My Dipole Has Gain!
WA5VJB was unavailable this month, so we asked regular contributors W1IS and KC1DSQ to step in as guest columnists with their excellent explanation of an often-mis-understood subject, antenna gain. – W2VU
Many antenna manufacturers specify gain figures in seemingly mysterious terms that are often difficult to interpret. This article provides a framework to assist you in understanding the terms.
The first question: Gain with respect to what?
The answer starts with a most basic antenna free from interference from nearby objects, including the earth. This basic antenna isn’t one that you can build or find in a catalog. It is an imaginary antenna that we can easily describe mathematically: A point source, or a really small antenna, that emits RF equally in all directions, floating in free space. This is called an isotropic antenna or isotropic radiator. As you can imagine, the radiation from this antenna is spherical, as shown in Figure 1. Even though this antenna doesn’t and can’t exist in reality, it provides us with a universal reference against which we can compare our earthly antennas.
Universal Reference – Isotropic Point Source vs. Reality
The first step is to simply describe the point source radiator mathematically. This, as the name implies, radiates equally in all directions. As the field travels away from the
