Magnetic Loop Antennas

Magnetic loop antennas (MLAs) are a type of antenna that is often used for shortwave listening and transmitting. 

They are large loops of copper or aluminum tube that are tuned with a variable capacitor. 

MLAs have a number of special properties:

• High directivity: MLAs have very high directivity, which means that they can focus their radiated signal in a very narrow beam. This makes them ideal for long-distance communication and for directional finding.

• Low noise: MLAs are very quiet antennas, which means that they pick up very little noise from the surrounding environment. This makes them ideal for reception of weak signals.

• Turning Range: MLAs can be tuned to cover a wide range of frequencies, making them very versatile antennas.

• Narrow Bandwidth: As high Q resonant circuits they can help improve receiver selectivity,

Design Factors

The most important design factors for MLAs are:

• Loop size: The size of the loop determines the antenna's frequency response and directivity. Larger loops have lower frequencies and higher directivity.

• Conductor resistance: The resistance of the conductor used in the loop affects the antenna's efficiency and bandwidth. Conductors with lower resistance are more efficient and have lower bandwidths.

• Radiation resistance: The radiation resistance of the antenna is the resistance that the antenna presents to free space which dissipates energy as electromagnetic waves.  The efficiency of the antenna is related to the ratio of radiation resistance to conductor resistance.

• Variable capacitor: The variable capacitor is used to tune the antenna to the desired frequency. It should be large enough to handle the power of the transmitter.

The magnetic field produced by a single turn loop 

Applying DC to a loop produces a north pole on one side and a south pole on the other.

The conductivity of metals at DC and Radio Frequencies (RF)

The normal flow of current in a conductor is greatly affected by magnetic fields a high frequency.

Resulting in the Skin Effect and Proximity Effect.

The skin effect causes the current to be concentrated near the surface of the conductor, rather than being evenly distributed throughout the conductor. This is because the high-frequency magnetic field of the current induces eddy currents in the conductor, which oppose the original current.

Radiation Resistance

Radiation resistance expresses the dissipation of energy supplied to an antenna in the form of radio waves. The radiation resistance of a magnetic loop antenna is very low. This means the other resistance losses from the limited conductivity of metal, the skin effect, the proximity effect and connection losses have to be very low for the loop to work.

The radiation resistance of a single turn loop antenna is proportional to the square of the area of the loop and the frequency of the radiation.

Inductance of single turn

Approximate values.

Loop diameter 0.5m: Inductance: 0.683 uH, radiation resistance: 0.006 ohms

Loop diameter 1m: inductance 1.7uH, radiation resistance : 0.083 ohms

Resonance

Since the loop is inductive and you need very high currents to transmit from the loop you can resonate the loop with capacitor for operation at a particular frequency.  This leads to resonant build-up of current and voltage over time. The power transmitted as radio waves is I²Rrad as per Ohm's Law. Where Rrad is the radiation resistance. The wasted power from the conductor losses is I²R, where R is the conductor losses including the impact of the skin effect and proximity effect.

Matching

There are a number of ways of matching a magnetic loop antenna to a transmitter, some example are:

A high Q factor in a magnetic loop antenna has several advantages and disadvantages.

Advantages:

• Narrower bandwidth: A high Q antenna has a narrower bandwidth, which means that it is more selective in the frequencies it receives. This can be a benefit in situations where there is a lot of interference, as it can help to reduce the amount of noise that is received.

• Higher selectivity: A high Q antenna is also more selective in the direction from which it receives signals. This can be a benefit in situations where it is important to point the antenna in a specific direction.

• Higher gain: A high Q antenna can have a higher gain than a low Q antenna. This means that it can receive weaker signals more effectively.

Disadvantages:

• Lower bandwidth: A high Q antenna has a narrower bandwidth, which means that it is less tolerant of changes in frequency. This can be a problem in situations where the frequency of the signal being received is changing rapidly.

• Higher selectivity: A high Q antenna is also more selective in the direction from which it receives signals. This can be a problem in situations where it is important to receive signals from a wide range of directions.

• Sensitivity to the environment and hand capacitance. Materials (particularly iron) nearby can reduce the efficiency of the antenna. Likewise current can be induced into the ground causing losses. If tuning a receive antenna by hand stray capacitance can make tuning difficult.

• High voltages and currents make the antenna dangerous to be near when used for transmitting. 

Practical tips for a magnetic loop antenna.

• Use large diameter copper or aluminum tube to construct the antenna. Alternatively use copper or aluminum flashing or copper foil tape.

• Make all connections very low resistance.

• Use a tuning capacitor with very low internal resistance.

• Keep all ferromagnetic (iron) material away from the antenna and high loss dielectric material (eg. PVC) away from the high voltage (tuning capacitor) region of the antenna.

• Don't mount the antenna too near the ground.