VE7CNF – VHS Propagation: Radio Long Delayed Echoes and Video Recorders
Toby Haynes
July, 2015
Long delayed echoes (LDEs) are part of radio lore. Ham operators have reported hearing echoes of their own radio transmissions, usually with short delays, but others delayed by hours or days. Various theories involving natural phenomena are proposed to explain echo delays up to about 40 seconds. Explaining longer delays by natural phenomena is unlikely, since a very long delay means the signal must propagate over a great distance. Such echoes should be attenuated so far below background noise levels as to be undetectable.
In a discussion with Steve, VE7SL, he described an experience with very long delayed echoes and a theory to explain them:
“In all my years on the air (since 1963) I've not heard anything delayed more than a few msec from long path echo but I had a friend (VE7BLF now SK) that had some on 160m CW one winter when using a cloudwarming low 80m zepp with strapped feeders. He recorded it several nights in a row but these were like hours or one day later that he was hearing them. The signals had the strangest sound to them, like they had been to hell and back...all unstable and distorted tonally. He never did figure it out but I always suspected a wayward VCR somewhere in the neighborhood somehow being involved. It was very eerie listening to them.”
VCR’s have enough bandwidth to record HF radio signals along with a video signal. Video luminance bandwidths are approximately 3 MHz for VHS, 4.5 MHz for Beta, and 5 MHz for S-VHS. Perhaps RF can leak into and out of an analog VCR that’s located near an HF antenna. The VCR would record any incoming HF leakage at the same time it’s recording video. During playback the video luminance signal would include the HF signal, which could leak out and be heard by the nearby ham station. This could explain some LDEs.
It’s common for VCRs to connect to video sources and TVs through cables using RCA phono connectors. The cable outer shield path through RCA connectors is unreliable, so there’s an opportunity for RF leakage. Poor shielding of video equipment can allow for RF leakage to internal circuits. Cablevision coax connected to a VCR’s antenna port can conduct HF signals into the box, where poor internal shielding could allow coupling to the video signals. With a neighbor’s video equipment very close to one’s 80m or 160m antenna, a little leakage can couple a significant signal.
I had to test this idea. I hooked up my old VHS recorder (Magnavox MSC-455) with RCA video and audio cables on the input and output. I pulled out the video RCA connectors a little to disconnect the shields but leave the center conductor in contact. This allowed some RF leakage. The audio cables provided the ground path. These bad connections had no effect on video quality.
My Icom IC7410 transceiver was connected to a dummy load through a “T” connector. Another cable from the “T” led to a short whip antenna on top of the VHS unit. Transmit power was 100W to the dummy load, which produces about a 200VPP signal on the whip antenna. The antenna is far too short to radiate at 3.5 MHz, but will produce an RF electric field to nearby objects. [Don’t panic! I’m an electrical engineer and can guarantee that the physics of this situation cannot produce any RF exposure hazard. However, don’t try this with a 100W VHF or UHF transmitter as the radiation from the whip will be very strong!]
I transmitted while recording video on the VCR. When I played it back I could hear my signal, pretty much on-frequency. Best results were with the whip antenna close to the video cables. I chose frequencies where I couldn’t hear much HF noise from the VCR. There was no noticeable interference to the TV picture on playback.
I tried first using CW at 1820 kHz and the playback signal had about +/-200Hz of frequency flutter. Perhaps the RF was interfering with the VHS recorder’s power supplies or tracking. At around 3510 kHz the playback signal was stable enough for SSB. Playback signal strength was about S2 on 160m and S4 on 80m. Here are some recordings:
3513 kHz SSB playback from VCR
Note that the VCR must be recording video at the same time it records the HF signal. The VCR relies on the presence of video sync signals for proper operation. Trying to record HF alone without video probably won’t work.
I sent the audio files for Steve VE7SL to listen to, and here’s his response:
“The 160m one sounds very similar to Tommy's recordings although his were much weaker...probably coming from a nearby neighbor's home. I kind of thought that's what it might be at the time as it was the only logical thing I could attribute the (sometimes two days delayed) LDEs to.”
So, a nearby video recorder with RF leakage can explain radio LDEs where the delay is very long. This works for signals below about 4 MHz. Now there’s a little less need for time warps, wormholes, extra dimensions, mischievous aliens at the L5 Lagrangian point, and all the other fun stuff.