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204 ANTENNA BASICS

same or very close frequencies). Evidently, the latter increases the communication channel
capacities at the cost of an auxiliary system that has stabilized the antenna mutual
orientation in space. Besides, all antennas in the system shall possess a very low level of
cross polarization.

Linear Polarization.

3. Human-made EM noise signals are predominantly Vertically Polarized (VLP) toward the
earth’s surface. If so, the selection of Horizontal Polarization (HLP) might provide some
protection against interference from such noise. If some high-level RF signals of particular
polarization are detected in a communication area, the smart decision would be to switch
your polarization to the opposite polarization to provide some isolation. Taking into
account that the spectrum, intensity or space distributions of interfering signals are close
to random, it is also wise to test the projected link with VLP and then HLP to see which
yields the best performance (if any).
4. Medium wave (535-1705 kHz) broadcast stations use exclusively VLP because ground
wave propagation over the earth is considerably better using vertical polarization.
Meanwhile, HLP shows a marginal improvement in long distance communications based
on the reflection EM waves from the ionosphere at short wave (3-30 MHz).
5. VLP antennas such as a simple electric dipole or current loop are often used for mobile and
portable radio communication between moving objects like vehicles. Looking back at
Figure 4.3.2 we can see that such antennas can be omnidirectional in the horizontal planes
(XY-planes in Figure 4.3.2) that promise more reliable communication link between
objects moving over uneven terrain. In some cases, HLP antennas are superior especially
if they are mounted on vertical towers like antennas for TV broadcast at relatively low
frequencies of 54 – 200MHz. This choice has a simple explanation: the tower itself can be
considered a VLP antenna and have relatively low interference with HLP antennas that
might be mounted on its surface.
6. Most radar systems operate with a single polarization VLP or HLP that provides more
positive detection of metal targets such as ship or aircraft. This phenomenon is based on
the fact that the waves reflected from metal objects are practical of the same polarization
as an incident. Evidently, the detectable target surface must be mainly perpendicular to the
front of EM wave sent by radar, and the tangential component of the E-field on the highly
conductive metal surface becomes close to zero (see Boundary Condition in Chapter 2).
Subsequently, the wave incident on the metal target and backscattered by it should be

almost zero-sum on the target surface. It means that the reflection leads mainly to a 180°
phase shift (a phenomenon is known as specular or mirror image reflection) between the
incident and backscattered wave without polarization change. As a result, the radar antenna
receives the signal of the same polarization for processing. Note that targets of complex
geometric shape can impose depolarization effects that lessen the signal received by an LP
antenna. To avoid such effect, many modern radars use a wide variety of polarizations (i.e.
polarization diversity) drastically increasing the information collected about the targets
including their size and shape.
7. Generally speaking, LP antennas are more frequent because of their simplicity. The LP
antennas tend to be smaller, cheaper and easier to build. That is why most handset antennas