How to Choose Coaxial Feed Line for Ham Radio: Loss, Connectors, and Weatherproofing

2026-10-18 · Equipment & Gear

The counterintuitive part

Most beginners judge feed line by thickness and price. Neither is wrong exactly, but neither is the point.

The point is total loss. Keep it in the 1–2 dB range for the whole run and stop worrying.

Why loss and not power handling? Because at 100 W on HF, power handling is essentially never the problem — even RG-58 handles hundreds of watts there. What decides how strong your signal is, is how much of it the line converts to heat on the way to the antenna. The question is never “is this cable tough enough” but “at what frequency, over what distance, how much do I lose.”

Numbers worth memorizing

These are typical matched-loss figures in dB per 100 feet (about 30 m). Real values vary by manufacturer and installation, but the magnitudes are what matter:

• RG-58 (5.0 mm OD): ~0.8 dB at 7 MHz, ~1.1 dB at 14 MHz, ~4.4 dB at 144 MHz
• RG-8X (7.4 mm): ~0.5 dB at 7 MHz, ~0.8 dB at 14 MHz, ~3.1 dB at 144 MHz
• RG-213 (10.3 mm): ~0.4 dB at 7 MHz, ~0.5 dB at 14 MHz, ~2.1 dB at 144 MHz
• LMR-400 (10.3 mm): ~0.1 dB at 7 MHz, ~0.2 dB at 14 MHz, ~0.9 dB at 144 MHz
• LMR-600 (15.2 mm): ~0.1 dB at 14 MHz, ~0.4 dB at 144 MHz
• 1/2-inch hardline: ~0.06 dB at 14 MHz, ~0.2 dB at 144 MHz
• 450-ohm ladder line: ~0.03 dB at 14 MHz, ~0.12 dB at 144 MHz

Published tables agree on the ordering even where individual numbers differ; ARRL’s QST measurements, for example, put RG-213 at about 0.8 dB per 100 ft at 14.15 MHz versus roughly 1.9 dB for RG-58A.

From that table, the selection rules write themselves:

• HF (below 30 MHz), runs up to about 30 m: RG-213 or LMR-400 both do the job, and RG-213 usually wins on value.
• Long HF runs or high power: start at LMR-400.
• RG-58: short jumpers of a few meters, nothing more.
• 2 meters (144 MHz) beyond about 10 m: RG-213 at minimum, LMR-400 or equivalent low-loss cable preferred. Do not use RG-58.
• 70 cm (430 MHz) and up: low loss is mandatory — LMR-400 is the entry ticket.
• Portable and QRP: RG-8X or Aircell 7, balancing loss against weight and flexibility. Nobody wants to wrestle a stiff LMR-400 up a hillside.

One more law hides in that table: loss rises with frequency. The same cable can lose a dozen times more at 430 MHz than at 7 MHz. “This cable works great on shortwave” tells you nothing about 70 cm.

The nastiest trap: low SWR does not mean a good antenna

This is the part worth tattooing on the bench.

An antenna that measures 2.0:1 SWR at the feedpoint can read 1.3:1 at the radio. That is not measurement error — it is feed line loss flattering your meter. Reflected power has to travel back down the line to be measured, and the longer and lossier the line, the less of it arrives. The loss is lying to your SWR meter.

The consequences are real. You can have a badly mismatched antenna and never know, because the reading at the shack looks fine — while a meaningful fraction of your power is being dissipated in the cable. The fix is to measure at the antenna end. A compact vector network analyzer such as a NanoVNA lets you set the calibration reference at the feedpoint and see the true impedance and SWR. That single habit saves more stations than any cable upgrade.

Related arithmetic: mismatch amplifies loss. At 3:1 SWR, multiply the matched loss by roughly 1.5 to 2. At 10:1, the additional loss can exceed 3 dB — which is half your transmit power gone, radiated as heat instead of signal.

Connectors

Three connector families cover almost everything in amateur service:

• PL-259 / SO-239 (the “UHF” type): the standard from HF through 2 meters. Rugged, high current capacity, easy to solder. It is not a true constant-impedance 50-ohm connector — irrelevant at HF, measurable at 70 cm. Silver-plated versions behave better at HF.
• N-type: a true 50-ohm connector, usable into the GHz range, and genuinely weatherproof. The default choice for VHF/UHF and every outdoor connection.
• BNC: quick bayonet lock, true 50 ohms (beware 75-ohm versions in the wild). Right for test gear, handhelds, and QRP; wrong for high power and exposed outdoor use.

Connector quality matters more than people expect. A poorly installed PL-259 costs about 0.1 dB at HF and more at VHF/UHF — and at UHF, two bad PL-259s can easily outweigh the savings from thinner cable. Install a sloppy connector at each end and you have spent the entire upgrade budget on nothing.

Weatherproofing: two layers, every time

Every outdoor connector gets sealed. This is the most common slow failure in the whole station.

Once water wicks into the braid, loss climbs day by day, and you see nothing abnormal until SWR suddenly jumps or the cable dies outright. The standard treatment is two layers:

1. Self-amalgamating tape wound tightly around the connector body, stretched as you wrap so it fuses to itself.
2. A UV-resistant outer layer over that — plain electrical tape cracks and peels within months outdoors.

Two easy-to-miss points:

• Burial: do not assume “outdoor rated” means “direct burial.” LMR-400 has a dedicated direct-burial version (LMR-400-DB) with water-blocking construction. Ordinary cable takes on water in the ground.
• Bend radius: stiff low-loss cable deforms its dielectric if bent too tightly, creating an impedance discontinuity. Follow the manufacturer’s number, not your judgment.

The mistakes that keep happening

• Running long VHF/UHF feeds in RG-58 — the classic way to throw away most of your power.
• Skipping connector weatherproofing — water in the braid is permanent damage.
• Buying copper-clad steel center conductors to save money — mechanically weak; solid copper is what serious use requires.
• Judging a cable by its DC resistance as a “ground” — at HF, the inductive reactance of a long run dominates its resistance, so a long cable is nearly useless as an RF ground.
• Upgrading to fat cable while keeping the same tired connectors — the bottleneck did not move.

A one-minute selection method

Ask three questions:

1. What is the longest run, in meters?
2. What is the highest frequency I will use, in MHz?
3. Will any of this run be exposed outdoors?

Pick the smallest cable that keeps total loss inside the 1–2 dB budget, seal every outdoor joint with both tape layers, and measure true SWR at the antenna end at least once. Do those three things and your feed line will never be the weak link again.

References

• ARRL — “Feed Lines”, arrl.org (accessed 2026)
• R. Hallas, “Higher SWR means more loss, but more loss can mean lower SWR”, QST, March 2005
• Radio Society of Sri Lanka — “Coax Quick Reference Data (Amateur Radio Use)”, rssl.lk (accessed 2026)
• KB9VBR Antennas — “Coax Feedline Attenuation Chart”, jpole-antenna.com (accessed 2026)
• National Amateur Radio Alliance — “Coax, connectors and chokes”, wiki.usnara.org (accessed 2026)

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