Figure 19. Evolution of a Broadband
Antenna
A coaxial transmission line gradually diverges in such a
way as to hold constant the natural impedance of the
line (A). The wave traveling along the line will expand
smoothly over a larger and larger area and, when reach-
ing the open end of the line, will pass into free space
with little reflection. This infinitely broad structure can
be modified (B) while still holding to the concept of
gradual dimensional change per unit of wavelength,
now resembling a broadband conical antenna working
against a modified ground plane. More severe modifica-
tion (C) produces a true conical antenna of moderate
bandwidth and more severe change in system cross sec-
tion. The ultimate modification is reached when the
center structure is reduced to a monopole (D) having a
very restrided bandwidth and minimum reflection only
over a restricted frequency range.
For very practical reasons it is economical to hold the
volume occupied by any antenna to the very minimum.
;. Wideband antennas such as those discussed are un-
economical, except in the uhf region, since they occupy
more space than other designs that have acceptable
bandwidth. Smaller antenna structures can be built by
permitting a greater degree of reflection to occur in the
transformation of radio energy from the guided to the
free state, and then compensating for the undesired
reflection by introducing a compensating reflection
somewhere in the feed system, or transmission line.
In the hf and vhf spectrums, in particular, very thin
wire or tubing elements are commonly used to assemble
relatively narrow-bandwidth antenna systems having
high gain, suitable only for operation over a quite re-
stricted frequency region.
Electromagnetic Wave Propagation
Radio waves may be propagated from a transmitting
antenna to a receiving antenna along the surface of the
earth, through the atmosphere, or by reflection or scat-
tering from natural or artificial reflectors. At the lower
end of the communications spectrum, the groundwave
may be propagated for several hundred miles. At high
frequencies, however, the ground losses are so great
that the groundwave can be propagated for less than
one hundred miles. Propagation in the medium and
high portion of the hf band is therefore primarily by
ionospheric reflection.
The refractive index of the atmosphere is an impor·
tant factor in radio propagation, especially above 100
MHz. Scattering of the radio waves by inhomogeneities
in the atmosphere is used to provide satisfactory com-
munication up to several times the line-of-sight dis-
tance. At higher frequencies, atmospheric absorption
limits propagation to an extent, but the use of high-gain
beam antennas makes the use of such frequencies prac-
tical.
Propagation up to 2000 kHz
Propagation of very low frequencies (vlf) radio waves
over short distances is by a ground or surface wave.
Attenuation of the wave is quite low. At great distances
the field intensity falls rapidly because of losses in the
ground and because of the curvature of the earth.
These losses increase with frequency.
At sufficiently great distances propagation is chiefly
due to propagation in the earth-ionosphere
"waveguide" composed of earth and ionospheric mul-
tiple reflections.
Propagation above 30 kHz is a combination of sur-
face and sky waves reflected from the ionosphere. The
attenuation of surface wave propagation over land is
shown in Figure 20. Skywave reflection causes fading at
Figure 20. Propagation Loss
Basic propagation loss expected for surface waves prop·
agated over a smooth spherical earth.
medium distances, particularly at night and on the
lower frequencies during the day. Skywave field
20-12
DISTANCE IN STATDTE MIlES
strength is subject to various irregular fluctuations due
to changing properties of the ionosphere.
Propagation-2 to 30 MHz
At frequencies between about 2 and 30 MHz and for
distances greater than 100 miles, transmission depends
chiefly on sky waves reflected from the ionosphere.
This is a region high above the earth's surface where the
rarefied air is sufficiently ionized by ultraviolet light
from the sun to reflect or absorb radio waves. The
ionosphere is considered to be that region lying between
30 to 250 miles (50-400 km) above the surface of the
earth and consists of a number of layers:
The Fz Layer
The higher of the two major reflection regions of the
ionosphere is called the Fz layer. This layer has a virtual
height ranging from 130 to 250 miles (200-400 km) and
is the principal reflecting region for long-distance high-
frequency communication. Height and ionization den-
sity vary diurnally, seasonally, and with the sunspot
cycle. At night, the Fz layer merges with the F, layer
and reduction in absorption of the E layer causes night-
time field intensities and noise to be generally higher
than during daylight hours.
The Fz layer appears about sunrise, local time, the
critical frequency rising sharply, reaching a maximum a
few hours after the sun is at its highest elevation, then
decreasing exponentially from this value, reaching min-
imum during nighttime hours (Figure 21).
The E Layer
Below the F layer at a height of about 60 miles (100 km)
is an absorptive layer termed the E layer, which exists
during daylight hours, reaching a diurnal maximum at
noon. For all practical purposes, the E layer disappears
at night, although weak traces of it are often observed.
This layer is important for daytime hf propagation at
distances less than 1000 miles (1600 km), and for occa-
sional medium-frequency nighttime propagation at dis-
tances in excess of 100 miles (160 km). Irregular cloud-
like areas of unusually high ionization, called sporadic
E, may occur up to more than half of the time on
certain days or nights. A large percentage of sporadic E
propagation is attributed to visible bombardment of the
atmosphere by the sun.
Layer height and electron density of the atmosphere
determine the skip-distance of sporadic E propagation
for a given signal angle (Figure 22), and distances of
400 to 1200 miles (650-1930 km) are common on 50
MHz. Multiple-hop propagation is often possible up to
about 2500 miles (4000 km) on occasion. Sporadic E
propagation has been observed in the 144 MHz band,
but is not as common as on the lower frequency bands.
z
a
c
>
Figure 21. Representative Hour-to-Hour Figure 22. E Layer Scatter Range
Changes in the lonosphere
E layer scatter range may be as great as 1400 miles for
lonized regions are referred to as layers, but they are not low angle, single-hop transmission. A high antenna (sev-
completely separated from one another. Each region eral thousand feet high, such as on a mountain top),
overlaps the adjoining one, to some extent, forming a combined with a sea level horizon, is ideal. The scatter
continuous but nonuniform area with at least four levels occurs at layer height of about 36 to 60 miles.
of peak density designated D, E, F, and Fz layers. Sum-
mertime Fz critical frequencies are lower than winter E layer propagation on the vhf bands is most com-
values but Fz nighttime critical trequencies during the
summer months are higher than in winter. Thus the mon during the summer months, with a shorter season
difference between day and night critical frequencies is during the winter, with the periods reversed in the
much smaller in the summer than during the winter. southern hemisphere.
The D Layer
Below the E layer, the D layer exists at heights of 30 to
50 miles (50-80 km). It is absorptive and exists in the
middle of the day during the warmer months. Not
much is presently known about the characteristics of
this layer, as it is so weakly ionized that the usual pulse-
probing techniques do not produce meaningful echoes.
y........)
absorption of signals in the medium- and high-fre-
quency range during the middle of the day.
The Critical Frequency
The critical frequency (f) of an ionospheric layer is the
highest frequency which will be reflected when the wave
strikes the layer at vertical incidence. Frequencies
higher than f pass through the layer. The critical fre-
quency of the most highly ionized layer of the iono-
sphere may be as low as 2 MHz at night and as high as
10 to 15 MHz in the middle of the day.
The critical frequency and height of the layers are
measured by a pulse technique, the pulse and its return
echo being observed on a cathode-ray tube, as in a
radar set. The virtual height, or point of reflection in
the ionosphere determined by this technique, is
presented in an ionogram, showing height as a function
of frequency for specific periods of time (Figure 23).
Figure 23. Virtual Height of lonosphere Is
Presented in an lonogram
Point of refledion of radar echo in ionosphere is mea-
sured and presented in graphic form, showing height as
a function of frequency for specific times. Frequencies
higher than a critical frequency will pass through the
ionosphere and not be reflected, when a vertical pulse is
used as a measuring device. At oblique angles, frequen-
cies higher than the critical frequency will be refleded
back to earth, creating a skip distance zone for a given
circuit.
The critical frequency is of interest in that a skip
distance zone will exist on all frequencies greater than
the highest critical frequency at a given time for a given
circuit. The higher the critical frequency, the greater the
density of ionization and the higher the maximum usa-
ble frequency.
The Maximum Usable Frequency (nnuF)
High-frequency radio waves travel from the transmitter
to a distant point by reflection from the ionosphere and
Figure 24. The Maximum Usable Frequency
In order for a radio signal to be refleded from T to R, the
electron density at B must be high enough to support
refledion. As the frequency of the signal is raised, at
some point the electron density will not be great
enough to bend the wave back to earth and it will
continue through the ionosphere into space. The upper
frequency limit, or maximum usable frequency, can be
calculated from ionospheric measurements by determin-
ing the critical frequency at point E. The vertical critical
frequency determined is multiplied by a factor to provide
the value of the oblique incident nnuF for a particular
distance (D) and layer height (h).
earth in one or more hops, as indicated in Figure 24.
For a radio signal to travel from T to R via the iono-
sphere, its frequency must be less than a maximum
value. Above this frequency, the electron density at B
will not be great enough to bend the signal back to
earth and it will continue on through the ionosphere
into space. There is, therefore, an upper limit to the
range of frequencies that will be reflected by the iono-
sphere between any two fixed points. This upper limit-
ing frequency is called the maximum usable frequency
(IvtuF) for a given circuit. The wtuF is highest near noon
or in the early afternoon and is highest during periods
of greatest sunspot activity, often going to frequencies
higher than 30 MHz (Figure 25). The n,cuF often drops
below 5 MHz in the early morning hours. Ionospheric
losses are at a minimum near the IvtuF and increase
rapidly for lower frequencies during daylight. tviuF data
is published periodically in radio amateur magazines.
The Lowest Usable High Frequency (wF)
The lowest usable high frequency (LuF) is the lowest
frequency that can be used for a satisfactory communi-
cation circuit over a particular path at a particular time.
The LuF depends primarily on atmospheric noise and
static at the receiving site for a determined signal-to-
noise ratio. At frequencies below the LuF, reception will
not be possible since the received signal is lost in the
prevailing noise level. As the operating frequency is
raised above the LuF, the signal-to-noise ratio im-
proves.
Unlike the IvtuF, which is dependent entirely upon
The F Layer
The F, layer has a virtual height of about 100 to 150
miles (160-240 km) and exists only during the daylight
hours. This layer occasionally is the reflecting region
for hf transmission, but usually waves that penetrate
the E layer also penetrate the F, layer, to be reflected by
the Fz layer. The F, layer introduces additional absorp-
tion of such waves. At night the F7 layer is nonexistent,
merging with the Fz layer to form the single nighttime F
layer.
