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Antenna gain: real signal intensity and theoretical reference values

Almost all radio amateurs begin their hobby using a basic setup consisting of a transceiver and a wire or vertical antenna. This setup can provide great satisfaction, but if it simultaneously sparks a strong thirst for knowledge, it’s a sign that passion is taking over. You might wonder how to improve your setup, and besides considering a radio upgrade (because aesthetics matter), your thoughts will turn to antennas.

The web grants us immediate access to an overwhelming amount of information, and as soon as we delve into antenna research, we are bombarded with a myriad of details that can lead to terrible confusion. One aspect that immediately captures our attention is antenna gain, often prompting questions about why a small and economical antenna might have higher gain than a larger and more expensive one.

Let’s clarify.

Antenna gain is measured in decibels (dB) and represents the ratio of an antenna’s gain to that of a reference antenna. One common reference is the ISOTROPIC ANTENNA (a “theoretical” point source radiating equally in all directions). When comparing the signal intensity of an antenna to an isotropic antenna, the value is indicated in dBi (dB+”i” for isotropic).
Another common reference antenna is the simple DIPOLE ANTENNA in free space. The term “free space” means it accounts for the “THEORETICAL” performance of the dipole without any nearby objects, including the ground. When comparing the signal intensity of an antenna to a “dipole”, the value is indicated in dBd (dB+”d” for dipole).
The free-space dipole exhibits a gain in preferred directions compared to the isotropic antenna of 2.15 dB.

You may have realized that both described antennas, the isotropic radiator and the dipole placed far from obstacles, including the ground, are “pure theory”. You might wonder: what use are these values? At first glance, they might seem “useless,” but in fact, these values allow us to make the right assessments.

The values obtained with reference to a “real ground” antenna are closely related to the environment and, most importantly, have a strong dependence on the installation height. Therefore, the same antenna will have different gain values when installed at heights of 1/4, 1/2, or 1 wavelength above the ground. Unscrupulous manufacturers might indicate the most stimulating gain value for potential customers, omitting to specify the installation height and sometimes even the reference antenna (dBd or dBi but only dB)!

Let’s take a simple example of a 3-element Yagi for 14 MHz. Here are the gain data for the same antenna measured in both free space and real ground.

Free space
Gain in “free space” dBd: 4.9 dBd
Gain in “free space” dBi: 7.04 dBi

Real ground at a height of 1/2 wavelength (10 meters above the ground)
Gain in real ground in dBi at 1/2 wavelength above the ground: 11.33 dBi
Gain in real ground in dBd at 1/2 wavelength above the ground: 9.19 dBd

Real ground at a height of 2 wavelengths (40 meters above the ground)
Gain in real ground in dBi at 2 wavelengths above the ground: 12.68 dBi
Gain in real ground in dBd at 2 wavelengths above the ground: 10.54 dBd

This helps us understand that if a manufacturer “forgets” to specify reference values and only uses the most convenient one, they might claim that their antenna has a gain of a whopping 12.68 dB. In contrast, the manufacturer with an “iron memory” will state the following characteristics:

Free space
Gain in “free space” dBd: 4.9 dBd
Gain in “free space” dBi: 7.04 dBi

Real ground at a height of 1/2 wavelength (10 meters above the ground)
Gain in real ground in dBi at 1/2 wavelength above the ground: 11.33 dBi
Gain in real ground in dBd at 1/2 wavelength above the ground: 9.19 dBd

It is entirely evident that the only values that remain unchanged and that we must use for necessary comparisons are those related to free-space gain, with the appropriate specification of the value expressed in dBd or dBi. Values referring to installations on real ground, having a strong correlation with the surrounding environment, require more careful consideration.

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