Hi Ron, I also tried to receive KAAY when I was in Vietnam, and was unsuccessful. Part of the reason, I think is that outside North America, AM stations are not spaced 10khz apart. Here's a chart of Vietnam radio frequencies in use today - I am reasonably sure many of these frequencies were in use when you and I were there.
The closer spacing of the frequencies would undoubtedly contribute significant additional adjacent channel interference, making KAAY's signal even more difficult to dig out of the "mud".
You are correct about the spacing. When I wrote the post about AM modulation and frequency response, I think I mentioned other parts of the world use 9 KHz spacing between channels. Many of us would have done just about anything to hear a signal from home. I didn't talk to my friends or family back in the USA for a year. My only communication was by way of mail or one of those small reel-to-reel tapes. At least our troops overseas now have some real-time communications. I never did make it over to one of those MARS stations to try to talk to my family via shortwave radio. If you did, can you tell me what it was like?
I heard many signals on the AM band, but I didn't have any means of identifying them. I was sleeping or on duty when there was darkness between Vietnam and the USA. I'm guessing 5:00 or 6:00 AM Central Standard Time would have been the perfect time to try to hear something from the states. I was just waking up about that time. What do you think?
I wonder what some of those foreign stations used for frequency control. Do you think they had the same plus or minus 20 Hz legal tolerance like in the USA? My guess is many of them were lucky just to have any signal on the air. My boss was telling me about when he worked at WMAQ as a summertime replacement, he saw a crystal oven in use and a spare sitting next to it. I never gave a second thought to an oscillator failing.