Antenna Concepts
Gain
This is a measure of the Directivity or Directionality of the antenna.
The higher the gain, the narrower its beam. The vast majority of
antennas have gain in the region 0 to 50 dBi (decibels
with respect to isotropic). An antenna having a gain of 0 dBi (or
unity ratio) radiates equally in all directions and is called Isotropic.
An antenna with a gain of 50 dBi (or a ratio of 100,000) has a very
narrow beam, of the order of half a degree. The gain (as a ratio)
is inversely proportional to the beamwidth.
Beamwidth
This is a measure of the Directivity or Directionality of the antenna
and is usually measured at the –3 dB or half-power points.
The tenth-power or –10 dBi point is also useful for assessing
how well a horn feed illuminates a reflector.
Horn Antenna
A horn is possibly the simplest type of microwave antenna, acting
rather like a funnel. However, the larger the horn aperture, the
narrower the beamwidth and the higher the gain. The beamwidth is
inversely proportional to the gain (as a ratio). The slant length
of a horn is longer than the axial length, and this gives rise to
an aperture phase error which degrades the beam shape and reduces
the gain. It is thus important to minimise the phase error by making
the horn as long as practicable. As the aperture gets wider, the
length must increase as the square of the increase in aperture width,
in order to maintain a given aperture phase error. In most cases,
horns cannot have gain much higher than 20 dBi, or the horn becomes
impracticably long (except in the case of millimetric horns, which
are quite small). A simple horn will generally have –13 dB
sidelobes in the E-plane. These can be reduced by adding chokes
around the aperture, or creating extra modes in the horn, or by
corrugating the walls of the horn.
Reflector Antenna
The reflector antenna takes over where the horn leaves off, at gains
of about 20 dBi. A small horn feeds the reflector, and the gain/beamwidth
is determined by the width of the reflector aperture. A reflector
antenna generally has lower sidelobes than a horn antenna, because
the radiation distribution on the reflector is tapered towards the
edge of the reflector, rather than being cut off abruptly. A reflector
with a long focal length has a relatively flat surface and a narrower
subtended angle, requiring a relatively large feed horn and consequently
higher aperture blockage. A reflector with a short focal length
is relatively steeply curved, and has a larger subtended angle,
requiring a relatively small feed horn, but it can be difficult
to achieve very wide beamwidths from feed horns.
Space Attenuation
Because reflectors are paraboloidal and not spherical, the radiation
from the feed spreads out more towards the edges of the reflector,
resulting in lower power density and a more tapered distribution.
This edge illumination is a function of the feed beamwidth, and
the space attenuation, itself a function of the focal length to
diameter ratio (f/D). The greater the taper, the lower the sidelobes
and the lower the gain. A flattish reflector (long focal length)
has very low space attenuation. Thus for a given reflector edge
illumination (e.g. –10 dB gives a good compromise between
high gain and low sidelobes), the feed horn’s –10 dB
beamwidth has to match the reflector’s subtended angle. Any
feed radiation below –10 dB is wasted as spillover.
On the other hand, a steeply curved reflector has high space attenuation,
e.g. 3 dB. Thus for the same –10 dB reflector edge illumination,
the feed horn’s –7 dB beamwidth has to match the reflector’s
subtended angle. Thus now any feed radiation below –7 dB is
wasted as spillover, and this is less efficient.
Spillover
This appears as sidelobes in the rear hemisphere of the antenna’s
radiation pattern, but does not acquire any gain from the reflector,
so is relatively low with respect to the antenna beam nose.
Far Field Range
This is the range at which the antenna behaves more or less properly, according to an arbitrary but reasonable level of phase error. The formula for this range is:
2D2/ λ
where D is the diameter or maximum width of the antenna aperture, and λ is the
wavelength. Most antennas will operate quite well at half this range,
and even at a quarter with some loss of performance, but below that
serious distortion can occur.
Field Strength
For EMC applications, it is often necessary to produce a given field
strength at a given distance (usually one metre). Field strength
is a function only of power, gain and range, and is independent
of frequency. However, the gain depends on frequency at distances
less than the far field range. The formula for this range is:
2D2/ λ
(see above)
of which D / λ is proportional to the gain. Thus for a given gain (necessary to produce a given field from a given power at a given range), the far field range is proportional to D. Thus for that given gain, the lower the frequency, the bigger is D, and the longer is the far field range. So there is a low frequency limit, below which the antenna will not develop the required gain at the required range. The only solution is to increase the power – increasing the gain will make matters worse!
