Nullling Unwanted Signals; Vertical Polarization - MFJ Box Fan -1780 Instruction Manual

MFJ-1780 Box Fan    Loop Antenna
Instruction Manual
loop as a wheel, the nulls are in the same directions
that the wheel's axle would run. Signals will be
attenuated more than 10 dB if they arrive within 15
degrees of the axis of the loop. See Figure 2.
- In general this antenna, like most others, should be
placed 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 placed more than 15 feet above
ground level the best placement arrangement will
be with the loop placed vertically (sitting on its
edge). Otherwise, (higher than 15 feet) it is
possible to mount the antenna horizontally.
Figure 2

NULLLING UNWANTED SIGNALS:

In the vertical position, the Box Fan loop nulls can be used to reduce interference (if receiving) from
undesired directions if the unwanted signal is coming from a fixed direction and wave angle, or to
minimize interference (if transmitting) caused by the antenna itself to other household electronics
equipment.
Other examples of using the null can be understood if we consider a loop antenna placed horizontally
on a second or higher floor of a building. By placing the antenna horizontally on the middle of the attic
the null can be positioned directly below and the antenna (through the building). This placing
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. See Figure 3.

VERTICAL POLARIZATION:

When the loop is placed 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 placed some distance
above ground.
The broadside horizontal radiation that occurs in a small vertical loop is mostly above 10 degree wave
angles and extends straight above the loop and to the opposite 10 degree elevation point. True vertical
polarization occurs only in line with the loop. As you move around the loop towards the sides, the
pattern skews and eventually becomes completely horizontal broadside to the loop. See Figure 4.
Unlike linear verticals, vertically polarized small loops also radiate straight up and down from the
antenna. This high angle of radiation can be used to cover short distances by sky wave. The high angle
horizontal radiation in a vertically polarized loop antenna occurs because the ground below the loop is
4