Nulling Unwanted Signals; Vertical Polarization - MFJ Super Hi-Q Loop Instruction Manual

MFJ Super Hi-Q Loop Antenna Instruction Manual
Signals will be attenuated more than 10 dB if they arrive within 15 degrees of the axis of the loop.
In general this antenna, like most others, should be mounted as far away from and as high above other
objects as possible. The "null" (or broadside axis) of the loop should be placed in line with the
direction that you do not want to transmit or receive. If the loop can not be mounted more than 20
feet above ground, or above a metal roof, the best mounting arrangement will be with the loop
vertical. If the loop is mounted over 20 feet above the ground (or ground plane) a horizontally
mounted loop will probably be the most satisfactory.

NULLING UNWANTED SIGNALS

The narrow loop nulls can be used to reduce
interference from undesired directions, if the
unwanted signal is coming from a fixed
direction and wave angle. The narrow null
pattern usually makes tilting the loop a
requirement to null sky wave signals. Because
of the constantly changing angle and direction
of sky wave signals the nulls of the loop are
probably most useful for eliminating ground
wave interference, although they can be
effective on some sky wave signals.
Other examples of using the "null" can be
understood if we consider a loop antenna
located on a roof. By mounting the antenna
horizontally above the middle of the roof the
"null" can be positioned directly below the
antenna (through the building). This mounting
arrangement helps receiving by reducing the
noise pick-up from devices in the building and
helps transmitting because energy is not
coupled into the building's "lossy" structure.
RFI in the building will also be reduced
because the signal transmitted into the building
is weaker.
Figure 3

VERTICAL POLARIZATION

When the loop is mounted to provide vertical polarization the pattern is vertically polarized in line with
the loop element. There will still be a large amount of horizontally polarized radiation broadside to the
loop, especially if the ground under the loop is less than perfect or if the loop is mounted some distance
above the ground.
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