Fall 2007 Issue

Moondata Update 2008 and Related Comments


One of the most important factors in EME communications is knowing
when it is best to communicate via moonbounce. W5LUU presents a
summary and table of the best and worst conditions for EME in 2008.


By Derwin King,* W5LUU

The Earth-Moon distance and the cosmic (sky) noise temperatures in the direction of the Moon are predictable, cyclical variables that set the basic quality of Earth-Moon-Earth (EME) communications for frequencies below 1.0 GHz. The best conditions occur when: (1) the Range Factor (Earth-Moon distance) is at the absolute minimum, and (2) the Sky Temperature toward the Moon, as seen from Earth, is the coldest along the moon path. While the Range Factor is independent of frequency, Sky Temperature decreases with frequency, up to ~1 GHz, and then levels out. The EME signal-to-noise ratio, in dB, is usually degraded from the ideal by a factor (DGRD, see below) which varies with time over hourly, daily, weekly, monthly, and yearly periods. The DGRD, in dB, for 144 and 432 MHz, and other pertinent EME data, are listed in the W5LUU Weekend Moon Data for each Sunday at 0000 UT and provide a guide for the basic EME weekend conditions (see the accompanying table). Random variables such as ionospheric disturbances, local noise, and polarization mismatch will increase the “apparent” DGRD.

EME conditions generally will improve in 2008, with all moon perigees at north declinations, but on many weekends the moon is at right ascension, where Sky Noise is 1 to 4 dB above minimum, and five good weekends are negated by New Moon. Many weekdays will be good or better. Over the next one to two years, as perigees occurs near the best cold sky region, conditions will improve. During the annual ARRL EME Contest period there are no ideal, high declination weekends for VHF due to the high sky temperatures. Dates will have to be a compromise. For 1296 MHz and up, several high declination dates are near perigee and near ideal.

Definitions

DEC (deg): Moon declination in degrees north and south (–) of the equator. This is cyclical with an average period of 27.212221 days. The maximum declination during a monthly cycle, plus and minus, ranges from 18.15 up to 28.72 degrees with a period (maximum to minimum and back to maximum) in about 19 years. The last maximum was on 09/15/2006.
 

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