How far do Earth radio/TV transmissions reach?

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Profile Bob DeWoody
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Message 1877927 - Posted: 12 Jul 2017, 8:53:51 UTC

I pick " They don't want to be found" especially by us.
Bob DeWoody

My motto: Never do today what you can put off until tomorrow as it may not be required. This no longer applies in light of current events.
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Message 1877937 - Posted: 12 Jul 2017, 9:44:31 UTC - in response to Message 1877924.  

>The way I interpret it is what they're saying is Arecibo is small ...

Oh, good point.


>We do the best we can with what we have.
>... it is socialising with a group of like minded people.

I certainly agree.
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Profile William Rothamel
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Message 1877953 - Posted: 12 Jul 2017, 13:22:19 UTC - in response to Message 1877927.  

The "They"' , if they exist, don't know about "Us" since they are not in our close neighborhood; and we are not sending any type of repeating beacon to promising directions in the sky.
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Profile William Rothamel
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Message 1877982 - Posted: 12 Jul 2017, 16:03:30 UTC - in response to Message 1877953.  

At 1000 feet (335 meters) across it is hardly small. There is at least one other that is larger.
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Profile Chris SCrowdfunding Project Donor
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Message 1877986 - Posted: 12 Jul 2017, 16:17:15 UTC

we are not sending any type of repeating beacon to promising directions in the sky.

Because mankind simply does not know what if anything is out there, how can we? There is no consensus at present across the scientific community nor internationally to actively advertise our presence. In theory we could get visited by a warlike race that simply takes over the Earth or kills us all for food. If they are technologically advanced enough have conquered FTL space travel, then our weapons would be of little use against them.

That as I understand it is the current situation.

We can be pretty sure that there is no ET in our Solar system, I think we can all take that for granted, even allowing for Nibiru or planet X. Whether there is in our Milky Way Galaxy, again we don't know. Nearest star system to us is Alpha Centauri (same spectral type) at 4.3 light years, Barnards Star at 6 light years, and Sirius at 8.6 light years. We keep discovering new exo planets but with no means of getting to them.

A signal travelling at the speed of light would take 8.6 years minimum before we got a reply back, 300 times faster in a pulsed laser beam? We would get a reply in 10 days! Watch this space .......

http://physicsworld.com/cws/article/news/2000/jul/19/laser-smashes-light-speed-record
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Message 1877990 - Posted: 12 Jul 2017, 16:31:51 UTC - in response to Message 1877986.  
Last modified: 12 Jul 2017, 16:43:39 UTC

Whoever wrote that article got the whole thing completely backwards:

Wang and colleagues begin by using a third continuous-wave laser to confirm that there are two peaks in the gain spectrum and that the refractive index does indeed change rapidly with wavelength in between. Next they send a 3.7-microsecond long laser pulse into the caesium cell, which is 6 centimetres long, and show that, at the correct wavelength, it emerges from the cell 62 nanoseconds sooner than would be expected if it had travelled at the speed of light. 62 nanoseconds might not sound like much, but since it should only take 0.2 nanoseconds for the pulse to pass through the cell, this means that the pulse has been travelling at 310 times the speed of light. Moreover, unlike previous superluminal experiments, the input and output pulse shapes are essentially the same.


Light in a vacuum does indeed cover 6cm in 0.2 nanoseconds; since it took 62 nanoseconds, the pulse is traveling 310 times slower, not faster.

In other news, snail smashes land speed record travelling 310 times slower than walking. :^)

Edit: Article is also 17 years old. No faster-than-light communications after all this time so I think I am correct.
“Never doubt that a small group of thoughtful, committed citizens can change the world; indeed, it's the only thing that ever has.”
--- Margaret Mead

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Profile Chris SCrowdfunding Project Donor
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Message 1877999 - Posted: 12 Jul 2017, 17:15:55 UTC

Upon the face of it your maths "seem" to be correct, but how likely is it that a worldwide respected Journal like Nature would misprint it or get it wrong.? Indeed read this report where even Berkeley is happy about it. I haven't got the knowledge to explain it further, perhaps others have.

Before the pulse had fully entered the chamber it had gone right through it and travelled a further 60ft across the laboratory. In effect it existed in two places at once, a phenomenon that Wang explains by saying it travelled 300 times faster than light.

Dr Raymond Chiao, professor of physics at the University of California at Berkeley, who is familiar with Wang’s work, said he was impressed by the findings. “This is a fascinating experiment,” he said.

http://electrogravityphysics.com/scientists-break-speed-light/
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Profile Gordon Lowe
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Message 1878152 - Posted: 14 Jul 2017, 1:10:12 UTC - in response to Message 1877999.  

I haven't got the knowledge to explain it further, perhaps others have.


Way over my head. Where is Dr. Hawking?
The mind is a weird and mysterious place
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Message 1878197 - Posted: 14 Jul 2017, 8:12:59 UTC

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Profile Chris SCrowdfunding Project Donor
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Message 1878198 - Posted: 14 Jul 2017, 8:29:20 UTC - in response to Message 1878197.  
Last modified: 14 Jul 2017, 8:33:12 UTC

Who would want to travel faster than light, anyway? The show it puts on is just too good to miss.

Rather silly statement. Mankind would need to be able to do that if they wish to visit the stars, and mankind DOES!!

I also believe that Einstein was right and also wrong. He formulated his theories according to scientific knowledge at the time, plus his own thoughts. If he were alive to day with 21C knowledge I believe he would modify them.

I firmly believe that things can travel faster than light, and not just massless particles. We just haven't discovered how to do that yet. But we will in time to come I am sure. [/url]
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Message 1894850 - Posted: 12 Oct 2017, 14:38:14 UTC - in response to Message 1871315.  

Thursday October 12 2017, 7:37 AM

I’m having a hard time reconciling these sentences:

“The signals radio/TV that are emitted from earth have a range of about 100 light years before the dissipate into the noise background.” [from https://setiathome.berkeley.edu/forum_thread.php?id=80585&postid=1833000#1833000]

"Am I to understand that whereas Earth leaks radio/TV transmissions as far out as 100 light years, the process of detecting such (weak? scattered?) signals is so difficult that we couldn’t do it ourselves right now (that is, we have no instrument that could pick up/identify radio/TV leakage from 100 light years away. In fact not even 1 light year away.) ?"


In regard the two paragraphs above, I would like to comment as follows:

I have not checked the "100 light years" figure quoted above, but will assume it was given in good faith, and calculated using normal radar range equations. These work fine down to the thermal noise floor of -174 dBM/Hz.

As a microwave engineer and former consultant for NASA, who was involved with the DSN (Deep Space Network) Voyager probe design, I know that signals from our distant probes can be routinely tracked, when they are far out, BELOW the thermal noise floor, at least 20 dB, and perhaps more, below. That would mean that if the "100 light year" number is correct, and based on a -174 dBM noise floor, that the distance would be even further out, 20 or more dB further, however many light years THAT translates to.

The methodology behind this "gain" of 20-odd dB, or more, is as follows: Noise is RANDOM, repetitive signals are NOT. If you look at the precise frequency a probe is transmitting on, over time, and integrate, the below the noise signal CAN be seen.

Of course the "catch" is you must know the precise frequency to "look," and "integrate," at. In the case of a probe of ours, this is possible, but in the case of alien transmissions it is not. Another flaw in the ointment is what is called a "CRC," or "Cyclical Redundancy Code." Our probes send out a "number" telling the earth-bound receiver WHAT to expect to hear, i.e, how many bits will be sent. The receiver then listens, and if the CRC matches the data, the data is correct. If not, the data is ignored as it's "scrambled," and the receiver listens again, over and over, until the probe transmission is correctly received. Of course a "CRC" is NOT what an alien signal would send to us, AND, at the crux of the matter, the alien signal will likely be non-repetitive, and not in "plain English." :)

"Or, in other words, right now we are only able to detect signals that were sent more or less directly to us (in our direction)?"

I have to assume the above is TRUE. In addition, to use this as an example, imagine the alien SAYS "Hello," in English. Haha, fat chance, huh? If she/he/it says it once, we would likely ignore it. But if she/he/it transmits "Hello, Hello, Hello, Hello, Hello", repetitively, and the signal was above the thermal noise floor, we could hear it. If below the floor we would not .

This is an interesting topic.

Stay here on Earth. It's the only planet with DARK CHOCOLATE !!


Sorry if I misunderstood something. It has happened before.

River Song (aka Linda Latte on planet Earth)
"Happy I-Phone girl on the GO GO GO"
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Message 1894866 - Posted: 12 Oct 2017, 15:38:54 UTC

The "one hundred light years" is based on the fact that we've been using radio transmissions for somewhat over 100 years and radio waves travel at the speed of light.
Too many people fail to recognise the difference between "detecting" a signal and "decoding" a signal. One can detect signals that are far weaker than those that can be decoded. Thus one may be able to detect radio waves from earth at a distance of say 100 light years, but one can almost certainly not decode the same signal. I don't know what the noise floor of the radio telescopes that provide us with our data is, but both of the use cryogenic cooled receivers.
Bob Smith
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Message 1894879 - Posted: 12 Oct 2017, 16:40:24 UTC - in response to Message 1894866.  

The "one hundred light years" is based on the fact that we've been using radio transmissions for somewhat over 100 years and radio waves travel at the speed of light.
Too many people fail to recognise the difference between "detecting" a signal and "decoding" a signal. One can detect signals that are far weaker than those that can be decoded. Thus one may be able to detect radio waves from earth at a distance of say 100 light years, but one can almost certainly not decode the same signal. I don't know what the noise floor of the radio telescopes that provide us with our data is, but both of the use cryogenic cooled receivers.


Thursday October 12 2017, 9:35 AM

Hi Bob,
Very interesting.... Yes, transmission, earth to probe, or to an alien her/him or 'it' are limited to light speed. However, as you know, they are attenuated greatly when radiating outward, and subject to the radar range equation. Sadly so. :(

When I first learned of our ability to 'see' signals far below the thermal noise floor I was truly anamzed. To me, prior to that, you could NEVER 'see' a signal unless it was at least 3 dB stronger than -174 dbm/Hz. I gave no thought, back then, to noise being random, and transmissions, sent repetitivly, and using a CRC code, as being viable. :)
River Song (aka Linda Latte on planet Earth)
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Message 1895033 - Posted: 13 Oct 2017, 14:31:40 UTC - in response to Message 1894866.  

The "one hundred light years" is based on the fact that we've been using radio transmissions for somewhat over 100 years and radio waves travel at the speed of light.
Too many people fail to recognise the difference between "detecting" a signal and "decoding" a signal. One can detect signals that are far weaker than those that can be decoded. Thus one may be able to detect radio waves from earth at a distance of say 100 light years, but one can almost certainly not decode the same signal. I don't know what the noise floor of the radio telescopes that provide us with our data is, but both of the use cryogenic cooled receivers.


Friday October 13 2017, 6:18 AM

The "noise floor' of a radio telescope goes hand in hand with the "noise figure" of the receiving apparatus. The noise figure of any really GOOD earth-bound receiver LNA, (Low Noise Amplifier), is usually in the range of 1.5 to 2 dB or so. Cooling of critical parts of the receiver input certainly will lower the noise figure, and thus the detection noise floor, but not below "0 db." So, it's reasonable to assume that the noise figure of a radio telescope receiver is surely lower than anything in "normal" commercial use, but not lower than zero.

If you do a search on "noise figure of a radio telescope" you will find many references. One that I saw at the top of a search using Bing on Mozilla Firefox indicated that the noise figure of one was 0.2 dB. This falls in line with my assertion that a noise figure less then zero is not possible. :) See https://www.rtl-sdr.com/radio-astronomy-0-2db-noise-figure-lna/

One source I found says that "lowering the noise figure of a satellite receiver by 1 dB is equivalent to increasing the size of the large receiving antenna by about 40%." Given the SIZE of a typical radio telescope dish, (they are humongous), a 40% increase is dramatic and very significant.

We are in total agreement that there is a BIG difference between "detection" and "decryption." :) THAT makes GOOD sense indeed! You can DETECT noise but to DECRYPT a signal below -174 dBM/Hz requires the signal to be transmitted repetitively by the probe, and CRC coded to assure error-free decryption.

There is another aspect to low-noise receiver design that is not obvious to many, and that is the LNA input VSWR. The noise figure calculations assume a "matched load." Simply stated, the noise source and the LNA are assumed matched impedance-wise. There is NO commercially available single-stage LNA that has an input VSWR of 1.0 . It is not at all unusual for a low-cost LNA to have a VSWR = 2, 3, or higher. So, the "dynamic," or "working" noise figure is different than the "static," or measured noise figure. To compensate for this, more expensive LNA's use TWO identical LNA's coupled together with a low-loss hybrid power combiner. The net effect of such a combiner is to present a matched load to the noise source. But, the combiner used has an "insertion loss," typically 0.3 dB, and THIS loss ADDS to the final input noise figure. You never get "something for nothing." A sad thing. :(

A 2nd point rarely mentioned is "transmitter noise." It is entirely possible for a transmitter to add noise to the intelligence being amplified and sent to its antenna. This noise has to add to the difficulty of decryption below the thermal noise floor. A deep space probe transmitter HAS to be very clean.

I've never been involved in the design of an LNA for a radio telescope BUT I have to assume that they MUST take into consideration input VSWR mismatch. I'm curious how they deal with this? Perhaps they gave found a "work around" for LNA input VSWR correction that doesn't involve a hybrid combiner? I doubt it tho. Hmmmm, an interesting topic for another day?
River Song (aka Linda Latte on planet Earth)
"Happy I-Phone girl on the GO GO GO"
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Message 1895062 - Posted: 13 Oct 2017, 16:48:51 UTC - in response to Message 1895033.  

Friday October 13 2017, 9:33 AM

Oops, sorry! I forgot to ADD the following to my previous post, and remembered it too late to do an 'Edit.' Deal with it, OK?

Here is something of possible interest in regard commercially available LNA's.

I feel that all LNA's have two different noise figures. One, and to me, the REAL one, is what you measure with a noise figure meter, which takes mismatch VSWR into account, and an IDEAL noise figure which is what you would measure with a noise figure meter IF, and only IF, you use two LNA's with a hybrid combiner. :)

As an example of this idea, say you have two identical LNA's, each with a 2.5 dB noise figure, as measured in the normal way. OK, now you use a hybrid combiner with a 0.3 dB insertion loss. I would think the noise figure at the combiner input, measured with a noise figure meter, would then be 2.2 dB because there is no longer mismatch VSWR to influence the measurement, only the combiner insertion loss.

I wonder, when commercial entities advertise LNA noise figure, if they use the 'single-ended' NF of one LNA, or if they advertise the somewhat lower number of a hybrid combined pair? mmmmmmmmmmm, huh?

Sorry to rattle on, but it is an interesting topic to some, I hope?

Question: if a tree falls in the forest, and there is no one there to hear it, does it make a sound? I wonder?
River Song (aka Linda Latte on planet Earth)
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Message 1895896 - Posted: 18 Oct 2017, 4:56:33 UTC

If anyone from far far away was that interested in looking for life in our part of the galaxy, they would likely deploy probes, which themselves would have the capability to transmit findings back home.
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Message 1895923 - Posted: 18 Oct 2017, 12:37:50 UTC - in response to Message 1895896.  

If anyone from far far away was that interested in looking for life in our part of the galaxy, they would likely deploy probes, which themselves would have the capability to transmit findings back home.


Wednesday October 18 2017, 5:36 AM

Makes good sense. :) But, depending on the method of transmission, it may take many years for the messages to reach back to the senders.

Stay here on Earth. It's the only planet with DARK CHOCOLATE !!
River Song (aka Linda Latte on planet Earth)
"Happy I-Phone girl on the GO GO GO"
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Message 1895962 - Posted: 18 Oct 2017, 15:28:30 UTC - in response to Message 1895062.  

This is a reply to my Message 1895062 dated Friday, October 13, 2017, 9:33 AM

[quote]Friday October 13 2017, 9:33 AM (in part)

As an example of this idea, say you have two identical LNA's, each with a 2.5 dB noise figure, as measured in the normal way. OK, now you use a hybrid combiner with a 0.3 dB insertion loss. I would think the noise figure at the combiner input, measured with a noise figure meter, would then be 2.2 dB because there is no longer mismatch VSWR to influence the measurement, only the combiner insertion loss.

I wonder, when commercial entities advertise LNA noise figure, if they use the 'single-ended' NF of one LNA, or if they advertise the somewhat lower number of a hybrid combined pair? mmmmmmmmmmm, huh?


==============================
Wednesday October 18 2017, 8:02 AM

I made a technical ERROR in my previous post, and I wish to CORRECT it. I postulated that, by combining 2 identical LNA's, (Low Noise Amplifiers), with a hybrid combiner having an insertion loss of 0.3 dB that, as a result of improving the impedance match to the source, that the combined noise figure would be DECREASED by the 0.3 dB. This is WRONG, you NEVER "get something for nothing." You WILL achieve a low input VSWR,, BUT to GET it you must suffer a worsening in the combined noise figure. The noise figure of the pair would RISE to 2.8 dB from 2.5 dB.

Tuff noogies, huh? So much for freebies. :)
River Song (aka Linda Latte on planet Earth)
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Message boards : SETI@home Science : How far do Earth radio/TV transmissions reach?


 
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