Choosing the Right Coax: Calculating Feedline Loss, Power Handling, and Length

Antenna performance starts at the feedline. Coaxial cable choice determines how much of your transmitter power actually reaches the antenna—and how much received signal makes it back to your rig. On HF you can sometimes “get away” with almost any 50 Ω line, but at VHF/UHF and above, the wrong coax can cost you multiple S-units. This guide walks you through the numbers: how to calculate loss vs. frequency and length, how SWR affects delivered power, how velocity factor changes electrical length, and how to weigh power handling, shielding, and connectors. By the end, you’ll be able to select coax with confidence and use HamCalc’s tools to verify your decisions before you buy or pull a single cable.

Understanding the Basics

Coaxial cable is a concentric transmission line comprising a center conductor, dielectric, shield, and outer jacket. For amateur stations, 50 Ω impedance (nominal) is standard to match most radios and antennas.

Key properties to know:

Attenuation (loss): Expressed in dB per 100 ft (or per 100 m) and increases with frequency. Datasheets list typical values at several spot frequencies.
Velocity Factor (VF): The ratio of wave speed in the dielectric to the speed of light. Solid PE often ~0.66; foam PE ~0.78–0.85; PTFE ~0.69–0.70. VF matters for stubs, phasing lines, and electrical length.
Shielding & leakage: Double-braid or foil+braid yields higher shielding effectiveness and lower leakage/common‑mode pickup than single braid. This improves noise performance, especially in dense RF environments.
Power handling: Limited by dielectric heating and breakdown; it decreases as frequency rises and with higher SWR. Larger diameter cables generally handle more power.
Environmental durability: UV‑resistant jackets, good water-blocking, and crush resistance matter for outdoor runs and rotor loops.
Connectors: PL‑259/SO‑239 (UHF) are common but not constant‑impedance above ~300 MHz. Type N and SMA maintain 50 Ω and lower loss at UHF/microwave. Proper installation and weatherproofing are critical.

Rule of thumb: loss is king above 100 MHz; power handling and mechanics often dominate on HF.

Key Concepts: Line Loss, SWR, and Electrical Length

Calculating line loss

Manufacturers specify attenuation A100 (dB/100 ft) at frequency f. For a length L (ft):

Loss_dB = A100 × (L / 100)

Delivered power at the antenna is:

P_ant = P_tx × 10^(−Loss_dB/10)

Example (typical values): At 146 MHz, RG‑8X ≈ 4.8 dB/100 ft; LMR‑400 ≈ 1.5 dB/100 ft. For 100 ft, Loss_dB is 4.8 dB or 1.5 dB respectively.

SWR and mismatch loss

With standing wave ratio SWR, the magnitude of the reflection coefficient is |Γ| = (SWR − 1)/(SWR + 1). The mismatch loss (independent of line attenuation) is:

ML_dB = −10 × log10(1 − |Γ|^2)

For SWR = 3:1, |Γ| = 0.5 → ML ≈ 1.25 dB. Reflected power also traverses the line twice, increasing effective loss. The exact increase depends on line attenuation and length; use HamCalc’s “Coax Loss with SWR” calculator for precise results.

Electrical length and velocity factor

Electrical length determines how coax sections act as stubs or phasing lines. A quarter‑wave shorted stub behaves as a notch filter near its design frequency. The physical length for a quarter‑wave stub is:

L_physical = (c / (4f)) × VF

where c ≈ 3×10^8 m/s and f is frequency.

Example: At 146 MHz, λ ≈ 300/146 ≈ 2.055 m; λ/4 ≈ 0.514 m. With VF = 0.66 (solid PE), L ≈ 0.34 m (34 cm). With VF = 0.80 (foam), L ≈ 0.41 m.

Practical Application

Step‑by‑step selection

Define band(s) and run length. Loss rises with both frequency and length.
Check datasheets for attenuation at your highest operating frequency; interpolate if needed.
Compute Loss_dB and delivered power using the formulas (or HamCalc’s Coax Loss tool).
Consider SWR in the real world; add mismatch loss and recognize added traversal losses.
Verify power handling at your duty cycle (SSB vs FT8/RTTY) and ambient temperature.
Choose connectors appropriate for frequency and weather exposure; plan for strain relief and weatherproofing.
Balance cost vs performance; price per dB saved is a useful metric at VHF/UHF.

Example 1: 2 m base station, 100 ft run

You run 50 W FM at 146 MHz to a rooftop vertical.

RG‑8X (4.8 dB/100 ft at ~150 MHz): Loss_dB ≈ 4.8. Efficiency = 10^(−4.8/10) ≈ 0.331. P_ant ≈ 50 × 0.331 ≈ 16.6 W.
LMR‑400 (1.5 dB/100 ft): Loss_dB ≈ 1.5. Efficiency = 10^(−1.5/10) ≈ 0.708. P_ant ≈ 50 × 0.708 ≈ 35.4 W.

Result: Upgrading saves ~18.8 W at the antenna—often a full S‑unit on simplex. If SWR is 3:1, add ≈1.25 dB mismatch loss (~25% more power lost) before considering additional traversal loss; fixing the antenna match or adding a proper choke/balun matters.

Example 2: 80 m dipole, 100 ft run

At 3.5 MHz, typical losses are small.

RG‑8X ≈ 0.5 dB/100 ft → Efficiency ≈ 89%. From 100 W, P_ant ≈ 89 W.
LMR‑400 ≈ 0.2 dB/100 ft → Efficiency ≈ 95%. From 100 W, P_ant ≈ 95 W.

HF takeaway: mechanical durability and power handling (plus cost) may outweigh small loss differences. RG‑213 or LMR‑400‑class cable offers robust jackets and higher power margins for tuners and near‑resonant antennas.

Connectors and installation

Use Type N above 300 MHz for better return loss and weather sealing. PL‑259 works on HF but can add measurable loss and mismatch at UHF.
Keep bend radius ≥10× cable diameter (≥5× for brief, non‑repeating bends). Avoid kinks and tight rotor loops.
Bond shields to a grounded entry panel with arrestors; seal all outdoor connectors with butyl rubber + UV tape.
Add a ferrite choke (e.g., several turns through mix‑31/43 cores) near the antenna to suppress common‑mode currents, reducing noise and RFI.

Common Mistakes to Avoid

Underestimating VHF/UHF loss: Choosing small‑diameter coax for long runs on 2 m/70 cm can waste most of your power.
Ignoring SWR’s compounded effects: A 3:1 SWR isn’t just 1.25 dB of mismatch; reflections add extra traversal loss in a real, lossy line.
Using UHF connectors at UHF: PL‑259/SO‑239 are not constant‑impedance. At 440 MHz, poor connectors can add significant loss and ringing.
Skipping weatherproofing: Water ingress raises loss and detunes lines. Always seal outdoor connectors and provide drip loops.
Overlooking duty cycle: Modes like FT8/RTTY are near‑continuous; ensure the cable’s power rating at frequency and temperature supports your duty.
Too‑tight bends and crush points: These deform the dielectric, raise loss, and can cause intermittent faults.

Tips and Recommendations

For runs over 50 ft at 2 m/70 cm, budget for low‑loss cable (e.g., LMR‑400/Belden 9913F7 class) and Type N connectors.
On HF, prioritize robust jackets (UV/abrasion), good braid coverage, and power handling; loss is usually secondary.
Use HamCalc’s Coax Loss calculator to compare candidates, the Stub Length tool to cut phasing/stub sections using VF, and the RF Exposure tool to verify compliance with FCC Part 97.
Price performance by dollars per saved dB at your frequency and length.
Install a choke at the feedpoint and another at the shack entry when needed to reduce RFI and noise pickup.

Conclusion

Selecting coax is an engineering tradeoff among loss, power handling, mechanical durability, and cost. Do the math for your band and length, account for SWR and connectors, and install with care. With a few calculations—and a check in HamCalc—you’ll preserve precious dB on transmit and receive, making every contact easier.