The purpose of this article is to provide radio amateurs with enough background information to understand the technical challenges involved in “small-station” digital EME on the 144 and 432 MHz bands. Suggested configurations, approximate costs and operational potential will be included, all with the goal of encouraging amateurs to consider EME by showing that it is neither excessively complex from a technical perspective nor prohibitively costly when compared to other amateur activities. The article also includes a brief overview of the author’s operational success to date.
A bit of EME history
Since the first successful amateur radio two-way EME (Earth-Moon-Earth or “moonbounce”) communications in the early 1960s, EME has been regarded as the pinnacle of technological challenge as well as an activity requiring substantial financial resources. The latter is due to the necessity of overcoming the huge path loss (~250 dB) involved in sending a radio signal to the Moon and back. Initially this meant a massive antenna array, full legal transmit power, and state-of-the-art technology to attain the lowest possible receiver noise figure. Achieving all of these goals still meant that only CW contacts were possible because of the faint return signals, sometimes buried in noise, which required excellent hearing to detect.
Beginning in the 1970s, however, several things happened to begin to make EME possible for those with somewhat lesser capabilities (and with less expenditure):
- The gradual development around the world of EME “super-stations” having huge arrays and the best state-of-the-art equipment that money could buy, allowing them to complete EME CW contacts with many less-well-equipped stations.
- The development of the gallium-arsenide field-effect transistor (GAsFet) and other devices that made significantly lower noise figure VHF/UHF preamplifiers practical.
- The development of high-quality affordable coaxial cable with lower loss at VHF & UHF.
- The publication of practical designs for VHF and UHF kilowatt amplifiers that a reasonably technically-astute ham could build.
While the above technological improvements made EME more affordable and “do-able” by more hams willing to take on the challenges, it wasn’t really until the development in the late 1990s of the WSJT series of digital transmission protocols by Joe Taylor, K1JT, that “everyman” use of the more esoteric weak-signal VHF/UHF communications modes – meteor scatter and EME – started to become more practical and affordable. These digital protocols allow the reception and accurate decoding of signals far below the noise level, as low as -24 dB and sometimes beyond.
While VHF meteor scatter was always possible during the major meteor showers – especially on CW – for anyone with a good Yagi antenna and 100 watts of power, the WSJT JT6M and FSK441a protocols fulfilled the dream of 50 and 144 MHz QSO completions using random daily meteors; i.e., the thousands of “grain of sand” micrometeoroids that enter the atmosphere every day and create usable ionized trails of 100 milliseconds or less. The near-simultaneous development of the WSJT JT44 and JT65 (a, b and c) protocols did much the same for EME that JT6M and FSK441a did for meteor scatter. It is now possible for an amateur with 100 watts and a single Yagi with at least 12 dB gain to work any of the really large EME stations and, when conditions are favorable, some of the more modest stations as well. For a single-Yagi EME station to successfully contact a 4-Yagi station still requires excellent conditions, skillful operating and a bit of luck, but it is being be done on 144 MHz.
Because of the increasing popularity of EME any ham with a modest station somewhat larger than the “minimum” to be described can, through perseverance, achieve EME DXCC and some operators have completed EME WAS. Even a station such as the one described should be able to accumulate enough “grid squares” by means of meteor scatter and EME to augment terrestrial operation and qualify for 144 MHz VUCC (and using EME, even 432 MHz!) without raising a tower or running full legal power.
EME Operational Characteristics and Lunar Availability
It is important that anyone interested in EME understand the “operational characteristics” of using the Moon as a reflector for two-way communications. Below is a short list of the major concerns.
- BOTH STATIONS MUST “SEE” THE MOON. This may seem like a superfluous statement but it bears repeating that the Moon *MUST* be above the horizon at both ends of an EME QSO.
- 2. The Moon’s position changes daily and the rising & setting times advance day by day by about a half-hour to an hour depending on the time of the month. This will impact potential operating times and depending on personal schedule may limit one’s “on-air” availability.
- 3. Due to the Moon’s rotational schedule relative to sources of celestial noise and the Sun the Moon is only available and useful for EME operation for about 20 days per month.
The two illustrations that follow should give a pretty good idea of how the Moon’s usefulness for making two-way EME contacts varies over a one-month period. The first illustration is a table of Moon rise and set times and the corresponding azimuth of the Moon for each. You can access the table for May, 2013, at the link below and just enter the desired month and year in the appropriate boxes.
The first thing you should note from the table is that the Moon rises later and later each day. This means that if you cannot adjust your antenna in elevation and/or wish to take advantage of any “ground gain” (described later) by operating at Moonrise or Moonset you will be doing it anywhere from approximately 30 to 60 minutes later each successive day depending on the day of the month.
The second illustration is a screen shot of the EME display window at the “Make More Miles on VHF” web page at http://www.mmmonvhf.de/eme.php. This particular shot is also for the month of May, 2013, but can be changed by entering a different month & year.
The screen shot shows the Moon’s Distance (yellow line), Declination (blue line) and Degradation (red line) and is useful for determining what days are best for attempting operation. The smaller degradation peak occurs when the Moon is roughly coincident with the Sun, while the very large peak occurs when the Moon is passing in front of the Milky Way, a *HUGE* source of galactic noise! The best days for small stations occur when the Degradation is 2.5 dB or less. It is possible to have some success with the larger stations at degradations of 3 dB or so but when the degradation is above 4 dB it is going to be very, very tough to make QSOs – not because you cannot copy stronger stations, but because your “small station” signal is normally right at the decoding limit and any small amount of additional noise will push your signal into oblivion. Using this description of limitations you can see that for the month of May, 2013, the Moon is potentially usable for a small station from May 1 – 8 (8 days), May 16 – 24 (9 days), and May 28-31 (4 days) for a total of 21 days, although actual “useful days” will likely be fewer.
While the above are useful tools for helping you to decide when to plan your EME operation, there are still propagation issues that can nullify your attempts even on the best of days. When you are running a single Yagi at low power there will be times when you will get absolutely nowhere, but don’t be too discouraged; just be patient and keep trying, and you *WILL* make QSOs!
Propagation Issues with EME
There is little debate among those who have “taken the plunge” in EME that the propagation challenges can be both formidable and unpredictable. Over the decades many hams have labored tirelessly to try and quantify, to the extent that such is possible, the vagaries of propagation associated with bouncing radio signals off of our nearest neighbor in space. What follows are some short descriptions of what these phenomena are and how they can affect the EME equation.
Orbit (Perigee – Apogee)
The Moon orbits the Earth approximately once every 28 Days in a slightly elliptical orbit. At Perigee (the closest the Moon approaches the Earth) the 144 MHz path loss approaches 251.5 DB; at Apogee the value reaches 253.5 DB. Believe it or not, this 2 dB variation can mean the difference between completing a QSO or not when other factors drive signal levels down.
As the signal passes through the Ionosphere it rotates in polarity both on the way up and on the return bounce. The amount and speed of the rotation are always shifting and are unpredictable. When using arrays of fixed polarity (such as horizontal, which is most common) it is necessary to wait for the polarity to rotate into phase for reception. At times this never happens and you are effectively locked out, regardless of how large your station antenna array may be. This is due to up to 20 dB difference between vertical and horizontal polarization. Attempting to contact another station complicates the situation even more as now the signal must pass through two different ionospheric areas before arriving at either antenna.
First proposed by KL7WE and K9XY in 1984, this phenomenon is the reason why stations are audible at one location and not another. Imagine you are on the Moon looking at North America; a station there using horizontal polarization is pointed at you and his wavefront arrives horizontal. Now look at the station in Europe using horizontal polarization and compare his wavefront to that of the North American station and you will see that they appear to be out of phase. At times the two polarities are 90 Degrees out of phase and thus 20 DB down from one another. That is far too much for the average EME station to overcome so no QSO takes place – EXCEPT for Faraday rotation, which can rotate the wavefront into the proper polarity and allow contact to be made. The fact is that due to the Spatial Polarity effect, without Faraday rotation most EME contacts would never happen.
There is a random fading effect on signals received off of the Moon caused by the rocking motion of the Moon and the signal wavefront bouncing off of the Moon’s jumbled surface and taking on an irregular shape itself. The distorted wavefront is now full of peaks and nulls which sometimes add up in phase although on the average they give a 7% Pi-R-Squared reflectivity. However, when the phase additions occur the overall path loss can be REDUCED by as much as 6 to 10 DB.
As the Moon travels in its orbit the surrounding sky is filled with the random radio frequency noise emitted by all of the stars and galaxies. Some celestial bodies are noisier than others and any additional noise adds up as so many DB of degradation to your system. Measured in degrees Kelvin it can vary from 170 or so to as much as 3000+ degrees. The Milky Way is by far the biggest contributor and when the Moon is in its vicinity communications is impossible even for the largest stations. When the Moon is near the Sun there is also more noise so those days may be unusable as well. It should be noted that on 432 Mhz and above celestial noise poses less of a problem as the sky temperature in degrees K goes down in proportion to an increase in frequency.
When a radio wave from a distant source such as the Moon reaches the ionosphere the phase surface of the wave is distorted by irregular patches of varying refractive index. Since these patches are constantly moving the result is an interference effect resulting in fading known as Amplitude Scintillation. This is analogous to the visual “twinkling” of the light arriving from stars. It is possible for the effect to be additive and when this occurs it can result in up to 10 dB of non-reciprocal enhancement of an EME signal.
At Moonrise the Doppler effect between the Earth and Moon at 144 MHz will cause the echos to appear 300 Hertz or so higher in frequency. As the Moon traverses the sky to a point due south the Doppler approaches zero, and as the Moon continues westward the echos shift up to 300 Hertz lower in frequency at Moonset. This can pose a problem for the operator who answers a CQ where he/she is hearing the station but is not allowing for Doppler and is calling a station using very narrow filter bandwidth. The solution is to always shift the receiver RIT to correspond to the Doppler (which is indicated by the JT65b operating window on the computer).
Moonrise / Moonset – 6 dB Ground Gain
In North America the best time to operate is at or near Moonrise, not only to take advantage of the extra 6 dB of ground gain (which will make a single Yagi perform like four) but also because that is the optimum time to work European stations. Europe has by far the largest number of EME capable stations in the world, many having eight Yagis or more, so from Moonrise to about +15 degrees elevation a single Yagi station in the eastern U.S. can hear and work many European stations with only 100 Watts and at least 12 dBd antenna gain, other propagation conditions permitting.
End of Part 1
In Part II we’ll take a look at how a “beginner”144 MHz EME station might be configured, what it will cost, and how the K4MSG EME station was initially configured.