EME

This was always something I wanted to do since I was an SWL 40 years ago and I heard two hams on 20m discussing it. Frankly it is absurdly addictive. The constant striving for improvements to the system means it isn't just the operating, The hobby never stops as you track down one more noise source, cut another 0.1dB off your losses before the preamp and read another article that gives you another idea. It is mechanical as you build your array and try to bodge an elevation system, Electronics as you try to get the system noise low. Political as you try to appease your neighbours about the metal tree  in the garden. There is nothing like the sound of your own echoes coming back off the moon , or the ghostly sound of JT65 over the headphones.

The thing is that actually moonbounce isn't difficult or even hugely expensive if you are careful. I spent less than a thousand on my first sytem and it worked well. OK I spent the money over a long period and got some great bargains- but it can be done - and from what I've learnt doing it , I could probably do it for a lot less. Over time I will document a beginners guide to moonbounce. There are a few good sites - but the information is often fragmented or outdated.

Initially this site is to document my Linear amplifier build to help others doing the same.

 I currently have the equipment for 2m, 70cm and I'm building the 23cm system at the moment.

However I can only run only one band at a time as I cant have more than one antenna up.

 

My systems are

 

2m 4x9 ele G0KSC design yagis, homebrew 1Kw Linear, VHF design preamp - Elecraft 2m transverter - Flex1500. All PLL's GPSDO locked.

 

70cm  4x 18 element G4CQM design Yagis, homebrew 500w Linear, VHF design preamp - Elecraft 70cm transverter - Flex1500 All  GPSDO locked.

 

23cm 2.4m homebrew dish, SM6FHZ patch feed, VHF design Preamp, SG-Labs 1296-144 transverter, Elecraft 2m transverter - Flex1500. All GPSDO locked. Still looking out for an amplifier 2W in 150W+ Out.

A basic introduction


 

Bouncing a signal off the moon so you can communicate with other people!

 

This isn't all that easy, but it isn't rocket science. Firstly it's a small target, a long way away. It isn't very reflective - only 7% of your signal bounces off. The signal that makes it back is tiny and swamped by the noise of anything electrical in your or your neighbours houses. Some days of the month the noise from the milky way behind the moon is enough to make EME impossible for the smaller stations. However with the aid of digital technology the entry level requirements have fallen to make this into an acheivable hobby.

What do you need

100W, a low noise preamplifier on the mast , a 7 element antenna and a computer will get you a contact with a bit of work. 2m is the best to start on as there are more stations.



What is better.

A rotator. More power, more or bigger antennas. You can get a lot of contacts with 200-300W and say 4x7 element antennas. I made 60 contacts in a month with such a system, just doing 4 nights operating. The other thing is that EME'rs seem to be a really helpful friendly lot. As long as you don't say things like CW is dead!



Really?

Yes - there re some big stations out there and with a bit of patience you can use their high power and receive sensitivity to do the hard work. Also as a low power station you are a desired target for the entusiasts.


Every bit helps

A standard rotator helps you point in the right direction. As the moon elevation increases you lose the 'ground gain' so if you have a small station there isn't really any point of having an elevation rotator as you will lose 10dB as soon as the moon goes above around 15 degrees. With a 4x7 or 2x12 station then you need an elevation rotator to get the best results.




Whilst moonbouncers can use CW or even SSB. To start with most stations use JT65B modulation which is a digital format like RTTY.


However unlike RTTY its data rate is very slow ( 1 minute periods of TX to pass two callsigns and a grid locator)  and uses encoding to enable it to work at signals measured with up to -26dB signal to noise ratio.


At that level you can't hear it on the phones or even see it on a waterfall on your receiver.


So the first thing you need it some software to encode the messages and to decode the incoming messages on your computer.


The most common and best regarded software for this is WSJT by K1JT.


Next you need a soundcard interface to get the audio in and out of your rig - just as with RTTY and other digital mode. There are heaps of guides on the internet on how to do this so I will skip over this. If you are looking at moonbounce you have probably already played ith other digital modes.


The next most important piece of the puzzle is the Low Noise preAmp. This is the crucial part to success and there should be as little between the LNA and the aerial as possible. Every connector and metre of cable, relay contact makes a huge difference to the sensitivity of your system.


When the signal is magnified by 20- 26dB by the LNA, that 'insignificant' 0.1dB loss on the cable or on a connector is multiplied up. There is a program EMECalc which is free and lets you calculate this.


You put in your design and see what signals you would get back from another station. Then you see what happens as you put in some cheaper cable or add an extra N type connection infront of the preamp and you soon start to see why EME'rs get obsessed over cables and relays!


For EME the 'quality' of the LNA is expressed as it's NF ( Noise Figure) good values range from 0.2dB to 0.5dB. There are other figures that are important if you  want to do VHF contests where there are going to be a lot of high power signals nearby. But this is EME - local stations to you will be most likely transmitting in the same 'Period' as you so are unlikely to cause issues.


The first period is the 1st minute past the hour (12:00)  and 'even' minutes the second period is the second minute of the hour (12:01) and odd minutes.


So if you've installed WSJT, the beam is pointing at the moon as it rises. Then off you go!


Well almost....


As I mentioned previously - the noise from the milky way and other galaxies can drown out the signal so go to the MMMonVHF page and see when the best day is going to be


 http://www.mmmonvhf.de/eme.php

http://www.mmmonvhf.de/eme.php



So after you have waited for the best day then......


Sign in to the EME chat room here and announce that you are going to transmit at what frequency and in which period.


http://www.chris.org/cgi-bin/jt65emeA


Normally by announcing something like :


=========CQ 144.120 FIRST======== ====== {M0VKK/4X9H/KW


If people hear you they may give you a signal report - or look at http://www.livecq.eu/


Keep perservering - the polarisation returning from the moon may be coming to them at the wrong polarisation in which casse you are locked out till the Faraday rotation comes back the right way.

Faraday rotation is where the polarisation of your signal gets slowly rotated to a new polarisation when it hits the earth on its return. So a signal cant be heard one minute comes booming in 10 minutes later. The big guns overcome this with either switxchable H/V X yagis or receive both simultaneously and recombine them to give a signal independant of the ploarisation.


Or in rare cases like me - I rotate my yagis around the boom axis to sleect the polarisation. Not recommended unless like me you have no other choice!


here is a good example of a typical QSO here:


 http://www.dxmaps.com/jt65banswer.html



One thing to remember in the chat room. NEVER comment on the progress of a QSO whilst it is happening. Only when it is completed can you give your signal report in the channel. If the information hasn't come directly via the moon it doesn't count.





Noise:


QRM - noise is the EME'rs worst nightmare. It drowns out the ridiculously small signals from the moon. It can make the difference in getting no contacts at all or a screen full of them.


This is a somewhat shortened version of what I had to do.



My noise floor was about -130db ( these are all relative measurement) with everything off and the receiver run off a 12V battery. When I say everything off - I mean the breaker in the house off and at 3am so that there wasn't much interference from the neighbours.


Then one at a time I switched things on seeing the effect on the noise floor - in my case the 12dB+ noise was caused by amongst other things.


1) A switchmode for a USB hub                     ( the shack)

2) 240V LED bulbs ( horrendous)                   (>20metres from the shack)

3) The power supply for the internet router (>20metres from the shack)

4) The old plasma TV                                       (>20metres from the shack)

5) My DECT phone base.                                 (>20metres from the shack)


Changing the LED's to 12V and buying a new TV set and replacing every switch mode in the house with analogue PSU's and fitting ferrites to every extension mains cable or device with an inbuilt power supply. Also buying some PSU's for the neighbours too.


The other thing to do is to rotate the yagi 20-30 degrees and repeat the breakers off trick to find noise sources that weren't in one of the side lobes when you did the first run through. Also repeat this for a few elevation positions too.


One final word of advice


DON'T TRY SWITCHING THINGS OFF ONE AT A TIME.

The 10db noise of the lights was being masked by the usb hub. So initially I thought the lights were fine - it was only when I switched off the hub that I happened to notice how bad the lights were - despite givign them a clean bill of health the first time round! After discovering 2 or 3 instances of this masking I bit the bullet and did it the everything off first way.



I also have mylar film over the backs of my computer monitors to cut out the sprogs from them when the moon is in a certain position and they are caught in the sidelobes. At one particualr AZ/EL combination I turn off the PC and run off a laptop - anything for a bit of quiet!