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
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.
very low readings in one direction and much higher readings in the opposite
direction because the director or reflector concentrates the power in one