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Deriving power from broadcast signals

Discussion in 'Electronic Design' started by George, Jun 21, 2007.

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  1. George

    George Guest

    I'm planning to develop a simple radio receiver module that must be small
    and have long battery life.

    It won't be required to produce audio output, and won't have user
    indicators, so I estimate power consumption will be well under one mW. It
    will have a small (say one foot long) antenna and will be used in cities
    where strong broadcast signals exist - say, 30 HF/VHF/UHF stations each with
    50 kW radiated power within a ten-mile radius.

    So the question is: Is it practical to use rectified ambient RF energy as a
    supplemental power source in order to extend battery life?

    Any pointers on where to look for additional info will be appreciated.

    George
     
  2. Phil Allison

    Phil Allison Guest

    "George"
    ** Forget it.

    The power received by a small antenna is only a microwatt or two - plus
    there is not practical way to convert it into a DC supply.


    ........ Phil
     
  3. Robert Baer

    Robert Baer Guest

    Best bet is to get power from AM stations; a simple crystal set for
    each station would do if you need a fair amount of power, but in (and
    near) large cities, tune to the most powerful.
     
  4. GregS

    GregS Guest

    I have a passive uA meter hooked up through diodes. It has a high indication
    with the tower across the street. I suggest you build the device and measure.

    greg
     
  5. mpm

    mpm Guest

    ANSI C95.1 Recommended Practice for RF Safety document has the
    equations for calculating RF power in the far field of antennas
    (300kHz to 3 GHz).

    Unless you are very, very close to the transmitting antenna (inches to
    maybe a foot or two), have efficient coupling, and the antenna itself
    is very high power, I seriously doubt you will convert and store
    enough energy to make the effort worthwhile. If you're just driving
    around town, the power will likely be in the microwatt range across
    the various bands.

    -mpm
     
  6. Phil Allison

    Phil Allison Guest

    "mpm"
    ** Spot on.



    ........ Phil
     
  7. Phil Allison

    Phil Allison Guest

    "GregS"

    ** Wot - AM band at 100 yards picked up by 30 yards of wire ?

    Dirty Harry would enjoy dems odds.




    ....... Phi
     
  8. MooseFET

    MooseFET Guest

    The answer is yes but not with a one foot whip.

    Why not use photovoltaic cells?
     
  9. However, there are devices being made and sold right now, that charge
    batteries, and other items (huge caps in a device) with RF (and
    associated circuitry, of course), while in one's home proximity area.
     
  10. Fred Bloggs

    Fred Bloggs Guest

    Sec. 73.184 Groundwave field strength graphs.



    (a) Graphs 1 to 20 show, for each of 20 frequencies, the computed

    values of groundwave field strength as a function of groundwave

    conductivity and distance from the source of radiation. The groundwave

    field strength is considered to be that part of the vertical component

    of the electric field which has not been reflected from the ionosphere

    nor from the troposphere. These 20 families of curves are plotted on

    log-log graph paper and each is to be used for the range of frequencies

    shown thereon. Computations are based on a dielectric constant of the

    ground (referred to air as unity) equal to 15 for land and 80 for sea

    water and for the ground conductivities (expressed in mS/m) given on the

    curves. The curves show the variation of the groundwave field strength

    with distance to be expected for transmission from a vertical



    [[Page 55]]



    antenna at the surface of a uniformly conducting spherical earth with

    the groundwave constants shown on the curves. The curves are for an

    antenna power of such efficiency and current distribution that the

    inverse distance (unattenuated) field is 100 mV/m at 1 kilometer. The

    curves are valid for distances that are large compared to the dimensions

    of the antenna for other than short vertical antennas.

    (b) The inverse distance field (100 mV/m divided by the distance in

    kilometers) corresponds to the groundwave field intensity to be expected

    from an antenna with the same radiation efficiency when it is located

    over a perfectly conducting earth. To determine the value of the

    groundwave field intensity corresponding to a value of inverse distance

    field other than 100 mV/m at 1 kilometer, multiply the field strength as

    given on these graphs by the desired value of inverse distance field at

    1 kilometer divided by 100; for example, to determine the groundwave

    field strength for a station with an inverse distance field of 2700 mV/m

    at 1 kilometer, simply multiply the values given on the charts by 27.

    The value of the inverse distance field to be used for a particular

    antenna depends upon the power input to the antenna, the nature of the

    ground in the neighborhood of the antenna, and the geometry of the

    antenna. For methods of calculating the interrelations between these

    variables and the inverse distance field, see ``The Propagation of Radio

    Waves Over the Surface of the Earth and in the Upper Atmosphere,'' Part

    II, by Mr. K.A. Norton, Proc. I.R.E., Vol. 25, September 1937, pp. 1203-

    1237.



    Note: The computed values of field strength versus distance used to

    plot Graphs 1 to 20 are available in tabular form. For information on

    obtaining copies of these tabulations call or write the Consumer Affairs

    Office, Federal Communications Commission, Washington, DC 20554, (202)

    632-7000.



    (c) Provided the value of the dielectric constant is near 15, the

    ground conductivity curves of Graphs 1 to 20 may be compared with actual

    field strength measurement data to determine the appropriate values of

    the ground conductivity and the inverse distance field strength at 1

    kilometer. This is accomplished by plotting the measured field strengths

    on transparent log-log graph paper similar to that used for Graphs 1 to

    20 and superimposing the plotted graph over the Graph corresponding to

    the frequency of the station measured. The plotted graph is then shifted

    vertically until the plotted measurement data is best aligned with one

    of the conductivity curves on the Graph; the intersection of the inverse

    distance line on the Graph with the 1 kilometer abscissa on the plotted

    graph determines the inverse distance field strength at 1 kilometer. For

    other values of dielectric constant, the following procedure may be used

    to determine the dielectric constant of the ground, the ground

    conductivity and the inverse distance field strength at 1 kilometer.

    Graph 21 gives the relative values of groundwave field strength over a

    plane earth as a function of the numerical distance p and phase angle b.

    On graph paper with coordinates similar to those of Graph 21, plot the

    measured values of field strength as ordinates versus the corresponding

    distances from the antenna in kilometers as abscissae. The data should

    be plotted only for distances greater than one wavelength (or, when this

    is greater, five times the vertical height of the antenna in the case of

    a nondirectional antenna or 10 times the spacing between the elements of

    a directional antenna) and for distances less than

    80f\1\/\3\MHz kilometers (i.e., 80 kilometers at 1 MHz).

    Then, using a light box, place the plotted graph over Graph 21 and shift

    the plotted graph vertically and horizontally (making sure that the

    vertical lines on both sheets are parallel) until the best fit with the

    data is obtained with one of the curves on Graph 21. When the two sheets

    are properly lined up, the value of the field strength corresponding to

    the intersection of the inverse distance line of Graph 21 with the 1

    kilometer abscissa on the data sheet is the inverse distance field

    strength at 1 kilometer, and the values of the numerical distance at 1

    kilometer, p1, and of b are also determined. Knowing the

    values of b and p1 (the numerical distance at one kilometer),

    we may substitute in the following approximate values of the ground

    conductivity and dielectric constant.



    [[Page 56]]



    [GRAPHIC] [TIFF OMITTED] TC13NO91.018





    (R/[lambda])1= Number of wavelengths in 1 kilometer,



    * * * * *



    fMHz=frequency expressed in megahertz,

    [GRAPHIC] [TIFF OMITTED] TC13NO91.019





    [egr]=dielectric constant on the ground referred to air as unity.

    First solve for [chi] by substituting the known values of

    p1, (R/[lambda])1, and cos b in equation (1).

    Equation (2) may then be solved for [delta] and equation (3) for [egr].

    At distances greater than 80/f1/3 MHz kilometers the curves

    of Graph 21 do not give the correct relative values of field strength

    since the curvature of the earth weakens the field more rapidly than

    these plane earth curves would indicate. Thus, no attempt should be made

    to fit experimental data to these curves at the larger distances.



    Note: For other values of dielectric constant, use can be made of

    the computer program which was employed by the FCC in generating the

    curves in Graphs 1 to 20. For information on obtaining a printout of

    this program, call or write the Consumer Affairs Office, Federal

    Communications Commission, Washington, DC 200554, (202) 632-7000.



    (d) At sufficiently short distances (less than 55 kilometers at AM

    broadcast frequencies), such that the curvature of the earth does not

    introduce an additional attenuation of the waves, the curves of Graph 21

    may be used to determine the groundwave field strength of transmitting

    and receiving antennas at the surface of the earth for any radiated

    power, frequency, or set of ground constants. First, trace the straight

    inverse distance line corresponding to the power radiated on transparent

    log-log graph paper similar to that of Graph 21, labelling the ordinates

    of the chart in terms of field strength, and the abscissae in terms of

    distance. Next, using the formulas given on Graph 21, calculate the

    value of the numerical distance, p, at 1 kilometer, and the value of b.

    Then superimpose the log-log graph paper over Graph 21, shifting it

    vertically until both inverse distance lines coincide and shifting it

    horizontally until the numerical distance at 1 kilometer on Graph 21

    coincides with 1 kilometer on the log-log graph paper. The curve of

    Graph 21 corresponding to the calculated value of b is then traced on

    the log-log graph paper giving the field strength versus distance in

    kilometers.

    (e) This paragraph consists of the following Graphs 1 to 20 and 21.



    Note: The referenced graphs are not published in the CFR, nor will

    they be included in the Commission's automated rules system. For

    information on obtaining copies of the graphs call or write the Consumer

    Affairs Office, Federal Communications Commission, Washington, DC 20554,

    Telephone: (202) 632-7000.



    [28 FR 13574, Dec. 14, 1963, as amended at 50 FR 18823, May 2, 1985; 51

    FR 45891, Dec. 23, 1986; 52 FR 36878, Oct. 1, 1987; 56 FR 64866, Dec.

    12, 1991; 57 FR 43290, Sept. 18, 1992]
     
  11. Phil Allison

    Phil Allison Guest

    "Spurious Response"
    "Phil Allison"

    ** Not "Powercast " ?

    Nothing to sell - reeks of being a total scam.



    ........ Phil
     
  12. mpm

    mpm Guest

    snip: Excerpt from FCC Rules CRF Title 47, Part 73.

    You could use this information, but it assumes a perfectly conducting
    earth and rounded obstacles. Ken Bullington's method would be another
    similar approach.

    A better alternative would be the Longley-Rice equations.
    You'll need a copy of (the very hard to find) original Tech Note 101
    from NIST, or a good primer on the method. Or, you can use any of
    several dozen commercially available radio prograpation prediction
    software models.

    Also, you could work the Bioelectromagnetics angle of this. ICNIRP,
    C95.1, Canada Safety Code 6, etc..., but they're all going to lead you
    to the same conclusion...

    -mpm
     
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