Cool Fading Visual...

The waterfall half of my new Console displays some interesting things of which I was completely unaware. Recently I was talking with some friends on 75 meters, and I noticed an interesting effect...

On some stations, the waterfall looked, over time, as though a "comb" had been dragged through it. That is, it looked as though the tines of a comb had been run diagonally through the signal. I assumed that what I was seeing was due to the pitch (and harmonics) of the talker's voice changing with time, and I didn't think much more about it, except to note that it looked interesting.

Several days later I was monitoring a local boatanchor AM net, and I noticed the same thing. Again, it looked like comb tines had been dragged diagonally through the entire AM signal. What was unexpected, though, was that the direction of the drag was the same on both sidebands. What did this mean? Well, it meant that my initial assumption that the drag marks were due to voice pitch changing was wrong, for if they had been due to voice pitch changes, then the drag marks would have had the opposite slope on the other sideband. That is, if the drag marks were moving towards the carrier (with time) in one sideband, (indicating pitch lowering over time), then they should also be moving towards the carrier in the other sideband (that is, the sign of the slope should have changed in the other sideband).

But, surprise! The drag marks continued in the same direction across both sidebands!

What did this mean?

After watching the signal for a bit, I noticed that the carrier on the panadapter display would drop slightly as a "drag mark" reached the carrier. Bingo! I suddenly realized that what I was seeing was an example of fading - specifically, "selective fading"as it moved through the AM signal (this is a phenomena anyone who has listened to shortwave AM broadcasts is familiar with).

Below is a screen shot showing the fading with time on a shortwave AM broadcast station. (Click on it to blow it up). Looks like a comb was dragged through the spectrum, doesn't it?

Pretty cool, eh?

(What was also interesting, but not shown well in this image, is that the AM signal to the left also has "drag marks," but that these drag marks have a different slope and spacing, indicating a different selective fading pattern due to the different propagation path.)