Deriving power from broadcast signals

"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

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.

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

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

"mpm"

** Spot on.

....... Phil

"GregS"

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

Dirty Harry would enjoy dems odds.

...... Phi

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

Why not use photovoltaic cells?

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.

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]

"Spurious Response" "Phil Allison"

** Not "Powercast " ?

Nothing to sell - reeks of being a total scam.

....... Phil

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