MFJ MFJ-1782 User Manual

MFJ-1782 Super Loop Antenna Instruction Manual

MFJ-1782

CAUTION! Do Not Attempt Operation Of This Unit Before Reading All Instructions

The MFJ Super Hi-Q Loop is the best performing and most convenient small space antenna available
to amateurs today. This antenna is only 36 inches in diameter and features an indoor tuning unit. All
tuning and control voltages are coupled to the antenna through the coaxial feedline for simple, neat,
one wire installation.

The loop antenna element is constructed from thick walled aluminum pipe. Every current carrying
joint is welded to eliminate high resistance pressure contacts that reduce efficiency. The loop element
is tuned with a low-resistance, high current, variable capacitor. The outdoor electrical and mechanical
components are protected by an attractive weather resistant molded cover.

WARNING! Never mount this, or any other antenna near power lines or utility wires!! Any
materials: ladders, ropes, or feedlines, that contact power lines can conduct voltages that kill. Never
trust insulation to protect you. Stay away from all power lines.

THEORY OF OPERATION

When resistive losses in a small loop antenna are kept low, a small loop antenna will transmit nearly as
well as a full size dipole. MFJ was able to make this small loop antenna radiate nearly as well as a full
size dipole by paying special attention to the electrical and mechanical construction of this antenna.

Because radio frequency currents primarily flow near the thin, outer edges of flat conductor loops, flat
conductor loops will have much higher RF losses. To avoid this problem the MFJ Super Hi-Q Loop
uses a thick wall, large diameter, round aluminum pipe for the radiating element. This construction
method results in much better performance since the RF losses in the round, large diameter pipe are
many times lower than the losses in a flat conductor.

MFJ forms the large diameter aluminum pipe into a circle on special machines and heli-arc welds all
joints to eliminate resistive pressure connections in the antenna. A specially constructed butterfly
capacitor using arc-welded construction has much lower loss resistance than conventional, less
expensive, pressure contact, air variable capacitors.

The care and expense used in selecting the best materials, not the most convenient materials, has
resulted in an extremely efficient small size antenna. Extensive "on the air" tests have confirmed that
most stations can detect little difference between the signal from the MFJ Super Hi-Q Loop and the
signal from a full size dipole at the same height.

Super Hi-Q Loop



Antenna

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MFJ-1782 Super Loop Antenna Instruction Manual

PATTERNS, POLARIZATION AND LOCATION

This loop antenna can be mounted to provide either vertical or horizontal polarization. To mount this
antenna for vertical polarization the loop should be mounted standing up. To mount the loop for
horizontal polarization the loop should be mounted so it lays flat. See Figure 1.

Figure 1

The radiation pattern of a small loop antenna is essentially omni-directional with the exception of two
very narrow nulls in the axis of the loop. If you visualize the 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 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.

Figure 2

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MFJ-1782 Super Loop Antenna Instruction Manual

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MFJ-1782 Super Loop Antenna Instruction Manual

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. See Figure 3.

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MFJ-1782 Super Loop Antenna Instruction Manual

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.

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 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 either too far away or is not a
good enough RF reflector to cancel
the horizontal radiation component
of the vertical loop.

This effect can be reduced by laying a screen or grid of wires that extend for at least 20 feet each
direction from the antenna just below the a vertically mounted loop antenna, or mounting the loop
vertically over a large metal roof.

Figure 3

Figure 4

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