Coax Comparison Guide — RG-58 vs RG-213 vs LMR-400 vs Heliax

Construction and characteristic impedance

Coaxial cable consists of a central conductor — copper or copper-clad aluminium — surrounded by a dielectric insulator, enclosed in a tubular outer conductor, and sheathed in an outer jacket for weather and mechanical protection. The characteristic impedance is determined by the ratio of the outer conductor diameter to the inner conductor diameter and by the dielectric constant of the insulating material between them. For solid polyethylene dielectric, the impedance is approximately 50 ohms when the diameter ratio is approximately 3.6:1. For foam polyethylene, a slightly different ratio achieves the same 50 ohms at a lower dielectric constant, which also produces a higher velocity factor.

The 50-ohm impedance standard for amateur radio is a practical compromise between minimum loss — which favours around 77 ohms for a given outer diameter — and maximum power handling — which favours around 30 ohms. The 50-ohm standard gives reasonable performance in both domains and has become universal in RF engineering. The alternative 75-ohm standard used in cable television and domestic video systems offers lower loss for a given outer diameter but is optimised for receive-only applications with lower power handling, making it less suitable for transmit applications above a few hundred watts.

What causes signal loss in coax

Loss in coaxial cable arises from two primary mechanisms. Conductor loss — also called resistive loss or I²R loss — is caused by the resistance of the inner and outer conductors. At DC and low frequencies the current flows through the full cross-section of the conductor. As frequency increases, skin effect progressively confines current to a thin surface layer whose thickness decreases with the square root of frequency. This means conductor resistance, and therefore conductor loss, increases proportionally to the square root of frequency. Doubling the frequency increases conductor loss by approximately 41 percent, not 100 percent.

Dielectric loss is caused by energy absorbed in the polarisation cycling of the dielectric material as the RF electric field reverses at each RF cycle. Dielectric loss is proportional to frequency — doubling the frequency doubles the dielectric loss. At HF frequencies below 30 MHz, conductor loss dominates for most practical coax types. At VHF and UHF above 100 MHz, dielectric loss becomes comparable to or larger than conductor loss, making the choice of dielectric material as important as conductor size. This is why low-loss cables like LMR-400 and Heliax use foam polyethylene or air-spaced dielectrics with very low dielectric loss factors, while standard cables like RG-58 use solid polyethylene that is adequate at HF but lossy at UHF.

Velocity factor and its importance

The velocity factor of a coaxial cable is the ratio of the speed at which the RF signal travels through the cable to the speed of light in free space. It is determined by the dielectric constant of the insulating material — for solid polyethylene (dielectric constant ~2.3) the velocity factor is approximately 0.66, meaning signals travel at 66 percent of the speed of light. For foam polyethylene (dielectric constant ~1.4) the velocity factor is approximately 0.78 to 0.84. For air-spaced Heliax the velocity factor is 0.88 or higher.

Velocity factor affects two practical aspects of cable use. First, the physical length of a cable corresponding to an electrical wavelength is shorter than the free-space wavelength by the velocity factor — a quarter-wave matching section or phasing stub must be cut to the physical length that corresponds to a quarter electrical wavelength, which is the free-space quarter wavelength multiplied by the velocity factor. Second, velocity factor affects the propagation delay through the cable, which matters when cables of specified electrical length are used in antenna phasing networks or stacked Yagi phasing lines.

Power handling and heating

Power handling capacity of coaxial cable is limited by two independent mechanisms: voltage breakdown and thermal heating. Voltage breakdown occurs when the RF voltage across the dielectric exceeds the breakdown field strength of the insulating material, producing a destructive arc that permanently damages the cable. Thermal heating occurs when resistive losses in the conductors raise the cable temperature to the point where the dielectric softens and the cable geometry — and therefore its characteristic impedance — changes permanently. Both limits depend on the cable type and the operating conditions.

For amateur radio operators, thermal heating is almost always the binding constraint rather than voltage breakdown. A 100-watt HF station running RG-58 will never approach the voltage breakdown limit of a properly constructed cable, but long continuous-duty digital mode operation in hot conditions could approach the thermal limit on a long run with high SWR. The thermal power limit published by manufacturers assumes matched conditions at 40°C ambient temperature. High SWR increases the standing wave voltage and current peaks, reducing the effective power handling below the matched-condition rating by a factor proportional to the SWR.

RG-174 — ultralight portable and short jumpers

RG-174 is the smallest and lightest common 50-ohm coax, with an outer diameter of just 2.8mm. Its conductor is a thin stranded copper-clad steel or tinned copper centre, and its solid polyethylene dielectric gives a velocity factor of 0.66. The matched-line loss is high — approximately 3.9 dB per 30 metres at 14 MHz — but at the short lengths used in portable operation this is manageable. A 3-metre RG-174 jumper from a SOTA transceiver to an EFHW UNUN loses about 0.4 dB at 14 MHz, which is entirely acceptable when the weight saving is critical.

RG-174 is suitable for short interconnects, patch leads, portable QRP operation, and receive-only applications where cable weight and flexibility matter more than low loss. It is completely unsuitable for fixed station feedlines longer than about 5 metres, for VHF or UHF operation at any significant power level, or for runs exposed to repeated flexing or outdoor UV exposure. The thin outer jacket degrades quickly in sunlight. Power handling is low — approximately 35 watts at HF — making it unsuitable for even modest transmit power on longer runs.

RG-58 — the universal entry-level coax

RG-58 is the most widely used amateur radio coax and the default choice for operators who need a compromise between cost, weight, flexibility, and adequate performance at HF. Its 5mm outer diameter makes it easy to handle and route through tight spaces. Solid polyethylene dielectric gives a velocity factor of 0.66. Loss is approximately 1.6 dB per 30 metres at 14 MHz, rising to 6.1 dB per 30 metres at 144 MHz. For HF use with reasonable feedline lengths — 15 to 20 metres — and power levels to 100 watts, RG-58 is entirely adequate.

The point at which RG-58 becomes a liability is clearly defined by frequency and run length. Above 50 MHz on runs longer than 10 metres, the loss is significant enough to warrant a better cable. At 144 MHz on a 20-metre run, RG-58 loses over 4 dB — nearly one-third of the transmit power — before the signal even reaches the antenna. This is the most common reason operators report poor VHF performance despite running adequate power: the coax is eating the signal before the antenna sees it. RG-58 should not be used for 2m or 70cm antenna feedlines of any significant length.

RG-8X (Mini-8) — the flexible upgrade

RG-8X — also sold as mini-8 or belden 9258 — uses a foam polyethylene dielectric to achieve a velocity factor of 0.78 while maintaining the same outer diameter as a cable mid-way between RG-58 and RG-213. The foam dielectric significantly reduces dielectric loss at VHF compared to solid polyethylene, and the larger conductor reduces skin-effect loss at HF. Loss at 14 MHz is approximately 1.1 dB per 30 metres — about 30 percent better than RG-58 — and at 144 MHz approximately 4.3 dB per 30 metres, roughly 30 percent better than RG-58 at VHF as well.

RG-8X sits in a useful middle ground for operators who need a cable that is better than RG-58 but more flexible than the larger LMR-400 or RG-213. It takes standard PL-259 connectors and coils easily into tight storage spaces. For POTA and portable operation where cable quality matters more than for SOTA but where pack weight and storage convenience still count, RG-8X is a sensible upgrade from RG-58. It is not suitable for serious VHF operation on runs longer than 10 to 15 metres, where LMR-240 or LMR-400 become the appropriate choice.

RG-213 / RG-8 — the traditional fixed-station HF cable

RG-213 is the classic fixed-station HF coax, with a 10.3mm outer diameter, solid polyethylene dielectric, and a stranded copper centre conductor. Its loss at 14 MHz is approximately 0.7 dB per 30 metres — less than half the loss of RG-58 over the same run — making it comfortable for HF feedlines of 30 to 50 metres at 100 to 200 watts. Power handling is approximately 800 watts at 14 MHz under matched conditions, making it suitable for legal-limit HF operation on most bands.

The trade-off of RG-213 versus LMR-400 is one of dielectric type: RG-213 uses solid polyethylene with its higher dielectric loss factor, while LMR-400 uses a foam dielectric. At HF frequencies where conductor loss dominates, the two are nearly equivalent in loss performance for the same outer diameter. At VHF and above, where dielectric loss matters, LMR-400 pulls ahead significantly. For an HF-only fixed station with no VHF ambitions, RG-213 at a lower cost per metre than LMR-400 is the rational choice. For a station that uses the same feedline from HF through 2m, LMR-400 is the better investment.

LMR-400 — the practical performance benchmark

LMR-400 from Times Microwave has become the benchmark low-loss coax for serious amateur installations. Its 10.3mm outer diameter matches RG-213 physically — PL-259 connectors fit with the same technique — but its foam polyethylene dielectric dramatically reduces loss at VHF and UHF compared to RG-213. Loss at 14 MHz is approximately 0.3 dB per 30 metres, at 144 MHz approximately 1.2 dB per 30 metres, and at 432 MHz approximately 2.4 dB per 30 metres. These figures are three to five times better than RG-58 at the same frequencies.

LMR-400 uses a solid copper inner conductor and a bonded foil plus braid outer conductor that provides excellent shielding and resistance to connector pullout. The outer jacket is UV-stabilised polyethylene suitable for direct outdoor burial or UV exposure. Power handling is approximately 1,500 watts at 14 MHz — well within legal amateur limits on HF. The cost is typically two to four times that of RG-58 per metre, which is significant for long runs but entirely justified for any application where feedline loss matters: VHF/UHF station feedlines, satellite work, EME, or any HF run longer than 30 metres where the accumulated loss of cheaper cable becomes operationally significant.

Heliax — the professional-grade option

Andrew Heliax — specifically LDF4-50A (1/2 inch) and LDF5-50A (7/8 inch) — represents the upper tier of practical amateur coax. Heliax uses an air-dielectric construction: the inner conductor is supported by a corrugated outer conductor, with air as the primary dielectric. The velocity factor is 0.88, the loss at 144 MHz is approximately 0.3 dB per 30 metres — one-quarter the LMR-400 figure — and power handling exceeds 5 kW. The outer conductor is corrugated aluminium tube, making the cable stiff and requiring special connectors.

Heliax is the feedline of choice for EME stations, high-performance 2m DX operations, microwave stations, and any installation where tower runs of 20 metres or more feed VHF or UHF antennas. The connectors are expensive and require experience to install correctly, and the cable itself cannot be routed around sharp corners or coiled tightly. For HF-only stations, Heliax is overkill. For a serious 2m EME station with a 30-metre tower run, the 3 to 4 dB of loss savings compared to LMR-400 represents a meaningful improvement in system performance that cannot be achieved by any other practical change.

HF-only stations (3–30 MHz)

For an HF-only fixed station with feedline runs of 15 to 30 metres, RG-213 or LMR-400 are both excellent choices. The difference in loss between the two is small at HF — less than 0.2 dB per 30 metres on most bands — and either is entirely adequate for 100 to 400 watts. If budget is the primary constraint and the feedline run is 20 metres or less, RG-58 is perfectly adequate for 100-watt HF operation with reasonable SWR. For runs over 30 metres or power levels approaching 200 watts, upgrading to RG-213 or LMR-400 is worthwhile. For the lowest-cost adequate solution on a tight budget, RG-58 at HF up to 30 metres is difficult to argue against.

VHF and UHF stations (50–1296 MHz)

The coax choice matters enormously at VHF and above. The difference between RG-58 and LMR-400 on a 20-metre 2m feedline is over 3 dB — the equivalent of doubling the transmit power and doubling the receiver sensitivity simultaneously. For any serious VHF or UHF operation, LMR-400 should be considered the minimum acceptable feedline for runs over 5 metres. For tower runs over 15 metres feeding 2m, 70cm, or higher bands, LMR-600 or Heliax is the appropriate choice. The connector and cable investment for a proper low-loss VHF feedline pays ongoing dividends in received signal quality for the life of the installation.

Portable and SOTA/POTA operation

For portable operation the weight and flexibility of the coax matters as much as its loss characteristics. RG-174 is the choice for ultralight SOTA setups where every gram counts — a 5-metre jumper weighs 20 grams, and the 0.5 dB loss at 14 MHz is a reasonable trade for the weight saving. RG-8X is the choice for POTA operators who drive to the park and want a cable that is noticeably better than RG-58 while remaining easy to coil into a bag. LMR-400 is inappropriate for pack-in portable use — its stiffness and weight work against portability — but is fine in a vehicle kit where it lives coiled in the back of the car.

Connector considerations

The coax connector is often the weakest link in the feedline system. A perfectly specified cable terminated in a poorly made PL-259 produces a system that underperforms what the cable specification implies. Silver-plated PL-259 connectors with a proper solder collar, installed with adequate heat to ensure full solder penetration, perform consistently over years of outdoor use. Crimp PL-259s and compression-fit designs are faster to install but require the correct die for the specific cable diameter and are more prone to failure under thermal cycling in outdoor installations.

N-type connectors are mechanically superior to PL-259 for cables larger than RG-213 diameter and for all VHF and UHF applications. The N connector's precision geometry maintains a consistent 50-ohm impedance through the connector body at frequencies where the PL-259 — designed for HF — begins to show significant impedance discontinuities. For any antenna system operating regularly above 100 MHz, switching from PL-259 to N-type connectors at the antenna end of the feedline is a worthwhile improvement even if the cable itself is adequate.