Tuesday, November 17, 2009

Grey Line DXing Revisited

Hi Bud,

During my last business trip, I was watching the moving map GPS display in the airliner and a thought occurred to me – that under very unique circumstances (and extremely difficult to reproduce, I might add) it might be possible for someone to listen to KAAY in Vietnam. Here’s how.

Gray line propagation is the unusual propagation of radio waves that happen twice each day during morning twilight and evening dusk. Many “hams” take advantage of this effect to communicate very long distances. The effect lasts only a few minutes, and fades as quickly as it begins. Don Payne has also mentioned gray line propagation in a couple of his posts.

Here’s a definition of gray line propagation from the website http://dx.qsl.net/propogation/

Quote: “The grey line is a band around the Earth that separates the daylight from darkness. Radio propagation along the grey line is very efficient. One major reason for this is that the D layer, which absorbs HF signals, disappears rapidly on the sunset side of the grey line, and it has not yet built upon the sunrise side. Ham radio operators and shortwave listeners can optimize long distance communications to various areas of the world by monitoring this area as it moves around the globe."

What I noticed on the airplane’s moving map GPS display is that both Arkansas and Vietnam could both be inside the gray line at the same time during fall or winter months. Daylight hours are shorter in the northern hemisphere during winter months, and if you plot the gray line from Arkansas to Vietnam following the wintertime Great Circle Route, you go up over northern Canada, and then down into south-east Asia. If this theory holds any water at all, early morning in Arkansas would be sundown in Vietnam, and vice-versa. A late afternoon listener in Vietnam, for example might be able to catch a few short minutes of the KAAY morning show, when they are both within the “gray line”.

Also, there would only be a few days each year when both Arkansas and Vietnam would be in or close enough to the gray line to make this possible. I have absolutely no way of testing the theory but perhaps some of the wiser readers would be able to expand the theory or label it as “bs”. Have fun discussing!

Also, regarding the KAAY listener report from a military base in England – military radios are known their far-above average sensitivity, excellent filters, and variable “Q” tuning sections and when coupled with an amplified, tunable and directional long wire military antenna, you could in theory be able to listen to almost anything. But also remember that KAAY (1090kHz) would have to be sharply filtered against the “local” European long wave frequency 1089kHz, and that would take some specialized equipment not usually available to John Q Public.

I’m just sayin’,


(Yessir, Dave, those military and commercial receivers are hot properties...but, the average listener can't afford a $15,000 radio...and some of the top-line Ham tranceivers can run $9,000 to $13,000 for those who are very serious about getting every contact they go after...and that's NOT including the antenna(s), feedline/coax, tower/antenna supports, power amplifiers, meters and what-have-you.  I'm just happy to have what I have and enjoy the time operating and listening that I do.

Some of us dream of that multi-acre antenna "farm"- and I'm not talking of just a couple acres, mind you!- for huge rhombic antennas, etc.  Or even the hundred-plus feet [some go hundreds!] of tower and the biggest antenna on top.  But, its technique and persistance that count heavily, as well. Bud)

1 comment:

  1. I think it is possible to pickup a USA signal on the AM broadcast band in Vietnam only under the rarest of conditions. One problem is most military radios in use in Vietnam did not cover the AM broadcast frequencies. I went to radio repair school at Fort Knox where I learned to repair the Collins R392 receiver. The R292 covered from 500 KHz to 30 MHz. It was meant for AM and CW only. It did not have switchable filters, but it certainly exhibited a high amount of selectivity. Too bad they didn't let me take one home.

    When I was in troubleshooting class, I could find the problems the instructors had created in our trainer receivers in a matter of minutes; however, each class was an hour or longer. We had random length antennas which weren't very long. I spent most of my extra class time listening to WAKY, but many times I tuned around the dial to see what I could hear on an expensive receiver. Since this class was during the afternoon, I was thrilled when I could hear KAAY during the day. I couldn't do that on my transistor radio. I also heard all of the Chicago 50, 000 watt stations and WLW in Cincinnati. When I arrived in Vietnam ten months later, I never saw an R392. The radios used to communicate at a unit level were all FM, and they covered 25 to 75.95 MHz.

    I was also trained on a very expensive shortwave transceiver which is the AN/VRC106, and it was primarily intended for the single sideband mode of operation. It did not cover the AM broadcast band. It is the type of radio which had the narrow filters Dave has talked about.

    After word spread around Vietnam about me having the ability to build antennas, I was lent out to other units which were having trouble communicating with their personnel at least 200 miles away. These infantry divisions were all using AN/VRC106 radios for single sideband communications.

    The AM band had static crashes all year long like we have here during the summer. Hearing a station is one thing, but receiving a signal and being able to listen to it is another story. If you look at this link, you will see what the R392 looked like: http://www.vk2bv.org/museum/r392.htm . Many military radios were meant for vehicular use on 28 volts DC. They required an additional power supply to use them in a base station configuration.

    I think I was very lucky to have a hidden 40 foot long wire to use as an antenna. The military frowned on us erecting anything for our own personal use.

    Most of us didn't have any real-time communications with home. I never talked on a phone to my family during my year over there, and we were told our outgoing mail might be censored. One could make an appointment to visit a Military Affiliate Radio Service installation to talk via shortwave radio to another station in the USA. The USA station would telephone the soldier's home and interface the phone with the radio, so the soldier could talk with loved ones. If you had five minutes to communicate, would you really spend it trying to call KAAY?

    Remember Vietnam really was a war zone, so a soldier was very limited for security reasons. My DX'ing was right around sunset; I never tried it at sunrise.