Most new hams soon discover that there are some disadvantages to using a hand-held radio. In particular, some repeaters cannot be reached or the signal is "scratchy" or "picket fencing" to those listening at the other end.

The dB is a unit that is used to indicate a level of power of a received signal. An increase of one dB is barely distinguishable by the human ear. The strength of a signal is commonly expressed as S2, S6, S9, 20 dB over S9, etc. Each increase in S number (S7 to S8 for example) represents approximately a 6 dB increase in power. An increase of 3 dB equates to a doubling of power. An increase of 10 dB is the same as a 10-fold increase in power. In other words, a signal of 10 dB over S9 means that a signal has 10 times the power of a signal of S9. 20 dB over S9 means an increase of 100 times the power of an S9 signal. Adding dB is equivalent to multiplying power levels. Hence, a signal which has been increased by 13 dB (13 = 10 + 3) has been increased by 20 times in power (10 x 2). Similarly, a decrease in power is expressed as minus dB. A signal which has 20 times less power has -13 dB less power. Increases or decreases other than 10 dB and 3 dB have to be determined mathematically using logarithms. The formula can be found elsewhere, and is too complex for this discussion.

Antennas are often rated in dBi or dBd, where dBi is 2.15 dB less than dBd. dBi is based on a theoretical antenna which is a point in space, whereas dBd is based on a perfect dipole or ground plane vertical antenna. Usually hams start with a vertical or dipole antenna, and it is easier to related to the gain of a new antenna over that of a vertical or dipole than it is to some theoretical point in space. Manufacturers usually give the gain of an antenna in dBi (salesmen like to use big numbers to make selling points), so one must remember to subtract 2.15 dB from the manufacturer specification to see what the gain over a ground plane or dipole antenna would be.

Some Practical Examples:

A hand-held radio using its "rubber duck" antenna loses about 4 dB of signal before it leaves the antenna. This is because a rubber duck antenna is not very efficient. that decrease in signal equates to a decrease in power by a factor of 0.316 (trust the math). Therefore, if a radio is said to have an output power of 5 watts, only 1.58 watts ever leaves the antenna (0.316 times 5 = 1.58). If the rubber duck were replaced by a simple ground plane vertical antenna with a short cable connected to the radio the power leaving the antenna would really be closer to 5 watts. This can make a tremendous difference in being heard at a repeater or in simplex operation.

As mentioned earlier, the gain of an antenna can be expressed as a certain number of dB over that of a ground plane antenna. A typical high gain vertical antenna may have a gain of 6 dB over a ground plane antenna. This means that the power of a station using this antenna is increased by a factor of approximately 4 times the power of the same station using a ground lane antenna. Think how much greater power this would be over using a rubber duck antenna!

There are many kinds of coax cables to use between the radio and an antenna. Each of the cables has a signal loss factor, usually expressed in dB per 100 feet of cable at some specific frequency of operation. Suppose you have just purchased some RG-58A cable to connect your radio to a new fantastic antenna to be placed on the top of your roof. Assume that the new antenna is supposed to produce a 6 dB gain in signal power when operated in the 440 MHz amateur band on which you intend to operate. You find that you will need 50 feet of this new RG58A coax cable to reach the antenna from your radio. RG58A coax cable has a loss factor of -10.6 dB per 100 feet of cable at 400 MHz. This means that the 6 dB gain of the antenna would be reduced by 5.3 dB by using the 50 feet of cable, and the antenna would effectively be worse than connecting a ground plane antenna directly to your radio. This information indicates that it may be better to operate the antenna at a level closer to the roof eaves near the operating position if this coax is all one has.


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