b. Now you know what we mean by directivity and radiation pattern.
There is one more antenna characteristic you should know about, the gain of
an antenna.
11. Antenna gain.
a. An antenna is not an amplifying device, so you do not get a real
gain as you do from an amplifier. In other words, if you put two watts into
an amplifier that has a gain of 10, you get 20 watts out. If you put two
watts into an antenna, it doesn't matter what the gain is; you still cannot
get more than two watts out, there is no increase of power.
b. Antenna gain merely tells you how good a certain antenna is as
compared to a simple dipole antenna. We use a simple dipole as a standard
because it radiates energy in all directions parallel to it. Antenna gain,
then, is a measurement of the effectiveness of a directional antenna as
compared to that of a simple dipole which is nondirectional. When we make
an antenna directional, most of the power it radiates or receives is in one
direction.
c. The gain of a receiving antenna, therefore, is a comparison between
the power absorbed by the antenna from a given signal and the power that
would be absorbed by a simple dipole under exactly the same conditions.
d. For example, a receiving antenna with a gain of 10 absorbs about ten
times the amount of power as a simple dipole absorbs. For transmitting, an
antenna with a gain of 10 needs only one-tenth as much power to produce a
given field strength in the desired direction as does the simple dipole.
Here's how it works with the antenna you have just learned about.
12. Gain of a dipole with a director or reflector.
a. If you measure the radiated power a certain distance from a simple
dipole antenna, as shown in Part A of Figure 138, you will measure the same
amount of power in all directions around the antenna. Now, if you measure
the radiated power the same distance away from a dipole that has a reflector
or a director, as in Part B of Figure 138, here's what happens.
You get
very low readings in one direction and much higher readings in the opposite
direction because the director or reflector concentrates the power in one
direction.
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