3
Not Your Father’s Loop Antenna
Based on the work of Dr. Carl Baum for the U.S. Air Force his “Moebius Strip Shielded Magnetic
Loop Antenna” architecture outperforms much larger antennas. Dr. Baum was a Senior Scientist at
the U.S. Air Force Research Laboratory and is the recipient of several awards from the IEEE
(Institute of Electrical and Electronic Engineers) for his work.
Developed for a classified U.S. Air Force project involving the measurement of EMP (Electro
Magnetic Pulse) from nuclear weapons, this loop design has wide application to antennas for low-
noise, interference-free radio reception over a wide frequency range. The designer of this magnetic
loop antenna version coupled this antenna to a custom made low-noise preamplifier originally
developed by Clifton Laboratories.
Magnetic Loop Antenna Advantages
Most active antennas are the whip type and respond mainly to the electrostatic-field portion of an
electro-magnetic radio wave. This Magnetic Loop responds primarily to the magnetic-field and this
ensures high rejection of nearby
electric-fields. The intensity of the
electric field is usually higher than
the magnetic-field when an antenna
is close to interference sources such
as TVs, florescent lamps, power line
wiring, etc. By rejecting the electric-
field there is a reduction in local
interference compared to other types
of active and passive antennas.
Interference reduction is further
enhanced by the deep nulls of the
antenna’s 'Figure-Eight' directivity
pattern that can be used to null out or
reduce interference coming from a
specific local or distant direction. The electrical field is often much stronger than the magnetic field.
When lined up properly the loop will enable better copy for weak signals which could not be copied
by other antennas.
Intermodulation
Some active antennas generate intermodulation products which can appear as spurious signals
interfering with reception. This interference or second and third order intermodulation is caused by
non linearity in the preamplifier, producing signals which are usually the sum and difference of
strong stations and their harmonics. The RF-PRO-1B
®
Loop (
DXE-RF-PRO-1B)
has been
specifically designed to reduce intermodulation products to a minimum. The third order intercept
point is typ48 dBm (OIP3) and the second order intercept point is greater than +100 dBm
(OIP2 typically 110 dBm). The 1 dB compression point of this preamplifier is +27 dBm making the
levels of the intermodulation products generally far below the atmospheric and man- made noise.
