Rf power & The Fermi Paradox — Where Are All The Aliens? - Page 1
Forum section: electronics / rf-microwave

author: GreggD
author_rank: Regular Contributor
timestamp: March 05, 2020, 04:42:37 pm
content: Was looking at Rf links and wondering how much power it would take to hear aliens.So I looked at Voyager 2 with a Tx power of 12.7 W - low power mode.3.7 m spacecraft antenna.160 bps (now)70m receive antenna.Found this 1989 calculation;https://yarchive.net/space/spacecraft/voyager_datalink_numbers.htmlJan 2020;Voyager 2's distance from Earth - about 11.5 billion miles (18.5 billion kilometers)Not a significant percentage of the distance to the 25 closest stars.Communications traveling at the speed of light take about 17 hours to reach the spacecraft.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 05, 2020, 06:06:44 pm
content: The short answer is basically what you surmised: it is really hard to get a radio transmitter above the thermal background (the CMB) at an appreciable distance.If you want to get a best case scenario, consider the Arecibo observatory which unlike most radio telescopes can also transmit with an EIRP of up to 20 TW and has a 300 meter dish. I've seen the calculations done before for this, and it is plausible that we could transmit a signal which as similar setup could receive to some nearby starsystems, but even then the range is quite limited.In principle you could do a lot better with an optical transmitter or even a THz transmitter as blackbody radiation drops to zero very fast due to the Planck law. At optical frequencies you would probably want to operate in one of the Frauhnhofer lines so that you don't have to outshine the sun.

author: ogden
author_rank: Super Contributor
timestamp: March 05, 2020, 06:42:04 pm
content: Quote from: ejeffrey on March 05, 2020, 06:06:44 pmWhere do you point your transmission at? For how long? Aliens perhaps sometimes aim their optical, EM or gravity antennas at our star, but most likely they are off by thousand, million or even beeelion years. Not so long ago we did not know what EM waves are, in 200 years our ancestors most likely will consider EM waves as most inefficient communication possible, especially to call aliens- if any.

author: chris_leyson
author_rank: Super Contributor
timestamp: March 05, 2020, 07:21:37 pm
content: @GreggD: Thanks for the link. Was going to say Phil Karn KA9Q worked out the link budget for Voyager but I couldn't find it on his web site. Thanks again for the link. Given the distances involved and the crazy path losses you have to wonder how SETI can justify their budget.@ejeffrey: Optical communications makes more sense but which Fraunhofer line or lines do you pick.

author: iMo
author_rank: Super Contributor
timestamp: March 05, 2020, 08:14:00 pm
content: I would build a TWatt lighthouse (beacon) on Mars, powered by material from our dismantled nukes, modulated by a message in Morse.Thus the aliens would see a variable star nearby our Sun, with rather uncommon spectral lines, modulated by something of artificial nature.

author: chris_leyson
author_rank: Super Contributor
timestamp: March 05, 2020, 09:07:58 pm
content: @Imo: Once you've built your Mars transmitter then you'll have a few light years to build a receiver to pick up any reply. Maybe you could use some of the planets in our solar system as a phased array and perhaps a few objects in the Oort cloud as well for a bit more gain, the possabilities are endless. Alien communications is more in the realms of science fiction than science fact given our understanding of modern physics. It would however make for some good novels or a Hollywood film or two.

author: dmills
author_rank: Super Contributor
timestamp: March 05, 2020, 09:26:07 pm
content: The Fermi Paradox pas postulated back when our RF emissions were massive and had low entropy.Increasingly that is no longer the case, we have gone from multiple hundred kW class TV transmitters putting out a signal that auto-correlates really rather well, and shortwave senders designed to burn thru cold war era jamming, again with signals with plenty of redundancy, to satellite services running highly directional beams at a few hundred watts pointed AT the planet, and broadcast transmitters where if you don't know the coding scheme the signal looks a lot like thermal noise.If we assume that any putative alien civilisation has about the same level of technological innovation then the duration over which a planet radiates a signal which would be detectable using a sane receiver array is probably only a hundred years or so...That really changes the numbers! An advanced civilisation while radio noisy close to the planet, looks like CMB from a distance simply because there radios are running really good data compression schemes. You can look at a NTSC or PAL signal in the time domain and there is obviously something there, ATSC or DVB-T not so much.

author: radiolistener
author_rank: Super Contributor
timestamp: March 06, 2020, 12:26:09 am
content: Quote from: GreggD on March 05, 2020, 04:42:37 pmThey're already here...

author: StillTrying
author_rank: Super Contributor
timestamp: March 06, 2020, 02:02:43 am
content: They're already here...Yes, why try to beam a signal over 100s of light years when we could just beam it to one of their ships instead, which are often only about half a mile away.

author: ogden
author_rank: Super Contributor
timestamp: March 06, 2020, 05:21:59 am
content: Quote from: StillTrying on March 06, 2020, 02:02:43 amSometimes they are even closer. - Disguised as your neighbors, coworkers or even members of family.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 06, 2020, 08:05:13 am
content: Easy way to get an idea is just to look at the Friss equation. It is usually written as a function of gains, but you can relate it back to the effective apertures of antennas too, iirc.Could start with using shannon to get an idea of RX power required for a given speed, then take a sensible size of TX and RX antenna (make them quite large, just because) and then see what kind of TX power you would need.

author: fourfathom
author_rank: Super Contributor
timestamp: March 06, 2020, 08:30:34 am
content: Quote from: dmills on March 05, 2020, 09:26:07 pmThis is why most SETI (re)searchers are looking for a deliberate beacon that is intended to be easily detected, rather than trying to eavesdrop on stray alien communications signals which are likely compressed, error-corrected, spread-spectrum, etc. (all of which tends to make them look like noise unless you know the particular coding.)As for path loss, an alien civilization that wants to put out a very strong signal might use gravitational lensing by putting their radio or optical transmitter some distance from a star and letting the star focus the beam. Of course that only works if the receiver and transmitter are well-aligned, and for us to pick it up it would have to be aimed directly at us (accounting for various motions and time delay), so that's not likely to happen by random chance.It's a very tough problem, but since the SETI budget is almost completely privately-funded these days I see no harm in listening.