Practical_Antenna_Handbook_0071639586

CHAPTER 2
Radio-Wave Propagation
To intelligently choose the right antenna for our purposes we first need an understanding
of how radio waves get from one place to another. Intuitively, radio
signal propagation seems similar to light propagation; after all, light and radio
signals are both electromagnetic waves. But the propagation of radio signals is not the
simple matter that it seems at first glance: Simple inverse square law predictions, based
on the optics of visible light, fall down drastically at many radio frequencies because
other factors come into play—especially in the vicinity of earth.
In the microwave region of the spectrum, for instance, atmospheric pressure and
water vapor content of the air through which a terrestrial electromagnetic wave moves
become more important than for visible light. Clearly, those differences do not exist
when comparing light and radio waves in free space. Similarly, near-earth microwave
propagation differs from that of the lower VHF and HF bands. But perhaps the most
amazing difference is found in the medium-frequency (MF) and high-frequency (HF) portions
of the electromagnetic spectrum, where solar ionization of the upper reaches of
the atmosphere provides a refracting layer that acts much like a mirror at these frequencies
and hence is capable of supporting long-distance “skip” communications essential
to such uses of radio as international broadcasting and amateur DXing.
This chapter examines radio propagation phenomena so that you have a better understanding
of what an antenna is expected to accomplish and which design parameters
are important to ensure the communications results that you desire.
Radio Waves
Today it is well recognized that radio signals travel in a wavelike manner, but that fact
was not always so clear. It was well known in the first half of the nineteenth century
that wires carrying electrical currents produced an induction field surrounding the wire
that was capable of exerting a force over short distances. It was also known that this
induction field is a magnetic field, and this knowledge formed the basis for the invention
of electrical motors. But it was not until 1887 that German physicist Heinrich Hertz
succeeded in demonstrating the ability to transmit and receive radio signals between
two separate sets of equipment in his laboratory. In so doing, he confirmed experimentally
Maxwell’s theoretical work of a quarter century earlier predicting the existence of
previously unimagined radio signals that, like light, were electromagnetic waves.
Like the induction field, the electromagnetic wave is created by an electrical current.
Unlike the induction field, however, the electromagnetic wave requires a changing
electric current. And once launched, the radiated field further differs from the induction
field in that it is self-sustaining and no longer depends on the existence of a conductor
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