Notices
Results 1 to 13 of 13

Thread: Receiving our broadcasts in space.

  1. #1 Receiving our broadcasts in space. 
    Forum Freshman
    Join Date
    Dec 2007
    Posts
    66
    I have read many times that our early broadcasts are still going out into the space and might be being picked up on distant planets. I have even seen it on a NASA website.

    I say this is nonsense. Early low frequency broadcasts wouldn't even have passed through the ionosphere without considerable attenuation.

    The distances are far too great for our flea power transmitters and I reckon VHF - FM broadcasts would last a matter of minutes before become too weak and TV a matter of seconds.

    Make me think that SETI is a waste of time unless some distant planet(s) have very powerful transmitters.


    Reply With Quote  
     

  2.  
     

  3. #2 Re: Receiving our broadcasts in space. 
    Guest
    Quote Originally Posted by Pumblechook
    I have read many times that our early broadcasts are still going out into the space and might be being picked up on distant planets. I have even seen it on a NASA website.

    I say this is nonsense. Early low frequency broadcasts wouldn't even have passed through the ionosphere without considerable attenuation.

    The distances are far too great for our flea power transmitters and I reckon VHF - FM broadcasts would last a matter of minutes before become too weak and TV a matter of seconds.

    Make me think that SETI is a waste of time unless some distant planet(s) have very powerful transmitters.
    There's a tiny little 5 watt transmitter 10 billion miles away that is still sending messages that we still recieve. Some of the early radio transmissions from earth were many hundreds of killowatts, radio waves (like photons) will propogate forever in free space. The question is whether anybody out there has a sensitive enough receiver, tuned to the right frequency, pointing in the right direction to detect thse waves - there is no evidence that anybody is listening. The planet Jupiter emits very low power radio frequencies which escape it's magnetosphere, penertrates ours and can be listened to even by radio amateurs. As a matter of interest, we can hardly discern individual stars from our nearest neighbour galaxy so we would never 'see' a radio signal unless it's power was at least equal to the entire output of a star with todsay's technology. SETI is nothing more than a bloody joke to anyone with even a primitive grasp of radio communications,


    Reply With Quote  
     

  4. #3  
    Forum Freshman
    Join Date
    Dec 2007
    Posts
    66
    That small transmitter is only 1/600 of a light year away and you need a large dish such as Jodrell Bank (76 Metre dish) to receive it and JB has an amplifier cooled to a very low temperature at focus of the dish. The spacecraft with have some aerial gain so will have an ERP somehat higher than 5 Watts. A transmitter at 1 lt yr would have to be 360,000 (600 sqrd) times more powerful. The recently discovered planet which might have life is 20 lt yrs away so that would require 400 times again more power. = 720 MW if they had a Jodrell Bank pointing in the right direction. If they had a smaller 7.6 metre dish then we would need a transmitter of 72 GW. The total output of all UK power stations is about 60 GW.

    As you say knowing where to point is a great problem.
    Reply With Quote  
     

  5. #4  
    Guest
    Radio waves will travel in free space forever. The factor (in free space) which absolutely governs distance over which successful (radio)communications may be achieved is field strength at the receiver. Think of a torch bulb without a reflector it may vuaguely light a room evenly throughout but, if a reflector is suitably placed near the bulb it's light may be 'useful' over many hundreds of feet. If the beam could be made such that it does not diverge (or spread out) then it's effective communication distance will be infinite but the receiver must be in the field of the beam. These principles are used in almost all modern radio communication systems the best example being satellites.

    Even a laser beam diverges (spreads out) such that a laser pencil beam shone at the moon may be 20 miles across when it gets there! My opinion is that IF there are aliens out there communicating by radio waves they will have found ways to ensure all the transmitter energy is received at the receiver (something we can only do in theory at present) this will allow them to send 'energy' to their craft and communicate with it. This being so we will never pick up their signals
    Reply With Quote  
     

  6. #5  
    Forum Freshman
    Join Date
    Dec 2007
    Posts
    66
    Signals become too weak to receive because they are below the (thermal) noise floor and there will be external interference such as noise from stars and the cosmic background radiation. How sensitive a receiver is well defined for a particular bandwith and mode of transmission. e.g Standard FM Broadcast receivers work down to around -120 dBm ... 10 to minus 12 down from one milliwatt...a tiny amount of power. The path loss in dB is easy to calculate given the frequency and distance. You work out the transmitter Effective Radiated Power in dBm and add in any receiving aerial gain. Just a matter of adding up all the dBs to see if the received signal is above the -120 dBm. There are even on line calculators. Some even work out the received signal to noise ratio given all the parameters of the transmitter, receiver and aerilas used. When the s/n is zero or negative then you have no receivable signal.

    The distances you come up with might be a few tens of million miles for FM radio and a million or so for TV which has a much wider bandwidth. These distances are a tiny fraction of a light year.

    Narrow band communications signals will travel much further.

    This signals may well travel long distances but will not be receivable. They can't travel forever because at some point there will be no electrons moving in an receiving aerial stimulated by the transmiiter on Earth. There must be a limit.
    Reply With Quote  
     

  7. #6  
    Guest
    Yes, essentially you are correct BUT I did indicate that it was only possible where the beam does NOT diverge, it is the divergence of the beam (in free space) which defines field strength density - in reality all beams diverge and there are obstacles in free space such as dust, BUT radio will in free space progagate for ever. If you still believe that I am incorrect explain where/how (in free space) a parallel beam loses it's power? My point is that if there is a civilisation out there which can 'planet hop' at will, it is likely it will have overcome the divergent beam problem. From the quality of your post it is clear you have a good grasp of radio theory. Most of the loss you refer to is due to beam divergence, atmospheric losses and terrain attentuation.
    Reply With Quote  
     

  8. #7  
    Forum Freshman
    Join Date
    Dec 2007
    Posts
    66
    You are dead right. The 'free space' loss is purely based on the divergence of the beam...inverse square law... 6dB increase in loss per doubling of the distance.. Any other loss due to the atmosphere has to be added. It is assumed that 'free space' is lossless but if there is any material in space even though the density might be tiny and the loss might be considered negligible over the vast distances it might add up to a few dB?? Sending a signal across the surface of the Earth is another matter as the curvature obvioulsy limits how far signals can travel and the terrain will feature.. The atmosphere makes signals curve and they do go further than the visible horizon and at times, particularly when we high atmos pressure, considerably further. This applies to VHF, UHF amd microwave signals which don't usually bounce off the ionosphere like Short Wave signals do.

    It is not possibly to generate a parallel beam. I think Jodrell Bank has a beam of 12 minutes of Arc at 2.4 GHz. If it transmitted at that frequency the beam would be 22 km wide when it reached a satellite in geo-orbit.
    Reply With Quote  
     

  9. #8  
    Guest
    So to get back to your original post my opinion is that unless there is an alien equivalent of 'SETI' out there with technology way ahead of ours, then I agree nobody is listening. Some radio energy will have reached stars up to 100 light years away but getting there and being strong enough to be recognised/demodulated are two very different things. One interesting possibility though would be that of 'gravitational lensing' that could enhance radio transmissions but the chances of everything being aligned must by almost zero. I believe that we will be able to transmit to a few nearby stars with perhaps powers as little as a few tens of MW but at frequencies closer to xrays from transmitters located in space once we have mastered modulation/demodulation and focussing large powres at such high frequencies - yes I know this needs breakthroughs but there are no physical laws preventing such transmissions (at least that I am aware of).
    Reply With Quote  
     

  10. #9  
    Forum Freshman
    Join Date
    Dec 2007
    Posts
    66
    The snag with near parallel beams which are much easier to do the higher in frequency you go to is the the chance of pointing in the right direction. Even a domestic satellite dish with a 3 deg beam could point at any azimuth 0 to 360 ..that is 120 possible azimuths.. It also could point 0 - 90 in elevation ....30 possible elevations .... 30 x 120 combinations = 3600

    If a alien on a distant planet had a similar dish also with 3600 possible directions the chances of both antennas aligning will be 3600 x 3600 ,.. nearly 13 million possibilities...about the chance we would have winning the Nat Lottery. AND these are just small antennas not the very big ones with much smaller beams. We may have some ideas where to point rather than random pointing.

    I have read about proposals for using very low frequencies... I Hz or less. Lauching satellites which would float long wire dipoles in space which pick signals from most directions. Clearly data rates with very low carrier frequencies will be very low... More like minutes per word rather than wpm if we are talking about old fashioned Morse.

    The lower the frequency you use the more signal a half wave dipole will receive simply by having more lenghth.. If you are in field of X volts per metre then a 2 metre dipole develops twice the voltage than a 1 metre diople (at double the frequency),...4 times the power.
    Reply With Quote  
     

  11. #10  
    Guest
    Somehow a 1Hz TX freq with a 45,000 mile long antenna all cooled to absolute zero if near a star (to cut transmission losses over the length of the antenna) seem a tad backwards compared against high energy high frequency beams - the point you make about direction is valid, my own opinion on this is that any aliens out there know where their base is and fire off signals in that direction. They would probably have control over beam divergence and their base station might easily then track the source and fire back along the same path, unless the earth is in direct line we will not receive any signals (a principle reason for my belief that SETI is a complete waste) if you are out in space you will need to use your resources very efficiently. Because we can look into dense star forming clouds using Xray telescopes (whereas optical scopes are useless for this) xrays are IMHO the way to go.
    Reply With Quote  
     

  12. #11  
    Administrator KALSTER's Avatar
    Join Date
    Sep 2007
    Location
    South Africa
    Posts
    8,245
    and fire back along the same path, unless the earth is in direct line we will not receive any signals (a principle reason for my belief that SETI is a complete waste)
    Would it be unreasonable to assume that an advanced alien civilization could have the technology to directly observe the earth from their planet and then aim their beams at us?
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
    Reply With Quote  
     

  13. #12  
    Forum Freshman
    Join Date
    Dec 2007
    Posts
    66
    It was a serious suggestion by some scientists to use very low frequencies. ...some suggesting it was the only way we could communicate over vast distances. Another idea would be a radio sation on the Moon where you could erect a very tall mast (low gravity, no wind) to form a ground- plane antenna and use carrier frequencies in the kilohertz range.

    I dont follow the bit about directing narrow beams. Problems is with thousands and millions of possible other worlds ...which way do you send a narrow beam?? With the Earth rotation and orbit and the movement of the target 'world' keeping a narrow beam on track would be tricky.
    Reply With Quote  
     

  14. #13  
    Administrator KALSTER's Avatar
    Join Date
    Sep 2007
    Location
    South Africa
    Posts
    8,245
    Another idea would be a radio sation on the Moon where you could erect a very tall mast (low gravity, no wind) to form a ground- plane antenna and use carrier frequencies in the kilohertz range.
    The technology exists to build a structure on the earth that could reach into space, sturdy enough to house a station on top and could certainly include an antenna.http://en.wikipedia.org/wiki/Space_fountain
    With the Earth rotation and orbit and the movement of the target 'world' keeping a narrow beam on track would be tricky
    They could simply spread the beam to r = 1AU.
    Problems is with thousands and millions of possible other worlds ...which way do you send a narrow beam??
    All of them? We already know what might be possible with quantum computers, nano-tech materials, nuclear fusion and an unlimited budget. A civilization that survived it’s technological infancy would probably have all of these, if not more. Imagine probe-telescopes spread over an area equal to or larger than a solar system and you could have the means to both look for candidate planets and send signals to the most promising ones.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •