How to Read Antenna Gain: Why a Little Negative dBi Is a Lot of Antenna

Antenna Physics · Answered

How to Read Antenna Gain: Why a Little Negative dBi Is a Lot of Antenna

Short answer: on our gain tables, 0 dBi is an ideal quarter-wave vertical with a full field of buried radials — the antenna almost nobody on a small lot can build. A Greyline at −0.5 dBi on 40 meters is within half a decibel of that ideal, with no radials in the ground at all. Do not be afraid of small negative numbers. They are the sound of an antenna working.

What the zero is

We modeled every Greyline height in NEC over average ground and compared it to a quarter-wave vertical over a full, tuned radial field. That reference sits near 0 dBi at its best angle. It is the antenna in the textbooks: a quarter-wave standing on dozens of buried radials. Rudy Severns, N6LF, measured what happens when you build one with only a handful of radials in real soil — you give back decibels to the ground. Most backyard quarter-waves never reach the zero on the chart.

So when a Greyline shows 0.0 dBi on 40 meters, read it as equal to the textbook quarter-wave, without digging a single radial. When it shows −0.5, read it as half a decibel shy of that ideal.

How big is a decibel?

1 dB About the smallest change most ears can pick out on a signal.
3 dB Your power doubled, or halved.
About 6 dB One S-unit on the other station's meter.

By that scale, the difference between −0.5 and 0 dBi is something no one on the other end will ever hear. The difference between having an antenna up and not having one is everything.

Why the shorter heights read lower on 40

You will see bigger negatives on the shorter models: the 12-foot reads about −3.8 dBi on 40 meters, the 16-foot about −2.7, the 20-foot about −1.6. That is not a flaw; it is the aperture lesson in one row. A 12-foot pole is a small fraction of a wavelength on 40, and the low bands reward height more than anything else. The same short antenna runs ahead of the quarter-wave up top, from 15 meters through 10. Height moves the whole row up; that is why we say buy the tallest your lot, your HOA, and your wind allow. What height should I buy? →

The low bands: any signal is a win

On 40, 60, 80, and 160 meters, every backyard vertical is electrically short, and every HOA lot is too small for the antenna the band would like. That is the honest starting line. From there, a Greyline puts a real signal on the air: operators work 500 to 1,000 miles on the low bands and are delighted to be in the game, and the founder routinely works about 1,000 miles on 160 and 2,500 on 80 from one. Some would call that a struggle. The people who were not on those bands at all call it a doorway.

Beside the strongest band, still ahead

Each height has a band where its bonus is strongest — the Skylines on 17 meters, up to about +4 dBi. But the bands beside it are not a cliff. +0.1, +1, +2 dBi on an adjacent band is still a real advantage over the full-radial quarter-wave, and even where the pattern turns multi-lobe (EDZ), plenty of that energy lands where DX and regional contacts live. These are benefits, not fine print.

Questions about a number on our tables? Call and ask. — Jon KL2A · 435-200-4902