RG58 vs LMR-240: Same Size, Very Different Loss

Published on 10/13/2026

A gateway sits 10 m from its antenna. The old RG58 feeder fits the conduit, so a replacement cable that looks only slightly thicker seems like a simple upgrade. The signal-loss benefit can be real, but connector fit and routing still need engineering attention.

LMR-240 usually has lower attenuation than conventional RG58, but they are not the same size. A representative RG58 is 4.90 mm in overall diameter; standard LMR-240 is 6.10 mm. That difference changes connector preparation, ferrule choice, bend space and entry seals.

The title's "same size" is best understood as a compact-cable comparison, not dimensional interchangeability. Use exact cable part numbers and current datasheets. RG58 is a family designation, so construction and attenuation vary between products.

Dimensions and construction are different

The representative RG58 data below describes a solid-conductor, PE-insulated RG58/U product, not every RG58 cable. The LMR-240 values describe the standard solid-conductor cable, not an ultraflexible or fire-rated variant. Do not transfer these numbers to a similarly named cable without checking its datasheet.

Compare attenuation at the same frequency and length

At 900 MHz, the reference RG58/U product lists 13.7 dB per 100 ft. The standard LMR-240 datasheet lists 24.8 dB per 100 m. Convert the units before comparing. A 100 ft length is 30.48 m, not 100 m.

The converted RG58 loss is approximately 0.449 dB/m; LMR-240 is approximately 0.248 dB/m at the same frequency. These are nominal or typical bulk-cable values under the respective datasheet conditions. An assembly includes connector losses and production variation. Use tested acceptance limits for an actual purchase.

Do not extrapolate a single-frequency comparison to all bands with a fixed loss ratio. Conductor and dielectric losses vary with frequency. Also avoid assigning a generic 2.4 GHz RG58 value when the selected product's published table stops at 1 GHz. Obtain supporting data or measure the assembly.

Why lower loss can cost routing space

The lower-loss cable uses a different geometry and construction. A larger center conductor, foam dielectric and combination shielding change the electrical and mechanical trade-offs. Do not simplify the improvement to "double shielding means half the attenuation." Shielding effectiveness and insertion loss are separate parameters.

Standard LMR-240 has a solid center conductor. A cable intended for repeated motion needs its own dynamic-flex qualification; a minimum installation bend radius does not establish a fatigue life. Compare the standard and flexible variants by their actual mechanical data.

A connector intended for RG58 is not automatically suitable for LMR-240. Cable diameter alone is insufficient: center-conductor diameter, dielectric diameter, shield termination and ferrule all matter. A connector may share the same mating interface while needing a completely different cable-end termination.

When each cable makes sense

RG58 can remain useful for short, moderate-frequency connections where cost, available connectors and a proven installation favor it. A lower-loss compact feeder is useful when a long run or higher operating frequency makes attenuation expensive in the link budget.

An antenna upgrade may deliver less benefit than a feeder upgrade if cable loss consumes the added gain. Compare the net antenna-system change rather than buying the highest-gain antenna first. For very long or high-frequency runs, a larger feeder or a different equipment location may be the better choice.

Read SigmaRF's RG58 Coaxial Cable Explained guide for the cable-family context and Understanding Coax Cable Datasheets for loss and velocity definitions. This comparison focuses on a compact feeder upgrade, not on every RG58 or LMR variant.

Worked engineering example

For a 10 m run at 900 MHz, use the datasheet values above and assume matched ports. This calculation estimates bulk-cable loss only.

The lower-loss run delivers about 1.59 times as much output power under these assumptions. It does not amplify the signal; it wastes less input power. Include connectors, adapters, mismatch and environmental conditions before setting a final link budget.

Mistakes to avoid

Frequently Asked Questions

Bottom Line

Treat this as a loss-and-routing comparison, not a drop-in replacement guide. The lower-loss feeder can be worthwhile, but the cable, connector termination and installation must change together.

Ask for loss at the real operating band, the exact cable variant and a tested finished assembly. A clear drawing and acceptance limit are more useful than a promise that a cable is "better than RG58."

SigmaRF VNA-sweeps every assembly for VSWR and insertion loss before shipment. Its testing capability spans 100 MHz to 40 GHz; each assembly is specified for its own operating band. Builds are supported by the Vadodara volume facility and Cleveland-area precision facility. Lead times can be as short as 2 days, depending on the build and component availability.