Practical_Antenna_Handbook_0071639586

64 p a r t I I : F u n d a m e n t a l s
means we are interested at a minimum in the MUF at the midpoint of each hop on the
path between the transmitting and receiving sites.
For the 7500-mi path previously cited, we will normally want to be sure the MUF is
above our chosen transmitting frequency in the vicinity of at least three places spaced
2500 mi apart from each other, each corresponding to the midpoint of one of the three
hops minimally required to establish communications over that particular path.
Just as you can’t determine the weather in New York City by sticking your head out
the window in Los Angeles, you can’t draw any conclusions about your ability to communicate
with an amateur in China by the signal strength of an amateur in a neighboring
state. As discussed earlier, the MUF at each specific place on this globe is a function
of many variables. Here are two specific examples:
• MUFs are greatest near the equator and decline with increasing latitude, both
north or south. As a general rule, on any given day noontime one-hop transÂ
equatorial communications having the midpoint of the hop close to the equator
will be possible on higher frequencies than can be supported by paths anywhere
else on the globe. Although multihop links are possible, the typically lower
MUFs at the midpoints of the outer hops will force use of a lower frequency.
• MUFs usually “follow the sun”, peaking shortly after noon. To maximize the
likelihood of being able to communicate on a typical midlatitude east/west
path, such as between Maine and Oregon or between California and Hawaii,
choose a time when it’s midafternoon at the eastern end of the circuit and midto
late-morning at the western end. For best signal-to-noise ratios, pick the
highest operating frequency that appears to be open that day.
Of course, these are guidelines for the daytime bands. For the nighttime bands, you
typically want to select a time when the MUFs at the hop midpoints (called control
points) are as low as possible, yet still above the desired operating frequency. This minimizes
signal loss from D-layer absorption.
Great Circle Paths
As airplane pilots and seasoned air travelers well know, determining the correct direction
in which to travel to get from one area of our globe to another is not always as
simple as it appears. Using the traditional wall map found in schools, it would appear
a traveler could board a jet plane one morning at Reagan International Airport outside
Washington, D.C., and, by traveling due east, arrive in Lisbon, Portugal, in time for dinner.
In truth, if you head due east from Washington, D.C., across the Atlantic, the first
landfall will be West Africa, somewhere near Zaire or Angola. Why? Because the great
circle bearing 90 degrees takes us far south of Portugal. Similarly, if we operate a Portuguese
language shortwave broadcast station near Washington, D.C., and want to maximize
the signal we deliver to listeners in Lisbon, our conventional wall map might
cause us to beam our signal due east when the correct direction is northeast. The geometry
of spheres, not flat surfaces, governs air travel and radio waves on our spherical
world.
The problem of accurately representing the surface of a sphere on a flat two-dimensional
surface such as a sheet of paper has confounded cartographers for centuries.
Even today, despite the best efforts of the National Geographic Society and other experts,
there is no one projection that is best for all possible uses.