MFJ MFJ-1786, MFJ-1786X User Manual

MFJ Super Hi-Q Loop Antenna

Instruction Manual

MFJ Super Hi-Q Loop Antenna

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. The MFJ-1786 covers 10 MHz to 30 MHz. The MFJ-1788 covers 7 MHz to 21 MHz. The antenna is only 36 inches in diameter and features an indoor semi-automatic tuning unit with a built-in cross needle wattmeter. 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.

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MFJ Super Hi-Q 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.

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.

Figure 2

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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.

VERTICAL POLARIZATION

Figure 3

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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MFJ Super Hi-Q Loop™ Antenna

Instruction Manual

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.

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.

Figure 4

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 a vertically mounted loop antenna, or mounting the loop vertically over a large metal roof.

HORIZONTAL POLARIZATION

Horizontal mounting of the loop antenna results in an omni-direction, horizontally polarized pattern that has a null straight up in the air and straight below the center of the antenna. This means that any ground reflection will tend to cancel the radiation along the horizon, and also at low wave angles unless the loop is mounted some distance above ground. Mounting a horizontal SMALL loop antenna just above a good reflecting ground will practically guarantee no useful signal in any direction!

Note: Do not expect the best results if you mount this loop antenna horizontally if it is less than 20 feet above a metal roof or other ground plane!

Since ground wave signals only propagate well along the earth when they are vertically polarized, a horizontally polarized loop may not respond to some local noise sources. Like all other antennas, a small loop is generally a quieter receiving antenna when horizontally polarized. This also means that a horizontally polarized loop is not a good choice for local ground wave communications. However, it is an excellent choice for medium to long distance sky wave communications.

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MFJ Super Hi-Q Loop Antenna

Instruction Manual

OUTDOOR LOOP INSTALLATION

The MFJ "Super HI-Q Loop" has two mounting clamps that are held in place by four 1/4-20 7/16" hex head bolts. The mounting clamps accept masts up to 1-1/2" outside diameter.

Before mounting the antenna read the section on "PATTERNS, POLARIZATION AND LOCATION".

The following rules must be followed when mounting the antenna:

1.) The black housing on the loop element has drain holes for moisture. Never seal the drain holes.

2.) The coax connector must always be at the bottom of the antenna when the antenna is mounted vertically or pointed towards the ground when the antenna is mounted horizontally.

3.) If the loop is mounted vertically do not extend the mast more than two inches beyond the mounting clamps or the tuning and operation of the loop may be compromised.

4.) If the loop is mounted horizontally the horizontal bracket must be installed. Use four 1/4-20 1/2 inch machine screws at the base of the bracket. Secure the mounting strap around the antenna with 2, #6 3/8 inch on each side. The mast saddles can now be attached to the side of the mounting bracket. Refer to Figure 1.

5.) Always support the coax by securing it to the mast just below the mounting clamps. Never let the weight of the coax hang on the connector. Some improvement in lightning protection of the loop electronics can be achieved by coiling and taping the coax into a two turn, one foot diameter, loop between the antenna and the mast. Be sure this lightning retarding loop is below the plane of the antenna and at right angles to it.

6.) If you use "coax seal" use it sparingly. Seal only the area near the top of the loop's female connector and the knurled area of the male coax connector. Sealing the bottom gap in the male coax fittings outer shell, or the gap between the black housing and the connector on the loop, prevents condensation from draining out of the connectors and the loop housing.

Note: The best material used to seal connectors is either GE Dielectric Silicone Compound or a clear silicone heat sink compound. Use sealing compounds SPARINGLY on the threads of coax connectors. This is the same type of waterproofing material used by CATV and commercial antenna installers with good success.

7.) The mast or supporting structure should be galvanized steel or thick walled aluminum mast and at least 1-1/4" OD. Do not use thin wall aluminum tubing or standard steel electrical conduit to support the loop. The mast should be supported or guyed as close to the mounting point of the loop as possible and Never more than five feet below the loop. Non-conductive guy lines or guy lines with insulators installed to break the guy into 13 feet or shorter lengths should be used. This is especially important within 20 feet of the loop.

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