Professional Communications Antennas, Filters and RF Combiners
Design, Development and Production
1. What is an antenna?
2. Antenna Types
2.1. Base Station Antennas
2.2. Mobile Antennas
3. How ”good” is the antenna?
4. Measurements of antennas
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Professional Communications Antennas, Filters and RF Combiners
Design, Development and Production
1. What is an antenna?
An antenna is a transducer
An antenna is a transducer, i.e. a ”converter” between
two media.
The antenna converts electromagnetic energy in a cable into
electromagnetic radiated power in free space. Various
characteristics of the antenna determine ”how efficient” this
conversion is performed.
P radiated
P cable
1. To have as much energy as possible carried on as
radiation in the room is of great importance, i.e.
the antenna should not be capable of consuming
energy itself.
2. It is important that the antenna concentrates its
radiated energy as efficiently as possible in the
required way. If so, either a wide coverage range or
a reduced transmitting power can be achieved, until
the range is exactly suiting the one required.
Omnidirectional Antenna
The antenna converts electromagnetic
energy in a cable into electromagnetic
radiated power in free space
Directional Antenna
Consequently, antenna technology is simply a question of economizing on the energy.
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Professional Communications Antennas, Filters and RF Combiners
Design, Development and Production
2. Antenna Types
Communications
antenna types can
be distinguished
between:
2.1. Base Station Antennas
2.2. Antennas for movable
(”mobile”) units
Base Station Antenna
”Mobile” antennas can be split up into:
1. Land Mobile Antennas
(vehicle antennas)
Land Mobile Antenna
2. Marine Antennas
(for ships)
Marine Antenna
3. Portable Antennas
(antennas for portable
communication equipment)
Portable Antenna
(4. Air Craft Antennas)
Portable Antenna
Air Craft Antenna
Colloquially, the word ”mobile antenna” is usually used as designation for a vehicle
antenna.
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Professional Communications Antennas, Filters and RF Combiners
Design, Development and Production
2.1. Base Station Antennas
Base station antennas can be omnidirectional, directional or have a radiation being specially
constructed, the latter yielding the antenna a quite specific coverage range.
Omnidirectional
LinkAntennas
Connection (Directional)
Omnidirectional
Omnidirectional antennas are mostly applied for communication with mobile units when the positioning is
unpredictable in relation to the base station antenna.
Directional antennas
Directional antennas are applied at areas or in a limited area where the receiver of the transmitted signal is.
Antennas with special custom-made directional characteristics
Antennas with special custom-made directional characteristics are applied for coverage of an accurately
limited area, possibly an unsymmetrical area, usually with the purpose of not interfering with or being
interfered by other radio systems.
Delimited coverage
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2.2. Antennas for Mobile Units
2.2.1. Land Mobile Antennas
Quarter-wave Antennas
The most common vehicle antennas utilize the
car roof as one half of the antenna system.
The antenna is a so-called ”monopoly on
ground plane”.
The basic type is a so-called quarter-wave
(¼ λ) antenna.
The fact that the antenna being close to 50 Ω is
utilized when the whip has a length of approx.
1/4 wave length at the operating frequency. By
lengthening the whip, the antenna gain will be
increased. The gain will, however, again be
reduced if the whip length is above 5/8 wave
length.
By lengthening the whip, the
antenna gain will be increased.
Many antennas utilize
the car roof as one half of the
antenna system.
Colinear Antennas
In case higher gain should be achieved, so-called ”colinear” antennas should be applied,
for which more radiating elements are stacked and operate together, thus concentrating the
radiation in the horizon. The optimum mounting site is in the centre of the car roof, at
which the optimum omnidirectional characteristics are achieved.
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2.2.2. Marine Antennas
Marine antennas are in a way very similar to omnidirectional base station antennas except
for the fact that usually intensified requirements as to mechanical sturdiness and
corrosionresistance are made.
Maritime communication is mainly taking place on medium wave, short wave and the
maritime VHF bands.
HF Wireantenna
HF antenna
HF antenna
Marine antennas are similar to omnidirectional base station antennas
but with intensified requirements of sturdines.
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2.2.3. Portable Antennas
Quarter-wave Antennas
Like mobile antennas portable antennas are mostly ¼ wave ”monopoly” antennas, which
utilize the chassis of the portable radio as ground plane, i.e. as one half of the antenna
system. This usually results in insufficient ground plane as well as the fact that the
radiation from the portable radios is mostly very badly defined owing to the presence of the
hand or body, and in general a low
efficiency for portable antenna systems
should be taken into consideration, both
because of disadjustment loss and loss
owing to ”overturned” radiation.
Portable antennas are mostly
¼ wave ”monopoly” antennas.
An improvement of 5 dB can be achieved
Half-wave Antennas
by applying a ½ wave antenna together with
If, however, antenna whips with a
an adaption circuit.
½ wave length are applied together with
an adaptation circuit the antenna being
independent of the cabinet can be achieved. The antenna functions ”in itself” and a considerable
improvement of 5 dB as average value can be achieved (compared to a ¼ wave antenna on the
same device).
2.2.4. Air Craft Antennas
Formerly, air craft antennas were always placed out of the fuselage, but nowadays the
technology being necessary for building-in the antennas as an integrated part of the
fuselage is mastered, thus still obtaining a satisfactory radiation characteristic.
Nowadays air craft antennas are an integrated part of the fuselage.
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3. How to describe how good an antenna is?
Four ”figures” describe how good an antenna is compared to the required performance:
SWR = Standing wave ratio
D = Directivity
G = Gain
BW = Bandwidth
(We will not deal with the term ”polarisation” in this respect).
3.1. SWR
If the impedance of the antenna is different from the impedance of the cable, the antenna will reflect
back some of the induced energy through the feeder cable to the transmitter, which naturally is
undesirable.
Normally, the impedance of the cable is 50 Ω. If Ra indicates the impedance of the
antenna, the standing wave ratio is defined as:
SWR = Ra/50 Ω (if Ra is more than 50 Ω)
SWR = 50/Ra Ω (if Ra is less than 50 Ω)
Examples:
If Ra =
If Ra =
If Ra =
50 Ω
100 Ω
25 Ω
SWR =
SWR =
SWR =
1.0
2.0
2.0
Consequently, it is of importance that the SWR is as close to 1.0 as possible thus obtaining the
highest power being transmitted from the cable to the antenna.
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3.2. Directivity D
The directivity D is an indication of the capability of the antenna
to conduct the radiated power ”to a certain site”.
Normally, omnidirectional or directional antennas are mentioned.
Omnidirectional
An omnidirectional antenna with high directivity has a radiation
being similar to a pancake.
Directional
A directional antenna with high directivity has a radiation being
similar to the cone of light from a projector.
Omnidirectional
Omnidirectional
Directional
Directional
3.3. Gain G
Omnidirectional
The gain of an antenna is defined as G = η ·D, where η indicates the efficiency of the antenna.
Consequently, in the gain value possible loss in the antenna is comprised.
The η -figure is always than the directivity.
For most antenna types the own loss is so low that G = D can be considered.
3.4. Bandwidth
The bandwidth of the antenna is the frequency range, in which it operates properly, i.e. both gain
and SWR are within the more specified limits.
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4. How to measure how good an antenna is?
4.1. Measuring SWR
Coaxial
cable
Standing Wave Ratio
The standing wave ratio is normally measured by inserting a
device in the feeder cable of the antenna. How much power
reflected back from the antenna compared to the power
transmitted forward is measured.
Such a device is called a directional coupler.
Forward
power
Reflected
power
Directional couplers
Directional couplers or SWR meters for frequencies lower than
150 MHz are relatively cheap and quite accurate.
For frequencies above 400 MHz exact meters are expensive.
The most wellknown universal instrument is a BIRD-wattmeter.
SWRmeter
Transmitter
Measuring the standing wave ratio.
4.2. Measuring gain
The gain of an antenna is measured in relation to
the gain of another further specified antenna
through a comparison measurement.
The latter antenna is called a reference antenna
and the reference antenna is different,
irrespective of wether base station antennas,
mobile antennas or portable antennas are
considered.
Direct radiation
Reflected radiation
Measurement setup/procedure
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EIA RS-329-1 Standard for Base Station and Mobile Antennas
EIA RS-329-1 is an internationally valid standard for measuring gain for base station and mobile
antennas. According to this standard the gain of base station antennas is specified compared to a
½ wave dipole and the gain of mobile antennas is specified compared to a ¼ wave whip in the centre
of
the car roof.
EIA RS-329-1 standard is used for
base station and land mobile antennas.
No international standard for portable antennas
No international standard for measuring gain of portable antennas is available. Normal practice is,
however, to apply a ” ¼ wave antenna on same device” as a reference antenna. Owing to the indefinable
ground plane, which a hand-held radio constitutes, it is necessary to perform statistical average value
calculations for a large number of measurements in order to be able to state the gain of a portable
antenna rather precisely.
CCIR 368.3 Standard for Medium and short Wave Antennas
Medium wave and partly short wave maritime antennas are measured in accordance with the
international standard CCIR 368.3. The reference antenna is in this respect a ¼ wave monopoly antenna
above water, as for which calculated field strength is determined as function of the distance from
the antenna when applied with a certain power.
Measuring and Calculating
By measuring the gain of an antenna, a signal of a known size is applied and the field strength is
measured at a certain distance. By calculating back it can be figured out to which degree the
antenna has (generally) a poorer performance compared to the reference antenna. Gain will in such
cases be specified as an efficiency. Antennas for maritime VHF are measured like base station
antennas.
Direct radiation
Reflected radiation
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4.3. Measuring BW
The bandwidth is generally specified as the
area of the lowest part of the frequency ranges,
in which SWR and gain observe the
specifications.
¼ wave and collinear at the car roof centre
A
1/4 wave on roof
B
Collinear
at the roof
centre
A
Collinear on
rear wing
Antennas
placed
onBthe
and on
Antennas
placed
on car
the roof
car roof.
Collinear on
the rear wing.
roof
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