6.2 Feed Lines, Impedance Matching & SWR

Getting radio frequency (RF) energy from your transmitter to your antenna (and vice versa) requires a transmission line, commonly called a feed line. The characteristics of this feed line, along with how well the entire system is electrically matched, determine how efficiently your station operates. Understanding feed lines, impedance, and Standing Wave Ratio (SWR) is crucial for building a safe and effective amateur radio station.

Types of Feed Lines

The two primary categories of feed lines used in amateur radio are coaxial cable and parallel-conductor line (open-wire or ladder line).

Coaxial cable (or coax) is by far the most popular choice for modern amateur stations. It consists of a center conductor surrounded by a dielectric insulating material, which is in turn surrounded by a conductive shield (braid, foil, or both), all encased in a protective outer jacket. The primary advantage of coax is convenience: because the electromagnetic field is confined within the cable by the shield, coax can be buried, run alongside metal objects, or taped to a tower without severely affecting its electrical properties.

However, coaxial cables exhibit loss—some of the RF energy is converted to heat as it travels through the cable. This loss increases at higher frequencies and with longer cable runs. Common types of 50-ohm coaxial cable include:

• RG-58: Thin and flexible, but high loss. Best for short runs at HF or VHF frequencies (e.g., mobile installations).
• RG-8X (Mini-8): Thicker than RG-58, offering a good compromise between flexibility and loss. Very popular for HF base stations with moderate cable runs.
• RG-213: A thick (nearly half-inch), heavy cable with much lower loss than RG-58 or RG-8X, suitable for higher power and longer runs, especially at VHF/UHF.
• LMR-400: A modern, low-loss alternative to RG-213, excellent for VHF and UHF installations where minimizing signal loss is critical.

Parallel-conductor line, such as ladder line or open-wire line, consists of two parallel wires separated by insulating spacers. Unlike coax, the electromagnetic field surrounds the wires, meaning ladder line must be kept away from metal objects and the ground to prevent detuning and losses. Despite this installation difficulty, ladder line has significantly lower loss than coaxial cable, especially when operating with a high SWR.

Impedance and Matching

In AC and RF circuits, the opposition to the flow of current is called impedance, measured in ohms (Ω). For maximum power transfer from a transmitter to an antenna, the impedance of the transmitter output, the feed line, and the antenna must all be the same.

Most modern amateur radio transceivers are designed to operate into a 50-ohm resistive impedance. Therefore, we generally use 50-ohm coaxial cable. If the antenna also presents a 50-ohm impedance at the operating frequency, the system is perfectly matched.

However, an antenna's impedance changes with frequency. A dipole might be 50 ohms at its resonant frequency, but if you try to transmit on a frequency far from resonance, the impedance might rise to hundreds of ohms and become reactive (capacitive or inductive).

Standing Wave Ratio (SWR)

When the impedance of the antenna does not match the impedance of the feed line, not all the RF power traveling down the feed line is absorbed by the antenna. Some of that power is reflected back toward the transmitter.

The forward power and the reflected power interact on the feed line, creating a standing wave of voltage and current. The ratio of the maximum voltage to the minimum voltage along the line is the Standing Wave Ratio (SWR).

• 1:1 SWR: This is the ideal. It indicates a perfect match (zero reflected power). All power reaching the antenna is radiated (minus minor ohmic losses).
• 1.5:1 to 2:1 SWR: Generally considered acceptable for most modern solid-state transceivers. There is a slight mismatch, but power loss is minimal.
• Greater than 3:1 SWR: This indicates a severe mismatch. High SWR is problematic for several reasons.

First, modern solid-state transceivers have protection circuitry that will automatically reduce transmitter output power (foldback) if it detects a high SWR, to protect the final amplifier transistors from overheating or being destroyed by the high voltages associated with reflected power. Second, a high SWR increases the loss in coaxial cable, meaning less power makes it to the antenna to be radiated.

SWR Meters and Antenna Tuners

To measure the mismatch, amateurs use an SWR meter placed in the feed line between the transmitter and the antenna. It indicates the ratio of forward to reflected power.

When faced with a high SWR, an antenna tuner (more accurately called a transmatch or an impedance matching network) is used. An antenna tuner is placed between the transmitter and the feed line. It uses variable capacitors and inductors to transform the complex impedance presented by the feed line and antenna into the 50-ohm resistive impedance the transmitter requires.

It is important to understand that an antenna tuner located at the transmitter does not fix the SWR on the feed line running out to the antenna; the high SWR and associated cable losses still exist on that line. The tuner simply provides a 50-ohm load to the transmitter, keeping the radio happy and allowing it to deliver full power into the tuner. To eliminate feed line losses caused by high SWR, the antenna itself must be adjusted to resonance, or the tuner must be placed directly at the antenna feed point.

Coax Characteristics, Connectors, and Weatherproofing

Choosing Coax

Coaxial cable attenuation rises with frequency. Foam-dielectric coax generally has less loss per foot than solid-dielectric cable of the same size, but solid dielectric is often more moisture-resistant if the jacket fails. Outdoor jackets should resist ultraviolet (UV) light; UV damage cracks the jacket and lets water contaminate the dielectric—moisture contamination is a leading cause of coax failure.

RF Connectors

Common amateur connectors include PL-259/SO-239 (UHF connectors) for HF/VHF and N-connectors for UHF/microwave, where weather sealing and lower loss matter more. Select connectors rated for your frequency and power, install them with proper shield contact, and weatherproof outdoor joints with vulcanizing tape or sealant so water cannot enter the cable.

Measuring SWR

A directional wattmeter or SWR meter in the feed line indicates match quality. A perfect match reads 1:1. An SWR of 4:1 indicates a mismatch; power lost in the feed line is converted to heat, not useful radiation. Solid-state transmitters often fold back power as SWR rises to protect final transistors. Use a dummy load (typically a 50-ohm non-inductive resistor on a heat sink) when testing so you do not radiate on the air.