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Two questions about Receive Only EME

Discussion in 'Satellite and Space Communications' started by KK4NSF, Nov 9, 2018.

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  1. KK4NSF

    KK4NSF Ham Member QRZ Page

    As a first step into EME, I'm looking into setting up a Receive Only EME Station (CW and Digital Modes) and I've got a few questions that some of you more experienced folks may be able to help me with.

    1. What is the minimum antenna gain for receiving an EME signal? I'm thinking either 2m or 70cm here, with one or two yagis. Since I'm never going to transmit with this set-up, the antenna requirements should be less. I know more is better.... but what can I get away with and still have a reasonable chance hearing anything?

    2. How sensistive does my receiver need to be in terms of dB above the noise floor to be able to pick out an EME signal? Right now I can easily pick out a distant signal that is ~2 db above the noise floor, and I see a number of ways that I can improve that further.... but how good does it need to be?

    3. I've got an old 70cm GaAsFET pre-amp that adds about 18dB gain. Would that help out? Or would I be better off not using it?

    OK... that is three questions, instead of two. BUT since I want to do this right, the more experience / knowledge I can put to use, the better my experiment will turn out. Thanks in advance for your answers.

    Dave
    KK4NSF
     
  2. AI3V

    AI3V Ham Member QRZ Page

    Without knowing your actual noise level it's impossible to predict.

    Start by searching this site for my post "measuring the effective sensitivity of a receiver"

    If you live in a very radio quiet area, a 2 wavelength boom antenna with a gasfet will copy a few superstations.

    Rege
     
  3. SM0AOM

    SM0AOM Ham Member QRZ Page

    You are more or less forced to use preamps.

    On 144 MHz your sky temperature seldom falls below 200K, but on 432MHz it may be only 30-40 K.

    The lowest noise preamps contribute around 30K to the antenna temperature.

    You have to figure out the lowest figure of merit G/T [dB/K] that may permit the reception of various modes and stations. Suggest that you download the VK3UM EME Calculator and plug in some numbers.

    Aural Morse would require at least 2 10 element Yagis to receive even the larger stations. Somewhat depending on sidelobe content and operator skill.

    73/
    Karl-Arne
    SM0AOM
     
    Last edited: Nov 9, 2018
  4. K9JKM

    K9JKM Ham Member QRZ Page

    The Graves radar located in Dijon, France (JN27si) on 143.050 MHz is often used as a predictable signal reflection off the moon (assuming you, the radar, and the moon are appropriately placed). Google for "graves radar eme".

    --
    73 de JoAnne K9JKM
     
    KK4NSF likes this.
  5. WB3BEL

    WB3BEL Ham Member QRZ Page

    The easiest way to receive EME is to start with WSJT JT65B on two meters.

    There is more activity there and the signals from the superstations there are readily receivable with very low performance equipment.

    If you don't have a lot of local noise you will be able to receive the superstations with just a multimode transceiver like an Icom706 or 7000 or Yaesu 857 etc. No preamp is necessary for the superstations if your feedline is relatively short < 1 or 2 dB loss.

    You need a single antenna of medium boomlength. Maybe a 6 element Yagi is the minimum that is likely to work under good conditions. An 8 or 9 element Yagi like an M2 2M9SSB is definitely adequate. An older cushcraft with a boomlength greater than 10 ft probably is good enough. I would not be surprised if it is possible to decode superstations with much smaller antennas if you live where it is quiet. But if you want something that will work most of the time, an antenna like one of these is enough to get started. As a sidepoint you can probably do 2 Way EME with the superstations using an antenna like this if you have 100W of transmit power.

    CW EME requires much more sensitivity. You must have an LNA, live where it is quiet, and have a very good ear for decoding exceptionally weak signals embedded in noise. Bigger antennas help a lot here. You can get an idea how much signal is needed by listening by ear to see if you can hear the JT65 tones from the superstations. Under good conditions you can hear these tones by ear with a single Yagi. It requires an SNR just a bit above -20 dB in SSB bandwidth just to hear the tones. CW EME is fun, but I recommend it to those who successfully get their feet wet using digital modes.

    A similar boomlength Yagi will likely allow 70cm reception. This means a Yagi of 15 or 20 elements or more or maybe two antennas combined together. An LNA is really more needed on this band. There are fewer active stations on this band.

    An easy way to give this a try is to mount an antenna on a fiberglass or wooden step ladder temporarily. Point it at the moon and connect it via a short cable to your transceiver. Connect the audio from your receiver to your computer via a soundcard interface. Run WSJT and syncronize your computer using an NTP client like meinberg or dimension4. Or set your computer's clock via listening to WWV. If your clock is within a second or so or the right time everything is good enough to receive.
     
    KS1G likes this.
  6. KK4NSF

    KK4NSF Ham Member QRZ Page

    I did that last night.... the article by G3YJR is interesting, and has some good info. I wonder if there is a similar possible target on 70cm?

    That seems doable to me.

    I've done a lot of studying over the last few weeks, and I'm leaning ultimately toward a 70cm for the reasons outlined in the Princeton Radio Club's paper: http://physics.princeton.edu/pulsar/k1jt/Optimized_Small-Station_EME.pdf However for this first listening experiment, I may go 2m. There are pros and cons with either band.

    Dave
    KK4NSF
     
  7. WB3BEL

    WB3BEL Ham Member QRZ Page

    The advice in that paper to use 70 cm while OK, is not the best route, in most EME operators' opinion. Two meters is easier. The minimum antenna boomlength is about the same for either approach. The radio needs to be more stable on 70cm. You will need an LNA on 70cm. Cable losses are higher on 70 cm. There are fewer operators on 70cm. There are many regions in the US where transmit power is restricted on 70 cm due to military radar uses. If you read that paper carefully, you will see that one of the goals which was to be able to contact other similarly sized stations on 70cm was never met.

    The best advice is to start on 2 meters and if you find it fun and want to try 70cm later ,when you have mastered the basics, go ahead.
     
    KS1G, KK4NSF and K4BAD like this.
  8. KS1G

    KS1G Ham Member QRZ Page

    A local club member, K4MSG, did a lot of backyard small station EME, started on 2M with a 9 element M2, later I believe moved to a pair of low-noise pickup loop fed yagis. He had a brick amp (with additional forced air cooling) out at the antenna, and used the built-in preamp in the amplifier. Started with 2M, then added 70cm. See his QRZ page for more info.
     
  9. K6CLS

    K6CLS Ham Member QRZ Page

    Good advice.

    Personally I think the OP should check noise and birdie levels at his QTH before committing to one band or another.

    6m is impractical for most folks.

    2m has a lot of noise and the antennas are large.

    70cm may have less noise... Or more! Because the 433 ISM is so close to 432.1. but the antennas are smaller. And Faraday rotation is worse, and can be stuck at the wrong angle for hours. So serious 70cm folks end up with arrays of Hpol and Vpol.

    23cm has many advantages. Circular polarized means no worries about Faraday. a lot less noise there but still, check first. Watts are same $$$ as 2m and 70cm. A 10 foot dish is plenty of gain.
     
  10. KK4NSF

    KK4NSF Ham Member QRZ Page

    what about crossed yagis to correct polarization problems?
     

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