Practical_Antenna_Handbook_0071639586

C h a p t e r 3 : A n t e n n a B a s i c s 107
Antenna Resistance and Losses
A current flowing in the antenna encounters three kinds of resistance:
• Radiated power expended in the form of radiation can be thought of as an I 2 R R
loss. R R (also written R RAD ) is called the radiation resistance.
• Current flowing along the antenna conductor dissipates a certain amount of
energy in the form of heat. In this I 2 R J loss, R J is the ohmic resistance. The ohmic
resistance is not the same as the wire’s dc resistance as measured on a
conventional ohmmeter or volt-ohmmeter (VOM); rather, it is somewhat higher
because of the presence of skin effect, which limits the flow of RF energy to the
surface of the conductor according to a complicated function of frequency and
the specific parameters of the conducting material.
• There is also an I 2 R SH loss because of the leakage, or shunt, resistance of dielectric
elements such as insulators. This effect of this resistance is often lumped in with
the ohmic resistance.
The purpose of any antenna system is to convert as much source energy as possible
to electromagnetic radiation. The energy dissipated by what we call the radiation resistance
R R is thus the useful part of the total power applied to the antenna system. Because
the actual power from the source is split between R RAD and the ohmic and shunt
resistance terms, the latter two resistances should be kept as low as possible. The radiation
resistance of a center-fed half-wave antenna in free space is 73 W; in a properly
designed installation, the radiation resistance should be large compared to the loss resistances,
and most of the available energy will be radiated as useful signal. The halfwave
antenna is, therefore, a very efficient radiator under most circumstances. In theory,
the very short dipole we used at the beginning of our analysis can have a radiated signal
comparable to that of the l/2 dipole (with the main lobe theoretically only 0.4 dB
less), but its R RAD is so low and its X C so large that it is virtually impossible to avoid dissipating
most of the transmitter output power in R O and R SH of the antenna as well as
similar or larger loss resistances in the matching network for such a short antenna.
Half-Wave Dipole Feedpoint Impedance
A half-wave dipole can be fed anywhere along its length; it does not have to be fed at
the center, even though there are many advantages to doing so. Because the voltage and
current vary along the length of the half-wave dipole, and because the impedance at
any point on the dipole is equal to the voltage at that point divided by the net current
passing through that point, the impedance will vary along the length of the antenna. If
V is divided by I at each point of the voltage and current curves of Fig. 3.13, the result
is the impedance curve Z. The impedance is a minimum of about 73 W at the center
point and rises to 2500 W or more at the ends. In general, however, the impedance is
complex (a mixture of resistive and reactive terms) throughout most of the antenna
length.
If the antenna is cut to a length corresponding to exact resonance the feedpoint impedance
at the center will be purely resistive. However, if the antenna is longer or
shorter than resonance, reactance reappears. When the antenna is shorter than its resonant
length, the feedpoint impedance will exhibit capacitive reactance; conversely,