Feedline Loss Explained Beginner Friendly Guide

If you are a new ham, you probably spent a lot of time picking your first radio and antenna. The coax between them often gets treated as an afterthought, but that "boring" piece of feedline can quietly throw away a big chunk of your power. In this guide we will look at what feedline loss really is, use simple decibel math, and walk through real numbers for common coax types so you can see how much power actually reaches your antenna.

Feedline loss sounds intimidating because it is usually shown in tables full of small numbers in dB per 100 feet. The good news is that you only need a couple of simple ideas: dB is just a way to measure how much power is lost, and you can convert those table values into "how many watts reach my antenna" with very basic math.

We will focus on common ham scenarios: a 50 watt radio on 2 meters, 100 watts on HF, and typical feedline lengths like 25, 50, and 100 feet. Once you see the pattern, you can apply the same steps to whatever coax and band you are using. If you are still piecing together your first station, you might also want to bookmark the Getting Started section so you can come back as you tweak things.

What is feedline loss in plain language?

Your feedline (coax cable) is just a long resistor and capacitor and inductor wrapped together in a neat jacket. Some of the RF energy that leaves your radio gets absorbed and turned into heat in the copper and dielectric as it travels down the line. The longer the cable and the higher the frequency, the more loss you get.

Most coax datasheets list loss as "dB per 100 feet" at different frequencies. To use it, you just scale by your actual length. For example, if RG-58 has about 4.5 dB loss per 100 feet at 150 MHz, then a 50 foot piece has about half that: ~2.3 dB. That number tells you how much weaker the signal is after the cable compared to before it.

Each 3 dB of loss cuts your power in half. 1 dB is roughly a 20% power drop, 6 dB is a 75% drop, and 10 dB is a 90% drop. On the air that might be the difference between full quieting and scratchy audio into a repeater. On HF bands, the losses are usually small for typical home-station lengths unless you use very thin coax or very long runs.

How to estimate feedline loss for your own station

You do not have to guess whether your coax is "good enough." With a datasheet or a simple online chart, you can estimate your loss in under a minute. Use this quick three-step process and you will know roughly how many watts your antenna actually sees.

• Step 1: Look up the loss in dB per 100 ft for your coax at the band you care about (for example, 146 MHz for 2 meters).
• Step 2: Multiply that number by your cable length divided by 100. So 4.5 dB/100 ft and a 50 ft run gives 4.5 × 0.5 = 2.25 dB.
• Step 3: Convert the dB loss into "power left" using a simple rule of thumb: 3 dB ≈ half, 1 dB ≈ 80%, 2 dB ≈ 63%, 4 dB ≈ 40%.

As an example, say you have a 50 W mobile rig feeding a 2 meter antenna on the roof through 75 feet of RG-8X. If the datasheet says 3.2 dB loss per 100 ft at 150 MHz, then your loss is 3.2 × 0.75 ≈ 2.4 dB. That is a little more than 40% loss, so your 50 W radio will look like about 30 W at the antenna.

If you swap that same feedline for LMR-400 with ~1.5 dB/100 ft on 2 meters, 75 feet gives 1.5 × 0.75 ≈ 1.1 dB. Now you are only losing about 22% of your power. Your 50 W rig effectively delivers close to 39 W. Same radio, same antenna, just better coax.

Real-world examples: HF vs VHF/UHF

Coax loss grows with frequency, so the same cable behaves very differently on 40 meters compared to 70 cm. That is why a roll of RG-58 might be fine for a short HF dipole run but a poor choice for a 440 MHz repeater antenna on a 70 ft tower. Here are some ballpark numbers for common coax types so you can see the pattern.

• 100 ft RG-58 @ 7 MHz (40 m): roughly 0.7 dB loss. You still deliver about 85% of your power to the antenna.
• 100 ft RG-58 @ 28 MHz (10 m): around 1.5 dB loss. You get about 70% of your power to the antenna.
• 100 ft RG-58 @ 146 MHz (2 m): roughly 4.5 dB loss. You are down to about 35% of your original power at the antenna.

Now compare that to a lower-loss cable like LMR-400 for the same 100 ft run:

• 7 MHz: about 0.3 dB loss, so over 90% of your power reaches the antenna.
• 28 MHz: about 0.6 dB loss, roughly 87% of your power at the antenna.
• 146 MHz: about 1.5 dB loss, a bit more than 70% of your power left.

On HF, both cables look pretty reasonable for typical lengths. On 2 meters, the difference is huge. This is why many hams use "whatever coax is handy" for HF but save the good stuff for VHF/UHF lines, especially on tall towers. If you are thinking about upgrading antennas, it is worth checking your feedline first. You might get more improvement per dollar there than buying a slightly fancier antenna. You can explore more antenna ideas in the Antennas section.

How much does feedline loss actually hurt your signal?

Decibels can feel abstract, but on the air the differences are often smaller than people expect. A 1 dB change is barely noticeable. 2 dB is a little bit. 3 dB is about half an S-unit on many S-meters. So going from 1 dB to 2 dB loss in your coax is not the end of the world. Where it really starts to sting is when you stack up several dB of loss, or when you are already working at the edge into a distant repeater or weak HF DX.

As a rough rule: keeping feedline loss under about 3 dB on your main band is a solid goal. Under 1.5 dB is great. If your calculation says you are losing 5 or 6 dB in the coax, that is where spending money on better feedline or re-routing the run is usually a better investment than an amplifier. Your future self will also thank you when you start playing with weak-signal VHF, satellites, or digital modes.

Practical tips for choosing and installing feedline

You do not need to become a coax nerd to make good choices. A few practical rules will get you 90% of the way there. Combine them with your budget, mechanical needs, and where the cable will live (indoors, outdoors, buried, etc.) and you will be in great shape.

1. Keep runs as short and direct as you reasonably can, and avoid unnecessary jumpers and adapters. Every extra connector adds a tiny bit of loss and one more failure point.
2. Use better coax for higher frequencies and longer runs. For a short HF run on a small lot, RG-8X might be fine. For 2 m/70 cm base antennas, especially over 50 ft, consider something like LMR-400 or a similar low-loss cable.
3. Protect your investment. Seal outdoor connectors with proper tape or boots, support the cable so it is not hanging by the connector, and avoid tight bends that can damage the dielectric. A small amount of care here can keep loss from creeping up over time.