User manual

Appendix A: Path Evaluation Information A-1
Moseley SL9003Q 602-12016 Revision G
Appendix A: Path Evaluation Information
Please visit www.moseleybroadcast.com and click on support for online Path
Evaluation resources or simply telephone Moseley Customer Services for help in this
area.
A.1. Introduction
A.1.1 Line of Site
For the proposed installation sites, one of the most important immediate tasks is to determine
whether line-of-site is available. The easiest way to determine line-of-site is simply to visit one
of the proposed antenna locations and look to see that the path to the opposite location is clear
of obstructions. For short distances, this may be done easily with the naked eye, while sighting
over longer distances may require the use of binoculars. If locating the opposing site is difficult,
you may want to try using a mirror, strobe light, flag, weather balloon or compass (with prior
knowledge of site coordinates).
A.1.2 Refraction
Because the path of a radio beam is often referred to as line-of-site, it is often thought of as a
straight line in space from transmitting to receiving antenna. The fact that it is neither a line, nor
is the path straight, leads to the rather involved explanations of its behavior.
A radio beam and a beam of light are similar in that both consist of electromagnetic energy; the
difference in their behavior is principally due to the difference in frequency. A basic
characteristic of electromagnetic energy is that it travels in a direction perpendicular to the plane
of constant phase; i.e., if the beam were instantaneously cut at right angle to the direction of
travel, a plane of uniform phase would be obtained. If, on the other hand, the beam entered a
medium of non-uniform density and the lower portion of the beam traveled through the denser
portion of the medium, its velocity would be less than that of the upper portion of the beam. The
plane of uniform phase would then change, and the beam would bend downward. This is
refraction, just as a light beam is refracted when it moves through a prism.
The atmosphere surrounding the earth has the non-uniform characteristics of temperature,
pressure, and relative humidity, which are the parameters that determine the dielectric constant,
and therefore the velocity of radio wave propagation. The earth’s atmosphere is therefore the
refracting medium that tends to make the radio horizon appear closer or farther away.
A.1.3 Fresnel Zones
The effect of obstacles, both in and near the path, and the terrain, has a bearing on the
propagation of radio energy from one point to another. The nature of these effects depends
upon many things, including the position, shape, and height of obstacles, nature of the terrain,
and whether the effects of concern are primary or secondary effects.