Why “Antenna Loss Calculators” Aren’t That Useful

and Where They Go Off the Rails

Most hams have used an “antenna loss calculator” at least once. You punch in coax type, length, frequency, maybe an SWR, and out pops a neat answer like:

• “Total loss: 2.1 dB”
• “Power out: 62 W”
• “ERP: 85 W”

It feels like you’ve quantified the problem. The issue is that these calculators often quantify a simplified slice of reality—and the simplifications aren’t small. When mismatch (SWR) is involved, the errors can easily be multiple dB.

What the KV5R calculator is actually doing

Using the KV5R coax loss calculator as a concrete example, the page is essentially a feedline loss + “SWR loss” + ERP calculator. You enter things like:

• Line type (coax family / entry in a list)
• Line length
• Frequency
• Load SWR (SWR at the antenna end)
• Power in

And it outputs:

• Matched loss
• SWR loss (an “additional loss” term)
• Total loss, plus power-out and percent power loss
• ERP if you supply an antenna gain number

So even before we get to real-world station behavior, this style of calculator is already resting on two fragile assumptions:

1. a simplified mismatch-loss method that may be inaccurate for your line length and load reactance, and
2. line-loss inputs that may not match your actual coax (brand, construction, aging, connectors, installation).

What many “loss calculators” are actually doing

Most web calculators combine two separate ideas:

• Matched (insertion) loss of the feedline (as if the load were perfectly matched), and
• “Additional loss due to SWR” (an SWR-based correction term).

The problem is not that matched-loss math is “bad.” Matched/insertion loss is a valid, useful planning number. The problem is when a calculator pretends it can reliably “fix” that number from SWR alone.

Where it goes off the rails: SWR is not enough information

The fundamental mistake

SWR tells you the magnitude of mismatch (how strong reflections are). It does not tell you the phase of the reflection coefficient, and it does not uniquely define the actual complex load impedance.

That missing information matters because a feedline is an impedance transformer. Voltage/current peaks and valleys move with:

• line length,
• frequency, and
• load reactance (the “jX” part).

And on a lossy line, where current and voltage sit along the line affects how much power ends up as heat.

Why “matched loss” isn’t always the baseline you think it is

This is where people get surprised: you can find real cases where the total loss from source to load (as a percentage of source power) dips below the calculator’s “matched line loss” value for certain reactive loads and certain line lengths.

(This does not “beat physics.” It’s a reminder that “matched loss” is one specific condition, not a universal minimum-loss reference. Once a reactive load is involved, power transfer through a lossy two-port is more nuanced than “matched loss + SWR penalty.”)

Another trap: confusing “loss” with “reflection”

A huge conceptual trap is calling mismatch effects “loss,” as if SWR itself burns power.

• Attenuation (real loss) is dissipated as heat in the line and cannot be “tuned away.”
• Mismatch/reflection is power being rejected by the load and sent back toward the source. It depends on where and how you match.

When calculators blend these into one bucket (“SWR loss”), they encourage wrong conclusions like:

• “My SWR is high, so I’m losing that much power in the coax,” or
• “If I fix SWR at the radio, feedline loss must be lower.”

Sometimes those statements correlate with reality, sometimes they don’t. A calculator that only knows SWR can’t reliably tell which case you’re in.

What gets forgotten in a real station

This is the part that matters most in practice: the big items calculators rarely model (or users rarely enter correctly).

You usually don’t know the load SWR

Many calculators ask for “SWR at the antenna end.” Most operators only know SWR at the radio end (and often through a tuner).

On a lossy line, SWR is not constant. Reflections are attenuated on the way back, so SWR often looks “better” in the shack than it really is at the antenna.

Manufacturer coax loss numbers assume a perfect match

Datasheet loss figures are typically quoted for a matched condition (insertion loss). Once mismatch exists, the “simple loss per 100 ft” number is no longer the whole story—and bolting on a questionable “SWR correction” is not a reliable fix.

“RG-8X” is not one thing

Generic coax labels hide real variation: different manufacturers, different constructions, different foams, different shielding, different aging and handling. Even “the same” coax type can differ enough to move the result by a meaningful amount.

The tuner changes what you think you measured

A tuner can make the radio happy, but it does not magically change the SWR on the line between tuner and antenna. If your calculator assumes “load SWR” equals whatever your rig shows, it’s wrong in many common station layouts.

Common-mode current and feedline radiation are ignored

Most calculators assume coax is a passive part that does not radiate. In real installations, common-mode current can make the feedline part of the antenna system. That changes pattern, changes SWR, changes noise pickup, and can completely dominate “why the antenna behaves weird.” A loss calculator won’t catch any of that.

Weather, aging, and workmanship change loss

Water ingress, UV aging, kinks, cheap connectors, corrosion, braid damage, and poor installation practices all move the real attenuation. Calculators assume “as new, lab conditions” unless you manually derate.

“ERP” is often fantasy math

ERP outputs often look impressively precise, but the “antenna gain” input is usually a single number. Real antennas have gain that changes with direction and elevation angle—and it is heavily distorted by height above ground, nearby conductors, and feedline currents.

So… are these calculators useless?

Not useless—just easy to misuse.

They’re useful for:

• quick matched-loss comparisons (especially for VHF/UHF planning),
• sanity-checking “is this coax run obviously ridiculous at this frequency?”,
• rough budgeting when you already know the system is close to matched.

They become unreliable when you use them for:

• “How much am I losing because my SWR is 4:1?”
• “Will a tuner make the feedline loss go away?”
• “What’s my real radiated power?”
• “Should I change coax length to fix SWR/loss?”

What a more useful approach looks like

Option A: Use tools that take complex load impedance (R + jX)

If you want trustworthy answers, start with the actual complex feedpoint impedance and use real transmission-line math (Telegrapher’s equations / two-port behavior). That’s the difference between “SWR guessing” and real modeling.

Option B: Measure, don’t guess

• Move your measurement reference closer to the antenna (or measure at the feedpoint if practical).
• If you can, measure insertion loss of your coax assembly (including connectors).
• If SWR/pattern/noise seems “weird,” assume common-mode is involved until proven otherwise—and fix it at the source with proper choking (ideally at the feedpoint).

Option C: Treat calculator results as a range, not a point value

Even with good math, the inputs vary in the real world. So instead of believing “1.7 dB,” think “roughly 1–3 dB depending on the exact cable and installation.”

Takeaway

Coax/antenna loss calculators are convenient, but they often mislead because they assume you know feedpoint conditions and because they try to predict mismatch-related loss from SWR alone. SWR does not uniquely define load impedance, and real loss depends on line length, complex impedance, and actual transmission-line parameters. If you need answers you can trust, use tools that accept complex impedance—or measure the system—rather than trying to back-solve reality from one SWR number.

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