Assume a text message of say 60 characters such as "CQ CQ Hi there, we've just read about you in the newspapers K"
Assuming about 10 bits per character, this message needs about 600 bits to be transmitted. If sent at 1 bit/s it would take 10 minutes of transmission time to get the message going. It would then take a further 20,000 years to go to the far end for reception.
The range required is a big problem so the transmitter power required will be enormous and the dish diameter is going to have to be huge. I've tried various numbers and the results looks a bit crazy, to be honest.
This just about works, in theory that is.
Transmit dish diameter = 200m
Transmit power = 1,000,000 watts
Frequency = 94 GHz.
Unfortunately the surface profile of the dish antenna needs to be about 1/10 of the wavelength. Wavelength for 100 GHz is 3mm. To make a dish 200m diameter to an accuracy of about 0.3mm sounds difficult. I'm not sure about getting hold of a 1 megawatt transmitter at 94 GHz either. But you could contemplate 1000 transmitters, each of 1000 watts, with each transmitter connected to its own dish of 6.5m diameter. There would still be a problem of getting all to transmit at the correct times so that all the signals added together in the far field, but not perhaps quite impossible.
My guess is an antenna 200m diameter built by perfectionists. For analysis, see my SETI radio range calculator and try your own ideas. It would be better if we could find some aliens living closer, like under 20 light years; then the chance of radio contact is better, we can send and receive data more quickly and there is a chance of a reply in a lifetime.
Just a thought: If the message was sent much more slowly, for example by just turning the transmitter full on for 12 hours and then on or off for the next off for 12 hours. That might send 1 bit of information per day, so it might take two years to send all 52 characters. It considerably eases the link budget calculation however, as the modulation bandwidth needed comes down to something like 0.0001 Hz.
Here is a web site that enables you to send a SETI message yourself http://www.outerspacemessage.com/
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