LDES - LONG DELAYED ECHOS There are stories told in the cult books of radio signals being heard again years after the radio station left the
air. One such story involved a broadcast AM station that had closed down, yet the radio signal was heard nearly two years after the station went off the air.
Those stories are like the UFO and ghost stories. They have no foundation. LEDs, though, are very real. What they are not, are attempts by some alien space craft to contact us.
What they are is an as-yet unexplained electromagnetic phenomena relating to radio waves. In the 1960s Stanford University's Physics Lab tried to find an explanation for them but LDEs have no apparent
useful purpose, so allocating much in the way of scientific time or funds is pretty unlikely. The study closed with only a few suggestions. There are some facts available about the standard
LDE. First, it does not show signs of Doppler shift. It is on frequency, it remains on frequency for the duration, and the pitch, if it is CW, remains steady. Secondly, it is rarely a
full transmission. It is segments, often very short segments, only five or six seconds, of a transmission. It may not begin at the beginning, and it may not end at the ending of the
original transmission. It is not complete. Third it does not fade in or fade out. It is "there." At least that is true of the one I heard that was so clearly identifiable. It
turns on, and then it turns off. Fourth, they have been showing up LONG before computer sound cards. While it is possible one or two could be a hoax, it is highly unlikely.
Fifth, they are not limited to the ham bands. Sixth, reports of them are mostly in the 3 to 18 mhz range. Occasionally one is heard above that, and on very rare occasions, one below
that. This could be because most radio activity is in that range, so it really isn't a defining fact, but it is a point to consider. MY OWN EXPERIENCE To my knowledge I have heard only
one LDE. It is possible I have heard others, but did not recognize them as such, perhaps due to crowded band conditions, or other situations. The one I heard, though, was unmistakable. To
explain it, I have to set the scene a bit. This was in 1967, in the Far East. I was a government radio operator/tech at a somewhat isolated radio relay site. We ran a CW net that was used almost
entirely for the purpose of establishing contact and setup procedures for RTTY operations. Hence it carried very little, and very intermittent, CW traffic. We ran three or four transmitters
simultaneously, depending upon time of day. We ran four receivers (Collins 51J3/R388) models in a console. There was a J38 straight key for our use. There was also a Frederick Electronics Baudot
to Morse converter, that converted 5-level punched paper (or plastic) TTY tape to Morse. We ran what is typically know as a "guard" frequency. You've heard them on the marine bands. It
sends something like: VVV VVV VVV DE KGP444 KGP444 KGP444 QSX 4/6/10 K And then there is silence. That is the listening period.
Our machine sent this (and this is a fictitious call sign, not the one we used) VVV VVV VVV DE AAA123 AAA123 AAA123 VVV VVV VVV DE AAA123 AAA123 AAA123 VVV VVV VVV DE AAA123 AAA123 AAA123 QSX 4/6/10
K (In the day time the freqs would change to 6/10/13) That sequence, at 18 wpm, took just about one full minute. The tape then looped (using "letters" characters) for another full minute,
during which time any station could call in. Because our transmitters were located a couple of miles away, we ran the receivers at full RF gain all of the time. There was NO muting! All
four receivers were wired to a speaker panel. That meant any signal heard would be heard throughout the building. Twice each hour, on the hour and on the half hour, one of us (there were usually
only two on duty) would actually sit down at the receivers for "close monitor." For that, we switched the speakers to a panel down near the desktop, and we would sit for two minutes and monitor.
At all other times, the operator was away from the position and doing other things, so he had to be able to hear the R388s above all the Teletype noise in the building - and we had a LOT of such noise!
Motors for printers and machines, plus air conditioning, fans, and the like. So the receivers were very loud. When our transmitters keyed, the AGC action of the receivers served to mute them, as we
kept the AGC slow. The first "beep" of the CW shut down the AGC, which prevented any full breakin. There was no other muting. Just pretty much receiver overload! Now the scene is
set. It is not close monitor time, and I, with my partner, am doing other work in the RTTY section. We grow accustomed to hearing the receivers go dead as our own transmitters overload
them. Then the background noise returns. Suddenly we hear: VV DE AAA123 AAA123 AAA123 VVV Then it's gone. It lasted perhaps five or six seconds. It did not
fade in, it did not fade out. It "turned on' and then it "turned off." It was our keying. At our speed, and automatic. It was strong, but nowhere near strong enough to kick the AGC
and overload the receivers. It was not strong enough to even "thump" them. It was simply a fairly strong CW signal. For an instant I thought it was someone calling us. Since that
did not happen unless there was a problem, I moved quickly to the console. I stopped our automatic keying and I listened. No one on any of the frequencies. I sent "QRZ DE AAA123 K" with the
hand key. Got no response. I tried that maybe three times. No one was out there. Of course I was keying all four transmitters simultaneously, so no matter what frequency he had called
in on, I would hear him. Then it dawned on me! I had heard "DE AAA123!" He had been identifying as my station! Not AAA123 DE AAB321, for example. It was "DE ME!" By
sheer luck, we had been running a slow speed monitor tape on another receiver in a different rack. That was a rack with two Collins 51S1 receivers, that we used to monitor other things, but
it had been set up to monitor a couple of frequencies, and I had, earlier in the afternoon, set one of those two receivers on one of our guard frequencies. The other was on some other frequency.
What if, I thought, the echo had been on that particular guard channel? I had no way of knowing which of the console receivers had picked up the signal but it was early evening, and we had been
about to close down the 13 mhz guard, as it was no good for us at night, so it wasn't likely it would be there. I could be on our 4, 6 or 10 mhz frequencies, and there was no way of knowing. But
the 51S1 was on the 6 meg guard frequency. What if! I ran the tape back. Yes, indeedy, it had been on the 6 mhz guard frequency. And I had a tape of it. Not only that,
because because the four channel recorder used one of its channels to permanently monitor a highly accurate 1 KHZ oscillator, I could now tell precisely what time the signal had been recorded, using our test
gear designed for that purpose. All that told me nothing. Except it was not one of our field stations. It was not our own transmitter being keyed by our keyer. It was a received
signal, and in ham language would have been RST589. We gave it a QRK5, QSA5. The only thing I knew for sure about it now was it had been on our 6 MHZ frequency, not on the four or 10 mhz
one. I made a copy of that segment of the tape, as it was something we wanted our technical staff to analyze for us. They did, and learned nothing more. I kept the tape for a few years
but finally erased it. Shortly after the actual occurance, (perhaps a few months) which neither my coworker or I understood at the time, I happened across a story on LDEs. And then we knew
what we had heard. Again I played the tape several times, but garnered no information from it. Except one thing. The "echo" had begun 16 seconds after the end of our transmission.
But it had not begun at the beginning. It had not echo'ed our entire transmission, only a few seconds of it. All this took place outside the USA. Some things we did know. It was
not ham operators. It was not a hoax. It was not foreign intelligence. It was not one of our own transmitters. It was not a recording. (thank about that one, and why you know it
was not an audio recording!) It was not one of our field stations. It did not fade. It had no Doppler. It was our keying, at our speed. It was reasonably strong, but not
enough to trigger AGC on the receiver. It was very clear, with no QRM. It was in the 6 MHZ band. I have never heard another one. I have some theories about LDEs, but I'm no
scientist, so I just keep them to myself, mostly. I think the key, though, is the word 'ducting.' Have Fun. . |
