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Monopole crystal radio antenna length

  1. Sep 7, 2010 #1
    Good Morning

    Can anyone help me understand why, apparently the physical length of a monopole end feed wire for a crystal set need to be ½ the wave length of the desired signal.

    The reason I ask is, that I understood that, if you have a variable coil in series with the antenna, the resonate frequency is the combination of the coil and antenna impedance, so you can compensate for any antenna length within the range of the variable coil.

    So what is the problem of a 3 meter antenna with the correct compensating coil?

    Is it due the fact that a crystal radio needs to raise it power from the antenna an a longer antenna can absorb more power?

    A pointer in the direction of some good practical explanation would be great

    Thanks


    David
     
  2. jcsd
  3. Sep 7, 2010 #2

    Born2bwire

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    Yes, you could match the antenna impedance for a desired frequency more or less regardless of the length (Steven Best does a lot of work with small antennas). However, that does not give any indication to efficiency. A matched antenna network matches the complex impedance with that of the antenna. This maximizes the power transferred. But the real part of the impedance is the radiation and conduction losses. Ideally, we would have a very high radiation resistance which would indicate that the power is being radiated. However, if we have a low radiation resistance, then what happens is that the input power gets trapped in the capacitive and inductive properties of the antenna (the near-field and any lumped matching elements you may have added). It appears that the network is matched but most of the energy is unusable.

    A proper resonant antenna tries to maximize the transfer of the inputted energy into propagating waves and this is done by controlling the length of the antenna. You can radiate at non-resonant frequencies, but it will not be as efficient.

    Also, I believe that you want the antenna to be 1/4 the wavelength. With a monopole antenna, it will approximate a 1/2 wavelength dipole antenna. A 1/2 wavelength monopole is a 1 wavelength dipole. This is a resonant mode but it is not very efficient because the current elements that are 180 degrees out of phase will cancel each other out in the far field (some people solve this by using a folded dipole but I believe that you also have problems with the input impedance regardless).
     
  4. Sep 7, 2010 #3
    Is a monopole crystal radio the same as the old fashioned crystal radio or crystal set?If so I think the best aerial(antenna) for these should be as long and high as possible.
     
  5. Sep 7, 2010 #4
    thanks for your reply

    Does this mean that on a receiving antenna the length of the antenna is proportional to the energy absorbed (intuitively I can see why it may be proportional to the energy that could be absorbed, the area has doubled that the radiation is striking , like doubling the area of a photoelectric array)

    Will 2 meter antenna resonating at x hertz transfer half the power into the lumped matching elements connected to the detector of an identically balanced 4 meter one at X hertz

    I guess what I am trying to find out is, is the length of the antenna linearly proportional to the amount of energy available to the detector.

    I suspect that it’s not, and there is an sinusoidal type decay of the power as the antenna length moves away from ¼ wavelength (regardless of any lumped matching elements) . If this is the case I would like to understand the mechanism better.


    thanks

    David
     
  6. Sep 7, 2010 #5
    Morning

    by monopole crystal radio , I just ment a cyrstal radio set with a long single wire as the antenna, using the ground/earth as the second pole.


    I am very restricted as to where I can put the antenna up, and am trying to work out some optimisation before felling any tree's !!

    thanks


    thanks
     
  7. Sep 7, 2010 #6
    I wouldn't fell trees.In fact you can fix the antenna to a tree.The antenna does not need to be enormously long so my advice is to experiment.Back in the day some people used to get good reception by using their bed springs as an antenna.
     
  8. Sep 7, 2010 #7

    Born2bwire

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    Yes and no. If we were to consider an ideal antenna, one that would perfectly absorb all incident radiation, then we see that the power received is directly proportional to the physical area of the antenna (called the physical aperture). That is because the signals from a radio tower look like very very large spherical wavefronts at the receiver. The more area that an antenna occupies then the more area of this wavefront that it can capture. However, the various losses and inefficiencies of real antennas mean that that actual antenna aperture is smaller than its physical. That is, the antenna aperture is the "apparent" size of the antenna and is that amount of area that the antenna effectively captures a theoretical isotropic wavefront.

    This can easily be imagined with a horn antenna which actually has a real physical apeture. Only the sections of the waves that are incident on the aperture are directed to the receiving antenna in the horn.

    But we cannot just willy-nilly make a large antenna because we have to consider how the entire structure operates in practice. For example, as I stated above a 1 wavelength dipole antenna is a poor choice. This is because there are current elements that are excited on the antenna that are 180 degrees out of phase of each other. In the far field, these two current elements cancel each other out. So while this antenna has a larger physical aperture, it has a much smaller antenna apeture than a 1/2 wavelength dipole. In fact, any wire antenna that is 1 wavelength in size or larger is going to have these kinds of problems. This can be solved, in the case of the 1 wavelength dipole for example, by "folding" the antenna. You spread out the wire and at the 1/4 wavelength point on each end of the dipole you fold it back in on itself. Thus, the physical size of the antenna is still 1/2 wavelength but because of the fold the 180 degree current elements are now in phase. But we have reduced the physical aperture and we also end up having problems with providing a good matching network because the antenna wants has a much larger impedance than the 1/2 wavelength dipole. But end we do get a larger radiation resistance.

    So with a simple whip antenna that you are describing, the best you can do is just do a 1/4 wavelength piece of wire. If the length is not feasible (like it would be with radio waves), just use the longest piece of wire that you can. In the end, you will just make what is known as a "small" antenna. It's not very efficient (and for very small ones the actual shape of the antenna does not greatly affect its radiation properties) and the best you can do is provide a proper matching network.

    EDIT:

    Dadspring brings up a rudimentary example of another antenna, a loop antenna, with the bedsprings. The loop antenna has the advantage that you can improve the radiation resistance by winding multiple coils. This is especially good for small antennas because you can wind a number of coils without adversely affecting the performance of the antenna do to the fact that the phase varies slowly over physical length (and thus you avoid the problems of opposing phase elements cancelling each other out in the far field). However, a loop antenna requires a balun to create a balanced feed. If you can use a simple whip antenna, then you do not have a balun as the whip antenna relies on the ground image to create a balanced feed from an originally unbalanced signal. Thus, I do not think you can just make a coil of wire and connect it between your signal output and ground points unless you had a balan to change the original output to a balanced feed.
     
    Last edited: Sep 7, 2010
  9. Sep 8, 2010 #8
    Many Thanks Born2bwire

    That's helped a great deal.

    A quick thought, if a 1/4 wavelength monopole antenna was resonating at its natural frequency (say a one meter antenna receiving a 4meter signal) since the resonate pattern is not symmetrical how do you know what end of the monopole to connect to?

    Thanks

    David
     
  10. Sep 8, 2010 #9
    Afternoon

    I think I may be able to answer this myself now.

    Physically it's a monopole but electrically it a dipole.

    The half that missing is the ground reference plane.

    So a monopole whip antenna has a sort of reflection with the ground plane making a dipole.
    http://en.wikipedia.org/wiki/Dipole_antenna#Quarter-wave_antenna

    Which end to connect to now is perhaps a mute point. (maybe not ?)

    Thanks

    David
     
  11. Sep 8, 2010 #10
  12. Sep 8, 2010 #11

    Born2bwire

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    That is correct. The image currents in the ground plane turn the monopole into a dipole and it offers a balanced signal using an unbalanced configuration. That is, if you took a true dipole and hooked up the signal to one contact and the ground to the other contact then it would be an unbalanced feed and the antenna would not work.
     
  13. Sep 9, 2010 #12
    For a crystal radio receiver on the AM band (550-1600 kHz), it is rarely possible to get an ideal tuned antenna. For example, a new 10 kW AM transmitter antenna is being built in our town, and it is a quarter-wavelength vertical tower, 165 feet high (for 1490 kHz). It would be ~250 feet high for a 1000 kHz center frequency. To get a receiving antenna this high, you will need a) a helium balloon, and b) a building height variance permit. Crystal radios will work on much shorter antennas, and the reactive impedance will be compensated for when you tune the resonant frequency of the LC circuit.

    Bob S
     
  14. Sep 10, 2010 #13
    Afternoon

    Can any shed any insite of on the faulty logic

    The propagated Magnetic field and Electric field are orthogonal and in Phase.

    The magnetic field is associated with the current and the electric field with the voltage

    Yet the voltage and current in the ¼ wavelength monopole are out of phase.

    thanks




    David
     
  15. Sep 10, 2010 #14
    A resonant circuit has both a reactive part (voltage and current 90 degrees out of phase) and a real part (current and voltage in phase). Resonant circuits include LC capacitors & coils (with resistances), antennas, and combinations of these. Only the real part of a complex signal in a resonant circuit includes available signal power, i.e., usable power induced by the propagating electromagnetic wave.

    Bob S
     
  16. Sep 13, 2010 #15
    Thanks Bob

    Does that mean that if you had a theoretically perfect antenna system with only Capacitance and Inductance you could not extract a signal from it?. Or propagate one?

    If you need some Real resistive component, how to you design it in?


    David
     
  17. Sep 13, 2010 #16

    Born2bwire

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    The resistive part is the antenna converting energy into propagating waves. So unless your antenna is completely non-radiating, then it will have a resistive component (in addition to the parasitic losses due to finite conductivity and such).
     
  18. Sep 13, 2010 #17
    Good Morning

    What is the Physics of the transfer mechanism between the energy in the antenna and the propagated wave?

    thanks

    David
     
  19. Sep 13, 2010 #18
    In antenna, electrons are constantly being accelerated and decelerated. If the acceleration is a, then the radiation (watts) per electron is (mks units, power into 4π sterads)*

    dW/dt = e2a2/6πε0c3.

    This radiation represents a real (not reactive) resistive impedance in the antenna. The impedance of free space is 377 ohms, and the radiation resistance of a simple dipole antenna is about 73 ohms.

    *Gaussian units: 2e2a2/3c3

    Bob S
     
  20. Sep 14, 2010 #19

    Born2bwire

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    Any accelerating charge radiates electromagnetic waves. The driven currents on the antenna are such an example of accelerating charges and thus produce electromagnetic waves. However, not all of these waves can propagate. Some of the fields that are produced stay within the local volume around the antenna in what are called near fields. These near fields represent the capacitive and inductive properties of the antenna because they represent stored energy in fields (stored electric field ~ capacitor, stored magnetic field ~ inductor). We can even incorporate elements into the antenna design to add capacitance or inductance to help tune the input impedance.

    To get propagating waves, what we need to do is provide a means of mediating the guided wave that is sent into the antenna into a propagating wave into free-space. In a way, an antenna is an impedance transformer. This can be readily seen physically with a horn antenna. A horn antenna starts out with the signal in a rectangular waveguide (excited by a monopole probe antenna) and thus a guided wave. Then the rectangular waveguide expands out in the horn until it is large enough that we simply terminate the horn. In this way, by slowly expanding out the size of the waveguide we are providing a smooth impedance transition from the impedance of the original waveguide to that of free-space (377 ohms). This is not a perfect transition though and thus some of the energy is reflected back, some gets trapped in the near field and the rest propagates out to the far field. We can pretty much make a similar physical description for most antennas despite the wide variety of forms that they can come in.
     
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