Antenna Briefs #4 - Interstellar Communication and SETI (Revised)

This episode looks at communicating across distances of several light-years (multiple parsecs), and at the big antennas and very large power levels needed. In this revised episode, we start with an errata (list of changes) to the previous Episode 4 video. We then proceed, as before, to the solution to the "homework" problem assigned in episode 3 and then apply the results to the Search for Extraterrestrial Intelligence (SETI). The goal is to illustrate the large power levels that must be assumed, the antenna's needed, and the narrow bandwidths that may be required, using the underlying theory developed in this series.
For anyone interested in the original episode 4 (with the error in the calculation), here's the link. Sadly I can't find a way to fix it using the KZbin editor. Hence, the replacement version "4A"... • Antenna Briefs #4 - In...

[@W1RMD]

This lesson was very humbling for me. A lot more to space communications than I've originally realized. One thing we have to assume is that there is any extra terrestrial life that is even as advanced as we are. Movies and tv portray "them" as superior beings, but we don't know the exact advancement of any life outside of earth that is close enough to communicate with us, or would use modes, or frequencies that detectable by us or them.

[@MegawattKS]

Yes - I had always wanted to look into this, so it was fun making this video. I agree that other life may not be as developed as us. But given 13B years for the universe and 4.5B for our little corner, it seems equally likely they are much more advanced, and have moved on (not sure to where...). Perhaps the real improbability is to catch them at the right time? Sadly, the background noise seems to rule out possible catches from lots of other distances (times), which could mitigate this issue.

[@therfnoob7697]

Hello! This is a great video and a fantastic series! Thanks! I have tried to do the exercise and came up with similar numbers! But I have a question. In these calculations it seems to me that we are assuming that the receiving antenna is isotropic (no gain) ? In the equation: P_r = P_t * G * (1/4*pi*d^2) * A_eff, G refers to the Gain of the transmitting antenna, and A_eff to the effective area of the receiver. But is defined as A_eff = pi(radius)^2, and so it only takes into consideration the area of the receiving antenna, but not it's "shape/gain" (in other words, it is considered isotropic). Am I missing something? It seems to me that we could get better bandwidth if both receiver and transmitter used the same high-gain parabolic antenna. Keep up the great work!

[@MegawattKS]

Thanks. On the question about receiver antenna gain: that is embedded in the formulation by Aeff. The classic Friis equation uses both G_t and G_r, but to me, the G_r term is tricky to understand. Where does this "gain" come from? G_t comes from focusing the beam. To me that makes physical sense. It gets brighter like a focused flashlight. Power tightly focused in one direction. But we can't really "focus" a receive "beam" because the received energy is a plane-wave as it passes us. The effective gain used in the Friis equation is actually caused by equations rather than any real physics. It increases as (D/lambda)^2, just as Aeff does. So one can formulate the received power expression either way. Hope that helps. It's tricky stuff. (I actually struggled with why the received noise doesn't increase as Aeff (or G_r) increases, but that's another story. I figured it out once. I think its related to your question, and bound together with Fourier and reciprocity and coherence... )

[@therfnoob7697]

Thank you for your quick comment!! Much appreciated. You said: "[...] It increases as (D/lambda)^2, just as Aeff does." I think this is what I am struggling to understand. I used the definition A_eff = pi(radius)^2 (taken from minute @3:40) which of course does not depend on lambda at all. It just denotes the area of surface of the parabola (well, a circle), so it's the "real area" not the "effective area". Anyway, supercool to know that we can have keyboard-to-keyboards chats with planets light-years away, although with a "LITTLE" bit of lag :-P I love this type of "back of the envelope" calculations! THANKS !

[@electronics.unmessed]

Interesting! 👍 I did some some similar calculations trying to find out, how far we could reach out into space with different technologies. The results are comparable. Happy New Year!

[@MegawattKS]

Thanks ! That's great to hear. Too bad we don't have a bigger aperture. Maybe in a hundred years we could create an automated assembly of a multi-km diameter phased-array antenna in orbit (once it gets cheap enough to do). I also suspect we'll have to start listening on long timescales. As biological folks, we're biased to 1Hz or faster. But that limits sensitivity, and if there's any data, it'll have to be modulated very narrowband. I suspect ET is very patient... :-)

[@electronics.unmessed]

@@MegawattKSYes, it seems that other solar systems are only in reach with a very narrow bandwidth. I also considered an array of big dish antennas, repeated messages, cooled receiver front ends, etc. The cost per transmitted MegaByte is becoming ridiculous. And finally I am asking myself, what question should I ask ET, when I can expect answer only 100 or 1000 years later? However, in our own solar system, other planets would be reachable. Even with ham radio.

[@BaldurNorddahl]

You have not seen enough Hollywood movies to assume that ET wants to be nice ;-)

[@MegawattKS]

I'm sure ET follows the Prime Directive ;-) "All I Really Need to Know I Learned from Watching Star Trek" www.amazon.com/Really-Need-Know-Learned-Watching/dp/0517883864

[@dominicestebanrice7460]

The insights from the original lecture #4 and this update are quite shocking.....it turns out SETI is a bit of a boondoggle isn't it? I've watched documentaries by big-name science communicators like Profs. Brian Cox and Iain Stewart, NDG et al, and basically grasped some of the philosophical and evolutionary difficulties with SETI (timing, simple life vs complex life, needing a planet with a magnetosphere to block cosmic rays, a planet with a 'big brother' a la Jupiter to block frequent asteroids, etc) all pointing to the conclusion that SETI is a long shot because the probability of complex life at this point in time is low BUT they all let the viewer assume we can "see" or "hear" basically the entire universe....which I now understand we obviously can't. Ans no journalist (to my knowledge) has ever pushed back on what is apparently elementary radio physics.....given the years of media interest in SETI, this is a sad state of affairs! I'm not a cynical person but it does makes me wonder about LIGO & JWST.

[@MegawattKS]

Thanks for bringing this up ! Yes - I was also a bit shocked and saddened to see what the limits are with our current technology. Of course ET has better capabilities - so they could put out planetary-levels of RF power in a near-zero bandwidth beacon signal, IF they wanted to. But motivation is everything. Why would they do this anyway? And two-way comm is off the table for us unless there's somebody/some-AI out there within a few 10s of light years using radio beacons - which apparently isn't the case. SETI has at least answered that question, IMO. Still, I enjoy the movie "Contact" and continue to support SETI. And I'm fine with JWST and LIGO. They are built for other science :-) My main focus now is on alien intelligence from 'within'. That darn AGI problem coming our way ! (not sure which emoji to put here...)

[@scollyer.tuition]

Have you removed episode 4 or has it been replaced by this video?

[@MegawattKS]

It's still on KZbin, but is 'unlisted' so you can only get to it if you know the link. Here it is: kzbin.info/www/bejne/mXW9q2WugquFr5o I unlisted it because it's largely the same as the new one - but I did like parts of it better (except for the calculator mishap of course) . Sadly, KZbin doesn't really let you edit a video other than deleting sections. No additions/fixes are possible as far as I can tell. So I had to do it over in version 4A...

[@scollyer.tuition]

@@MegawattKS Thanks. Yes, you're right. You can only remove and replace a video totally, even if the changes are trivial. It's a bit of a pain at times.

[@scollyer.tuition]

@@MegawattKS Great series, BTW. Useful information, but presented at a sufficiently relaxed pace that I don't have to think too hard while watching in the evening.

[@MegawattKS]

@@scollyer.tuition Cool. Great to hear. Thanks !