Fall 2007 Issue

PROPAGATION

The Science of Predicting VHF-and-Above Radio Conditions

By Tomas Hood,  NW7US

This past August did you see the Perseids meteor shower? Did you work any of the meteor plasma trails on amateur radio VHF (6 or 2 meters)? Iíve seen many reports of moderate success for operators in various regions of the world during the 2007 Perseids season. Along with working meteors, a mix of tropospheric ducting often occurs in some regions, and I wonder if the two propagation modes ever combine. Iím interested in hearing from you if youíve observed and worked such combinations of VHF propagation.

The 2007 autumn meteor season is open for DX hunters. While the Perseids meteor shower is one of the impressive yearly showers, partly due to the time of year in which it appears, the showers that will occur over the next few months are great rivals.

The Next Shower

One of the largest yearly meteor showers occurs during November. Appearing to radiate out of the constellation Leo from November 10 through November 23, the Leonids will peak on the night of November 17 and the early morning of November 18. This shower is known to create intense meteor bursts. Since the source of the Leonids, the Tempel-Tuttle comet, passed closest to the sun in February of 1998, the years following were expected to produce very strong displays. The greatest display since 1998 was the peak of 3,700 meteors per hour in 1999. Every year since then has been significantly less spectacular. However, a few (lonely) forecasters think that we still might have a meteor storm with an hourly rate of thousands of meteors sometime in the next several years. The more common forecast is that weíll only have a rate of about 15 meteors per hour.

The best time to work meteor scatter off the Leonids is around 11:30 PM, local time, in the Northern Hemisphere. The shower should increase in rate the closer you get to midnight, and then move down toward pre-dawn.

A Possible Meteor Storm in November?

Keep alert for the a-Monocerotids shower, which will occur from November 15 through November 25. The peak will occur on November 22 at 0310 UTC. Normally, this shower produces about five meteors per hour, but this year it may produce a burst as high as 400 per hour. The most recent big burst was in 1995 with a reported ZHR (zenith hourly rate) of about 420. It was expected that the ten-year cycle would result in a large storm in 2005, but that never occurred. That is why we should stand ready this year, as the cycle could be longer than anticipated.

Will this be the year of a return of the storm-level activity? If so, it will make the prospect for exciting meteor-shower radio propagation probable. We just cannot know for sure, since it takes a direct interaction with the comet dust trail by the Earth in order to see such a higher rate of meteors entering the atmosphere.

The chances of Earth hitting a dust trail that is so narrow and filamentary are slim. This has proven true for most meteor showers in recent years, when we have missed various meteor trails nearly completely. During these misses, Earth slips between the clouds, where there is only a sprinkling of meteoroids.

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