At AM radio frequencies, most of the radio waves are reflected back to earth or absorbed in the ionosphere. Same concept as how amateur radio HF/MF propagation allows individuals to communicate with people around the world, past the horizon. (The 160m ham band nearly backs right up to the top end of the AM radio band.) You can even sometimes hear your own echo, apparently, as the signals travel the entire way around the earth, although I've yet to experience this. (Recent Extra class licensed ham radio operator here. 73.)

Higher frequencies, on the other hand, are not reflected back by the ionosphere and go straight out into space. That's why you typically use VHF frequencies (30-300 MHz) for things like EME "earth-moon-earth" or "moonbounce" communication, or communication via satellite. So if your concern is about radio waves making our planet a beacon for aliens, you should be more concerned about FM radio towers than AM ones.

that is a good point, i always forget about the ionospheric plasma. however, there is going to be some amount of outbound re-radiation from the plasma. the sound quality by the time you get to alpha centauri is going to lousy but any alien civ capable of coming over here and sucking up our precious atmosphere is probably going to have good enough spectroscopy and computing to reconstruct our morning zoo broadcasts and judge that we are not worthy to have nitrogen.

I have a recording done long ago of my signal going around the world, I think maybe on 40m or 15m. It is pretty rare.

We'd need to blast a radio signal at about 100 million watts for it to hit the closest star with the same strength that Voyager is sending to us right now. Something like 4 light years away. If we consider how quickly signal strength decays, it's pretty easy to picture just how quiet we are. Our loudest tv stations are like 5% of that.

We think we're blasting a firehose off into space but we're spitting from a straw.

Would be true for omnidirectional transmission. Much much less for a directed transmission.

You'd have to know where the aliens are for that to matter!

Given the detection of exoplanets of approximately Earth mass orbiting in habitable zones, that's rapidly become tenable.

Laser / maser signalling would probably be about the best we could do for now.

Hitting a fixed target, from light-years away, with a signal stronger than cosmic background? Pretty sure a world-class team could get PhDs just for the attempt.

good points, but signal strength decay in the semivacuum of space really has to do more with the geometry of your emitter path vis a vis a fixed target (earth). so we get attenuation from the voyager but you can bet your sweet patootie that voyager is broadcasting our business to a gajillion other star systems even with a weak signal.

in which case deflections from the intergalactic ambient hydrogen is the only hope of mankind that they don't invert the signal back to our solar system with our primitive space force.

Do stars function as lenses for the AM band as well?

sure, they are just radio photons. i don't see why not!

Then the reach might be a lot further.

Military radars, su h as early missile warning systems, might achieve higher power signals, and their signals are much, much better focused.

that is a very good point. we should also shut down BMEWS and send a manned constellation up there to spot ICBMs visually. this is a good billet for the space force.

ICBM launch detection has been done since the 60s with satellites using infrared sensors to detect the heat emitted by the rocket engine.

https://en.wikipedia.org/wiki/Defense_Support_Program https://en.wikipedia.org/wiki/Space-Based_Infrared_System

> for national security

Don't you mean planetary security?

I stand corrected!

Planet America.

They'll hear the War of the Worlds prank broadcast and come rushing to our defense.

Watch Outer Limits, Galaxy Being episode!!!