loss of 0.05–0.15 dB per 30 m at 14 MHz — five to ten times lower than RG-213 at the same frequency. More importantly, that loss stays low even at SWR of 5:1 or 10:1, making it the ideal feedline for multi-band wire antennas.

The operating principle is that the antenna does not need to be resonant on every band when fed with open wire. A centre-fed dipole of any convenient length, fed with 450 Ω ladder line into an ATU with a 4:1 or 1:1 balun, will provide workable multi-band operation on all HF bands. The ATU's job is to transform whatever impedance the open-wire line presents at its input — which varies enormously across bands — into 50 Ω for the radio. The open wire's low loss means that even large transformation ratios cost very little power.

Installation constraints for open-wire line

Open-wire line cannot touch metal objects — doing so alters the line's characteristic impedance at the contact point, creating an impedance discontinuity that reflects energy and degrades the match. It must be kept at least 100 mm (4 inches) from metal gutters, downpipes, conduit, and building frames. It cannot be run through conduit or buried in the ground. Where the line must pass through a wall, a dedicated through-wall insulator or a short section of coax at the point of entry (with appropriate matching) is used. These constraints make routing in urban installations challenging but not impossible.

A dipole is a balanced antenna: its two feed arms are symmetrical with respect to ground. Coaxial cable is an unbalanced feedline: the outer shield is at ground potential and the centre conductor carries the signal. When you connect unbalanced coax directly to a balanced dipole without a balun, the shield is no longer truly at ground potential at the antenna — it becomes part of the antenna. RF current flows on the outside of the coax braid, turning your feedline into an unintended antenna element.

The consequences of this common-mode current problem are serious and often overlooked:

• The feedline radiates, distorting the antenna's designed radiation pattern — often tilting the pattern in undesirable directions
• RF enters the shack on the outside of the coax, causing RF in the microphone, TVI, and interference with other equipment
• The SWR measurement at the radio reflects both the antenna impedance and the common-mode impedance of the cable, making tuning unreliable
• On receive, noise picked up by the feedline is delivered directly to the receiver input, raising the noise floor

The solution is a balun — a balance-to-unbalance transformer — at the antenna feed point. For most dipole applications, a 1:1 current balun (also called a choke balun) is the correct choice. It presents a high impedance to common-mode current while passing the differential signal through with minimal loss. See the Baluns & Chokes Guide for full construction details.

Centre-fed dipole — direct 50 Ω coax

A resonant half-wave dipole in free space presents approximately 73 Ω at the feed point. At typical heights above ground, this drops to 50–70 Ω, making a direct 50 Ω coax connection viable with SWR of 1.0–1.5:1. A 1:1 current balun at the feed point suppresses common-mode current. This is the simplest and most common feed arrangement for a single-band dipole. For a multi-band dipole, the impedance varies widely and an ATU is required at the radio end.

Centre-fed dipole — open-wire to ATU

Run 450 Ω ladder line from the dipole feed point directly to an ATU in the shack. Place a 4:1 current balun (or a 1:1 current balun) at the ATU's balanced output terminals. This configuration allows the antenna to operate on any band where the total wire length is at least λ/2. The ATU transforms the widely varying balanced impedance to 50 Ω for the radio. This is the most flexible multi-band HF feed arrangement available to the amateur operator.

End-fed wire — 49:1 unun

End-fed antennas present very high impedance at the feed point — typically 2,000–5,000 Ω for an end-fed half-wave (EFHW). A 49:1 impedance transformer (7:1 turns ratio wound on an FT-240-43 toroid) steps this down to approximately 40–100 Ω for a 50 Ω coax connection. A short (0.05λ) counterpoise wire at the transformer's ground terminal suppresses common-mode current. EFHW antennas are popular for portable and stealth installations due to their single feed-point convenience.

Vertical — base feed

Quarter-wave verticals present approximately 35–50 Ω resistive at the base when fed against a good radial system. A direct 50 Ω coax connection works well. Because the antenna is asymmetric (vertical element above ground), it is an unbalanced system and coax is appropriate without a balun, though a choke at the feed point prevents current from flowing down the outside of the coax shield along the ground. For 5/8-wave verticals, a matching network (LC circuit or hairpin) at the base transforms the higher feed impedance to 50 Ω.

Yagi / beam — gamma or beta match

The driven element of a Yagi typically presents 20–30 Ω due to mutual coupling from parasitic elements. The gamma match uses a tap point on the driven element and a series capacitor to transform this low impedance to 50 Ω without modifying the element length. The beta (hairpin) match shortens the driven element slightly (to introduce capacitive reactance) then adds a shunt inductive stub across the feed point to cancel the reactance and raise the resistive component to 50 Ω. Both methods are implemented at the feed point with no balun required if the beam is mounted on a metallic boom — the boom acts as a common-mode choke for the coax.

Loop antennas — 4:1 or 1:1 balun

A full-wave square or delta loop fed at a corner presents approximately 100–130 Ω. A 2:1 or 4:1 balun brings this to 50–65 Ω for coax. Loops fed at a side midpoint present approximately 50–70 Ω and can feed directly with coax and a 1:1 current balun. Small magnetic loops are resonant designs with very low feed impedance (under 1 Ω resistive) and are fed with a small coupling loop typically 1/5 the diameter of the main element.

Every feedline entering a building is a potential lightning conductor. Proper lightning protection is not optional — it protects life, property, and expensive equipment. The fundamental approach is to bring all feedlines to a single grounding point on the outside of the building, connect every coax shield and open-wire line to a common bonding bus at that point, and run a heavy conductor from that bus to a dedicated ground rod. This gives any induced lightning surge a low-impedance path to ground before it reaches the equipment.

RF in the shack

RF feedback into microphones, displays flickering during transmit, and erratic transceiver behaviour are classic symptoms of common-mode current on the coax shield returning to the shack. The fix is a 1:1 current balun or choke at the antenna feed point. A coaxial choke (several turns of coax wound into a coil at the feed point) provides 500–1,500 Ω of choking impedance at HF with no additional hardware. If RF problems persist despite a balun at the antenna, add a second choke at the radio end of the feedline — sometimes called a shack entry choke or station choke.

SWR rises with rain or humidity

Water ingress at a connector or through damaged coax jacket causes the dielectric constant of the insulation to change, shifting the effective impedance and electrical length. A single drop of water inside an N-type or PL-259 connector can cause 3–10 dB of additional loss at UHF. Inspect all outdoor connectors after heavy rain; replace any connector showing green or brown corrosion inside the barrel. On open-wire line, rain that wets the insulating spacers slightly increases the effective dielectric constant and lowers the characteristic impedance — this generally produces a modest SWR shift rather than a severe loss increase.

SWR varies with coax routing

If the SWR reading changes when you move or reposition the coax run, common-mode current is the culprit — the coax is radiating and its routing is altering the effective antenna geometry. Install a current balun at the feed point and observe whether the SWR becomes independent of coax position. If it does, the problem is solved. If not, a more complex common-mode problem exists, possibly involving other cables or grounded equipment in the shack.

High SWR on all bands after a storm

A direct or nearby lightning strike can vaporise internal braid and dielectric without leaving visible external damage to the coax jacket. Disconnect the coax at the radio end and measure continuity: centre to shield should be open; centre to connector body should be the characteristic resistance through the load. Any short or unusual continuity reading suggests the cable has failed internally and must be replaced entirely.

Single-band HF fixed installation

Use RG-213 or LMR-400 from radio to antenna. Fit a 1:1 current balun at the dipole feed point. Use N-type connectors at VHF or where connector quality is critical; PL-259 is acceptable at HF on clean, well-made connections. Install a coax lightning arrestor at the wall entry and bond the chassis to the station ground. This setup provides low loss, low common-mode noise, and adequate safety for a permanent HF station.

Multi-band HF wire antenna

Run 450 Ω window line from the antenna centre to the ATU. Use a 4:1 current balun at the ATU's balanced output terminals or a 1:1 current balun if the ATU has a dedicated balanced output. Keep the ladder line away from metal structures along the run. This arrangement provides the lowest possible feedline loss across all HF bands and accommodates the widely varying impedances presented by a non-resonant dipole on multiple frequencies.

VHF/UHF fixed installation

Use LMR-400 or better for any run over 10 metres. Use N-type connectors throughout — PL-259 connections are not suitable above 300 MHz due to the connector's geometry introducing significant SWR errors. Weatherproof every outdoor connection. At 70 cm and above, even a short run of RG-58 produces noticeable loss and should be avoided for serious operating.

Portable and QRP operation

For portable use, RG-8X or LMR-240 balances acceptable loss with manageable weight and flexibility. Avoid solid-dielectric coax (RG-58, RG-213) in cold climates — the jacket stiffens below freezing and repeated flexing at connectors eventually cracks the braid. Pre-made coax assemblies with crimped connectors are more reliable than field-fitted PL-259s for portable use. A BNC or SMA adapter at the radio allows the use of lightweight thin coax runs to the antenna.