A, B, C's of Dx Fundamentals of the Art of DXing VIII
W5FKX, Don Boudreau
Radio Frequency Waves
What are RF Waves?
To begin, it is helpful to understand what are radio waves and how they move (propagate) from A to B. Since we all know a bit about water waves, they can serve as a beginning point. Consider a pebble dropped (kerplunk!) onto the calm surface of a pond. As the pebble breaks the tension of the water surface, it pushes aside (disturbs) the surface water around it, which then collapses back upon itself as the pebble sinks below. Because the surface tension acts as a sticky coupling between adjacent water molecules, any motion in one area will disturb nearby areas as well, and these will then disturb others beyond, and so forth, as illustrated in the 2-dimensional diagram below:
After the pebble breaks the surface, the disturbance propagates radially outward from the point of impact as a surface wave. This illustrates the fundamental definition of all wave motion:
Wave motion is the propagation of a disturbance through an elastically-coupled medium.
The term "elastically coupled" means that if you disturb one area of the medium, it will affect the adjacent areas. It is easy to understand this for various waves in media, such as water waves (water surface) and sound waves (air, solids, liquids), but what about radio waves? Although it is certainly true that they propagate through air in our atmosphere, we know that they also propagate through space between the heavenly bodies, where there is essentially nothing but a vacuum - so what "elastically coupled medium" are they using???? Hmmm....!!!!
Historically, the study of radio waves required a new concept of what the medium of propagation might be and scientists in the 19th century, grappling with this difficult question, laid the groundwork for our current understanding of what are now called Fields. While investigating the new phenomena of electricity and its possible relation to the older known phenomenon of magnetism, the electro-magnetic effect was discovered in which a current flowing in a wire produced a magnetic field around the wire; also that changing magnetic fields would produce electric fields that could cause a current to flow, that is:
A changing Electric field will create a Magnetic field, and a changing Magnetic field creates an Electric field.
Studies of this phenomenon led to the startling discovery that high-frequency alternating electrical currents in wires would produce "energy" waves that could travel through the air without the need for wires. It was soon understood that these waves were of an electromagnetic (EM) nature; that is, they consisted of time-varying electric and magnetic fields. For many years, early scientists struggled to extend their understanding of ordinary waves in solids, liquids, and gases to that of the "wireless" EM waves. The big question was: what was the elastic medium that supported the disturbance known as EM waves? Finally, they conceived of a medium that they called "ether" (no relation to the anesthetic!), which was thought to be an extremely low-density gaseous substance that permeated all of space and allowed the propagation of EM waves. The idea of a substantive ethereal medium was eventually shown to be erroneous and was replaced by concept of "vector fields" - that is, the potential for a directional force (electrical, magnetic, gravitational, nuclear) to arise at a point in space as the result of the presence of a physical object. As this abstraction of a "potential force at every point" is difficult for many, it will be easier for our purposes to imagine all of space as being permeated by an invisible 3-D grid of "field-lines" consisting of invisible lines that intersect at every point in space. In the figure below illustrated in 2-D, the lines exist at all points in space, within all atoms and molecules, and extending throughout the universe. These are not actually things; rather, they are an intrinsic attribute of space and time, representing the potential for directional forces (e.g., EM, gravity, or nuclear) to arise in the event of a disturbance to space-time caused initially by a physical body. For example, when a solid body is placed at a point in space, it gives rise to a gravitational force field (and if it accelerates, it will produce a Gravity Wave ... but we digress!).
Of interest to us is the fact that a high-frequency oscillatory motion of electrical current flow in a wire (our antenna), caused by a time-varying E-field in the wire, will simultaneously give rise to a time-varying M-field around the wire, and this amounts to a disturbance in the surrounding space. As the E-field in the wire rises and falls, it produces an M-field encircling the wire that also builds and collapses. The disturbance of the surrounding space caused by the changing M-field around the wire creates another E-field farther out around the wire, which in turn collapses, producing another changing M-field extending farther outward, and therefore a changing E-Field even farther out, and so on ... . One might be inclined to imagine this as a repeating sequence in which the changing E- and M-fields "leap-frog" outward .
In the illustration above, consider the charge to be part of an RF current oscillating back & forth along the wire, creating the outward ripple of EM fields. Each complete back-and-forth cycle of the current produces one wavelet around the wire that propagates outward. So it is that an EM wave disturbance is formed, rippling away from the antenna.
To better visualize the actual wave, imagine the sinusoidal representation as being rotated around the antenna in 3-D, forming an expanding doughnut shape with the antenna through its center, propagating outward at the speed of light as a spherical wave-front of increasing diameter. Each current cycle produces a wavelet that leaves the antenna, as seen in a top-view cross-section in the left figure below (dipole antenna at center), and in an end-view of the antenna in the right figure.
Notice that because the antenna is horizontal to the ground, all of the E-field vectors are formed horizontally. By definition, the "polarization" of the EM-wave is determined by the E-field vector. In this case, the wave is horizontally polarized. Can you guess what would be the polarization of a vertical antenna?
If we were to be able to actually see the wave as it emerged from the antenna, we would expect that there would be countless interactions with the environment at play within a very wide area around the antenna, including the effects of the ground, buildings, trees, and metallic conductors like house wiring, powerlines and fences. All of these factors in the near-field region of the antenna would be expected to affect the final shape of the wave as it expanded outwards. At some distance from the antenna, beyond the near-field region, we would expect to eventually see the "final form" of the wave-front. In fact, this theoretical region at a distance of many wavelengths from the antenna is called the far-field region.
We will discuss more of these issues in the chapter on "Antennas", but for our purpose here, let's just ask "What would the wave-front of the dipole antenna look like if there were absolutely no interacting structures, including the ground and the antenna supports?" Hmmm ... you might then ask ... "Where in the world would you go to get away from ALL structures?? ... Space???". Good guess!!! That is exactly what is done - of course, only theoretically at this point (although one day we may be able to do it in reality!). Using antenna modeling programs based upon the theory of how RF waves emerge from a conductor, we can have the computer software "draw" the predicted pattern of an antenna that is placed out in space, free from any interactions with physical objects. These abstract models are called "Free-space" patterns and are very useful for apples-to-apples comparisons of different antenna designs.
The theoretical free-space plot of the dipole far-field radiation pattern is seen shown below.
Free-space plots are mainly used in comparing antenna designs without concern for unknown local variables (antenna height, ground conductivity, structures, etc). As a rough rule-of-thumb, ground effects will "slice off" the bottom of the pattern below the antenna, absorbing some of the energy while reflecting some back to interact with emerging wave-fronts and alter the final pattern. As we shall see in the chapter on "Antennas", depending upon the height above ground (and surrounding objects), the actual radiation patterns may be considerably altered from that seen in the theoretical plots (see the chapter on "Antennas").
A nicely illustrated elementary lesson on EM waves is available online from the University of Colorado, Boulder (references). It is important to note that although the outwardly propagating wavefront is increasing in diameter, the frequency of the "ripple" (and therefore wavelength) do not change; however, since the energy is being spread over an increasing volume of space, it follows that the amount of energy that will be received in any given direction will be decreasing with distance from the source. In fact, the wave energy received in any given direction will be reduced by one-fourth with each doubling of distance. Also, we should always remember the inverse relationship between wavelength (lamda) and frequency (f ) :
lamda = velocity / frequency
lamda = velocity / frequency
where V is the speed of light in the the medium in which it is traveling. In free-space, V is approximately 300,000,000 m/s; however, it is a bit less in air and even less in solids. In fact, since we must so frequently calculate antenna dimensions, it is very useful to remember a convenient approximation of the above equation for wavelengths in copper wire:
lamda (in feet) = 968 / frequency (in Mhz)
Now that we have an idea of what radio waves are, let's take a look at the things that may affect their propagation, especially in the HF bands, between us and the DX!
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