Coaxial Cable Types and Loss

Walk into any ham radio store and you will find shelves of coaxial cable with different type numbers: RG-58, RG-213, LMR-400, RG-8X. They are all 50-ohm coaxial cables, they all use PL-259 or N-type connectors, and they all look superficially similar. But their loss figures — and therefore their suitability for different applications — vary by a factor of five or more. Choosing the wrong cable can mean losing half your transmitted power as heat before it even reaches the antenna.

Six common coaxial cable types shown at the same scale. Larger cables have lower loss because their larger conductor cross-sections reduce resistive heating. LMR-400 achieves its low loss through both larger conductors and a foam dielectric.

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How Coaxial Cable Loss Occurs

When RF energy travels through a coaxial cable, some of it is converted to heat. This conversion — called attenuation or loss — occurs through two distinct mechanisms: conductor loss and dielectric loss. The sum of these two is the total cable loss, usually expressed in decibels per 100 feet (or per 100 meters).

At HF frequencies (3–30 MHz) and lower, conductor loss dominates in most practical coaxial cables. At VHF and UHF, dielectric loss becomes increasingly significant, and the type of dielectric material in the cable matters more.

Conductor Loss and Skin Effect

You might expect that at RF, current flows evenly through the cross-section of the conductor — just as it does at DC. In fact, the opposite is true. At radio frequencies, current concentrates on the outer surface of conductors. This is the skin effect.

The skin effect arises because alternating magnetic fields inside a conductor induce eddy currents that oppose the flow of current in the interior. As frequency increases, these opposing currents push the net current density increasingly to the surface. The depth at which the current density has fallen to 1/e (about 37%) of its surface value is called the skin depth:

Practical skin depths in copper:

• At 1 MHz: 0.066 mm (66 μm)
• At 10 MHz: 0.021 mm (21 μm)
• At 100 MHz: 0.0066 mm (6.6 μm)
• At 1000 MHz (1 GHz): 0.0021 mm (2.1 μm)

As frequency increases, the current is confined to a thinner and thinner layer. This means the effective resistance of the conductor — and therefore the ohmic loss per unit length — increases with the square root of frequency: conductor loss ∝ √f.

This explains why larger-diameter cables have lower loss: a larger conductor has a larger circumference, so even though the current only flows in a thin skin, that skin has a greater total cross-sectional area. The conductor resistance per unit length decreases as conductor diameter increases.

Conductor material also matters. Silver has slightly lower resistivity than copper (1.63 × 10⁻⁸ vs 1.72 × 10⁻⁸ Ω·m), which is why some premium microwave coax uses silver-plated conductors. Aluminum has higher resistivity (2.65 × 10⁻⁸ Ω·m) but is used in some low-cost cables where weight or cost matters more than loss.

Dielectric Loss

The dielectric material between the conductors is not perfectly lossless. Polar molecules in the dielectric — molecules with a permanent electric dipole moment — attempt to align themselves with the alternating electric field, rotating back and forth at the RF frequency. This molecular motion generates heat, dissipating energy from the traveling wave. The loss tangent of the material (tan δ) quantifies this dissipation.

Dielectric loss increases directly with frequency (dielectric loss ∝ f), unlike conductor loss which increases with √f. This means that at higher frequencies, dielectric loss becomes increasingly significant compared to conductor loss.

Material loss tangent values at 100 MHz (lower is better):

This explains why foam-dielectric cables (like LMR-400) have lower loss than solid-polyethylene cables (like RG-213): the foam is part air and part plastic, and the effective loss tangent of the mixture is lower than solid polyethylene alone. Additionally, the higher velocity factor of foam dielectric (VF = 0.85 vs 0.66) means the inductance per unit length is slightly higher, which also contributes to lower conductor resistance per wavelength.

Loss Increases with Frequency

The total loss of a coaxial cable increases with frequency because both loss mechanisms — conductor loss (∝ √f) and dielectric loss (∝ f) — increase with frequency. The total loss curve is steeper than either alone:

Total loss ≈ A × √f + B × f

where A and B are constants that depend on the cable's physical dimensions and material properties. At HF (below 30 MHz), the A × √f term dominates. At VHF and above, the B × f term grows in importance. For solid-polyethylene cables, the crossover point where both terms contribute equally is roughly in the 200–500 MHz range.

Coaxial cable loss versus frequency. All cables show increasing loss at higher frequencies, but the difference between cable types grows dramatically at VHF and UHF. LMR-400 delivers five times less loss than RG-58 at 1 GHz.

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Common Coaxial Cable Types Compared

Loss values shown are typical at 68°F (20°C) for new cable under matched conditions (SWR = 1:1). Loss increases at higher temperatures and with higher SWR.

RG-58

RG-58 is a thin, flexible, inexpensive 50-ohm cable with a solid polyethylene dielectric. Its small inner conductor (0.9 mm AWG 20) makes it suitable for short runs, test cables, and applications where flexibility is more important than efficiency. At 14 MHz, RG-58 loses about 1 dB per 60 feet — meaning a 100-watt transmitter delivers only about 79 watts to the antenna. This is tolerable for a short run, but unacceptable for a 200-foot run from the shack to the antenna tower. RG-58 is also limited in power handling — typically 100 watts at HF — which makes it unsuitable for amplifier use.

RG-8X

RG-8X (also called mini-8) is a foam-dielectric cable with a similar outer diameter to RG-58 but significantly lower loss. Its foam dielectric gives it a velocity factor of 0.82 vs 0.66 for RG-58. It is an excellent choice for moderate-length HF runs where flexibility is needed and the extra expense of LMR-400 is not justified. Power handling is about twice that of RG-58.

RG-213

RG-213 is the workhorse of HF amateur radio. It is a thick, solid-polyethylene cable with a larger center conductor (2.26 mm) that gives it substantially lower loss than RG-58 or RG-8X. It is rated for continuous operation at 400 watts or higher at HF, and its robust construction handles outdoor installation well. The main disadvantages are its size (10 mm, difficult to bend around corners) and its limited VHF/UHF performance. Many stations use RG-213 for HF feedlines up to 100 feet and switch to LMR-400 for VHF/UHF work or longer HF runs.

LMR-400

LMR-400 (by Times Microwave, the original manufacturer; now also made by Belden and others under different names) is the preferred cable for low-loss applications in amateur radio. It achieves its excellent performance through a combination of a large aluminum-foil outer conductor, a thick outer copper braid, foam polyethylene dielectric, and a large center conductor. Its VF of 0.85 means cut-to-length applications need less physical cable than solid-PE equivalents, and its power handling exceeds that of RG-213 despite similar physical size. For 2-meter and 70-centimeter VHF/UHF installations especially, LMR-400 is the strongly preferred choice for runs over 20 feet.

RG-6 and RG-11 (75-ohm CATV)

These are 75-ohm cables designed for cable television distribution. They have quad-shielded construction (two braids plus two foil layers) that gives excellent shielding effectiveness, and their foam dielectric provides low loss. The 75-ohm impedance requires care in amateur radio use (there is a 1.5:1 mismatch with 50-ohm systems), but the connectors are inexpensive, the cables are easy to find, and the loss performance of RG-11 rivals LMR-400. Some operators use RG-6/RG-11 with 75-to-50 ohm matching networks for receive-only or low-power applications.

Coax Loss Calculator

How SWR Increases Feedline Loss

All the loss figures above assume a matched load (SWR = 1:1). When the load is mismatched, SWR is greater than 1, and the feedline loss increases above the matched-line value. This additional loss is called SWR-induced loss.

The total loss of a mismatched feedline can be calculated from:

The key insight is that SWR-induced loss is small when the feedline loss is small, and large when the feedline loss is already significant. A low-loss cable like LMR-400 with 1.35 dB of matched loss barely suffers at all from moderate SWR. But a high-loss cable like RG-58 with 6.5 dB per 100 feet at 100 MHz — already a very lossy situation — would be made dramatically worse by elevated SWR. This is why high-SWR operation is most harmful on long, lossy feedlines.

Power Handling and Temperature

The power rating of a coaxial cable is determined by the maximum temperature the dielectric and outer jacket can withstand continuously without degradation. The power is limited by resistive heating of the center conductor, and therefore depends on:

• The conductor's resistance per unit length (inversely proportional to conductor cross-section)
• The current in the conductor (related to the power and impedance: I = √(P/Z₀))
• The thermal resistance from conductor to ambient air

Power rating decreases at higher frequencies because the skin effect reduces the effective conductor cross-section, increasing resistance. The power ratings in the comparison table earlier are for 30 MHz. At 2 meters (146 MHz), power handling drops to roughly 40–50% of the HF value, and at 70 cm (432 MHz) it drops further still.

Operating in hot weather or in a confined space where the cable cannot shed heat to the air reduces the power rating. Many operators in hot climates derate coax by 25–30% to maintain safe margins.

Never coil excess feedline inside the shack unless you check that the coiled length can dissipate the heat it generates. A 50-foot coil of RG-58 running 100 watts can become warm enough to degrade the jacket over time.

How to Choose the Right Coax

Cable selection involves balancing loss, cost, flexibility, and power handling against your specific application requirements. Here is a practical decision process:

HF (3–30 MHz), runs under 50 feet

RG-213 is appropriate for most applications, including amplifiers up to 1500 watts. RG-8X is a good choice if you need more flexibility and the lower power rating is acceptable. RG-58 is acceptable for QRP (5 watts or less) only.

HF (3–30 MHz), runs 50–200 feet

RG-213 is standard. LMR-400 is preferred for runs over 100 feet, particularly if you are running legal-limit power and cannot afford to waste 2–3 dB in the feedline. The cost premium of LMR-400 over RG-213 is justified when feedline loss noticeably affects your signal.

VHF/UHF (50 MHz and above), any run

Use LMR-400 or equivalent for any run over 20 feet. At 432 MHz, even RG-213 loses nearly 50% of your power in 100 feet. The difference between LMR-400 and RG-213 at 432 MHz is about 4.5 dB — the equivalent of reducing your transmitter power by a factor of 2.8. This is not a minor convenience issue; it directly affects whether you can make contacts.

Portable or emergency operation

RG-8X offers the best compromise between low loss and flexibility/weight for portable use. LMR-400 is stiffer and heavier, making it less convenient in the field. RG-58 is acceptable for very short runs at HF.