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 75 atic Pacific region is often blocked by unsettled polar conditions, there are few HF operating experiences that compare with aiming the 40-m beam southeast on a winter afternoon and hearing an operator in the Far East answer a “CQ”. It’s magic. Ionospheric Scatter Propagation Ionospheric scatter propagation occurs when clouds of ions exist in the atmosphere. These clouds can exist in both the ionosphere and the troposphere, although the tropospheric model is more reliable for communications. Figure 2.41 shows the mechanism for scatter propagation. Radio signals from the transmitter are reflected from the cloud of ions to a receiver location that otherwise might not receive them. There are at least three different modes of scatter from ionized clouds: backscatter, side scatter, and forward scatter. The backscatter mode is a bit like radar, in that the signal is returned to the transmitter site, or to regions close to the transmitter. Forward scatter occurs when the reflected signal continues in the same azimuthal direction (with respect to the transmitter), but is redirected toward the earth’s surface. Side scatter is similar to forward scatter, but the azimuthal direction usually changes. Unfortunately, there are often multiple reflections from the ionized cloud (shown as “multiple scatter” in Fig. 2.41). When these reflections are able to reach the receiving site, the result is a rapid, fluttery fading that can be quite deep. Auroral Propagation The visible aurora produces a luminescence in the upper atmosphere resulting from bursts of particles released from the sun 18 to 48 hours earlier. The light emitted is called the northern lights (aurora borealis) or the southern lights (aurora australis). The ionized regions of the atmosphere that create the lights also form a reflective curtain at radio frequencies, especially at VHF and above, although auroral propagation is infrequently Higher layer Multiple scatter Backscatter Scattering cloud Side scatter Transmitting antenna Forward scatter Receiving antenna Figure 2.41 Various modes of scatter propagation.

76 p a r t I I : F u n d a m e n t a l s observed on frequencies as low as 14 MHz. Auroral effects are normally seen at higher latitudes, although a few blockbuster auroras have been seen (and heard) as far south as Mexico! At those times, listeners in the southern tier of states in the United States are often treated to the reception of signals from the north being reflected from auroral clouds. Reflection off the aurora curtain is truly mirrorlike, and radio communication out to 1500 mi or so is possible. Numerous Internet sites, including some operated by U.S. government agencies, provide early aurora alerting services based on the expected time delay between visual observance of a flare on the sun’s surface and the subsequent disruption of the earth’s magnetosphere by arriving solar particles. Meteor Scatter Propagation Scatter propagation has been exploited mostly at VHF—not just with the ionosphere, but from meteor trails and man-made orbiting objects as well. When meteors enter the earth’s atmosphere, they do more than simply burn up. The glowing meteor leaves a wide, but very short duration, transient cloud of ionized particles in its path. These ions act as a radio mirror that permits short bursts of reception—especially high-speed CW—between sites correctly situated. Meteor scatter reception is not terribly reliable. Its primary value for radio amateurs is to make communications possible over extended paths that are not normally available. However, at least two companies offer meteor scatter communications services for commercial users. Propagation Predictions Thanks to the relentless march of science and technology, propagation forecasting is gradually morphing from art to science. We now have a variety of solar sensors and monitors, both terrestrial and space-based, both simple and sophisticated, to help us better understand the ebb and flow of solar activity. Some propagation events—especially those triggered by the arrival of particles that can be seen leaving the sun hours before they arrive here—can be fairly well predicted in the short term, although we still may not be able to predict the timing of whatever caused the outburst of the particles themselves. We can reasonably consistently build a causality trail between the appearance of a sunspot or group of sunspots rotating back into view on the visible side of the sun and changes in the solar flux index over the next few days. We can follow weather fronts as they track across our continent and identify patterns likely to spawn clouds of sporadic E-skip in the next few hours. We can receive telephone alerts notifying us of the high likelihood we’ll be able to see visible aurora borealis tonight or tomorrow night, based on the fact that a flare-up on the sun has sent a bundle of charged particles streaming toward the magnetic belts surround our globe. But we still cannot predict with any certainty or reliability what the instantaneous state of our ionospheric bands will be like at any given minute of any given hour on any given day. Our MF and HF ionospheric frequencies continue to surprise us in the details, even as we have an everimproving ability to predict what we’ll find overall. In the decade following publication of the fourth edition of this book, distribution of propagation predictions has shifted almost totally from the print media to the Internet. Short-term propagation predictions that were a month old never were too great, anyway, but for looking at monthly averages, the family of prediction charts that appeared in QST every month (Fig. 2.42) was useful for seeing general trends. Today we are blessed to have not only near-instantaneous access to the forecasts of the best minds

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 75<br />

atic Pacific region is often blocked by unsettled polar conditions, there are few HF operating<br />

experiences that compare with aiming the 40-m beam southeast on a winter<br />

afternoon and hearing an operator in the Far East answer a “CQ”. It’s magic.<br />

Ionospheric Scatter Propagation<br />

Ionospheric scatter propagation occurs when clouds of ions exist in the atmosphere. These<br />

clouds can exist in both the ionosphere and the troposphere, although the tropospheric<br />

model is more reliable for communications. Figure 2.41 shows the mechanism for scatter<br />

propagation. Radio signals from the transmitter are reflected from the cloud of ions<br />

to a receiver location that otherwise might not receive them.<br />

There are at least three different modes of scatter from ionized clouds: backscatter,<br />

side scatter, and forward scatter. The backscatter mode is a bit like radar, in that the signal<br />

is returned to the transmitter site, or to regions close to the transmitter. Forward scatter<br />

occurs when the reflected signal continues in the same azimuthal direction (with respect<br />

to the transmitter), but is redirected toward the earth’s surface. Side scatter is<br />

similar to forward scatter, but the azimuthal direction usually changes.<br />

Unfortunately, there are often multiple reflections from the ionized cloud (shown as<br />

“multiple scatter” in Fig. 2.41). When these reflections are able to reach the receiving<br />

site, the result is a rapid, fluttery fading that can be quite deep.<br />

Auroral Propagation<br />

The visible aurora produces a luminescence in the upper atmosphere resulting from<br />

bursts of particles released from the sun 18 to 48 hours earlier. The light emitted is called<br />

the northern lights (aurora borealis) or the southern lights (aurora australis). The ionized<br />

regions of the atmosphere that create the lights also form a reflective curtain at radio<br />

frequencies, especially at VHF and above, although auroral propagation is infrequently<br />

Higher layer<br />

Multiple scatter<br />

Backscatter<br />

Scattering<br />

cloud<br />

Side<br />

scatter<br />

Transmitting<br />

antenna<br />

Forward<br />

scatter<br />

Receiving<br />

antenna<br />

Figure 2.41 Various modes of scatter propagation.

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!