UNDERSTANDING ANTENNA GAIN

What's This About Antenna Gain?


One of the most discussed topics in Amateur Radio is antenna gain. Every antenna that you buy will have some type of gain figure in its specifications telling you why their antenna is better than the other brand. Very few, however, use this specification in its proper context or do they tell you to what it has gain over. A good example of this is some of the cellular antennas advertised today. Often two very similar antennas will have specifications that are very different. One will specify that it has 3dB gain and the other will show 5dB gain. They look almost identical, so who is telling the truth?
Knowing how to decide which antenna to purchase can be quite confusing based on the information that the manufacturers provide. Manufacturers have sometimes been careless or outright deceitful about how they advertise their antenna specifications. I have actually compared an antenna that advertised a gain figure of 5.4 dB to one which advertised a figure of 6 dB only to find that the one with the lesser advertised gain actually produced the stronger signal of the two.
Gain is a comparative measurement, usually expressed in decibels (dB). By calculating this "gain" figure in dB we can express an evaluation of not just the antennas radiating efficiency, but the efficiency of the overall antenna system. In mobile systems "Relative Gain" is the best indicator of how an antenna will perform. Relative Gain is the ratio of power gain (in a given direction) for the antenna under test to the power gain of a reference antenna (in the same direction).
For example, in the case of familiar six and two meter mobile antennas there are two ways to express relative gain. These are (1st) using an "isotropic reference" (dBi) and (2nd) using a "quarter wave dipole reference" (dBd). Often the quarter wave ground plane is used as a reference since it is equivalent to the quarter wave dipole and is more common in mobile use. Reputable manufacturers like Larsen and some others, use the dBd reference. In Larsen's case, their comparison antenna is a quarter wave radiator mounted on an appropriate ground plane which is equivalent to the half wave dipole.
Some manufacturer's state gain figures (i.e. 3dB gain, etc.) without giving the reference source from which their figures were calculated. Many give their reference as dBi (or an isotropic source) which gives the illusion that the antenna has as much or more gain than their competitor who is quoting dBd (quarter wave dipole) figures. Unfortunately dBi figures are calculated values and effectively are 2dB less than the quarter wave reference antennas similar to those that we actually use. In other words the quarter wave dipole has 2 dB gain over the isotropic radiator (dBi) reference that many manufacturers calculate and have historically advertised. An isotropic source is merely a mathematical representation of a point of energy, radiating in free space equally in all directions, and without any loss (attenuation) figured in. Although there is no such thing as an actual "isotropic source" antenna, this is a great and theoretically valid way to make a cheap antenna spec. well against a well designed and built model without really lying about the numbers. Most people don't know or understand the differences in these two ways of calculating gain figures and assume that 6 dB simply means 6dB gain, period. It doesn't.
The truth is that because of the differences in actual patterns and theoretical patterns, a well designed quarter wave antenna (dipole or ground plane vertical) will exhibit about 2 dB gain over the isotropic source calculations every time. To put it another way, an antenna claiming 3dBi gain would only produce 1dB gain when compared to a quarter wave reference antenna (dBd). There are many factors affecting gain that come into play when an antenna is put into operation. Some of these are the materials used to build the antenna, their physical size and properties, and how they are assembled.
The most common radiator material in amateur mobile use is probably stainless steel. Stainless steel and Nichrome wire (which is used in heating elements) are in the same family of conductors. In other words stainless steel is not the best conductor but is chosen for its other characteristics. Losses from the "skin effect" you learned about from your study of AC circuits comes into account when a transmitter applies power to an antenna. Some of the power will be lost from heating in a poor conductor which produces some reduction in radiated signal. (I.e. Loss of gain or attenuation) That is why you find many reputable antenna manufacturers plating their stainless steel radiators with copper. Copper is a much more conductive material than stainless steel and thus will significantly reduce the antennas losses from "skin effect" and heating. Many quality antennas are "factory tuned" for a given band spread to further reduces losses from varying material quality and variances in reproducible part manufacturing and to squeeze all the radiation from their antenna that can be obtained. Hopefully more gain than their competitors. Other factors affect gain in actual radio installations such as antenna placement, quality and routing of the feed line, the final tuning and trimming of the antenna radiating element to resonance, and probably the most vulnerable, installation of the installation of the coax connector. If you do nothing else as a home brewer in radio amateur, learn how to properly install RF connectors, this is crucial to a good antenna system.
What all this really means is that an antenna with a true 3dB gain with reference to a quarter wave dipole or equivalent will effectively double the radiated power of your transmitter and also double the receive sensitivity as well. Remember that an increase of 3dB gain equals the same radiated power from your station as if you had doubled your transmitter power without the 3dB gain. An antenna with the same 3dB specifications referenced to an isotropic calculation will produce only about 1dB relative gain when compared to a quarter wave dipole reference. That is why you will notice strange differences say between a high quality 5dB gain antennas like the old "stationmaster" series when compared to a 6dB aluminum bargain brand antenna. Now you know one of the reasons why the 5.4 dB gain stationmaster will outperform its cheaper 6dB counterparts every time.
Gain is a relative term in its own right and can confuse us about what is actually happening with our signal to give it gain. No power increase is actually produced with antenna gain. The "isotropic radiator" we have been discussing can be illustrated by visualizing it as a light bulb suspended in free space and without the shadow of the socket interfering with its output. It radiates equally in all directions; even the directions in which there are no receivers, straight up and straight down. We produce gain in antennas by altering the pattern of radiation into one which benefits our use. For instance if you held a parabolic mirror behind that "isotropic" light bulb you would be able to see farther with better light, but only in the direction in front of the mirror. We haven't produced more light but have "focused" its radiation onto a pattern that serves our purposes better. In this illustration we have produced forward gain in the light radiated and since there is little light behind the mirror, we have produced a "front to back ratio" on an order of magnitude as well. This is a good illustration of how a yagi or a parabolic antenna works, nothing new is produced; we just bend it or "focus" it to our advantage. The reason that a quarter wave antenna exhibits 2 dB gain over the "isotropic" antenna is that the real world quarter wave dipole or vertical radiates more of its signal parallel to the radiating element(s) rather than equally in all directions as in the case of the isotropic radiator. This, in essence, is "focusing" more of the signal in useful directions at the expense of directions we can't use. Producing this same effect is the purpose behind the yagi, parabolic, quad, and other directional antenna designs we are familiar with.
Perhaps this information will serve to clear up some things that antenna advertisements have spent years complicating. At least none of will have to scratch our heads when comparing antenna specifications for that next install. The discussion in this article is one of several reasons why some of the amateur radio magazines stopped printing manufacturer specifications in their antenna ads several years ago. Perhaps they have made an impact in the way manufacturers develop and advertise their gain figures and that they are all on the same page with their advertised figures now. In any event this is a synopsis of one of my study adventures and was not meant to be a text on antenna gain. At least it puts me on a better footing when comparing all the antenna specifications I've been trying to understand all these years. I hope that it will do the same for you.

Keith Carter, KF4BI


References: Larsen Antenna Education publications, (circa. 1980)
ARRL Handbook, (2007)
ARRL Antenna Book, (1995)
Standard Communications Dealer Kit, (circa. 1981)
Lots of personal experience, (1970 - 2007)
USMC Field Antenna Handbook, (circa. 1970)
Thanks to Larsen Antenna Mfg. for providing much of the invaluable information for this article in their
1980 circa. Land Mobile Dealer publications.

 

Contents Copyright © 2008 Coastal Plains Amateur Radio Club Inc.

Web pages by W4KN and WD4LYV