Wednesday, June 5, 2013

EME and Antenna Patterns

I'm trying to further optimize my small EME station.  Based on the noise plots in the posts below, it's easy to see that raising the antennas to higher elevations will significantly decrease the radio noise that they "see" from the surrounding houses.  What I have noticed over the last six months or so is that by raising the antennas out of the noise (and pointing them slightly away from the moon), I can get significantly improved reception of EME signals.  This is because the main lobe of the antenna array is fairly broad, so even if the array is not pointed directly at the moon, there's still enough gain to receive signals.  Similarly, the steep sides of the main lobe help to selectively null out much of the surrounding terrestrial radio noise.

So, if the reception of moon signals at low moon elevations can be improved by pointing the antennas above the moon, does this have any negative effect on the transmission of signals to the moon?  If so, how much?  Is there a way to find out?

I have found that an easy way to test this is to call CQ, and monitor the reports of my received signal strength at a distant station.  The site makes this relatively easy, as members of the MAP65 spotting network automatically upload signals that they have received from the moon.  In my case, station KB8RQ has a very good receive system and (when online) automatically will report my signal, in real-time, as it's received.  KB8RQ also has the advantage of using an adaptive-polarity dual-receive system, which helps to eliminate the effects of signal strength variability due to Faraday rotation.

By alternately raising and lowering the antennas every second CQ cycle, it's possible to do A/B comparisons between the two antenna angles.  There's a significant natural variation in signal strength on EME signals, but over time it seems possible to do some rough calculations as to whether one antenna elevation is significantly different from another.

My first test was to alternate a +0 degree elevation offset with a +7 degree elevation offset.  The offset is the number of degrees in elevation above the moon.  I was able to record received signals at KB8RQ over the period of about half an hour of calling CQ (with three brief interruptions as new inits from Hungary, Germany, and France replied to the CQ calls!).  Plotting the reported signal strength over time gave the following plot:

In the plot above I've recorded the signals received at the DX station.  The pink colored dots are the +7 degree elevation and the blue colored dots are +0 elevation.  From the plot, visually at least, there's not much difference between the two.  Both are highly variable and both are "about" the same.  Mathematically, it's possible to compare each cycle with its adjacent one, and see if it was "better" or "worse".  This is the value in the delta column, and with a mean delta of 0.31 dB and a standard deviation of 4.06 dB, the statistics confirm that there's no apparent difference in transmit signal between having the antennas pointed straight at the moon, versus pointed 7 degrees higher than the moon.

What about higher elevations?  Some draft antenna gain modelling of the M-squared (M2) 2M7 yagi antennas by W6YX suggests that it should be possible to raise the antennas 15 degrees above the target with only a -1dB penalty in signal.  The plot below shows the predicted gain of the 2M7 antenna in both the Horizontal (H) and vertical (V) planes:

My second test was therefore to alternate calling CQ at +0 and +15 degree elevations above the moon.  Results from this test are below.  

The second test indicates, again, a relatively small difference in transmit signal strength between having the antennas pointed at the moon versus raised 15 degrees above.  Mathematically, the pairwise comparisons indicate that elevating the antennas results in -1.6dB signals with a standard deviation of 3.2dB.  This actually fits very closely with the model predictions!

Subsequent tests the next day indicated similar results:
In the above plot, elevating the antennas 15 degrees above the moon causes a decrease in the received signal strength at the DX station of -1.7dB with a standard deviation of 2.7dB.  This fits nicely with predictions and the previous day's data.

In addition to KB8RQ, station S52LM in Slovenia was reporting signal strengths at the same time.  The S52LM station has less receive capability than KB8RQ so the signal strengths are lower:
This plot shows similar results to the last two:  relative received signal strength is down -1.7dB with the antennas elevated 15 degrees.  What is also of interest is that there are fewer total decodes with the elevated antennas (5 decodes at +15 degrees versus 8 decodes at +0 degrees).  This is probably a result of the slight signal loss pushing these (already marginal) signals past the limit of MAP65's Wide Graph detection (about -25dB).

Overall, what does this all mean?  Basically, it provides some early evidence that it's okay to aim the antennas well above the moon, in order to reduce the effects of the surrounding terrestrial noise and improve received signals, with little impact on the transmitted signal itself.

How much should the antennas be raised away from the moon in my situation?  It really depends on the moon's proximity to the surrounding terrestrial noise.  If the moon is at 20 degrees elevation, then raising the antennas +15 degrees will get them to a part of the sky with approx -3dB less noise, substantially increasing receive capability, with only an estimated -1.7 dB reduction in transmit signal strength.  If the moon is at 50 degrees in elevation (already in quiet sky), then there's no benefit to aiming the antennas off of the moon.  I haven't tested it yet, but based on the +7 degree tests, the calculated shape of the main lobe, and the fact that the +15 degree test shows a -1.7dB reduction, I suspect that a +10 degree elevation offset will not have a measurable impact on transmit signal strength.  My gut feeling is that using a "permanent" elevation offset of +10 degrees will allow me to take advantage of lower noise levels without any significant impact on TX signals.

(much thanks to W6YX for the M2 2M7 draft antenna plots!)


  1. Dave,

    I love this stuff ! ! !

    _Rick WA6RAI

  2. Dave,
    thanks for this information and your effort to investigate it.
    It could have helped to finish our today's QSO with my array elevated above the trees. The bursts you mentioned might be caused by the branches moving in the wind.
    Even after more than 2 years on 2m EME still new ideas to learn.
    Hope to work you soon, best 73 Ivan OK1IL

  3. Thanks Ivan! I enjoyed the QSO attempt, was quite interesting to see how the signals varied so quickly over time! 73 de Dave KB5WIA

  4. Hello Dave,

    many tnx for your tests, vy helpful.
    Made the same experience with my setup, in certain
    noisy directions its better to try to reduce
    the man-made noise, a loss of 1-2dB is bearable..

    vy 73 de Bodo/DL2FCN

  5. Dave thanks for the helpful tips like your small 6 meter yagi antenna you make it !
    i have bin looking for something small for six meters thanks again
    getting setup for EME this year with a little help
    ted / KD2ARD

  6. oh Dave do you have info in that six meter Yagi thanks ted


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