Coax Loss Calculator: How Much Power Your Feedline Wastes

How coax loss is calculated

Coax loss is rated as decibels of loss per 100 feet, measured separately at each frequency. To find the loss for your run, scale that rating to your actual length:

loss_dB = (length_ft / 100) * loss_per_100ft

Then convert decibels into the fraction of power that survives the trip down the cable:

P_out = P_in * 10^(-loss_dB / 10)

A quick way to build intuition: 3 dB of loss is half your power, and 6 dB is a quarter. So 50 feet of a cable rated 6 dB per 100 feet loses 3 dB, meaning 5 watts in becomes about 2.5 watts out. Every 3 dB you add halves what is left again. Because the loss is exponential, doubling the length does not double the pain — it squares the survival fraction — but long runs of lossy cable still bleed power fast.

Why 70cm loses far more than 2m

Coax loss rises with frequency because higher frequencies crowd the current into a thinner skin on the conductor and lose more energy to the dielectric. In round numbers, the same cable loses roughly 1.5 to 2 times as much at 440 MHz (70 cm) as it does at 146 MHz (2 m). That is why a feedline that feels fine on the 2-meter repeater can quietly sabotage you on 70 cm.

The practical lesson: pick your coax for the highest band you plan to use, not the lowest. If you run dual-band VHF/UHF, size the cable for 440 MHz. A run that costs you a tolerable 1 dB on 2 meters might cost 2 dB on 70 cm, and that difference is the gap between a solid contact and a scratchy one.

Why 50 feet of RG-58 on UHF wastes most of your power

RG-58 is thin, cheap, and everywhere, which is exactly why beginners get burned by it. At 440 MHz it loses roughly 11 to 12 dB per 100 feet. Run 50 feet of it and you drop about 5.5 to 6 dB before the signal reaches the antenna:

At 440 MHz that 50-foot RG-58 run throws away roughly 70% of your transmit power as heat in the cable. Your 5-watt HT effectively becomes a 1.3-watt radio at the antenna. Worse, the same loss hits your receive signal on the way back in, so weak stations that a better feedline would let you hear vanish into the noise. For any UHF run longer than a jumper, RG-58 is a false economy.

RG-58 vs RG-8X vs LMR-240 vs LMR-400

These are the four cables most beginners choose between. The numbers below are approximate loss per 100 feet at the two ham bands you will use most; always confirm against the manufacturer's datasheet for the exact cable you buy, since "RG-58" and "RG-8X" quality varies widely between brands.

RG-8X is a real step up from RG-58 while staying flexible and easy to route. LMR-240 fits the same connectors and roughly the same diameter as RG-8X but with noticeably lower loss, making it a strong middle choice. LMR-400 is the low-loss standard for base-station and long mast runs, but it is thick, stiff, and harder to bend into tight spaces. Match the cable to the run rather than defaulting to the biggest or the cheapest. For specific product recommendations, see our guide to the best coax cable for ham radio.

Connectors: PL-259/SO-239 vs N

The connectors at each end matter too, especially on UHF. The classic ham connector is the PL-259 plug that mates with an SO-239 socket (the UHF connector family). It is cheap, rugged, and standard on nearly every VHF/UHF mobile and base radio. Its weakness is that it is not a constant-impedance design, so it adds a small amount of loss and mismatch that grows with frequency but stays negligible at 2 m and acceptable at 70 cm for most beginners.

The N connector is a weatherproof, 50-ohm, constant-impedance design that performs better at UHF and above. If you are building a permanent outdoor run at 440 MHz or higher, N connectors are worth it. For a typical beginner mobile or base install on 2 m and 70 cm, quality PL-259/SO-239 connections are fine. Whatever you use, the biggest real-world losses come from poorly soldered or corroded connectors, not the connector type — a bad PL-259 will cost you more than the choice between UHF and N ever would. Adapters let you bridge SMA, BNC, and SO-239 hardware without buying new cables, which is handy when feeding a handheld into a mobile antenna or bench-testing an install. See ham radio antenna basics for how the feedline and connectors fit into the whole system.

Short runs vs long runs: when to upgrade

Loss is length times rate, so the right cable depends entirely on how far your antenna is from your radio.

• Under about 10 feet (jumpers, an HT to a nearby antenna): almost any coax is fine. RG-58 or RG-8X loses a fraction of a dB and the upgrade is not worth the money or stiffness.
• 10 to 30 feet (typical mobile or attic run): use RG-8X or LMR-240. This is the sweet spot where the cheap cable starts costing real power on UHF but flexibility still matters.
• 30 to 75 feet (base station to a roof or short mast): use LMR-240 for lighter installs or LMR-400 if you want to minimize loss, especially on 70 cm.
• Over about 75 feet (tall tower, long outdoor run): LMR-400 or better. At this length cheap coax can eat more than half your power on VHF and far more on UHF.

A good target is to keep total feedline loss at or below about 3 dB (half your power) at your highest operating frequency. If the calculator shows more than that, either shorten the run, move the radio closer, or step up to lower-loss cable. Remember the loss applies to receive as well as transmit, so lower-loss coax quietly improves both sides of every contact. Feedline loss also feeds into your overall reach, so pair this with the radio range calculator when you plan a base or mobile install.