Technically a "power density" meter,
the RF Field Strength Meter detects the electric field of radio and
microwaves (RF) from .5 MHz to 3 GHz, and expresses the field strength
as power density (.001 to 2000 microwatts/cm2).
This is an extremely sensitive meter which
can accurately measure RF background even in rural areas far from any
transmitters. The meter reads true power density directly on the
display. Unlike other low-cost field strength meters, this meter¡¦s
frequency response does not depend on the characteristics of an external
antenna; the internal detection system yields a flat response over a
very wide range of frequencies.
Accuracy in the FM, TV and cell tower
frequency range (30 MHz ¡V 2.4 GHz) is +/-25%. Sensitivity is low by 50%
(-3 dB) at the frequency limits .5 MHz and 3 GHz. (Sensitivity is 25% at
5 GHz. That is, you must multiply the reading by 4 when measuring
microwaves at that high a frequency. At 10 GHz, sensitivity is about
A High-Pass selector switch allows you
to measure either the full bandwidth ("Wide" = 0.5 MHz ¡V 3 GHz) or to
apply a high-pass filter ("Narrow" = 6 dB/ octive rolloff with a knee at
100 MHz) that effectively allows only 100 MHz to 3 GHz through. In
practice, this high-pass selector function can be used to estimate one
additional parameter: the average frequency of the RF (if it is in the
range 10 MHz ¡V 500 MHz).
The RF Field Strength Meter is
directional and it detects only the component of the electric field
which has the same polarization as the long axis of the meter. That is,
if only a vertically-polarized RF wave is present, but you turn the
meter in the horizontal direction, it will essentially read zero. If you
subsequently rotate the meter to vertical, it will then read the full
power density of the RF wave. Most RF radiation has only vertical
electric field, so the full strength can be read by holding the meter
vertically. (At the end of this page is more information on how to read
radio waves with other polarizations).
The meter has a 4 ½ digit display which
reads in three ranges: .001 to 19.999, .01 to 199.99, and .1 to 1999.9
microwatts/cm2. For comparison, a low power 100 milliwatt dipole
transmitter (typical 49 MHz cordless phone) produces about .010
microwatts/cm2 at a distance of 50 feet. This is 10x the minimum
sensitivity of the meter. A FAST/SLOW update switch is normally set in
the FAST position so you can quickly measure changes in the RF level.
However, if the field strength is fluctuating rapidly, this switch can
be set to the SLOW position, which averages the reading over several
The "zero" level will shift slightly
with temperature. This shift is no more than +/- .010 microwatts/cm2
over the meter¡¦s operating temperature range of 30¢X F to 110¢X F (-1 to
43¢X C). Two controls on the right side of the meter correct for this: a
button, when pressed, turns off the pre-amplifier, so it is the
equivalent of zero field. Then an offset control is rotated until the
meter reads zero in the most sensitive "19.999" setting. After one
minute or more of warm-up, this should be adjusted. Once adjusted, this
need not be readjusted unless the temperature changes by more than 5¢X F.
(Then a shift of about .001microwatt /cm2 will occur).
The RF Field Strength Meter comes
with a standard 9-volt battery. A low-battery indicator shows on the
display when approximately 10 minutes of battery life remain. Electric
current consumption from the battery is about 15 ma, with low battery
indication at about 7.6 volts.