"My Antenna Works Great!" — Compared to What?

You hear it all the time. Someone builds an antenna, keys up, makes a few contacts, and declares: "It works great!" And maybe it does. But how would you know? Compared to what — a coat hanger? A fork? The gutter on your roof?

Without a reference, "it works" doesn't tell you much. You made contacts — sure. But would a wet noodle hanging from your balcony also have made those contacts on a good propagation day? Probably. The ionosphere is generous sometimes. That doesn't mean your antenna is good.

This isn't about being negative. It's about having a way to actually know whether your antenna is doing its job, and how well. And for that, you need two things: a reference to compare against, and a unit that makes the comparison meaningful. That's where decibels come in.

Why "It Works" Isn't Enough

Say you build a dipole for 20 metres, string it up between two trees, and call CQ. Someone in Germany comes back to you with a 59 report. You're happy. The antenna works.

But here's the thing — on 20 metres with decent propagation, you could probably make that same contact with 100 watts into a random piece of wire. Or into a non-resonant vertical. Or into a dipole that's 3 metres too short and way off resonance. The ionosphere was doing the heavy lifting, not your antenna.

Now imagine you have two antennas: your dipole and a 3-element Yagi, both at the same height, same coax, same radio. You switch between them and the Yagi is consistently 2 S-units stronger on every station you hear. Now you know something real. The Yagi is better than the dipole — by a measurable, repeatable amount. That's a grounded comparison.

The key word is "compared to." Without it, any statement about antenna performance is just a feeling.

The Decibel: A Way to Compare Things

The decibel (dB) is not a unit of power or voltage. It's a ratio — a way of expressing how much bigger or smaller one thing is compared to another. That's it. It always compares two values.

The formula for power is:

dB = 10 × log₁₀(P₁ / P₂)

If P₁ is twice P₂, that's +3 dB. If P₁ is ten times P₂, that's +10 dB. If P₁ is half of P₂, that's −3 dB.

Some useful values to remember:

The reason we use decibels instead of plain ratios is that they add up nicely. If your coax loses 2 dB and your antenna has 7 dB of gain, the net is 7 − 2 = 5 dB. Try doing that with raw power ratios and it gets messy fast.

Because plain dB is just a ratio — no fixed reference — it works with anything. You can add or subtract dB values freely regardless of what they refer to. Coax loss in dB, antenna gain in dBi, filter attenuation in dB — they all combine just fine, because you're just stacking ratios on top of each other.

But here's the important part: a dB value is meaningless without knowing what it's compared to. "This antenna has 6 dB of gain" — 6 dB more than what? A rock? A dipole? An isotropic radiator? The answer changes everything.

dBi — Compared to an Isotropic Radiator

When you see gain expressed in dBi, the "i" stands for "isotropic." The reference is an isotropic radiator — a theoretical point source that radiates equally in all directions. It doesn't exist in the real world (you can't build one), but it's a mathematically perfect reference because it has no preferred direction.

An isotropic radiator has 0 dBi of gain by definition. Every real antenna has some gain in dBi, because every real antenna concentrates energy in some directions more than others. Even a simple dipole doesn't radiate equally in all directions — it has a figure-8 pattern with nulls off the ends and maximum radiation broadside. That concentration gives it about 2.15 dBi of gain.

dBi is the standard in antenna simulation and engineering. When 4NEC2 or any other antenna modelling software reports gain, it's in dBi. When you see antenna specifications from manufacturers (the honest ones, at least), it's in dBi. It's the universal reference.

dBd — Compared to a Dipole

When you see gain expressed in dBd, the "d" stands for "dipole." The reference is a half-wave dipole in free space — a real, buildable antenna with a well-known pattern and gain.

A half-wave dipole has 0 dBd of gain by definition. Since a dipole has 2.15 dBi, the conversion is simple:

dBi = dBd + 2.15

So a Yagi with 5 dBd of gain has 5 + 2.15 = 7.15 dBi. Same antenna, same performance — just a different reference point.

Why does dBd exist? Because for many hams, the dipole is the practical baseline. It's the simplest real antenna you can build. Saying "this Yagi has 5 dBd" means "it's 5 dB better than a dipole" — which is immediately useful. You know that switching from your dipole to this Yagi will give you roughly 5 dB more signal. That's almost one S-unit.

The Trap: Mixing Up dBi and dBd

This is where people get burned, and where some manufacturers get sneaky.

A Yagi with 7 dBi of gain and a Yagi with 7 dBd of gain are very different antennas. The 7 dBd antenna actually has 9.15 dBi — it's over 2 dB better. That's a meaningful difference.

Some manufacturers advertise gain in dBd when the number looks bigger, and in dBi when it suits them, without always being clear about which one they're using. Some don't specify the reference at all — they just say "7 dB gain." That's like saying "the store is 5 away" without saying 5 what — kilometres? miles? minutes?

When you see a gain number, always check the reference:

• dBi → compared to isotropic (the universal standard)
• dBd → compared to a dipole (practical, but 2.15 dB lower than dBi)
• dB with no suffix → ask. Don't assume.

If a manufacturer claims "12 dB gain" for a small antenna without specifying the reference, be sceptical. They might be using dBd to inflate the number, or they might be measuring under unrealistic conditions, or they might just be making it up.

dBm — Compared to 1 Milliwatt

You'll also see dBm in radio work. This one is different — it's not a ratio between two signals, it's an absolute power level referenced to 1 milliwatt.

0 dBm = 1 mW

From there:

• +10 dBm = 10 mW
• +20 dBm = 100 mW
• +30 dBm = 1 W
• +40 dBm = 10 W
• +47 dBm = 50 W
• +50 dBm = 100 W

dBm is useful because it lets you do link budget calculations entirely in decibels. Your transmitter puts out +47 dBm (50 W), the coax loses 2 dB, the antenna adds 7 dBi — the effective radiated power is +47 − 2 + 7 = +52 dBm. Clean and simple.

The key thing to understand: plain dB is just a ratio. It has no fixed reference — it just means "this much more" or "this much less." Because of that, you can add or subtract dB to anything. Coax loss? That's a ratio. Antenna gain? Also a ratio. They combine freely.

Think of it this way: you order a pizza and your friend orders one that's twice the size. His pizza is +3 dB bigger than yours. That 3 dB doesn't care whether it's pizza, water, or RF power — it just means "double." Always just a ratio.

dBm and dBi are different. dBm is an absolute power (referenced to 1 mW). dBi is a gain (referenced to an isotropic radiator). They measure different things. You can't add 20 dBm + 7 dBi and get "27 dBm-i" — that's meaningless, like adding kilograms to metres.

What you can do is treat the antenna gain as a plain dB ratio and apply it to a power in dBm. That's what a link budget does: +47 dBm power, minus 2 dB cable loss, plus 7 dB antenna gain = +52 dBm out. The dBi value tells you the gain is 7 dB relative to isotropic — and that 7 dB ratio gets applied to the power, because this is what antenna gain is: the power gain on one direction compared to other antenna (in this case isotropic antenna). The result stays in dBm because you started with dBm and only added ratios.

dBW — Compared to 1 Watt

Same idea as dBm, but referenced to 1 watt instead of 1 milliwatt:

0 dBW = 1 W

So +30 dBm = 0 dBW. The conversion is just dBW = dBm − 30.

You'll see dBW more in professional RF engineering and EMC work. In amateur radio, dBm is more common.

Why Grounded Comparisons Matter

Back to the original question: "My antenna works great — compared to what?"

Here's a real scenario. Someone builds a ground plane vertical for 2 metres and reports: "I'm hitting the repeater 30 km away, full quieting. This antenna is amazing."

Is it? Let's think about it:

• The repeater is on a hilltop with a high-gain antenna and a sensitive receiver.
• The repeater is designed to be easy to reach — that's its whole purpose.
• With 50 watts on 2 metres, you could probably hit that repeater with a piece of coax with the shield stripped back 50 cm.

Hitting an easy repeater doesn't tell you the antenna is good. It tells you the repeater is accessible. To know if the antenna is actually performing well, you'd need to compare it to something:

• A/B test: Switch between your ground plane and a known reference (a dipole, a J-pole, a different vertical) and compare signal reports on the same stations.
• Simulation comparison: Simulate your antenna in 4NEC2 and compare the predicted gain to a reference dipole at the same height. If the simulation says your ground plane should have 1.5 dBd of gain and the dipole has 0 dBd, you know the ground plane is about 1.5 dB better — a small but real improvement.
• Absolute measurement: Use a calibrated field strength meter at a known distance. This is the gold standard but requires equipment most hams don't have.

Without one of these, "it works" is just "I made contacts." Which is fine — making contacts is the point of ham radio. But it doesn't tell you anything about the antenna.

How to Make Meaningful Comparisons

If you want to actually know how your antenna performs, here's what works:

Simulate first. Before you build, model the antenna in 4NEC2 or similar software. The simulator gives you gain in dBi, pattern, impedance — all referenced to the isotropic standard. You can compare any two designs on equal footing. A simulated 7.2 dBi Yagi is better than a simulated 5.1 dBi Yagi, period. No propagation variables, no S-meter inaccuracy, no guessing.

A/B switch on the same station. If you have two antennas, use a coax switch and compare them on the same signal, back to back, multiple times. The one that's consistently stronger is the better antenna for that direction and frequency. Note the difference in dB if your radio has a calibrated S-meter (or use an SDR with a power meter).

Compare to a known reference. If you only have one antenna, compare your real-world results to what the simulation predicts. If the simulation says you should have 7 dBi and you're getting signal reports consistent with that, the antenna is working as designed. If you're consistently weaker than expected, something is wrong — bad coax, poor connections, wrong height, ground losses.

Use the right units. When you write down or share your results, always specify the reference. "7 dBi" is clear. "7 dB" is not. "2 dB better than my dipole" is useful. "It works great" is not.

The Honest Antenna Evaluation

Here's what a grounded antenna evaluation looks like:

"I built a 3-element Yagi for 145 MHz. The simulation predicted 7.2 dBi gain with a 14 dB front-to-back ratio. On the air, it's consistently 4–5 dB stronger than my half-wave vertical on signals from the north (where the Yagi is pointed), and about 10 dB weaker on signals from behind. The SWR is 1.2:1 at 145 MHz, matching the simulation. I'm satisfied that it's performing close to the model."

Compare that to: "I built a Yagi and it works great."

Both people might be equally happy with their antennas. But the first person actually knows what they have. They can troubleshoot if something changes. They can compare it to other designs. They can make informed decisions about upgrades.

The second person just knows they made contacts. Which is fine — until the antenna develops a problem and they have no baseline to compare against.

Summary

Every statement about antenna performance needs a reference. "It works" means nothing without "compared to what." The decibel system gives us a clean, universal way to make those comparisons:

• dB — a ratio between two values. Always needs a reference.
• dBi — gain compared to an isotropic radiator. The universal standard.
• dBd — gain compared to a half-wave dipole. Practical, 2.15 dB less than dBi.
• dBm — absolute power referenced to 1 milliwatt.
• dBW — absolute power referenced to 1 watt.

Simulate your antennas. Compare them to known references. Use proper units with clear references. And the next time someone tells you their antenna "works great," ask them: compared to what?