by Rick Fearns, K6VE
Let’s begin by talking about what is meant by “antenna gain.” Antennas do not amplify a signal, but merely re-direct energy in a specific pattern instead of evenly distributing it in a perfect circle. The basic antenna design against which most antennas are compared, is the DIPOLE. To build a dipole antenna, you’ll need just three insulators, a coil of wire (the length of which is determined by the frequency for which it’s made), and some transmission line. Don’t underestimate the dipole antenna. It’s a great performer, and believe it or not, it has over 2.15 db of gain!!! Really? I thought that antennas are referenced to the dipole, so shouldn’t a dipole have a gain of 1? If it doesn’t, what antenna does it have gain over?
Perhaps you bought a VHF or UHF antenna recently and made your choice based upon comparing the gain figures listed on the outer wrapper of the package. Let’s say, for example, that you chose an antenna whose manufacturer claimed a gain of 3.5 dBi. That doesn’t mean your new antenna has 3.5 db gain over a dipole! It doesn’t. Look at the package and check the gain figure very carefully. Notice that the claimed gain figure reads 3.5 dBi? What is dBi? Well, dBi means that the antenna you purchased has an actual gain of just 1.35 db over a dipole antenna. Yes, that’s right. Your super $100 fiberglass beauty has just about the same gain as a three dollar wire dipole! (Three insulators @ $1 each. Wire and transmission line free from your junk box). 1.35 db gain is hardly enough signal to move an “S” meter! What happened to the 3.5 db you expected would move the receiving station’s meter half an “S” unit higher?
The answer as to what happened to the “missing” 2.15 db lies in knowing the difference between dBi and dBd. Simply put, the answer is 2.15 db. But what is dBi and why do manufacturers prefer to reference their antenna gain figures against it rather than to a basic dipole antenna? After all, isn’t the dipole antenna the standard against which antenna gain measurements are made? “No”, not always!
Let’s examine the manufacturer’s gain figure on their whizbang fiberglass antenna more carefully. The claimed 3.5 dBi figure includes the letters dBi. dBi means decibels over isotropic. Simply put, isotropic is a theoretical pinpoint radiation source (that means it really doesn’t exist) in space that disperses a signal in a perfect sphere. “i” means isotropic radiator. A dipole antenna (referred to simply as “d” in the specification) is a real antenna and offers about 2.15 db more gain than an isotropic radiator. So there you have it. When a manufacturer measures the gain of an antenna versus an isotropic radiator, they are adding 2.15 db to their claimed gain figure, without any other design changes.
To convert between dBi and dBd, simply add or subtract 2.15 db as appropriate, depending upon which way you’re going (dBi to dBd or vice versa). Remember that when choosing an antenna based upon its gain, you should subtract 2.15 db from the manufacturer’s claimed dBi figure in order to learn how much gain you can expect to have over a simple wire dipole antenna. Then, you’ll know how much gain you’re really paying for.
