4-12
Advantages of Horizontal Polarization
A simple horizontal antenna is bi-directional. This characteristic is useful when you desire to
minimize interference from certain directions. Horizontal antennas are less likely to pick up man-made
interference, which ordinarily is vertically polarized.
When antennas are located near dense forests or among buildings, horizontally polarized waves
suffer lower losses than vertically polarized waves, especially above 100 megahertz. Small changes in
antenna locations do not cause large variations in the field intensity of horizontally polarized waves.
When vertical polarization is used, a change of only a few meters in the antenna location may have a
considerable effect on the received signal strength. This is the result of interference patterns that produce
standing waves in space when spurious reflections from trees or buildings occur.
When simple antennas are used, the transmission line, which is usually vertical, is less affected by a
horizontally mounted antenna. When the antenna is mounted at right angles to the transmission line and
horizontal polarization is used, the line is kept out of the direct field of the antenna. As a result, the
radiation pattern and electrical characteristics of the antenna are practically unaffected by the presence of
the vertical transmission line.
Q10. What type of polarization should be used at medium and low frequencies?
Q11. What is an advantage of using horizontal polarization at high frequencies?
Q12. What type of polarization should be used if an antenna is mounted on a moving vehicle at
frequencies below 50 megahertz?
RADIATION RESISTANCE
Radiated energy is the useful part of the transmitter's signal. However, it represents as much of a loss
to the antenna as the energy lost in heating the antenna wire. In either case, the dissipated power is equal
to I2R. In the case of heat losses, the R is real resistance. In the case of radiation, R is an assumed
resistance; if this resistance were actually present, it would dissipate the same amount of power that the
antenna takes to radiate the energy. This assumed resistance is referred to as the RADIATION
RESISTANCE.
Radiation resistance varies at different points on the antenna. This resistance is always measured at a
current loop. For the antenna in free space, that is, entirely removed from any objects that might affect its
operation, the radiation resistance is 73 ohms. A practical antenna located over a ground plane may have
any value of radiation resistance from 0 to approximately 100 ohms. The exact value of radiation
resistance depends on the height of the antenna above the ground. For most half-wave wire antennas, the
radiation resistance is about 65 ohms. It will usually vary between 55 and 600 ohms for antennas
constructed of rod or tubing. The actual value of radiation resistance, so long as it is 50 ohms or more, has
little effect on the radiation efficiency of the antenna. This is because the ohmic resistance is about 1 ohm
for conductors of large diameter. The ohmic resistance does not become important until the radiation
resistance drops to a value less than 10 ohms. This may be the case when several antennas are coupled
together.
RADIATION TYPES AND PATTERNS
The energy radiated from an antenna forms a field having a definite RADIATION PATTERN. A
radiation pattern is a plot of the radiated energy from an antenna. This energy is measured at various
angles at a constant distance from the antenna. The shape of this pattern depends on the type of antenna
