2 mA LED
25 mA bulb
15 or 20 feet long (for antenna)
(for ground connection)
Or full wave rectifier bridge
1,000 µF or similar
10 megohm input impedance
Or 3 SPST and 3 SPDT switches
Like RadioShack 276-044
Like RadioShack 272-1139
20:1 or similar
A small neon or argon bulb
clocks that are in most people's houses. This is especially true in an urban or suburban area, if there are power
lines up on telephone poles near the house — the amount
of EMF radiation that they generate might surprise you.
At any rate, you can sense such fields with the apparatus
of Figure 1 and judge for yourself.
Inside a typical house or office building, quite a lot of
our environmental EMF is 60 Hz AC or harmonics of it.
Weaker radiation is at the extra-low frequencies generated
by thunderstorms all throughout the world, as readers of
Nuts & Volts already know from the article "DX
Thunderstorms in Africa" in the October, 2003 issue
(page 44). A wide variety of radiation is being suggested
as sources of energy to be harvested, as described in the
publications that any Internet web search engine will turn
up, if you enter "harvest electricity" in the dialog box.
However, more and more RF will be at microwave frequencies, as we make increased use of things like the RFID
tags that Wal-Mart and other big businesses are putting in
place by the millions, and as Wi-Fi gets into more public
places and even into home networks.
of 60 Hz content visible in the received signal. In some
cases, there is much distortion of the basic sine wave, and
there is more harmonic content than fundamental, particularly at 120 Hz, due to various phase delays. There is
almost always a great deal of continuously varying, high
frequency hash riding on top of the steadily repeating signals. Some of this is at extremely high frequencies and consists of digital pulses, even though the simplified antenna
biases the apparatus towards picking up low frequencies.
From observations such as these, it can be seen that
our electronic devices need good shielding against EMFs
these days. Maybe the shields will have to be improved in
the future, as more and more of these invisible fields
become commonplace. In many cases, it is already necessary to use "guards" as well as "shields," for devices
such as op-amps with very high amplification factors.
(The difference between a shield and a guard is explained
in a textbook I wrote, Industrial Electronics for
Engineers, Chemists, and Technicians, William Andrew
Publishing, 2001.) It also seems sensible to re-evaluate the
possible effects on human beings from time to time, as
these fields get more intense, and their frequencies go up.
Accumulating the Energy
A Few Measurements
NUTS & VOLTS
With a 15 foot or longer antenna snaking along the
floor from room to room in a typical suburban house, and
using a grounding rod outside, you are likely to see from
two to four volts of AC (RMS). Surprisingly, this might not
vary much as a high current appliance such when a
clothes dryer is switched on and off. Taking the antenna
outdoors, draped horizontally over some beach chairs
over a backyard lawn, you can expect a lower voltage,
possibly only a few hundred millivolts. Arranging the
antenna vertically up a tree, the measured voltage is not
usually much different, in spite of the fact that many radio
transmissions are vertically polarized. Moving the whole
apparatus to a front yard, near power lines on telephone
poles, the measurements are likely to be more like what is
seen inside a house, possibly a few volts. On the other
hand, in a rural setting, away from houses and power lines,
the voltages are likely to be lower.
Looking at the input with an oscilloscope attached in
place of the AC voltmeter of Figure 1, there might be a lot
With the attachment of a rectifier, DC can be stored
in the 1,000 µF capacitor. The full wave diode bridge in
the figure is efficient, but slower accumulation can be
obtained with just D1, as mentioned above. By switching
S2 (or just using a clip lead), the interrupted DC flow can
be estimated, and typical readings inside a house might
be (surprisingly!) as high as a few milliamperes, although
a tenth of that is more common.
After approximately an hour of charging, the 1,000
µF capacitor voltage can be measured via S3. This will be
the RMS AC voltage times the square root of 2, and four
volts is typical when measured inside a suburban house.
In an electrolytic capacitor, it is enough energy to light a
low current LED, through S4. In fact, a brief flash can be
visible in a low current Tungsten bulb, via S5.
Using S6 or the equivalent, the capacitor's charge
can be suddenly dumped into the primary of a step-up
transformer. (As mentioned above, a power transformer
that is normally step-down can be used "backwards" for
this experiment.) A ratio of 10:1 or 20:1 will usually light
a small neon or other gas discharge bulb for an instant.
On the oscilloscope, this pulse can register as high as 100
volts. Thus, a fairly high voltage can be harvested from
our environment, by very simple means. NV
Dan Shanefield was a retired Bell Labs scientist, then became
a Distinguished Professor at Rutgers University, and is now
retired from that job, as well. You can visit his website at