Posts Tagged ‘LDE’
Tommy happened upon the LDE when tuning around the band one December evening. He heard a weak, warbly ... "CQ CQ CQ de VE7BLF VE7BLF K". A chill went up his spine as he had actually called CQ about 30 minutes earlier, and was answered by a K7. Sure enough, the phantom LDE soon came back to the K7 while Tommy listened to himself send the report, name and his QTH information on the next transmission! Listening to his own signals, 30 minutes later, really shook my friend up as he had no idea what to make of the incident.
Over the next few weeks, Tommy heard several LDEs, some with separations as much as 3 or 4 days and during this time made several recordings of the events. He made a copy of the recordings for me to listen to and the sound of the signal really did send shivers up my back. It was tormented, warbly, with a bit of flutter and very weak. It sounded as though the signal had been ripped apart on a trip to hell and back, adding further to the mystery.
The LDEs stopped as quickly as they had begun and were never heard again but in the meantime, Tommy and I speculated on what might be the cause. Were they genuine LDEs? ... they certainly sounded as if they had been on a very long trip ... or was the explanation something far less sinister? We never did solve the mystery but I had suggested that one likely explanation might be a faulty (or normally operating) VCR, somewhere in the neighborhood. Someone that happened to record at around the same time that Tommy was operating and playing the program back when Tommy happened to be listening. It seemed a far-shot and one that we never tested, preferring to think of it as another one of the mysteries of radio. Unfortunately Tommy became an SK several years ago, never knowing the cause of his mystery signals.
During a recent discussion with Tony (VE7CNF) and Mark (VA7MM) about the newly imagined plasma tubes, the subject of their possible link to LDEs also emerged. I related the above story and Toby immediately went to work with his old VCR! In his own words:
Yes, old VHS recorders could cause long delay echoes. When Steve mentioned it, I had to try it. I attached a photo and some recordings.
I hooked up my old VHS recorder with BNC video and audio cables in and out. I pulled out the video cables a bit to disconnect the shields, so RF could leak into and out of the video ports. My IC7410 was connected to a dummy load and split, through a T connector, to a whip antenna on top of the VHS.Transmit power was 100W to the dummy load.
I transmitted while recording video. When I played it back I could hear the CW signal pretty much on frequency. I tried this at 1820 kHz and the signal had a lot of frequency flutter. At 3510 kHz the playback signal was stable enough for SSB.
So, RF leaking into and out of the video cables of a nearby VHS recorder could explain LDE's where the delay is hours or days.
Mystery solved ... the signals that Toby reproduced had the exact same characteristics as I recall from Tommy's old tape recordings although his signals were much weaker, making them seem as if they had been on a very long journey.