Practical_Antenna_Handbook_0071639586

C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 61
employed and from which layers the reflections (or refractions, to be precise) probably
occurred. Although it might seem obvious (especially for distances under 2500 mi) that
fewer hops are better than more hops, in fact there are some tradeoffs to be considered.
High-angle waves travel through less of the ionosphere during the process of refraction,
so the path loss per reflection in the ionosphere is often less for a high-angle wave.
Under normal conditions, the ground reflection loss is large enough to favor takeoff
angles leading to fewer hops, but if the path is almost totally over saltwater, that may
not be so true—especially if the ionosphere is disturbed (as indicated by higher A- and
K-values).
Another factor in determining which takeoff angle will result in the strongest received
signal is the relationship between the operating frequency and the MUF at all
points along the path between the transmitter and the receiver. The emphasis is added
because the MUF at any given instant varies with geographical location. For two sites
located 2500 mi or less from each other, the simplest mode is one-hop. For the ionosphere
to support single-hop propagation between those two sites, the MUF at the midpoint
of the path, 1250 mi or less from the transmit site, must be greater than the
operating frequency. The MUF at either end of the path is not the parameter of interest
for single-hop propagation, since the only region where the radio wave is in contact
with the ionosphere is near the midpoint of the path. As a general rule, the MUF is highest
where average temperatures are highest—namely, in the equatorial regions—so
higher-frequency communication is often possible on transequatorial paths when polar
paths and midlatitude east-west paths are nonexistent.
For distances greater than 2500 mi, or for those occasions when the height of the F
layer is lower than normal, multihop propagation will normally be required. For the
two-hop path of Fig. 2.33, there are two MUFs that control whether the transmitted signal
is heard at the receive site. Both MUFs must be greater than the operating frequency;
if either one drops below it, the path will fail. If the sketch in Fig. 2.33 represents a transatlantic
20-m path in the northern hemisphere, most likely the MUF on the right side of
the page (closer to Europe) will fail first, since the sun sets over the eastern Atlantic at
least two or three hours before it does so on the western side.
As additional propagation modes, employing increasing numbers of hops, are analyzed,
increasing numbers of MUF locations (often called MUF control points) must be
examined. These additional modes cause the software to calculate predicted MUFs for
a series of locations between the transmitter and the receiver. Hence, the previous statement
that it is necessary to know the MUF “at all points along the path”.
The specifics of the transmitting and receiving antenna installations also play a role
in determining how many hops may provide the best signal over a given path. For
ionospheric propagation paths (i.e., frequencies up through the lower VHF range), both
the antenna itself and its height above local ground contribute to determining the relative
amounts of energy radiated at each possible takeoff angle or received at each possible
arrival angle. An antenna such as a ground-mounted vertical monopole has good
low-angle radiation and minimal high-angle radiation. Although an infinite number of
simultaneous propagation modes may exist between the two sites, the relatively strong
low-angle radiation from the vertical will tend to make modes with the minimum number
of hops the most probable paths to success.
In contrast, a horizontal dipole suspended l/8 above ground will send most of its
radiated energy nearly straight up, thus favoring multihop modes employing relatively
high takeoff and arrival angles.