Best Times for EME

Best Times

Undoubtably the best time for EME is during one of the various EME contests, and if the contest is on 144MHz so much the better.
Contest organizers always choose weekends when conditions are likely to be above average, and contest stations are always looking for new contacts to boost their score.
VK and ZL stations are among those in particular demand, because there are not many of us, and contesters are always looking for multipliers from a new prefix.
Contests also bring out lots of smaller stations, so the band can be quite busy with many stations to work!
 
And when are all the EME contests?
Go to the Contests page on this website for an up-to-date listing of all EME contests along with the major VK contests.
All this information is of course available elsewhere, but it is convenient to have it all together in one place. I try hard to keep this page current.
 

Worst Times

There are two particularly difficult times each month for operating EME.

The first is within a day or two either side of the New Moon, because your antenna is then pointing almost directly into the Sun with its high temperature (and noise).
It is possible for stations of reasonable size to conduct digital QSOs into the sun, but the higher background noise level is obviously limiting, and you need to have enough margin of signal strength to overcome that.
 
The second is the 3 or 4 days each month when the moon passes through the Milky Way, another region of high sky temperature (and high background noise level).
 
It is helpful to have some software which will give you an overview of these bad times over a whole month. WSJT displays the information, but for the current time only.
MoonSked is a very good program for getting an overview of EME conditions - see the Software page - not yet written!
 

Other Considerations

Europe is densely populated with 144MHz EME stations, North America too but less so than Europe. There are also significant numbers of Japanese and Russian stations.
It is therefore important to be aware of the local time in Europe and the USA. While you might, and should, be prepared to get up sometimes at 3am local time to look for a rare Dx station, or for an arranged sched, you would probably hesitate to do this on spec just in case someone is on. The same is true of the Eu and US stations, though some Eu stations do stay up surprisingly late!
You are unlikely to find many US stations around on our moonrise when it is 3am over there, and the same is true for Europe on our moonset.
This need for the local time to be reasonable at both ends of the link is surprisingly limiting in VK.
 
While it is not hard to do the rough sums in your head, it is very useful to have some automated way of being aware of the time in other countries.
For Firefox users I highly recommend the Firefox add-on FoxClocks, which will show you the local time anywhere in the world (and UTC) in the taskbar.
 
And if the local time at the remote station is during working hours on a weekday, then of course there will also be fewer stations around!
 
Putting this all together, moonrise in our local morning is good, because it will be afternoon/evening in the USA.
Moonset in our local late afternoon/evening is also good because it will be morning (and moonrise) in Europe.
And weekends are usually best.
 

The Degrade Parameter

At  times when the moon is in a reasonable part of the sky for EME, there are two significant factors (apart from polarization) which affect the signal/noise ratio of EME signals:
 
1. The orbit of the moon around the earth is not quite circular; the earth-moon distance varies from about 360 000 km at perigee to 405 000 km at apogee, a ratio of 1.125.
Since the strength of the EME signal varies inversely as the 4th power of the distance, that gives a variation of signal strength by a factor of 1.60, or about 2dB.
 
2. The temperature Tsky of the background sky varies considerably as the moon moves in its orbit around the earth. The lowest value of Tsky is around 170 K.
This variation in Tsky affects the background noise level and therefore the Signal/Noise ratio of a signal. The Signal/Noise ratio of course determines the readability of any radio signal.
 
These two factors are commonly combined in a single Degrade (Dgrd) figure expressed in dB.
The figure quoted may vary slightly between different software programs, but this is only because their authors have used slightly different values for the lowest value of Tsky or the largest value of the earth-moon distance, which together determine the 0dB level.
Note that some programs such as WSJT express Dgrd as a negative number like -3.5dB, whereas others like Skymoon and MoonSked express the same Dgrd as +3.5dB. This is not significant, but rather reflects whether the author views Dgrd as the effect on S/N ratio (negative) or the amount of the degradation (positive). A small magnitude of Dgrd is good, a large magnitude is bad.
 

Frequently Asked Questions

1. Are EME signals strongest when the moon looks biggest at full moon?
Answer: Almost half of the moon's surface is visible from the earth at any time, but how much of that happens to be illuminated by the sun is irrelevant. So no, the phase of the moon makes no difference.
What does make a difference is the distance from the earth to the moon, and that varies somewhat through the month. Closest is obviously best, and the moon does then look slightly bigger.

VK2KU - 22 November 2009