Practical_Antenna_Handbook_0071639586
Share Embed Flag
Download ePaper

Practical_Antenna_Handbook_0071639586

SV1FKA
24.07.2018 Views

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 59 Ionospheric Sky-Wave Propagation Sky-wave propagation occurs because signals in the ionosphere are refracted sufficiently to be bent back toward the earth’s surface. To observers on the ground, the signal appears to have been reflected from a radio “mirror” at the virtual height somewhere above the junction of the stratosphere with the lower boundary of the ionosphere. The skip distance is the shortest distance along the surface of the earth from the transmitter site (A in Fig. 2.32) to the nearest of all points on the earth (point C in Fig. 2.32) for which the ionosphere is capable of refracting the transmitted signal back to earth. At shorter distances, the higher-angle radiation from the transmitted signal either remains in the ionospheric layer or continues on out into outer space. The ground-wave zone is the distance from the transmitter site (A in Fig. 2.32) to the locus of points where the ground wave fades to an unusably low level (point B in Fig. 2.32). The skip zone is the distance from the outer edge of the ground-wave zone to the skip distance, or the distance from B to C in Fig. 2.32. Note carefully that only one of these distances, the ground-wave zone, is strongly dependent upon transmitter power level (or, more precisely, effective radiated power). The other distances are a function primarily of transmitter frequency and the state of the ionosphere at any given time. At some frequencies and power levels the sky wave and the ground wave may interfere with each other. When this happens, the sky wave arrives at the receiver with a signal whose amplitude and phase relative to the ground-wave signal depend on the specifics of its path up to the ionosphere and back down. Since the ionosphere is in constant motion (think slow undulations, like a snake), the sky wave will arrive with randomly varying amplitude and phase relative to the ground wave. Thus, the sky wave can selectively strengthen or cancel the ground wave, giving rise to a class of fading mechanisms discussed further under “Ionospheric Fading”. Angle of Incidence and Radiation Angle One of the factors determining skip distance is the angle at which the radio wave enters the ionosphere. If the transmit antenna is relatively omnidirectional, it “sprays” the Ionosphere Sky wave not effective Sky wave Ground wave A Ground-wave zone B Skip distance Skip zone C Figure 2.32 Ground-wave zone and skip zone.

60 p a r t I I : F u n d a m e n t a l s lower ionospheric boundary with energy at all angles. As discussed earlier, the radiofrequency (RF) energy in angles nearly perpendicular to the boundary may or may not be refracted back to the earth, depending on the relationship between the transmitter frequency and f C , the critical frequency. Nonetheless, for a low enough frequency, transmitted energy will, in fact, be refracted from the ionosphere at all transmit antenna takeoff angles from perfectly vertical to slightly below horizontal—that is, tangent to the earth’s surface. (Exactly how far below horizontal depends on the height of the transmitting antenna above the spherical surface of the earth. For all practical purposes, however, horizontal is close enough.) If the transmitted frequency is f C or below, the transmitted energy for all takeoff angles will be returned to the earth, thus eliminating the skip zone. When that is the case, the ionosphere supports communication over a wide range of distances from the transmit antenna, potentially all the way out to where the refracted wave hits the earth some 2500 mi (on average) distant. The practical effect of this is shown in Fig. 2.33. High-angle waves from the transmitting antenna are returned to the earth closer to the transmitter than low-angle waves are. When they hit the earth’s surface (whether land or water) they are again refracted (or reflected) and once again head up to the ionosphere. Although Fig. 2.33 shows signal paths for only two specific takeoff angles, in fact there is an infinite number, all of which potentially bounce off the earth’s surface and go back to the ionosphere. For extremely high angle waves, this process can occur tens of times before the wave ultimately reaches the receiving antenna far away. In theory, the composite signal at the receiving antenna is the phased sum of all these different waves. In practice, each ground reflection introduces enough additional loss that waves encountering more than a handful of bounces (or hops) are far weaker at the receiving antenna than those that arrive after only a few hops. Many of the propagation prediction programs readily available to the hobbyist calculate probable signal strengths at the receiving site for various propagation modes. The user can often bring up an auxiliary window that shows how many hops were Ionosphere Transmitter Single-hop transmission Two-hop transmission Receiver Figure 2.33 Single-hop and multihop skip communications.

60 p a r t I I : F u n d a m e n t a l s<br />

lower ionospheric boundary with energy at all angles. As discussed earlier, the radiofrequency<br />

(RF) energy in angles nearly perpendicular to the boundary may or may not be<br />

refracted back to the earth, depending on the relationship between the transmitter frequency<br />

and f C , the critical frequency. Nonetheless, for a low enough frequency, transmitted<br />

energy will, in fact, be refracted from the ionosphere at all transmit antenna<br />

takeoff angles from perfectly vertical to slightly below horizontal—that is, tangent to<br />

the earth’s surface. (Exactly how far below horizontal depends on the height of the<br />

transmitting antenna above the spherical surface of the earth. For all practical purposes,<br />

however, horizontal is close enough.)<br />

If the transmitted frequency is f C or below, the transmitted energy for all takeoff<br />

angles will be returned to the earth, thus eliminating the skip zone. When that is the case,<br />

the ionosphere supports communication over a wide range of distances from the transmit<br />

antenna, potentially all the way out to where the refracted wave hits the earth some<br />

2500 mi (on average) distant.<br />

The practical effect of this is shown in Fig. 2.33. High-angle waves from the transmitting<br />

antenna are returned to the earth closer to the transmitter than low-angle waves<br />

are. When they hit the earth’s surface (whether land or water) they are again refracted<br />

(or reflected) and once again head up to the ionosphere. Although Fig. 2.33 shows signal<br />

paths for only two specific takeoff angles, in fact there is an infinite number, all of<br />

which potentially bounce off the earth’s surface and go back to the ionosphere. For extremely<br />

high angle waves, this process can occur tens of times before the wave ultimately<br />

reaches the receiving antenna far away.<br />

In theory, the composite signal at the receiving antenna is the phased sum of all<br />

these different waves. In practice, each ground reflection introduces enough additional<br />

loss that waves encountering more than a handful of bounces (or hops) are far weaker at<br />

the receiving antenna than those that arrive after only a few hops.<br />

Many of the propagation prediction programs readily available to the hobbyist calculate<br />

probable signal strengths at the receiving site for various propagation modes.<br />

The user can often bring up an auxiliary window that shows how many hops were<br />

Ionosphere<br />

Transmitter<br />

Single-hop<br />

transmission<br />

Two-hop<br />

transmission<br />

Receiver<br />

Figure 2.33 Single-hop and multihop skip communications.

logo

products

Resources

Company

Terms of service
Privacy policy
Cookie policy
Cookie settings
Imprint
Change language
Made with love in Switzerland
© 2024 Yumpu.com all rights reserved

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!