|
Summer 2004 Issue |
|
50-MHz Long-Path
Propagation |
|
Operation over long distances on 6 meters can
be very challenging. The fact that F-layer ionospheric propagation is
relatively rare provides interesting opportunities to observe some
propagation modes in isolation that, while they may occur more often at
lower frequencies, are often masked by other propagation modes occurring
at the same time. Among the most interesting cases are Transequatorial
Propagation (TEP) and two TEP-related beasts—Transpolar Long-Path
Propagation (TPL) and Transequatorial Long-Path Propagation (TEL). Under
suitable conditions these latter modes can produce spectacular 6-meter
openings spanning well over halfway around the world, provided that one is
lucky enough to be in the right place at the right time. TEP is a propagation mode that can allow VHF stations located in the magnetic tropics on one side of the Earth’s geomagnetic equator to communicate more or less on a north-south line with similarly placed stations on the other side of the magnetic equator over distances of several thousand kilometers, generally in the afternoon or evening. The ionospheric skip points are located in the F2 layer near the equator.1 This effect is very well documented at 50 and 144 MHz, and has occurred less frequently at 222 and even 432 MHz. The first recognition of this effect on 6 meters appears to have occurred in late August 1947, near the peak of solar Cycle 18.2 On 25 August, KH6/W7ACS at Pearl Harbor worked VK5KL in Darwin, Australia to set a new 6-meter DX record. Two days later XE1KE in Mexico City worked a stunned LU6DO in Argentina.3 At about the same time, stations in England and the Netherlands worked stations in Southern Rhodesia and South Africa.4 These patterns have been observed repeatedly since then, especially during solar maximum. While a total mystery at first, the basic mechanism began to come into focus as the result of amateur and professional studies, beginning with the International Geophysical Year (1957– 58), and it continues to be the subject of study today. During that time, it has become obvious that stations with the good fortune to be located within about ±40° of the magnetic equator can enjoy rather good propagation, often in the dead of night, with their neighbors on the opposite side of the equator. Notably, this effect occurs for a month or two in the Spring and Fall around the equinoxes, centered on March and October.
Moreover, the paths need not be strictly
north-south. Stations in South America and Hawaii work each other
frequently, and South American stations often also work into southern
Europe. While these stations are indeed on opposite sides of the magnetic
equator, there is a very significant east-west component to these paths,
in addition to north-south. Click here to return to Summer 2004 highlights Click here to subscribe to VHF _________________ © Copyright 2004, CQ Communications, Inc. All rights reserved. This material may not be reproduced or republished, including posting to a website, in part or in whole, by any means, without the express written permission of the publisher, CQ Communications, Inc. Hyperlinks to this page are permitted.
|