|World Prefix List||G3YRC Website Award||Club Information||Members Cards||Club Log||Members||Club QSL|
Figure 5 shows the radiation pattern of a dipole antenna as if we were up in the sky looking directly down on the antenna from above. A radiation pattern is a visual representation of how an antenna concentrates (distribute is another way of saying it) its signal. The plot of the 1/2 wave Dipole (sometimes called the polar plot) is shaped like a figure 8 pattern. This antenna actually has gain, that is, signals coming from the front and rear are stronger than signals coming in off the ends.
Figure 5 - Radiation pattern for a 1/2 wave dipole antenna.
Gain - Often misunderstood by radio operators!
Antenna gain is used to indicate the increase in power of one antenna (when transmitting or receiving) as compared to another antenna. Gain is actually a ratio of power levels and is stated in decibels, often abbreviated "db". So how do we use this number? Keep in mind I said "used to indicate the increase in power of one antenna as compared to another antenna.". So, how much gain does your antenna have compared to say, an actual coat hanger? Probably a lot! We know that a coat hanger could not be much of an antenna. But when using db gain to rate an antenna you must know what the reference antenna is!
It is important to note that antennas are passive, they do not "amplify" signals (or effect transmitted audio levels!), they merely re-distribute the power that they get to achieve gain. They have no effect on the "modulation" or audio quality of your radio! There are a few antenna manufactures that claim things like their beams, "have the best modulation". This is plain silly, over imaginative advertising.
Now, knowing something about the dipole (one of the most basic antennas) we can use it to compare antennas. If someone said, it has "6db over a dipole" or "6db, reference antenna is a dipole", it would have meaning! Lets look at two radiation patterns at the same time, to see how one antenna achieves gain. It will become obvious how it works once you see it, check out figure 6.
Figure 6 - How antennas achieve gain. You can see that the dipole antenna concentrates it signal, making it stronger in two directions. The vertical on the other hand, spreads the signal out evenly, resulting in a weaker signal, but with even coverage. The increased range comes from the dipole having gain over the vertical! The dipole uses energy off its sides and puts it in the two main lobes (the two parts of the "figure 8" pattern) You can see how gain translates into better signal range, by looking at the "increased range" marking on the diagram.
There is another "antenna" that people use give good honest gain figures over. It is known as the isotropic radiator, an antenna that exists only in theory. If we visualize the radiation pattern in 3D, we can see this antenna is radiating equally well in every direction. It is even transmitting straight down, straight up....all with the same intensity. You could picture it like a sphere (a perfectly round ball), the signal emits from the antenna (which would be at located at the exact inside center of the sphere) equally well in all directions. On planet earth however, this is not possible! The effect of the ground and other object alter the pattern of antennas on earth, and even if we designed an antenna to radiate like this, it would never work unless we took it out to space where there would be no objects (like the ground for instance) to interfere with it. Since it does not favor any particular direction, an isotropic radiator has a gain of Zero (0db).
So then, even our lowly dipole has gain over an isotropic radiator. How much you ask? Better sources say that a dipole has 2.1 db gain over an isotropic radiator. Then, if I tell you I have a antenna that has 6db gain over a isotropic radiator then, you know also that it has 3.9db over a dipole (6 - 2.1).
Great! So, how can I use this term gain? I have read many discussions on gain. The rule of thumb is that for every 3db of improvement you add to your antenna, it results in an effect that is noticeable to receiving stations (when transmitting, this goes for receive too). Any db less than 3 indicates an improvement that not appreciable (really detectable). Do not discount gain improvements under 3db...sometimes .5 of a db means hearing or not hearing a station!
Lets look at an example, say your current antenna has 9db of gain over a dipole. You really want to get a new antenna that is advertising 12 db over and isotropic. Really, your not going to see an improvement because we know that the advertised antenna really has a gain of 9.9db (12 - 2.1) compared to a dipole. Therefore, an improvement of .9db for the new antenna! Not a worthwhile gain especially if the new antenna is costly! A word to the wise...do not use advertised gain figures, instead consult this page or an antenna book to see what type of antenna design it classifies as and really see how much gain it has.
So, when you go to examine gain figures, manufactures should be stating a reference antenna. Usually if the gain figure is over a dipole you will see "dBd" for gain over dipole and "dBi" for gain over an isotropic. In all honestly, use all manufactures gain figures with care. Its better to figure out the antenna type (what specific design they are using), then consult this website or an antenna handbook to find out the real gain of the antenna. Gain figures today are so over inflated they are practically useless!
As another point, doubling your power (how many watts you use) results in improvements of 3db! We are not comparing antennas here, but this 3db is an improvement compared to our original signal running half the wattage. So, why bother with big antennas? Well, you might be transmitting further, but your receive range is unchanged! You could actually "out talk your ears", meaning people can hear you but you can not hear them! High gain antennas improve both transmit and receive! You can't talk to people you can't hear!
An example, if you are running 20 watts, you would need to jump up to 40 watts for anyone to hear any difference (a 3db improvement) compared to when you were using 20 watts. So as you can see, once you are running higher wattage (over 100 watts), you must make large increases for other stations to notice a difference.
Not only is this a bad idea because it gets expensive, it is not necessary when using a good antenna! How is this possible? Well, if you are using 500 watts and you would like to get a 1000 watts amplifier, you could alternatively use an antenna that has 3db more gain than your current antenna and achieve the exact same effect, a signal that is louder to the receiving station!
As a precaution, I am going to cover most types of commercial CB antennas, and even if i do not, you will be able to pick out its type and figure out its gain. Unfortunately antenna technology on the HF bands (where 11 meters is located) has not improved all that much over the years. Antennas are constructed better today and optimized, but many of the antennas used in the 1970s are still the best!
Home | Radio Waves | How Antennas Work | SWR | Bandwidth | Dipole | Radiation Pattern | Polarization | Theory Index