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.
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