ANTENNA DB GAIN

The gain of an antenna is a measure of its performance as compared to a standard antenna. A gain figure is meaningless without the reference being stated. Common references are: dBi - Isotropic radiator. A theoretical antenna, which radiates signal equally in all directions (or would if it existed). dBd - Reference Dipole. As an antenna cannot transmit more power than you put in, this gain means more power in one direction at the expense of less power in other directions.

Directivity
This is the ability of an antenna to direct more signal in one direction. The diagram on the right compares the signal from omni-directional and directional types, as viewed from above. While the directional type will transmit further, it can be seen that there are places it will not get your signal to. All practical antennas are directional to some degree. Several factors can affect the directivity - including design, height above ground, and other nearby objects. This kind of directivity is not always desirable. Although it is perfect for applications such as TV, where you know which direction the signal will come from, general communications could come from different directions.
As well as horizontal directivity, an antenna will exhibit some degree of vertical directivity. This may be referred to as the radiation angle, vertical angle or elevation. The diagram on the left shows a view from the side, displaying how increasing the vertical directivity can improve the gain. As the intention is to communicate with or receive stations on earth, any upward signal is wasted. So it follows that the lower radiation pattern (light grey in the diagram) is far better.

Directional Beams
A directional beam is designed to give a large degree of horizontal directivity. This type is useful for directing the signal where you need it, but can also be used to reject unwanted signals or noise before they reach your receiver. As well as dBi or dBd gain, a directional beam will also have a front to back ratio which is expressed as a dB figure. This is simply a comparison between the amount of signal transmitted in the desired direction, and that going out the back.

dB to gain conversion chart.

dB		power gain		dB		power gain		dB		power gain
1	=	1.26		3.5	=	2.24		5.5	=	3.55
1.5	=	1.41>		4	=	2.5		6	=	3.98
2	=	1.58		4.5	=	2.82		6.5	=	4.46
2.5	=	1.78		4.77	=	3		7	=	5.01
3	=	2		5	=	3.16		8	=	6.3

Remember that, as the dB figure is a comparison, it must include details of what it is better than. Also remember that the antenna only puts out what goes in, so that extra power is in a certain direction - there will be a drop in some other direction.

Antenna SWR.

SWR
SWR (Standing wave Ratio) is a measure of how well the antenna is matched to the transmitter. A poorly matched antenna will not perform as well as if it were correctly tuned. The Standing Wave Ratio is equal to the ratio of radio and antenna impedances. This means that for a 50 Ohm radio, an impedance of 75 Ohms will give a SWR reading of 1.5:1.
The SWR meter

The typical CB SWR meter does not actually measure standing waves. The standing wave ratio is calculated by measuring the amplitude of the forward and reflected signal.

What is a good SWR reading ?
The best reading possible is 1:1, but a reading below 2:1 is often acceptable. It must also be noted that many CB antennas are not exactly 50 Ohms and, as noted above, this will mean that the antenna will not give a 1:1 reading even if tuned correctly. Examples include the standard dipole, which has an impedance of 75 Ohms - giving a 1.5:1 reading when correctly tuned.

Why bother?
There are several reasons why SWR is important. It is well known that a high SWR means that not all the power is being transmitted, and so a good SWR will ensure that you get the most signal out that you can. There are also the dangers of damage to your transmitter and interference to nearby equipment. This is discussed in more detail on the other pages in this section.

Coax length
Most radio enthusiasts have heard about the coax length issue. So let's clear this one up now... As long as your antenna is correctly tuned, COAX LENGTH DOES NOT MATTER. If changing your coax length appears to change your SWR, then you have a problem with the antenna. Rather than worrying about coax length, sort out the problem with the antenna. For those wanting to understand why coax length may appear to change the readings, this topic is covered in more depth on the other pages in this section and the feeder section.

SWR and efficiency.

Chart. SWR Radiated % Loss 1 100 ------- 1.3 98.3 .08dB 1.5 96 .18dB 1.8 91.8 .36dB 2 88.9 .51dB 2.5 81.6 .86dB 3 75 1.25dB 3.5 69.1 1.61dB 4 64 1.94dB 5 55.6 2.55dB When using this chart, you should keep it in mind that reflected signal is only one factor involved in antenna efficiency. Antenna design, damage and aging can also make a large difference to efficiency. Some of these may not show up on an SWR meter, as some do not change impedance while others may be hidden by detuning the antenna to compensate for the problems which you do not know exist. This may sound strange but, as the meter is really only showing impedance ratio, detuning the antenna changes it's impedance which may then cancel out the change caused by the damage. The SWR may appear OK, but the antenna will still not be working at it's best. It can be important to also visually check the antenna for damage or water. Effects of a high SWR. Audio circuit feedback. As the transmitter attempts to deal with reflected signal, which it was not designed to cope with, this energy may get into other parts of the transmitter. The effects of this can be squealing, buzzing, warbling and other unwanted noises on your transmitted signal. RF circuit feedback. In really bad cases, or where a transmitter has a poor design (many CBs), reflected signal can also cause the transmitter to lock on transmit or to go into self-oscillation. In this situation, the output is no longer under the control of the internal circuits - and it may reach levels high enough to cook the output and any supply feeding it. Large supplies have disappeared in an equally large cloud of smoke due to this kind of problem. Transmitter damage. Any energy reflected back must be dissipated in some way. That energy which reaches the transmitter may be converted to heat. The transmitter, being designed to be used with the correct antenna, may not be able to get rid of this heat effectively. The resulting increased temperatures reduce the life span of the semiconductors used in the output stages, and extreme temperatures can instantly destroy such components. Some components may also be unable to cope with the higher peak voltages created by the combination of the transmitted and reflected wave. Radiation pattern. A poorly matched antenna can cause the feeder to radiate, resulting in a change to the radiation pattern. This may not be helpful, as your signal may now be sent into a nearby hill or even up into space. More about feeder is included in the Feeder sub-section within this site. Interference. As any unwanted radiation may be at lower levels, closer to other electronic equipment, interference to other devices is a real possibility. Movement. Moving or touching any part of your system may change any of the above effects. Even touching nearby objects, while you have the microphone in your hand, can make the above effects change in some way. Changes may involve reduction, increase, or additional symptoms occurring.