The other 90% of my weekend

I did a deep dive into an area of ham radio that I've known about for as long as I can remember, but never tried to get into because the price of entry has been just too high (in time, money, and effort). The common name for this is moonbounce, but the more technical guys tend to call it EME - for Earth Moon Earth.  

There were two reasons for trying more this weekend.  First, this weekend was a major EME contest, put on the American Radio Relay League, and these contests tend to bring out a lot of activity.  More activity means more people to listen for. The second reason was some online chatting with a guy who does quite a lot of it. He doesn't know about this blog, and I didn't ask for permission to talk about him so I won't. But he asked me if I was going to try to listen in this weekend and after some chatting about my station by email he gave me some hints on setting things up to try to listen and contact some of the guys who have invested a lot in EME activity.

Let me just put the "Bottom Line Up Front" (BLUF): I didn't hear a single station in the couple of hours I thought my station might be able to turn the trick.  Neither Friday or Saturday around moonrise. 

So let's start at the beginning. For newbies, I did a post about trying to communicate with the two Voyager satellites. The concerns are identical but the numbers are vastly different.

This was all done at 50.2 MHz. The idea is simple: you point an antenna at the moon, 250,000 miles away (not exactly) and listen for signals coming from the moon. If you've got a really good station, you can hear your own signal after you wait for the echo from the moon.  The speed of light (which is radio) is 186,000 miles/second.  Remember, your signal has to go from your station to the moon and back or 500,000 miles. That means you'll hear the echo 2.69 seconds later.  

The next big concern is the same as every communications link everywhere else: the amount the signal attenuates - weakens - over that 500,000 miles. The term for this is path loss, and back in the Voyager article, I used a handy form that gets you within less than half a dB of the more theoretically-backed equation.  

Path loss in dB = 37 dB + 20log(f) + 20log(d)  where,f is the frequency in MHz and d is the distance in miles.

So PL = 37 + 20log(50) and 20log(500,000) or 185 dB. 

Wait.  There's a nasty assumption hidden in there, that the reflection from the moon is perfect. No signal loss, it just changes direction. That implies the signal reflected back has an angular diameter less than the moon - or some would be lost  around the edges.  The diameter of the moon is just over 0.5 degree, which is very tight for an antenna beam. OK, let's just keep a note on that. Maybe there's a useful approximation people have made. Maybe someone said just add (some number) of dBs to your path loss.

Where does this leave us?  Let's say we put 1000 W out of our antenna (it could be less power in the transmitter and more antenna gain, or a simpler antenna and more out of the transmitter).  That's +60 dBm (power compared to 1 milliwatt in 50 ohms) or one million milliwatts.

Signal coming back is  +60dBm -185dB path loss or -125 dBm at our receiver input. 

So what? It's a good time to say, "what does that mean?  Is that a useful signal?  Do I need more power, or more gain, to get more signal at the receiver? 

At this point, we have to dive into the improvements that have made EME more accessible to more hams than when we operated voice or CW (Morse code).  That's a topic for another day.

A graphic from a guy who's among the biggest names in EME, especially 6m EME. Lance Collister, from Montana. From there, he links to his main web page on EME.