Coax Loss Calculator

Understanding Your Coax Loss Calculator Results

The calculator above estimates how much RF power a coaxial feedline loses before the signal reaches the antenna. Use its main result to judge the electrical loss, its power-flow display to see the consequence at your selected transmitter power, and its cable comparison to decide whether changing cable size would make a useful difference.

It is a planning tool, not a verdict on an entire station. Antenna placement, antenna efficiency, connectors, weatherproofing, installation quality, and the surrounding terrain all matter, but they are outside the calculator’s basic feedline-loss score.

What coax loss means

Real coaxial cable is not lossless. Some RF energy becomes heat because its conductors have resistance and its dielectric is not perfect. The remaining energy continues toward the antenna. This reduction is called attenuation or feedline loss and is normally stated in decibels (dB) for a specified cable length and frequency.¹

The calculator makes that one loss result readable in three ways:

• dB loss describes the fraction of power lost in the feedline.
• Percentage delivered shows the same ratio as a percentage.
• Watts delivered and lost apply that ratio to the transmitter power you selected.

For a matched line, the delivered fraction is calculated from the loss in dB:

delivered fraction = 10^(-loss in dB / 10)

That relationship produces several useful landmarks:

The calculator’s six score bands use dB rather than watts: Excellent through 1.0 dB, Good above 1.0 through 2.0 dB, Acceptable above 2.0 through 3.0 dB, Marginal above 3.0 through 4.0 dB, Poor above 4.0 through 6.0 dB, and Unsuitable above 6.0 dB. Those labels describe electrical loss in the modeled feedline or RF path; they do not declare the whole installation good or bad.

Why frequency, length, and cable construction matter

Longer cable means more attenuation

Manufacturer attenuation figures are commonly published in dB per 100 feet or dB per 100 meters. For a uniform cable operating under the same conditions, attenuation scales with run length. A 50-foot run has half the cable attenuation of a 100-foot run; connectors and other components are separate unless entered in Advanced system details.

Loss generally rises with frequency

At higher frequencies, conductor loss and dielectric loss increase. Canonical transmission-line references describe conductors as imperfect and the dielectric as lossy, and note that coaxial-cable attenuation rises with frequency.² This is why a cable that performs comfortably at 27 or 144 MHz can give a noticeably different result at 464 or 915 MHz.

Construction matters, not just the family name

Cable diameter often tracks lower attenuation because a larger conductor geometry can reduce conductor loss, but it also changes weight, bend radius, connector choice, and installation difficulty. Published Times Microwave data, for example, shows different diameters, bend radii, and attenuation among the LMR cable sizes.⁴

Traditional RG designations require extra care. Two products described as RG-58, RG-8X, or RG-213 type can use different conductors, dielectric materials, braid coverage, or jacket constructions. Belden’s own product data demonstrates that a specific part number defines the construction and its frequency-by-frequency attenuation—not the short RG family name alone.⁶ Treat the calculator’s RG entries as documented reference constructions, then check the datasheet for the exact cable you plan to install when precision matters.

How the calculator obtains attenuation values

The cable dataset retains source provenance and the manufacturer’s underlying data instead of keeping only a few precomputed service-frequency values. Its source preference is:

1. A manufacturer-published attenuation equation.
2. A manufacturer attenuation table, with documented interpolation when the requested frequency falls between published points.
3. A documented secondary engineering source only when suitable manufacturer data is unavailable.

Times Microwave Systems is the primary source for the LMR series, Belden for the calculator’s traditional RG reference constructions, and CommScope/Andrew for LDF4-50A and LDF5-50A HELIAX cable.

Values calculated from a manufacturer equation

When a manufacturer supplies a cable-specific model, the calculator evaluates that model at the requested frequency. Times Microwave publishes attenuation coefficients for LMR products and identifies attenuation as a frequency-dependent expression with conductor- and dielectric-loss terms.⁵ A value calculated from those published coefficients is calculated from the manufacturer’s model; it should not be described as a table value printed at that exact frequency.

Values derived between published points

Manufacturers cannot print a table row for every possible frequency. If the exact requested frequency is present, the calculator uses that published value directly. Otherwise, when the requested frequency lies between two supported points, it places the frequency between those points in logarithmic-frequency space and interpolates the attenuation:

t = [log(f) - log(f1)] / [log(f2) - log(f1)]

A(f) = A1 + t(A2 - A1)

This produces a traceable derived value that follows the shape of frequency-spaced manufacturer data more appropriately than treating frequency as a simple linear axis. For example, Belden publishes discrete attenuation points for its 8262 RG-58 reference and 9248 RG-6 reference, together with their nominal impedance and velocity factor.⁶⁷

The calculator identifies whether a result came from a manufacturer equation or interpolation. It should not silently call an interpolated value manufacturer-published, and it should not silently extrapolate beyond the supported range. A result outside published coverage needs an explicitly supported model or a clear limitation warning.

Choosing a cable without automatically choosing the largest

The comparison section holds length, frequency, and transmitter power constant while showing the selected cable beside practical neighboring choices. Read it in this order:

1. Compare dB. This is the stable measure of electrical improvement.
2. Compare watts delivered. This shows the practical consequence at your chosen power.
3. Check whether the score band changes. Crossing from Marginal to Good can be more consequential than shaving a few tenths of a decibel while remaining Excellent.
4. Consider installation costs. Diameter, bend radius, weight, routing space, connector availability, and termination skill can outweigh a small electrical gain.

A larger cable is worthwhile when the recovered power or improved loss band matters to the installation. A smaller cable can remain a reasonable engineering choice for a short run, a lower frequency, a portable installation, or a route where flexibility is important. The calculator’s recommendation intentionally describes the size of the improvement rather than automatically recommending the biggest cable.

What about LMR-400 and LMR-600 on GMRS?

Both are 50-ohm low-loss cable families, but suitability depends on the actual run. Times Microwave’s published coefficients and tables show lower attenuation for LMR-600 and a larger cable diameter and installation bend radius than LMR-400.⁴ Enter 464 MHz and your real run length, then compare the recovered dB and watts. A long run can make the difference meaningful; a short run may show only a modest benefit.

Is RG-58 suitable for UHF?

RG-58 is not automatically wrong at UHF, but loss can become substantial as length grows. Belden 8262, one documented RG-58 reference, is specified at 11.5 dB per 100 feet at 400 MHz and 17.0 dB per 100 feet at 700 MHz.⁶ That does not mean every cable labeled RG-58 has those exact values; it illustrates why the exact construction and run length matter.

Why 75-ohm cable is modeled differently

RG-59 and RG-6 are labeled 75 Ω in the selector because their characteristic impedance differs from the nominal 50-ohm source and load used by most two-way-radio systems. Belden 9248, the calculator’s preferred RG-6 reference construction, is explicitly a 75-ohm cable and has a nominal velocity of propagation of 82 percent.⁷

That does not mean 75-ohm coax can never carry a signal between a 50-ohm radio and antenna. It means the system is no longer a simple matched 50-ohm line. The result can depend on source impedance, load impedance, the cable’s characteristic impedance, electrical length, frequency, velocity factor, and attenuation. Transmission-line references also show that cable attenuation and standing-wave behavior interact, so assigning one fixed “mismatch loss” at each connector would miss important behavior.³⁸

The calculator therefore models the total feedline-system effect for RG-59 and RG-6 automatically under the selected conditions. The value may change with frequency or length in ways that a matched-line attenuation-only calculation would not.

Using Advanced system details

Basic mode uses the calculator’s nominal source/load assumptions and keeps cable choice quick. Advanced system details lets the model better resemble an installed RF path:

• Load SWR represents the antenna-side mismatch.
• Source impedance and load impedance define the nominal terminations.
• Connector / adapter loss adds measured or specified insertion loss without pretending every connector has one universal value.
• Additional components can account for a jumper, lightning arrestor, duplexer, filter, combiner, splitter, adapter, or another device with a known insertion loss.
• Custom cable supports a documented impedance, velocity factor, and known attenuation data.

Advanced results distinguish feedline loss from total RF path loss. That matters because low-loss coax should not receive the blame for insertion loss added by a duplexer or filter, while the antenna still receives only the power remaining after the complete path.

Common questions

How much coax loss is acceptable?

There is no single limit for every installation. The calculator’s bands make the tradeoff visible: 1 dB delivers about 79 percent of the applied power, 2 dB about 63 percent, and 3 dB about half. Use the band together with run length, installation constraints, and the benefit available from a realistic alternative.

Is 3 dB of coax loss a lot?

Three decibels means roughly half the power entering a matched feedline reaches the other end. That is a meaningful loss, especially when a practical cable change can reduce it. It may still be an accepted tradeoff in an installation where size, routing, or cost prevents a lower-loss line.

Does transmitter power change coax loss in dB?

Not in the calculator’s normal operating model. The same cable, length, and frequency produce the same dB loss and delivered percentage. Increasing transmitter power increases both the watts delivered and the watts dissipated, subject to the cable’s power and operating limits.

Why can two cables with the same RG name have different losses?

An RG designation identifies a family or type, not every construction detail of every commercial product. Conductor material and size, dielectric, shield, and manufacturing choices can differ. Use the exact manufacturer’s part number and datasheet when comparing a purchase with the calculator’s reference entry.

Why can the result differ slightly from a printed manufacturer table?

Your frequency may lie between the table’s published rows, in which case the calculator derives a value by logarithmic-frequency interpolation. A manufacturer equation can also produce a more precise value than a rounded catalog table. Temperature, production tolerances, connector loss, cable age, and installation quality can make a real system differ from a nominal planning value.

Does this calculate antenna gain, ERP, or radio range?

No. It estimates the RF feedline path through the coax and any Advanced-mode components you enter. Antenna gain and efficiency, propagation, terrain, receiver performance, ERP/EIRP, and a complete link budget are separate subjects.

Methodology and source transparency

The calculator’s data records retain the manufacturer, reference part or model, nominal impedance, velocity factor, published frequency/attenuation points, official equation coefficients where available, and source metadata. Its displayed source note tells you whether attenuation was calculated from a published manufacturer model or derived from published points by interpolation.

Primary product sources include:

• Times Microwave Systems, published LMR electrical specifications and attenuation coefficients.⁴
• Belden, product-specific data for traditional RG reference constructions, including 8262 RG-58 and 9248 RG-6.⁶⁷
• CommScope/Andrew, official LDF4-50A and LDF5-50A HELIAX product documentation.⁹¹⁰

Nominal manufacturer data is appropriate for planning, but it is not a field measurement of a particular installed cable. For critical systems, verify the exact purchased construction and assess the completed feedline with suitable RF test equipment.

References