Fall 2008 Issue

Moondata Update 2009 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 2009.


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 the Earth-Moon-Earth (EME) communications path for frequencies below 1.0 GHz. Best conditions occur when: (1) the Moon is at the absolute minimum perigee distance from the Earth and (2) the Sky Temperature behind the moon is the coldest along the moon path. The effect of distance is independent of frequency, but 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 over hourly, daily, weekly, monthly, and yearly time periods. As a guide for the basic weekend conditions for 2009, the W5LUU Weekend Moondata 2009 lists the DGRD, in dB, for 144 and 432 MHz, and other pertinent EME information for each Sunday at 0000 UT. Station, location, and factors such as ionospheric disturbances, local noise, antenna beamwidth, side lobes, polarization, etc., can increase the “apparent” DGRD.

EME conditions during 2009–10 will be the most favorable of the 9-year cycle. Now is the “best ever” time to take advantage of this mode. Ten weekends of 2009 are rated as Good to Excellent. Thirteen other days have 2-meter DGRD <1.0 dB. On May 1 it dips to 0.08 dB, and on Nov. 9 to 0.07 dB. However, during the traditional ARRL EME Contest period, sky noise at VHF is a problem for high north Moon declinations. Weekends around Oct. 11; Nov. 1, 8 (Good), and 30; and Dec. 6 (Excellent) are possibilities. For 1296 and up, the weekend around 9–13, high declination near perigee, should also be considered for the contest.

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) of about 19 years. The last maximum was on 9/15/2006.

RA (hrs): Right Ascension, in hours, gives the east-west position of the Moon against the sky background. Average period of RA cycle is 27.321662 days, but it can vary by a day or so due to effects of the Sun on the Earth and Moon motion.

144 MHz Temp (K): The 144-MHz cosmic (sky) noise in direction of the Moon expressed as absolute temperature.

Range Factor (dBr): The additional EME path loss, in dB, due to Earth-Moon separation distance being greater than absolute minimum (348,030 km surface-to-surface). Varies from a low of 0 to 0 .7 dB at perigee up to 2.33 ±0.1 dB at apogee.

DGRD (dB): The degradation in EME signal to noise, in dB, due to: (1) the excess sky-noise temperature, in dB, at the stated position of the Moon compared to the lowest cold sky temperature and the system noise temperature (all at the frequency of interest); plus (2) the Earth-Moon range factor, dBr, for the listed time and date. The tabulated DGRD is referenced to the lowest possible sky-noise temperature along the Moon path, for a system noise temperature of 80°K at 144 and 60°K at 432, an antenna beamwidth of ~150, and to the absolute minimum Earth-Moon (surface-to-surface) distance.

The dBr affects DGRD equally at all frequencies, but sky noise decreases rapidly as frequency increases. During a monthly lunar cycle DGRD can vary by 13 dB on 144 and 8 dB on 432. DGRD varies less with small antennas than with large ones.

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© Copyright 2008, CQ Communications, Inc. All rights reserved. This material may not be reproduced or republished, including posting to a website, in part or in whole, by any means, without the express written permission of the publisher, CQ Communications, Inc. Hyperlinks to this page are permitted.