Loop antenna

- an overview, summary, tutorial about the basics of the loop antenna, a form of RF antenna design that can be used in a varety of applications.

Loop antennas, or more correctly, closed loop antennas are widely used in many applications, often providing advantages over other types of RF antenna design. Loop antennas can be placed into two categories:

  • Small loop antennas
  • Large loop antennas

The terms refer to the size of the Rf antenna when compared to a wavelength of the frequency in use.

Small loop antennas

Small loop antennas can be likened to coils, as they have the same current distribution as ordinary 'circuit' coils, having the same phase and amplitude through the whole coil. To achieve this the total length of the conductor used in the loop antenna design must be no more than about 0.1 wavelengths long. Any longer than this and the current phase and amplitude will start to vary over the length of the conductor and some of the properties start to change.

Small loop antennas may also be split into those that us a single turn, and those that have a multi-turn loop, as in the case of a coil. One common form of multi-turn small loop antenna is the popular ferrite rod antenna that is used in many domestic portable radios and is also starting to be used in applications such as RFID devices. Another form of this antenna was the frame antenna or aerial found in many domestic radio sets of the 1940s and 1950s. Here a multi-turn coil about 30 centimetres or more square was built into the set to act as the antenna.

Multi-turn loop antennas are nor normally used for transmitting because the losses are high and the level of heat dissipated can give rise to rapid temperature increases. Instead single turn loop antennas may be used if a loop antenna is needed. These antennas have a number of advantages and disadvantages.

The main advantages of loop antennas are their size and directivity. Often a single turn small loop antenna is much smaller than a wavelength by its definition. They are also quite directive, and this can be used to direct the radiated power in the required direction. Both these advantages can be very useful in many applications. They find uses for transmitting and receiving, particularly on the MF and HF or short wave bands. Here they provide very compact antennas for applications such as amateur radio and shipping, etc. as well as receiving antennas for MF or medium wave receivers.

There are naturally disadvantages to this type of RF antenna design. The first is that the Rf antenna can have a very low radiation resistance, and this results in very high levels of current flowing in the RF antenna. In turn this means that even small levels of 'DC' resistance can result in significant levels of power being lost as heat. It is for this reason that single turn small loop antennas are made of very thick wire, or more often made of a tubular conductor. Additionally this means that they must have an effective form of antenna matching if the energy is to be efficiently transferred to and from the RF ntenna.

A further disadvantage of this type of RF antenna design is that it can have a very high Q. Not only does the RF antenna require tuning to bring it to resonance at the frequency of operation, but it may have such a narrow bandwidth, on frequencies such as the medium waveband or even a little higher, that it may be insufficient to accommodate the carrier and its sidebands.

Large loop antennas

Large loops tend not to be quite as widely used in many applications, although in some areas they may be popular. Their size can mean that they are only used in limited applications.

One popular form of loop for HF applications is a full wave loop. This consists of a full wavelength loop of wire which is fed at a break in the loop. This type of loop has a much higher radiation resistance and as a result the losses are very much lower, making it a far more efficient antenna, although one that is much larger.

By Ian Poole

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