EME basics

Radio hams have been bouncing signals off the moon, and hence having QSOs across the world via the moon, for the past seventy years. It’s termed moonbounce or EME (Earth-Moon-Earth communications).

Bouncing signals off the moon - an introduction

The attraction of moonbounce is obvious: just to be able to say, “I bounce signals off the Moon”, has kudos. It’s got to at least get the response, “Really, how interesting”, before the listener changes subject.

Bouncing signals off the moon

Moonbounce is a very eudaemonic pursuit. There’s no hedonic pleasure about it, save in the beers you drink while assembling the kit. It’s all about huge personal achievement and pride because it’s so hugely technically challenging.

Then once you’ve done it, you can move on to work more stations on more bands before going off to seek another obscure interest.

I’m saying all this, but I’ve yet to do it! But on the various pages under the topic of EME, here’s my analysis of the issues and about my journey.

In the first sixty years, the stations at either end of the link needed to be substantial – low noise amplifiers, stacked and bayed long Yagi’s or big dishes, and kilowatts of transmit power. The reason is easy to see – the System Value minus total loss for a pair of modest terminals is zero at 144MHz, hence received signals are on the noise. Simply, it will only just work with a modest station.

Recent technology improvements

About ten years ago radio hams developed narrow band data transmission and clever coding methods. These improved the System Value from that available from the slow Morse transmission that offered the best of the state of art then. These new transmission systems have been developed now such that, as an example, the JT65 data method developed by Joe Taylor K1JT, allows detection of signals at around -28dB below the 2.5kHz noise level. This is enough to overcome the various losses that previously thwarted success.

The 2.5kHz bandwidth noise level is the typical reference – indeed the only reference. In this bandwidth, an SSB voice signal would need around 10dB signal-to-noise ratio to be intelligible. JT65 allows successful decoding at around 38dB below the 10dB receiver threshold (28dB below the noise floor) – a huge improvement in System Value.

Q65, JT65B or other narrow band data modes now make QSOs via the moon possible between modestly constructed stations.

Getting to the truth

Sadly, there’s a lot of myth online in ham web sites. The result is that it’s difficult to understand why bouncing signals off the moon works, and hence how to best configure a station. When I asked about the path budget on one Facebook forum, well-meaning hams there advised that I should ‘not over think it – just assemble some kit and try it’. Sadly, that’s not my style.

This part of this site defines all the losses and gains, and the propagation mechanisms. This aims to define the ‘why’ behind the stories. It’s not overthinking. It’s explaining.

But a warning. This section is under development and will be updated when I discover more in the coming months and years.