Loop Antenna Dipole Antenna Marconi Antenna 11-41 Figure 11-91.  The V-shaped VOR navigation antenna is a common  dipole antenna.  Figure 11-92.  On a metal-skinned aircraft, a ¼ wavelength Marconi  antenna is used. The skin is the ground plane that creates the 2nd  quarter of the antenna required for resonance (left). On a non- metallic-skinned aircraft, wires, conductive plates or strips equal  in length to the antenna must be installed under the skin to create  the ground plane (right).  the antenna the proper conductive length. The same practice  is also utilized on ground-based antennas.  [Figure 11-92] The dipole antenna is the type of antenna referred to in the  discussion of how a radio wave is produced. It is a conductor,  the length of which is approximately equal to half the  wavelength of the transmission frequency. This sometimes is  referred to as a Hertz antenna. The AC transmission current is  fed to a dipole antenna in the center. As the current alternates,  current flow is greatest in the middle of the antenna and  gradually less as it approaches the ends. Then, it changes  direction and flows the other way. The result is that the  largest electromagnetic field is in the middle of the antenna  and the strongest radio wave field is perpendicular to the  length of the antenna. Most dipole antennas in aviation are  horizontally polarized.  The third type of antenna commonly found on aircraft is  the loop antenna. When the length of an antenna conductor  is fashioned into a loop, its field characteristics are altered  significantly from that of a straight-half wavelength antenna.  It also makes the antenna more compact and less prone to  damage. Used as a receiving antenna, the loop antenna’s properties  are highly direction-sensitive. A radio wave intercepting  the loop directly broadside causes equal current flow in  both sides of the loop. However, the polarity of the current  flows is opposite each other. This causes them to cancel out  and produce no signal. When a radio wave strikes the loop  antenna in line with the plane of the loop, current is generated  first in one side, and then in the other side. This causes the  current flows to have different phases and the strongest signal  can be generated from this angle. The phase difference (and  strength) of the generated current varies proportionally to the  angle at which the radio wave strikes the antenna loop. This  is useful and is discussed further in the section on automatic  direction finder (ADF) navigational aids.  [Figure 11-93] A common dipole antenna is the V-shaped VHF navigation  antenna, known as a VOR antenna, found on numerous  aircraft. Each arm of the V is one-fourth wavelength creating  a half wave antenna which is fed in the center. This antenna  is horizontally polarized. For a dipole receiving antenna, this  means it is most sensitive to signals approaching the antenna  from the sides rather than head-on in the direction of flight.  [Figure 11-91]  A Marconi antenna is a one-fourth wave antenna. It achieves  the efficiency of a half wave antenna by using the mounting  surface of the conductive aircraft skin to create the second  one-fourth wavelength. Most aircraft VHF communications  antennas are Marconi antennas. They are vertically polarized  and create a field that is omnidirectional. On fabric skinned  aircraft, the ground plane that makes up the second one-fourth  wavelength of the antenna must be fashioned under the skin  where the Marconi antenna is mounted. This can be done with  thin aluminum or aluminum foil. Sometimes four or more  wires are extended under the skin from the base of the vertical  antenna that serve as the ground plane. This is enough to give  Metal aircraft skin  ground plane Ground plane under skin in non-metallic  aircraft Antenna 4 4