I Have read about this in the February 2005 CQ magazine, by Mac Obara and this is very fascinating. Basically we are dealing with radio transmissions coming back to us after having been reflected off some ionized belt circling the solar system at about 297 AU from the earth, or 298 AU from the sun. One AU is the Sun-Earth distance, or 93,000,000 Miles. This belt seems to be beyond Neptune. There seem to be other belts too. This explains the various echo times.
The farthest echo seems to be from the 297 AU distance, and it was returned after 82 hours!!!! This occurred on 160 meters, of all places, a band not known for DX!! At least not like on 20 meters, so it is all the more amazing!! The feat was accomplished on January 18, 2004, for the transmit time. Most of these LDE receptions were done by JA's in Japan.
How can this happen? Would the path loss not be astronomical??? NO! The reason why is because the signal gets reflected off the circular belt, and refocussed back to the origin. The only loss would be the 2 transit losses thru the earth's ionosphere, and the loss in reflection of the belt, and the absence of the belt in most of the sky, off the target plane. Say the belt only reflects back one-millionth of the signal. That is a 60 db loss. And if the belt covers only one-thousandth's of the sky, another 30 db loss. Add two transit losses through our ionosphere of 20 db each, 40 db total, we get a grand total of 130 db "path loss". If you transmit with ONE WATT you will be at 0 dbw outgoing, and the return signal will be at minus 130 dbw, or S-5. Actually, in practice it may work out this way, the signals seem to be stable at RST 559, at levels around 100 watts. The loss for MOONBOUNCE signals is MUCH MORE than for this mode, because the moon reflects back only a tiny portion of the signal it gets, due to reflection inefficiency. And it also intercepts only a tiny portion of all the outgoing power, because it is only a tiny spot in the sky.
Does this happen all the time? No. Only from October to February, and throughout the solar cycle, except 2 years centered around the solar peak. One has to transmit at 2000 to 2200 Hours local time, but reception is okay from dusk to dawn. It is reported that the receptions are only of about 20 minutes duration. Only frequencies below 4 MHz seem to work for these extreme LDE's of several hours plus. You may have heard these signals, without knowing it, because if you hear one, you cannot verify if it is an LDE echo unless you contact the station, but he will not answer because he was on the air hours ago!! SWL's cannot really find out, for they would have to phone up the station to see if it is transmitting right at this time. And hams do not get answers to some calls they make. So, many of these signals get mistaken for real ordinary signals. But if you do hear a G station in W8 land at dawn, on 160 meters, you know that something funny is going on!!
W6BNB reported that W6XXX got LDE's on his signals ( 3555 khz ) as well, coming back to him about 30 minutes later, his entire net control session coming back. But he could not hear any of the stations he contacted during NCS. Very strange indeed.
W6BNB said that when sending a series of dots on CW with fast break- in, he could hear the echo coming back, which would just be a once round the world signal. This can also be quite common, but still exciting. From his article in the June 2004 CQ magazine.
What does this have to do with Radio? It is the sheer excitement of hearing one of these signals, which are extreme DX, your best chance at extraterrestrial radio work.
With computers and robot receivers, radio hams can set up a test station to probe this phenomenon more thoroughly. This is absolutely the farthest you can "DX" without expensive "SETI" like gear and big dish antennae and lab equipment.
July 24, 2007
