What Is a Feed Line and Why It Matters in Ham Radio
Definition and Role of Feed Lines in Antenna Systems
The feed line, also called the transmission line, is the RF power conduit between your radio and your antenna. The feed system begins at the radio's 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.
Every watt your transmitter generates must travel through the feed line before it can radiate into the air. This journey is never perfect. All real transmission lines absorb some fraction of the energy they carry, converting it to heat in the conductors and dielectric material. The goal of careful feed line selection is to make that journey as efficient as possible given your operating frequency, run length, power level, and installation constraints.
How Feed Line Choice Affects Signal Loss and Performance
Coax loses signal primarily through conductor resistance at HF and dielectric absorption at VHF and above. A 3 dB feedline loss throws away exactly half your transmit power, so you would need to double transmitter output just to break even. This relationship between decibels and power is the foundation of feed line evaluation. The scale is logarithmic - 1 dB is barely noticeable, 3 dB means half your power is gone, and 10 dB means 90% has been lost.
Feedline loss hurts receive as much as transmit, which is why a 15 dB masthead preamp ahead of 4 dB of cable nets only 11 dB of improvement. The damage is bidirectional. Weak signal operators on VHF and UHF understand this acutely - a noisy, lossy feed line can destroy the advantage of even the most carefully designed antenna system.
Overview of Feed Line Types Covered in This Guide
Ham radio operators have access to several fundamentally different feed line technologies, each with distinct electrical characteristics, physical properties, and ideal applications. This guide covers coaxial cable (in its many forms from RG-58 to LMR-400), 300 ohm and 450 ohm ladder line, 300 ohm twin-lead, true open wire feed line, and hardline and HELIAX. We also address impedance matching, baluns, connectors, weatherproofing, lightning protection, and how to select the right feed line for your specific station situation.
Coaxial Cable: The Most Common Ham Radio Feed Line
How Coaxial Cable Works and Its Basic Construction
The most common type of feed line is coaxial cable, or simply coax. It is called coaxial because there are two circular conductors positioned co-axially on the same axis, one inside the other. The inner conductor is surrounded by a solid or multistranded outer conductor commonly called a shield. There is also insulating material between the center conductor and the shield, which can be hard plastic, foam plastic, or even air.
The advantages of coax are that it is easy to route, weather-jacketed, and inherently shielded from electric field pickup. Its disadvantages are that loss rises with frequency and especially with SWR, and the braid can carry common-mode current if not choked. The shielded construction of coax is simultaneously its greatest strength - immunity to external interference and ease of routing - and a contributor to its loss, because the solid dielectric between center conductor and shield introduces energy-absorbing dielectric losses that climb rapidly with frequency.
Popular Coax Types: RG-8, RG-213, RG-58, RG-6, LMR-400
Not all coaxial cable is equal. The differences in diameter, dielectric material, and construction quality produce dramatically different loss figures at amateur radio frequencies.
- RG-58: RG-58 (50 ohm) is about 0.195 inches in diameter, quite lossy, and suitable only for mobile installations typically under 20 feet and 150 watts. RG-58 runs 2.2 dB per 100 feet at 14 MHz but 12.3 dB at 432 MHz, which makes it usable only for short HF runs and patch cables.
- RG-213: RG-8 and RG-213 are the standard 50-ohm ham radio cables for general HF use. At 14.2 MHz, RG-213 loses approximately 0.2 dB per 100 feet, so a 100-foot run gives about 0.2 dB total loss. A 100-watt station delivers about 96 watts to the antenna - almost negligible loss. This is why RG-213 is a popular and economical choice for moderate HF runs.
- RG-8X: RG-8X (50 ohm) is about 0.24 inch in diameter, suitable for medium power around 350 watts, HF, and low-VHF. Its smaller diameter and better flexibility than full-size RG-213 make it a popular choice for portable and mobile setups.
- RG-6: RG-6 (75 ohms) is about 0.332 inches and is typically used for cable and satellite TV. The impedance mismatch between 75Ω coax and 50Ω radio equipment creates a 1.5:1 SWR, causing about 4% of power to be reflected - often acceptable, especially considering RG-6's advantages: lower loss than RG-58 and very cheap availability at any hardware store.
- LMR-400: LMR-400 is a popular low-loss RG-8 type, suitable for VHF with approximately 1.5 dB loss per 100 feet at 146 MHz. Flexible versions like LMR-400UF are preferred particularly for rotatable antennas.
Impedance: 50 Ohm vs 75 Ohm Coax Explained
Common coaxial cable impedances are 50 Ω for HF/VHF gear and 75 Ω for TV/CATV and some receive applications. The 50-ohm standard was adopted by the military and subsequently by amateur radio because it represents a practical compromise between minimum loss (which occurs around 77 ohms for common dielectrics) and maximum power handling (which occurs around 30 ohms). Nearly every ham radio transceiver on the market uses 50-ohm output impedance, making 50-ohm coax the natural and universal choice for transmitting applications.
Coax Loss per 100 Feet at HF, VHF, and UHF Frequencies
The following loss figures illustrate the dramatic increase in coax attenuation as frequency rises:
- At 14 MHz (20 meters): RG-58 ≈ 1.1 dB/100 ft; RG-213 ≈ 0.2 dB/100 ft; LMR-400 ≈ 0.1 dB/100 ft
- At 144 MHz (2 meters): RG-213 loses 3.6 dB per 100 feet against 2.2 dB for LMR-400.
- At 432 MHz (70 cm): At 432 MHz the gap widens to 6.5 dB for RG-213 versus 3.9 dB for LMR-400.
- At 450 MHz: LMR-400 is 4.7 dB/100m versus RG-213 at 10.5 dB/100m.
At 446 MHz, RG-58 loses over 7 dB per 100 feet, throwing away more than 80 percent of your transmitter power before the antenna sees it. Switching to LMR-400 on the same run drops loss to around 1.5 dB, recovering the vast majority of that power and dramatically improving both transmit and receive performance.
When to Choose Coaxial Cable for Your Station
Coaxial cable is the right choice when ease of installation matters, when your antenna operates near resonance on a fixed band, when you need shielding from RFI in an urban environment, or when your feed line must be routed close to metallic structures. Upgrade to LMR-400 for VHF or UHF runs over 50 feet, HF runs over 150 feet, QRP operation where every dB counts, or when feeding a masthead preamp. Stick with RG-213 or RG-8 for HF runs under 100 feet, tight budgets, or where the smaller bend radius of the older cable matters.
Ladder Line: Low-Loss Balanced Transmission Line
What Is Ladder Line and How It Differs from Coax
A popular type of feed line for HF use is ladder line. In fact, at HF frequencies it is the most common feed line for random-length dipoles and other antenna designs. Ladder line consists of nothing more than two wires in parallel separated by insulating material. Unlike coaxial cable, ladder line is a balanced transmission line - both conductors carry equal and opposite currents with no shield. This balanced nature makes ladder line inherently immune to common-mode interference pickup but also means it must be kept away from metallic objects that would disrupt the current balance and increase loss.
The key characteristic that sets ladder line apart from coax is its behavior under high SWR conditions. Coax has low loss when SWR is approximately 1:1, but with SWR and long runs, attenuation and heating increase quickly. Ladder line loss remains very low even at high SWR, allowing a tuner in the shack to handle the match.
300 Ohm vs 450 Ohm Ladder Line Comparison
Traditional VHF television installations used 300-ohm ladder line. The standard in ham radio for HF operation is 450-ohm. The distinction matters in practice. The 450-ohm variety uses wider conductor spacing and less dielectric material, giving it lower loss than 300-ohm window line. At 7 MHz, RG-58 shows a loss of 1.0 dB per hundred feet, RG-8 coax shows 0.6 dB, 300-ohm twin lead 0.25 dB per 100 feet, and window line (either 450 or 300 ohm) less than 0.1 dB per hundred feet.
A 450 Ω window line typically has matched-line loss of 0.05 - 0.15 dB per 30 meters 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 multiband wire antennas.
Advantages of Ladder Line for Multiband HF Antennas
Ladder line does not suffer from high losses at high SWR, so it may be effectively used to feed an antenna that may, at various frequencies, present the feed line with any SWR from 1:1 to roughly 12:1. With ladder line you can completely forget about resonance and SWR until you get to the radio.
This property makes ladder line the ideal partner for a center-fed doublet or G5RV-type antenna used on multiple HF bands. 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 multiband operation on all HF bands.
Handling High SWR with Ladder Line and a Tuner
For HF ham radio operation, the loss of window or ladder line is so low that even at high SWR, the excess loss is rarely noticeable. Balanced line, even with a high SWR, can outperform coax that is perfectly matched. Because balanced transmission line has relatively low loss even at high SWR, the match at the feed point is not critical for HF operation.
With ladder line, you can completely forget about resonance and SWR until you get to the radio, where you use a tuner to make the match to 50Ω. The antenna tuner at the shack handles all the impedance transformation. If the balun or tuner gets hot, it is wasting power. Traditional baluns such as the coax-wound toroidal 4:1 Guanella are not designed to handle the extreme impedance variations of all-band doublets and tend to arc or saturate at high power. Modern balun manufacturers have discovered this and now
