This page offers definitions of terms that might be of interest to anyone buying, installing or using communication antennas.
Bandwidth
Colinear
Communication Antenna
Directivity
Efficiency
Gain
Glassmount
Ground Plane
Unity
VSWR
Bandwidth
The set of frequencies over which an antenna operates within a specified level of efficiency.
GSM900 equipment is required to transmit between 890 and 915MHz , and to receive between 935 and 960MHz. It could be said that the 'centre frequency' is 925MHz and the bandwidth is 70MHz.
A typical Panorama GSM900 glass-mount antenna has a bandwidth of 90MHz. It can operate between 870 and 960MHz with VSWR better than or equal to 1.5:1 at transmit frequencies; and better than or equal to 2:1 VSWR at receive frequencies.
Colinear
A stacked element antenna with the elements joined by phasing coils. (Phasing coils ensure the currents in the elements are in phase.)
The elements may be of various wavelengths (such as 3/8, 1/4, and 5/8). For example, the '5/8 over 1/4 wave', shown in Figure 1, is a two element colinear antenna because it has a 5/8 wave element joined end to end with a 1/4 wave element.
Different types of antenna construction produce different performance characteristics.
For example, if the performance of a colinear antenna is compared with the performance of a 1/4 wave antenna (assuming frequency, power and efficiency are the same) a colinear antenna will appear to flatten the radiating pattern along the horizon. As the amount of signal is the same for both antennas, the flattening effect pushes the signal further along the horizon than 1/4 wave antennas. This is known as the 'gain effect'.
Depending on the application, it may or may not be beneficial to 'flatten' the signal along the horizon. Antenna manufacturers can engineer the 'gain effect' by careful design and testing to produce antennas for different applications.
Communication antenna
A device for receiving AND transmitting radio signals. Used, for example, with 2-way radios and cellular telephones. Designed for use at a specific bandwidth.
(Not to be confused with broadcast receivers such with FM/AM radios which operate across much wider frequency ranges.) The power of the signals generated by most communication equipment is much weaker than broadcast signals. Therefore, the size, design and position of antennas for communications applications are critical - in fact the antenna is one of the most important parts of a communications installation. Without an efficient antenna, even the best quality radio will fail to perform properly.
Communication antennas are often described by their length. The length of an antenna relates to the wavelength of the radio transmitter it is used with. For example a '1/4 wave' antenna is as long as a quarter of the wavelength it operates at.
The most common types of antenna whip used in mobile communications are: 1/4 wave, 1/2 wave, 5/8 wave and colinear.
These types all have different radiating pattern properties which means that we can select the type of antenna that best suits our requirements.
Communication antenna
The shape and angle of elevation of the radiating pattern are known collectively as directivity.
Each type of antenna creates a characteristic three dimensional radiating pattern.
The radiating patterns of Panorama antennas are measured in both the E and H planes using the anechoic chamber.
All Panorama's antennas produce radiating patterns that have vertical polarisation, and are omni directional in the H-plane. This means that (if the antenna is vertical) the signal will be transmitted equally in all directions along the horizon.
Directivity is determined by the type of antenna (such as 1/4 wave or colinear).
The shape and angle of elevation of the radiating pattern are known collectively as directivity.
If the radiating patterns of a ¼ wave and a colinear were compared, (for a given frequency and power output) the radiating pattern of the colinear antenna would be more squashed and stretched along the horizon.
Directivity is difficult to measure, so we normally talk about gain. (In theory, if an antenna is 100% efficient, then directivity is equal to gain.)
Efficiency
The electrical performance of the communications antenna system (including radiating element, base and cable). Efficient antennas are expected to transmit all the signal available to them. In practice, antennas can never be 100% efficient. Inefficiency can be caused by many factors which are often inter-related, for example:
- DESIGN (such as in accurate pitch and turn of phasing coils.)
- CONSTRUCTION (such as poor electrical connection.)
- QUALITY OF MATERIALS (such as poor electrical conductivity.)
- IMPEDANCE MATCHING between the communication equipment and the antenna. (Most communication equipment has 50Ohms impedance. The impedance of the antenna should be as close to this as possible.)
- OBJECTS CLOSE TO THE INSTALLATION can have a de-tuning effect (such as the support frames of 'spoilers' on heavy goods vehicles or metal roof racks).
The most common way to measure efficiency is VSWR, or return loss. (A return loss of 14 dB is equivalent to 1:5 VSWR.)
Gain
a product of the efficiency and directivity of an antenna.
(Note that the highest gain antenna is not always the best for the job. Depending on the operating environment, such as remote or built up areas, you need to choose an antenna with a suitable gain value.)
Gain is a RELATIVE VALUE measured against a 'standard' that has the same input power and efficiency. Antenna manufacturers usually quote gain values as 'dBd' or 'dBi' to indicate which 'standard' is being referred to (1dBd = 3dBi):
dBd - dBd is relative to a centre fed 1/2 wave dipole
dBi - dBi is relative to an isotrope (i.e. it is a theoretical value)
For a given efficiency and power input, the difference in performance of antennas can be attributed to the 'gain effect'. The gain effect is determined by the construction of the antenna.
Generally, for a given frequency and power input:
Increasing the length of an antenna with a phasing coil tends to make the relative gain value higher. A higher gain antenna tends to receive and transmit signal further along the horizon.
Conversely, the lower the antenna gain the better the performance in built-up areas.
Glassmount
A type of antenna that attaches to vehicles without the need to drill a hole in a ground plane (i.e. glassmounts are 'ground plane independent'). They are simply 'stuck on' during installation, and, although securely fitted, can be removed without trace. The signal passes through the glass like a charge passes through a capacitor.
Panorama's cellular Glass Mount is a patented design. It was the first glass mount to apply electronic solid state technology to the coupling circuit.
Ground Plane
A metallic surface (such as a vehicle roof or body panel) on which antennas are installed. Standard panel mount antennas need to be fitted on a ground plane. For best performance the ground plane should be horizontal and have a radius of at least 1/4 wavelength of the antenna's operating frequency.
Ground plane independent (GPI) antennas have design characteristics which enable them to operate without a ground plane. For example: ½ waves, glass mounts, and 'elevated' antennas.
Unity
Antennas with a gain value of 0dBd are sometimes called 'unity' or 'unity gain' antennas. Unity antennas usually have quarter wave whips.
VSWR
(Voltage Standing Wave Ratio) one of several ways to measure efficiency.
VSWR is an indication of the effectiveness of the match between the impedance of the antenna at its feed point with the impedance of the transmitter (which is usually 50 Ohms).