Practical_Antenna_Handbook_0071639586

C h a p t e r 5 : a n t e n n a A r r a y s a n d A r r a y G a i n 165
Horizontal dipole
(end view)
4
+
Distant receiving antenna
Figure 5.6B Combined fields from antenna
and image at a distant receiver.
–
Image dipole
r 1
Outer circle =
5.3 dB greater
than for a
dipole in
free space.
4
r 2
Figure 5.6C Total received field strength versus
elevation angle broadside to dipole of (A).
phase at the other antenna, and the line of maximum field strength from this particular
array is directly above the two dipoles. At elevation angles between straight up (90
degrees) and the horizon (0 degrees), the faraway field strength takes on intermediate
values, as shown in Fig. 5.6C.
Because the maximum radiated signal strength for a horizontal dipole at heights of
l/4 or less above a ground plane is straight up in the air, the antenna is often called a
cloud burner, and it is ideal for near-vertical incidence skywave (NVIS) communications.
However, the broadside pattern is actually quite robust until the vertical elevation (or
takeoff) angle drops under 30 degrees (the 3-dB point). There are far, far worse antennas
for all-around use on 80 and 40 m than a l/2 dipole at a height of roughly l/4 (35 ft)!
Above 30 degrees, at those elevation angles that result in the radiated fields from
the original horizontal antenna and its image arriving at a distant point in phase, or
nearly so, the resulting signal at the receiver is twice the amplitude of the signal that
would have come from the original antenna in free space. The image antenna—which
is really a way of looking at the ground reflection effect—has increased the received
signal strength of our original antenna by nearly 6 dB relative to the free-space case!
In general, r 1 and r 2 (the distances from the dipole and its image to a remote receiving
point) are not equal, so the phase shift between the two radiated signals is something
other than zero. The degree of reinforcement or cancellation at any given elevation
angle above an arbitrary horizontally polarized antenna depends on the height of that
antenna above ground. As the height of the antenna above ground is increased, eventually
the phase shift of the wave from the image antenna on its way to distant receiving
points increases to where the maximum radiated field is no longer straight up in the air.
With increasing height above ground, the elevation angle corresponding to maximum