I figured just having a plain ol' binary counter would be the simplest and easiest way for one technological planetary civilization to say "Hi!" to another. None of this fancy primes or encoded message stuff. Nothing to get lost in the noise. All that can come in later if there's ever a reply. The message is nothing more than going 1... 2... 3... etc. And repeating it after a certain amount. (I'd say 100, because we count in base 10 and that's the square. But any reasonable number may do.) Now sure, some recipients may only notice the least significant digit (bit?) and dismiss it as a pulsar or similar repeating natural source. But if somebody bothered to check a bit more of the spectrum from the same emission source and notice that there is some synchronicity in pulses on other frequencies... And then notice how convieniently the frequencies are spaced out... And then takes a look at exactly how they line up... The obvious artificial nature and context of mathematical and electronic understanding should be there and in plain sight. I don't think any natural radio sources count using binary numbers, do they? And since the signal is sent in pulses, it should be fairly efficient. Ramp up peak power, without worrying about the average power load that a constant carrier signal would require. Seems this way would would have reaching power vs. costs involved. At least I would think so. I'm just saying this is what I'd send or look for if I wanted to say "Hi!" or see if anyone else with the technology is friendly enough to do so.