Or, making sense of confusing gain claims......
One of the most confusing and embellished aspects of CB equipment advertising, are antenna gain ratings. Most CB'ers, by definition, are continuously looking for ways to better their signal and, by extension, their status among their radio peers. One of the easiest and most effective ways to improve one's station is through the antenna. A well made gain antenna, can wake up a formerly marginal signal. Manufacturers are well aware of this, and they also exploit the general lack of solid technical background possessed by most CB'ers. They take advantage of this void in technical know-how to make inflated, questionable, and vague claims about their products for the sole purpose of attracting people and making sales. Some antenna makers use little gimmicks to justify their extra gain numbers. While more knowledgeable people may question the ethics of such practices, it has nonetheless continued even to this day. When evaluating a CB antenna purchase, it is helpful to make comparisons to similar amateur and commercial antennas. Do these antennas utilize these same "gain" enhancing "gimmicks"? Shouldn't it be logical to assume that any tried and true method for increasing performance would be utilized in antennas for other services? So why then, does this "magic" only work on CB?
At this point, it is also helpful to separate base station from mobile antennas. Since mobile antennas become structurally impractical if they exceed a physical 1/4 wavelength, it becomes difficult to achieve any appreciable gain, so gain ratings are rarely given for a mobile antenna, (since there usually isn't any) other than vague references such as; "Transmits 20% farther" (Farther than what?). For this reason, this discussion will be limited to base station antennas, where gain figures are commonly (mis)given.
Like I've said before, CB'ers are drawn to the biggest and best equipment. So with this in mind, many antenna manufacturer's marketing departments took a broad amount of liberty in making performance claims. Since it was not likely that the average CB'er would be taking the companies to task for misleading ratings (Another reason why ham and commercial antenna makers are less flamboyant with their gain claims), it was open season for wild advertisements to commence, much to the chagrin and consternation of the CB consumer, who's only looking for the best possible performance for the money he has.
Will the real db please stand up.....
One of the most common terms that is associated with antenna gain is the "db". db stands for "decibel", and it is a unit of relative power. For a db to be meaningful however, it has to be compared against a known standard of some sort. Simply saying that a particular antenna has "18 db of gain" is meaningless unless we know what it was compared against. 18 db compared to a standard 1/2 wave vertical? A 1/4 wave GP? A coat hanger? A wet noodle? Hopefully you get the idea.
It is this little ambiguity which allows some manufacturers to inflate their claims, and yet still claim to be telling the truth even when the smoke and mirrors of their measurement methods, and of their reference is revealed.
There are some engineering standards which are commonly used when comparing antennas. The 3 most common standards are dbi, dbq, and dbd.
dbi is in reference to an isotropic, theoretical antenna which radiates equally in all directions in a spherical pattern. This however, is not possible in practice.
dbq is a reference compared to a quarter wave vertical. The "gain" of a quarter wave falls somewhere between that of an isotropic radiator, and that of a half wave dipole. This standard is not as popular as dbi and dbd.
dbd is the most truthful of standards. It is referenced to a real 1/2 wave dipole antenna, which is one of the most basic antenna designs. A dipole has a "gain" of about 2.14dbi, or 0 dbd..
With these standards in mind it should be easy to see that an antenna with a rated gain of 4dbd is "stronger" than another antenna with a rated gain of 6dbi. Yet without the reference figures, one could be fooled into thinking that "more is better" and that the higher number is automatically the stronger antenna.
Because of the nature of CB and the desire of manufacturers to make their products look as big (or bigger) as they can be, most CB antennas are rated in terms of dbi (Of those who are actually somewhat honest).
And if you're not already confused.......
So now that we know what these standards mean, what does this mean in the real world?
Vertical antennas are normally derivatives of either 1/4, 1/2, 5/8, or 3/4 wave antennas. We've already determined that a 1/2 wave dipole has a gain of about 2.14 dbi. So why then do other 1/2 wave designs (such as the 1/2 wave ground plane), advertise a gain of up to 3.75 dbi? Much of this can be explained by another phenomenon of antennas; radiation angle or pattern. An antenna does not radiate equally in all directions (unless it's a theoretical isotropic radiator), so there will be some places where it will be stronger than others. Generally speaking, the higher the gain of a vertical antenna, the narrower its radiation pattern will be. When comparing the realized gain difference between 2 different antennas, gain figures could vary considerably depending on which angle the comparison is made at. Manufacturers can then compare apples to oranges by comparing the gain of the reference in a place where it is not at its peak gain, against their product, which performs better at that angle. This may or may not translate to better performance for the average user, but it does give them some grounds to defend their often inflated gain claims.
The other factor to consider is radiation angle. This refers to the angle in degrees above the horizontal plane, which the major radiation lobe is concentrated. Generally speaking, the lower the radiation angle, the more useful power is being sent in the desired horizontal direction, and not into the sky. But this is not always cast in stone. Because of irregular terrain, and things like mountains and valleys, one particular antenna, with a higher radiation angle, may perform better against a superior design with a lower radiation angle. For those who pursue DXing, a higher radiation angle may actually be preferable for shorter skip zones.
The highest gain vertical configuration, which still offers a fairly low radiation angle is the 5/8th wave vertical GP. The 5/8th wave has about a 1.85dbd gain, which equates to about 4dbi. 3/4 wave antennas can achieve slightly better gain, but at the cost of a higher radiation angle, which is why they are seldom seen. On CB frequencies, be especially skeptical of any non directional vertical which advertises anything over about 4.5dbi gain. You just can't get there from here.
On VHF and UHF frequencies, where primary radiator length is greatly reduced, there is another technique which is commonly employed, to increase gain, and that is known as a "colinear" configuration. A colinear antenna is simply multiples of either 1/4, 1/2 or 5/8 wave elements stacked on top of each other, and kept in phase by utilizing a coil between sections. This allows the antenna to achieve gain values of up to around 7 or 8 dbi or more depending on the number of segments in the array. The number of segments is normally limited only by physical antenna size. You can have 16, 1/2 wave segments at 900 Mhz, and still be under 9 feet long. However, the downside to a colinear antenna is that they achieve their extra gain at the expense of greatly narrowing the E-Plane radiation pattern, which can then produce dead spots in areas which fall above or below the main lobe of the pattern. To visualize this phenomenon, think of a 1/2 wave dipole antenna as radiating in a pattern in the shape a donut. Then take a flat piece of metal, and slowly squish the donut nearly flat. Notice that the diameter (gain) of the donut will increase, but the height (radiation pattern) will become narrower. This is what essentially happens when you utilize a colinear antenna array.
This technique is not practical for a CB antenna, due to the size required. If a 5/8th wave is around 20 feet at 27 Mhz, imagine a colinear antenna, which has 4, 5/8 wave segments. I don't think an 80' tall antenna would be very practical or structurally sound.
So, in conclusion, if you believe that an Antron 99 (which is simple end fed 1/2 wave) really has 9.9 "db" of gain, or that an A/S Starduster (which is only a 1/4 wave radiator) actually has 5 db (At even the dbi standard) of gain, then I have some swamp land in Florida that I'd like to sell you.