Page 11
Gain
Gain (directivity) is realized by the placement one Vertical element above the four NVIS elements close to the
ground. This enables the combined output to form an onmi-directional pattern. As the antenna has equal gain
in all directions, the aggregate results in a negative gain pattern over a dipole. Fortunately, gain does not only
come from antenna design. There is also “ground gain”, which can add up to 6 dB of additional gain. Ground
gain is signal reflection that is transformed into gain. By rule, horizontal elements achieve a lower radiation
take off angle with increased height over the ground, where at great heights the ground conductivity has little
influence on gain.
By design, the DX-Mil places both its’ Horizontal and Vertical elements close to the ground to take advantage
of ground gain. The results are the gain figures that occur above the Brewster angle, which is where no signal
is present and where the ground gain has come from. This angle, and the associated gain, is dependent of the
conductivity of the reflecting ground. For example, a greater amount of ground gain would be realized when
the antenna is placed over a salt marsh than a desert floor.
Gain pattern over real ground
Frequency (MHz)
Gain (dBi)
2
-27 + 6 = -21
7.5
-15 + 6 = -9
15
-9.4 + 6 = -3.4
22.5
-8.2 + 6 = -2.2
30
-6.5 + 6 = -.5
