Well, folks, its about time for a "Mythbusters Moment". Behind the scenes, Ron H., Dave M. and I have been discussing the feasability of our troops hearing mediumwave (AM broadcast band) signals in Vietnam from the U.S.A.
In theory, it might be possible...in practical terms, its not probable. Ron and I are Ham radio operators (and Ron currently works in the field of radio and is degreed in several areas), while Dave is an electronic engineer; all of us have different training and life experiences and have been fortunate to have good discussions and learning from one another (boy, have I been learning from these guys!).
I've been drawing on both Ron's and Dave's experiences while in Vietnam, re: radio reception.
One time, I believe it was Clyde Clifford who mentioned that he was getting calls from soldiers hearing the station; A. J. also mentioned to me that audio was possibly piped in via an unused phone line. On still another source, the KXOK tribute website, someone mentioned hearing that particular station for an hour, from an airport. Ron pointed out here that one could pretty much count on a lot of interference from different things in an airport and not reliably hear a station from the U.S.A.
Now, the "feasability" portion of an arguement would be that a signal from the nighttime portion of the planet would bounce off the ionisphere into the daytime portion and be heard. The "probability" portion against such a thing happening on the mediumwave band is that, outside North America, channel spacing is at 9 kHz- North America's channel spacing is 10 kHz. The factors working against being able to hear these stations lay in not only co-channel interference from the 9 kHz-spaced stations, but from daytime interference generated from a variety of other local sources and atmospherics...and pure distance from the station to the reception area.
Dave also mentioned that, while working with Felix McDonald, they would take signal strength readings at KAAY when the foliage was in full on the trees; vegitation acts like a large sponge, soaking up signal and diminishing said signal going outward. However, this mainly "skews" the ground wave propagation, not sky wave propagation (thanks, Dave, for the correction! bs). Also, during this time, atmospheric noise is higher. The opposite is true in the wintertime: less foliage to soak up the signal and clearer, quieter atmospheric conditions. That's why a radio signal sounds so LOUD during the wintertime, local-wise on ground wave. Dave also mentioned that sky wave propagation is a variable phenomenon, not as predictable as ground wave propagation, so many things could "go against" sky wave propagation.
Sorry folks, but we're all doubtful on this one. There were times that tapes were sent from home and were rebroadcast on what were local on-base low-powered stations, from what Ron has mentioned to Dave and I. I would say that the solder in the airport may have heard such a station during the hour-long layover mentioned. To hear anything with constant reliability had to be borne from a local station. Also, the average soldier did not have access to a huge, high-quality sensitive and extensive radio system and antenna.
The only thing that did work with any reliability and regularity was when MARS (Military Affiliate Radio Service) stations operating on both sides of the Pacific would communicate "health and welfare" messages, BUT, they were not restricted to one frequency in one band; they were able to operate on a multitude of frequencies on many bands, in what many know as the shortwave band, that would "make the trip", according to conditions and times of day or night. Senator Barry Goldwater had such a station that was manned 24 hours a day by volunteers providing communications (and this was before cell phones and other consumer-grade worldwide communication), but that is another story for another time.
If anyone- ANYONE- claims to have heard a mediumwave radio station from the U.S.A. in Vietnam, I would challenge them for physical proof; radio can be unpredictable sometimes, but we'll stand pat on this one....
Bud S. (firstname.lastname@example.org)