How far do Earth radio/TV transmissions reach?

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puh32

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Message 1871315 - Posted: 5 Jun 2017, 20:23:49 UTC

I’m having a hard time reconciling these three sentences:

1. “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]

2. "there isn't an instrument on Earth that can detect an Earth-like planet with Earth-like leakage of electromagnetic radiation." [from https://www.engadget.com/2017/05/25/listening-to-starlight-our-ongoing-search-for-alien-intelligenc/]

3. “It should be noted however that the most sensitive radio telescopes currently available on Earth would not be able to detect non-directional radio signals even at a fraction of a light-year,” [from https://en.wikipedia.org/wiki/Fermi_paradox]

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.) ?

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)?

Sorry if I misunderstood something. It has happened before.
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Profile Bob DeWoody
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Message 1871413 - Posted: 6 Jun 2017, 3:51:26 UTC

I have detected contradictions too, both here in posts and in science programming on TV.
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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puh32

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Message 1871635 - Posted: 7 Jun 2017, 22:58:10 UTC - in response to Message 1871413.  

>I have detected contradictions too, both here in posts and in science programming on TV.

I hear you here. Even so, this strikes me as a field where somebody would actually know the correct answers to these questions.

Specifically, the question “are only able to detect signals that were sent more or less directly to us (in our direction)?” strikes me as being at the very heart of the SETI@home effort. Surely, someone will know?

(Again, I’m very open to the possibility that I’m missing something obvious here.)
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Profile Ghan-buri-Ghan Mike

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Message 1871681 - Posted: 8 Jun 2017, 3:37:02 UTC

It can be confusing. My understanding is that the Earth is "brighter" than the sun in the FM bands (lots of stations pushing signals) , but that is different than capturing a single discrete transmission. Aliens are not going to be watching a single episode of "I Love Lucy", no matter what the popular press has written. Analog TV was designed to push an earth-surface hugging signal to ranges of ~ 160 km. These were typically low powered transmissions for practical commercial reasons: (1) power costs money and (2) commercial spheres of influence. In the US it was set up so stations in Boston wouldn't step on the NY stations, etc. Nearby cities had staggered channels to limit interference. In my youth NY had VHF channels 2,4,5,7,9,11 and 13; while Philadelphia and 3,6,8,10 and 12. It was a frequency management issue. The typical single VHF TV broadcast signal will peter out quickly in space (brightness follows the inverse-square distance law). The figure I've seen is about 0.6 LY. UHF TV typically broadcast even less power. But TV isn't the only signal source. Cold war military radars were sky oriented and some pushed considerable power. The sensitivity of the receivers has grown by leaps and bounds, as has computing power. We have a chance of success. We are also constrained by the earth's atmospheric window, which blocks large portion of the RF spectrum. And no exoplanet is likely to have precisely the identical atmospheric window on their end. They could be broadcasting and we might never know it. Even if BTL comes up empty this time around, we have by no means exhausted the search possibilities

Remember, we cannot see a single red dwarf star optically here from earth. Proxima Centauri is a little over 4 light years away, it is a massive fusion power source whose output dwarfs (no pun intended) any transmission man has made, and we can't see it without the aid of technology.
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Profile Gary CharpentierCrowdfunding Project Donor
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Message 1871706 - Posted: 8 Jun 2017, 6:00:56 UTC

One has to look at the assumptions behind the statements.

An analog TV transmission isn't going to be detectable at a large distance because it is a very wide band transmission. But the audio portion even though lower power will be detectable at a farther distance because the signal has a much narrower bandwidth. Also while these are not omnidirectional signals, they are not beam signals.

A military radar signal, essentially a short high power carrier wave pulse into a very directional antenna can be detected several orders of magnitude farther.

The planetary radar at Arecibo operates by sending a radar pulse to another solar system body and listening to the reflection. Those reflections are extremely weak so we have considerable confidence we would pick up a similar radar out to a large volume of the galaxy.

So ET won't be watching I Love Lucy, but they could be listening to our radar and even our Morse code transmissions.

One other thing they may be able to detect is our power grid. Not directly, but all the transmitters on earth do vary slightly with the power grid so the sum total of our transmissions also does.

Looking for ET is science and you should look to science papers, not general news for a answer to your question.
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Profile Gordon Lowe
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Message 1871840 - Posted: 9 Jun 2017, 0:13:43 UTC - in response to Message 1871706.  

One other thing they may be able to detect is our power grid.


I agree; an Earthly power signature as a whole makes a lot more sense to me as something detectable from afar than random isolated transmissions.
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Profile Chris SCrowdfunding Project Donor
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Message 1871912 - Posted: 9 Jun 2017, 12:05:09 UTC - in response to Message 1871840.  

I am not sure that all the electricity grids in the world (only overhead ones?) would give an "earthly power signature" as it has been termed. The nearest star to earth is Alpha Centauri at 4.3 light years away. Would anyone like me to ask the UK National Grid to boost their transmission power from from 33,000V transmission line to 35,00V just in case any Alphas are listening??
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Profile Gordon Lowe
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Message 1871988 - Posted: 9 Jun 2017, 18:05:51 UTC - in response to Message 1871912.  

Would anyone like me to ask the UK National Grid to boost their transmission power from from 33,000V transmission line to 35,00V just in case any Alphas are listening??


Yes! ;~)
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Profile Chris SCrowdfunding Project Donor
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Message 1871991 - Posted: 9 Jun 2017, 18:23:39 UTC - in response to Message 1871988.  

Ah my mistake, the National grid is 133,000 volts. Oooops!

I'll have a word Gordon :-)
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Profile Gary CharpentierCrowdfunding Project Donor
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Message 1871995 - Posted: 9 Jun 2017, 18:47:34 UTC

Not the voltage, it is AC. Everything goes up and down, on your side of the pond 50 Hz, North America 60 Hz. That produces a rather obvious modulation.
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Message 1872102 - Posted: 10 Jun 2017, 0:57:20 UTC - in response to Message 1871995.  

Not the voltage, it is AC. Everything goes up and down, on your side of the pond 50 Hz, North America 60 Hz. That produces a rather obvious modulation.

The power grid doesn't use modulation.
No carrier wave needed.
Radio waves does like AM and FM.
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Message 1872133 - Posted: 10 Jun 2017, 5:28:07 UTC - in response to Message 1872102.  

Not the voltage, it is AC. Everything goes up and down, on your side of the pond 50 Hz, North America 60 Hz. That produces a rather obvious modulation.

The power grid doesn't use modulation.
No carrier wave needed.
Radio waves does like AM and FM.

It, the power grid, modulates everything!
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Profile Chris SCrowdfunding Project Donor
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Message 1872152 - Posted: 10 Jun 2017, 8:31:16 UTC
Last modified: 10 Jun 2017, 8:33:30 UTC

It seems I gave incomplete info regarding the UK power grid. There are in fact 3 levels of transmission from power stations to sub-stations.

Length of 400 kV grid: 11,500 km (circuit)
Length of 275 kV grid: 9,800 km (circuit)
Length of 132 kV (or lower) grid; 5,250 km (circuit)
The grid uses AC transmission at a low current which needs a high voltage. The advantage of AC has always been that it is easy to change the voltage up and down with a transformer. DC requires more equipment and some losses to convert. There have been proposals to change to DC transmission, pdf document here. DC requires more equipment and some losses to convert.

DC Grid

Electricity down a cable can be likened to water down a garden hosepipe, the longer the hose the less pressure at the nozzle. Voltage = transmission pressure which drops over distance. Hence the high voltages of the grid.



Whether converting the worlds power grids to DC would make squiddly dat difference to a so called planet power signature I'll leave others to comment upon.
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Message 1872156 - Posted: 10 Jun 2017, 9:09:07 UTC - in response to Message 1872152.  
Last modified: 10 Jun 2017, 9:18:24 UTC

Yes Chris. You are right.
If you have to deliver power offshore HVDC is better than AC.
Britain have some islands like Sweden have.
I know that HVDC cables are connected to both Gotland and Poland.
In many cases, offshore transmission is better suited to HVDC applications than traditional AC.
Using AC, offshore cable voltages are limited to 220kV, limiting power transfer capabilities unless many cables are used.
Also, lengths of AC cable are very difficult (and consequently expensive) to joint offshore and reactive compensation cannot be installed mid-route, so transmission distances are restricted.

You could also could connect those cables to Internet:)
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Profile William Rothamel
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Message 1872605 - Posted: 12 Jun 2017, 18:47:30 UTC

It all depends on the antenna gain, transmitter power, absorption , polarization, signal to noise ratio, the atmosphere and a few other things as to the distance.

In any event: after 50 years of listening it appears that eavesdropping on this type of signal is not possible since no one is in range of our detectors. It also means that If a SETI effort depends on repeated signal locations they aren't going to find any. The exceptions to my hypothesis may just be that planet in Alpha Centauri that has recently turned on their first radar or tv station; or the planet a few thousand light years away that regularly beams a high power message our way.

For this reason I have said that every strong signal must be recorded and examined for content --I think that promising candidates could be screened right at the antenna as they were scanned and then sent on for further study for message type if any. I am sure that there are SETI practitioners, signal processing gurus and communication experts that would know exactly how to do this far better than I. I sincerely hope that we are or will soon apply this type of thinking to SETI efforts
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Message 1877766 - Posted: 10 Jul 2017, 19:22:45 UTC - in response to Message 1871315.  

From "An Introduction to Astrobiology" (ISBN 978 1 107 60093 5), p 288:

"The myriad of weak radio transmissions from inside Earth's atmosphere is, in principle, detectable from space ... but the range would typically be no more than a few tens of light-years."
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Message 1877881 - Posted: 11 Jul 2017, 23:49:52 UTC - in response to Message 1877766.  

However,

"SETI estimates, for instance, that with a radio telescope as sensitive as the Arecibo Observatory, Earth's television and radio broadcasts would only be detectable at distances up to 0.3 light-years, less than 1/10 the distance to the nearest star." (https://en.wikipedia.org/wiki/Fermi_paradox)

I don't know what to make of all of these seemingly conflicting statements. Anyone?
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Message 1877893 - Posted: 12 Jul 2017, 1:01:27 UTC - in response to Message 1877881.  

From "An Introduction to Astrobiology" (ISBN 978 1 107 60093 5), p 288:

"The myriad of weak radio transmissions from inside Earth's atmosphere is, in principle, detectable from space ... but the range would typically be no more than a few tens of light-years."

However,

"SETI estimates, for instance, that with a radio telescope as sensitive as the Arecibo Observatory, Earth's television and radio broadcasts would only be detectable at distances up to 0.3 light-years, less than 1/10 the distance to the nearest star." (https://en.wikipedia.org/wiki/Fermi_paradox)

I don't know what to make of all of these seemingly conflicting statements. Anyone?

The way I interpret it is what they're saying is Arecibo is small. A much larger antenna from much farther away, could in theory pick up Earth's broadcasts.
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Message 1877906 - Posted: 12 Jul 2017, 1:49:40 UTC - in response to Message 1877893.  

If true about arecibo then what are we doing here at SETI trying to eavesdrop.
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Message 1877924 - Posted: 12 Jul 2017, 7:44:41 UTC - in response to Message 1877906.  

We do the best we can with what we have. If we don't look at all thinking it is futile, we will never find anyway. Russian billionaires are not noted for wasting their money. Bear in mind also that we are LOOKING not deliberately ADVERTISING our presence.

There may came a stage sometime when we say, OK we've found nothing in 20-25 years looking for THEM, now lets transmit our presence and see if THEY can find US. The jury is still out on that one. A Klingon Bird of Prey de-cloaking on the White House lawn would cause a hiccup or two.

The bottom line William is this

There maybe nothing out there to find
There is but we are using the wrong method to look
We are using the right method but it isn't powerful enough
We have already found ET but our algorithms aren't good enough to know that
They don't want to be found
They could be quietly watching us as THEIR hobby

Any or all of the above. And in any case the Seti community is more than just crunching workunits, it is socialising with a group of like minded people. It's up to you if it ticks enough boxes for you.
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Message boards : SETI@home Science : How far do Earth radio/TV transmissions reach?


 
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