RF Antennas
RF signals that are transmitted through the air consist of
electro-magnetic waves that travel (propagate) at the speed of light (300
million metres/second). These electro-magnetic waves are generated by applying
an alternating voltage (e.g. the carrier with the information signal modulated
onto them) to a transmitting antenna.. Antennas are most efficient when the
wavelength (lambda) of the carrier is twice as long as the antenna. For a given
carrier frequency the wavelength can be calculated using the following formula:
lambda = c / fc
where lambda is the wavelength of the carrier, c is the
speed of light, and fc is the carrier frequency. For example, DAB uses carrier
frequencies around 220Mhz, so the wavelength of this carrier is:
lambda = 3 x 108 / 220 x 106 = 1.36 metres
Therefore, a typical DAB aerial will be half this length at
68cm long. The different carrier frequencies in the band used by DAB will all
have slightly different wavelengths so an antenna whose length is approximately
equal to half the wavelength of the centre frequency of the band is the best
choice and 220MHz is around this centre frequency value.
Aerials have the property of reciprocity. This means that
they have the same properties for transmission as they do for reception.
Therefore, if the transmitter consisted of a single antenna then it would be the
same length as a user’s antenna. Transmitting aerials may be a very different
shape though to the normal aerials that users have and they will usually consist
of multiple elements to improve the effective radiated power (ERP). The ERP is a
way of comparing the input power with the actual radiated power. Using arrays of
antennas, the antennas can be positioned so that the electromagnetic radiation
is made to be directional. For example, there is no point in transmitting power
above the transmitter so the array elements are positioned in such a way as to
increase the radiated power in the horizontal direction and to minimize the
power transmitted upwards. This results in an increased power in the horizontal
direction, which is what broadcasters want to achieve. Broadcast transmitters
are usually made to radiate omni-directionally. This means that the power
transmitted at one angle in the horizontal direction will be the same as it is
for all other directions.
Aerials for receiving radio signals such as DAB can either
be omni-directional or directional. An example of an omni-directional aerial is
the half-wave dipole, or just dipole, which is named because of its maximum
efficiency being at half the length of the wavelength of the frequencies it is
meant to receive as mentioned above. The half-wave dipole is the most basic type
of aerial used for radio reception and just consists of what looks from the
outside like a single rod that has a connection at the centre for fixing to the
outside of a house. The aerial is actually two rods that are half the total
length and each are connected to a different wire so that an electric current
can flow when it receives the electromagnetic waves.
Just as for transmitting aerials, multi-element aerials can
be used to increase the power gain of the aerial at the receiver. In this case
though the aerial will be ‘aimed’ in a certain direction to increase the
power gain in that direction. This is the same as for TV aerials which are
pointed at the nearest TV transmitter. A common multi-element aerial is the
3-element Yago aerial. This consists of a half-wave dipole as the centre
(receiving) element and a slightly shorter element in front called the director
and a slightly longer element behind it called the reflector. The addition of
the reflector and director increase the power gain of the Yagi over that of the
single dipole in the direction in which the Yagi aerial is pointing. The spacing
between the reflector, dipole and director is important but will not be
considered here.
More elements can be used than three, as can be seen on any
TV aerial, and the more elements that are used the higher the power gain in the
direction that the aerial is pointing. Another result of using a multi-element
aerial is that the beamwidth narrows with the more elements that you add. For
example, when you have a single element dipole, this receives signals from all
horizontal directions (ignoring signals that come from above or below). Such a
dipole has what is called a radiation pattern which shows the relative signal
strength for different reception angles. For a dipole this radiation pattern is
circular. When you have a 3-element aerial the radiation pattern looks like a a
wide ‘lobe’ in the direction that the aerial is pointing and for example a
value of zero for angles at right angles to the direction that it is pointing
to. When more elements are added the width of this ‘lobe’ reduces but
becomes higher in the direction it is pointing in. This has two effects, as well
as the increased power gain in the direction that the aerial is pointing, the
aerial receives far lower power from signals that do not fall within the region
covered by the aerial’s ‘main lobe’. This has its advantages and
disadvantages. An advantage apart from the high gain in the pointed to direction
is that interference from the angles that don’t fall within the main lobe will
have a far lower power and therefore will not disrupt the signal from the main
transmitter. The disadvantage is that the aerial cannot receive well from the
angles where the radiation pattern is low, so the aerial will not be able to
receive satisfactorily from such transmitters.
| 