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Question about radio basics.

  1. Jan 27, 2010 #1
    Say I make a simple crystal radio using a 1N34A diode.
    If I use it to tune in a radio station 100 miles away,
    would it be possible in theory to detect an infinitesimal
    increased load at the station when I tune it in with my
    crystal set?
     
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  3. Jan 27, 2010 #2

    vk6kro

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    You wouldn't get an increased load because the signal has left the antenna and is radiating away from it..

    However, you might get a VERY small signal radiated back to the transmitter from a resonant tuned circuit with an antenna on it.

    This is a little like the "chaff" that aircraft eject to confuse radar systems. Each piece of "chaff" is a resonant antenna length and radiates a small part of the radar signal back to the receiver.

    It is called a "passive reflector".

    To have the remotest chance of hearing it, you would have to turn off the transmitter, of course.
     
  4. Jan 27, 2010 #3

    berkeman

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    No, I don't think so. The energy still radiates away as usual from the transmitting (TX) antenna. You are just intercepting some of the energy that would have kept on going.

    You probably can detect a drop in energy at points past your antenna, since you have absorbed some of the energy with your antenna.
     
  5. Jan 27, 2010 #4
    The tiny signal from the crystal set radiated back to
    the transmitter should generate a minute opposing
    EMF in the transmitter's antenna. In theory, would
    you not be able to detect this at the station?
     
  6. Jan 27, 2010 #5

    berkeman

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    That's a different question. An ideal RX antenna and receiver will not put out any signal from the RX antenna. To the extent that some local oscillator energy in a heterodyne receiver makes it out the RX antenna, and to the extent that the RX antenna is not well matched to the receiver's feed line, then yes, you could detect something at the original TX station. But it's due to either a partially reflected signal at the RX antenna, or due to other noise being generated by the receiver. Not because of the energy that is absorbed by the RX antenna and receiver. Makes sense?
     
  7. Jan 28, 2010 #6

    sophiecentaur

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    The guy with a receiver 'downstream' of you might notice a small difference in his received signal power. But you'd need a large, well matched antenna and be very close to him.

    AS far as the transmitter is concerned, your antenna would constitute a small Radar target, of course but radar systems always have to switch off the transmit signal (i.e. pulse) to avoid drowning out the received reflected signals (pulses).
    Your clandestine listening is quite undetectable - relax.
     
  8. Jan 28, 2010 #7
    I'm taking about a simple crystal set (germanium diode AM radio). When you tune to the
    broadcast station, the two circuits will be resonating. When in this state the crystal set
    will be drawing power from the transmitter. Nikola Tesla understood this stuff.
     
    Last edited: Jan 28, 2010
  9. Jan 28, 2010 #8

    berkeman

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    They do not resonate together. The RX station circuit is being excited by the incident radiation, which has nothing to do with the TX station after it leaves the TX antenna.
     
  10. Jan 28, 2010 #9

    Averagesupernova

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    I'm not so sure. If you get enough antennas I would assume you will load the transmitter differently. Granted, in the real world this doesn't occur for obvious reasons, but my point stands.
     
  11. Jan 28, 2010 #10

    berkeman

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    Even at 100 miles away? I was picturing the question in his first post.
     
  12. Jan 28, 2010 #11

    sophiecentaur

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    The mutual impedance between transmit and receive antenna is negligible. The receiver is not "drawing power" from a distant transmitter - it is merely intercepting the wave which has been launched previously. Let's face it, for a distant receiver, the transmitter could have been switched off and the antenna dismantled long before the receiver actually gets the signal.
    Nikola Tesla strikes again, I notice. Will he never lie down?
     
  13. Jan 28, 2010 #12
    Actually, to my way of thinking, if the receiving antenna is matched and tuned for maximum power absorption, it must also re-radiate. In fact, in the ideal case, as much power will be re-radiated outwards as will be absorbed at the load. In practise this type of system is not normally seen in AM crystal radio receivers because the parasitic lossed dominate all other circuit parameters.
     
  14. Jan 28, 2010 #13

    sophiecentaur

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    If no power is absorbed by the load then there is no received signal.
     
  15. Jan 28, 2010 #14

    berkeman

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    Why do you say that? How much power is reflected off of a correct termination resistor at the end of a transmission line?
     
  16. Jan 28, 2010 #15
    I'm sorry, I didn't mean to contradict you. I didn't notice you were a group moderator when I posted.
     
  17. Jan 28, 2010 #16

    Averagesupernova

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    Yes even at 100 miles. It may take antennas right next to each other in a 360 degree sphere, but I still stand by my original point.
    -

    You can say the same thing about sending a pulse down a transmission line and unhooking the pulse generator before the pulse is reflected off of the other end of the line.
    -
    We all seem to forget that the transmitter relationship to the receiver is quite simply a very lossy pad. An tiny tiny tiny tiny tiny tiny tiny tiny tiny tiny tiny portion of the transmitted signal gets to the receiver, so naturally, one lone receiver is not going to do much to the transmitter. Try this: Run a signal generator at 0 dBm into a directional coupler and sample the reflected port with a spectrum analyzer. Short the output of the coupler and you will have alot of reflected power as shown on the spectrum analzyer. Properly terminate the directional coupler and you will notice no reflected power on the spectrum analyzer. Well, maybe some due to not being able to perfectly match impedances. Now properly terminate the output but do so through 50 or 60 dB of attenuation. You will again notice very little if any reflected power. Now short the output of the pad. Shorting the output of the pad now doesn't seem to do much to the reflected power as shown on the spectrum analyzer. This is similar to what happens when we transmit from an antenna into space. The space is a natural attenuator. So no matter what we try to do at the receiving end, the isolation between transmitter and receiver minimalizes any effect on the transmitter. But if we put enough receiving antennas out there (similarly, if we hang enough short circuits through enough attenuating pads) the transmitter should see it. It certainly isn't a perfect analogy, nothing is.
     
  18. Jan 28, 2010 #17

    berkeman

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    No, no, don't worry about that. If I'm wrong, then I learn something new. Not an issue.

    I just honestly didn't understand your statement. I think it's wrong, but I wanted to ask about it in case my understanding is missing something.

    To my understanding, the EM wave going past the RX antenna causes current to flow in the antenna, and through the feed coax to the receiver, which has an input impedance well-matched to the Zo of the feed coax. I'm not seeing where there is reflected energy off of the antenna, but I suppose it's possible.
     
  19. Jan 28, 2010 #18
    Thanks, because sometimes I find I have to watch myself around here.

    It's actually a very very cool analysis which I did myself quite a few years ago. Of course to receive power there has to be SOME current flowing in the receiving antenna; and once current is flowing, you have a transmitting antenna, so there must be outgoing power. But I want to show that this is not a trivial or inconsequential aspect of the process, but absolutely essential.

    This analysis applies to an ideal antenna without any lossy materials around (e.g. the ground). The antenna doesnt have to be long, it just has to be lossless (a superconductor). You tune it with a coil and choose the resistance that absorbs the most power. That's what we call matching. If you now decide to short out the load, the antenna becomes a pure reflector. The current is exactly double the matched load current.

    Think about this and see if it makes sense to you. I can explain it better if necessary. The funny thing is that it doesn't matter how short your antenna is...you can still match it with the right lossless coil. The matching resistance changes as you get shorter in length, but there always is a matching resistance whatever length you work with.
     
  20. Jan 28, 2010 #19

    berkeman

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    Interesting thoughts. My understanding is that the current in the RX antenna is consistent (via Maxwell's equations, etc.) with the EM field flowing past it from the source, at least if the antenna is well matched to the coax feed line. Yes there is current flowing in the antenna, which means there is a field, but I think it is consistent onloy with the original EM wavy going by, not generating an additional reflection.

    But I'm not sure... Maybe Averagesupernova or vk6kro or another E&M/antenna person can chime in...
     
  21. Jan 28, 2010 #20

    sophiecentaur

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    I think that resistance is referred to as the radiation resistance - when treating the antenna as a transmitter, which always seems easier to visualise - to me. It represents the resistive component of the impedance that the feeder sees, or the energy lost out into the aether which could be replaced by putting a sphere of resistive material of 377 Ohms - the 'impedance of free space' around the antenna. The other way round is a bit harder because you have to get a current induced in a receiving antenna in the first place, before it works.
    I got round that problem by 'believing in reciprocity'.
     
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