Wednesday, June 23, 2010

I enjoy 6 meters, even when the band isn’t open ...

Q I enjoy 6 meters, even when the band isn’t open. I’ve found that it is a terrific band for “local” communications up to a few hundred miles away. Some of the local signals, however, have an odd fluttering characteristic. What causes this? Does the fact that I live near an airport have anything to do with it?

A Take a look at Figure 2. The energy traveling directly between the horizontally polarized transmitting station antenna and receiving station is attenuated to about the same degree as in free space. But unless the antennas are very high or quite close together, an appreciable portion of the transmitted energy is reflected from the ground as well as from buildings and towers. These two signals combine at your antenna, and that’s where things get particularly interesting.

When the signal strikes another surface, its phase is reversed. If the distances traveled by both signals were exactly the same, and if the reflection phase reversal was exactly 180°, the signals would arrive out of phase with each other and cancel completely. This never happens in the real world or you would hear nothing at all! Instead, the reflected signal travels a little farther. Combine this with the less-than-180° phase reversal and you have partial cancellation at the antenna, not total. Your statement about living near an airport provides an important clue. Signals bouncing off aircraft can arrive at your receiver with rapidly varying phase and amplitude, causing considerable flutter.

Figure 2—Part of the signal energy takes the direct path to the antenna, but another portion arrives as a wave reflected from the ground or other objects. There is a phase reversal with each reflection, and the distance the wave travels is greater as well. The signals combine at the antenna, adding and subtracting from each other.

From QST April 1999