Down to basics with antennas - Part II
Monday, 26 September, 2011
Antennas are the heart of every radio transmitter and receiver. So vital are they that it seems appropriate to look more closely at them. And whether you are experienced in their use or are coming to them for the first time, there is bound to be information here that could be useful.
Antenna tuning is the process whereby the resonant point of an antenna is adjusted. In most instances this is accomplished by physically adjusting the antenna length.
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While simple range tests can be used to blindly tune an antenna, a network analyser is a virtual necessity for serious characterisation. In some cases external inductive or capacitive components may be used to match and bring the antenna to resonance.
Such components can introduce loss. It should be remembered that match and resonance do not necessarily translate into effective propagation.
In addition to broad concepts of antenna function, there are specific issues of antenna performance that are equally important to consider.
The term radiation pattern is used to define the way in which the radio frequency energy is distributed or directed into free space.
The term isotropic antenna is commonly used to describe an antenna with a theoretically perfect radiation pattern. That is one which radiates electromagnetic energy equally well in all directions.
Such an antenna is, of course, only theoretical and has never actually been built, but the isotropic model serves as a conceptual standard against which ‘real world’ antennas can be compared.
In the real world, an antenna will efficiently radiate RF energy in certain directions and poorly in others. The point(s) of greatest efficiency are called peaks while the areas of no field strength are called nulls.
The overall distribution characteristics of the antenna make up the radiation pattern. In many applications it is advantageous to have the antenna perform equally well in all directions.
In these instances a designer would choose an antenna style with an omnidirectional radiation pattern as such characteristics would be desirable. In instances where highly directional antenna characteristics are needed, an antenna style such as a Yagi would be chosen.
The term gain refers to the antenna’s effective radiated power compared with the effective radiated power of some reference antenna. When the isotropic model is used, the gain will be stated in dBi (meaning gain in dB over isotropic).
When gain is being compared with a standard dipole, the rating will be stated in dBd (meaning gain over dipole). The generally accepted variation between isotropic and a standard dipole is 2.2 dB.
Thus, an antenna rated as having 15 dBi of gain would indicate the antenna had 15 dB of gain over isotropic or 12.8 dB of gain as compared with a standard single-element dipole. Gain is commonly misinterpreted as an increase in output power above unity. Of course, this is impossible since the radiated power would be greater than the original power introduced to the antenna.
A simple way to understand gain is to think of a focusable light source. Assume the light output is constant and focused over a wide area. If the light were refocused to a spot, it would appear brighter because all the light energy is concentrated into a small area.
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Even though the overall light output has remained constant, the light will have a gain in lux at the focus point over the original pattern.
In the same way, an antenna that focuses RF energy into a narrow beam can be said to have gain (at the point of focus) over an antenna that radiates equally in all directions.
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In other words, the higher an antenna’s gain the narrower the antenna’s pattern and the better its point performance will be.
The effective polarisation of an antenna is an important characteristic. Polarisation refers to the orientation of the lines of flux in an electromagnetic field.
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When an antenna is oriented horizontally with respect to ground it is said to be horizontally polarised. Likewise, when it is perpendicular to ground it is said to be vertically polarised.
The polarisation of an antenna normally parallels the active antenna element; thus, a horizontal antenna radiates and best receives fields having horizontal polarisation while a vertical antenna is best with vertically polarised fields.
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If the transmitter and receiver’s antennas are not oriented in the same polarisation, a certain amount of power will be lost. In many applications there is little control over the antenna orientation; however, to achieve maximum range the antennas should be oriented with like polarisation whenever possible.
In the VHF and UHF spectrums, horizontal polarisation will generally provide better noise immunity and less fading than vertical polarisation.
Not all the power delivered into the antenna element is radiated into space. Some power is dissipated by the antenna and some is immediately absorbed by surrounding materials.
Forward power - the power originally applied to the antenna input.
Reflected power - a portion of the forward power reflected back towards the amplifier due to a mismatch at the antenna port.
Net power - the power applied to the antenna that actually transitions into free space is called the net power or effective radiated power. Net power is usually calculated by finding the difference between the actual forward and reflected power values.
Multipath fading is a form of fading caused by signals arriving at the receiving antenna in different phases.
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This effect is due to the fact that a signal may travel many different paths before arriving at the antenna. Some portions of the original signal may travel to the receiver’s antenna via a direct free space path. Others, which have been reflected, travel longer paths before arrival.
The longer path taken by the reflected waves will slightly delay their arrival time from that of the free space wave. This creates an out-of-phase relationship between the two signals.
The resulting voltage imposed on the receiving antenna will vary based on the phase relationship of all signals arriving at the antenna.
While this effect is environmental and not related directly to the antenna, it is still important to understand the role multipath may play in theoretical vs realised antenna performance.
A whip-style antenna provides exceptional performance and stability. A straight whip has a wide bandwidth and is easily designed and integrated. Many designers opt for the reliable performance and cosmetic appeal of professionally made antennas, such as those offered by Linx.
These ‘off-the-shelf’ whip designs are generally made from a wire or cable encapsulated in a rubber or plastic housing. A whip can also be made by cutting a piece of wire or rod to the appropriate length.
Since a full-wave whip is generally quite long and its impedance high, most whips are either a 1/4 or 1/2 wave. The correct length can be found using the formula in the section entitled ‘How is Antenna Length Determined?’.
A helical element is a wire coil usually wound from steel, copper or brass. By winding the element its overall physical length can be greatly reduced. The element may be enclosed inside the antenna housing or exposed for internal mounting.
A helical antenna significantly reduces the physical size of the antenna; however, this reduction is not without a price. Because a helical has a high Q factor, its bandwidth is very narrow and the spacing of the coils has a pronounced effect on antenna performance.
The antenna is prone to rapid detuning especially in proximity to objects. A well-designed helical can achieve excellent performance while maintaining a compact size.
Helical antenna design is a bit more complex than that of a straight antenna. It is possible to calculate the length of a helical once the diameter, material type and coil spacing are known.
In most cases, however, it is just as easy to arrive at a design empirically by taking an excessively long coil and tuning it by clipping until it is resonant at the desired frequency. The length may then be calculated by the turns and radius values or simply by straightening the coil and measuring it.
The last style of antenna to discuss is the loop trace. This style is popular in low-cost applications since it can be easily concealed and adds little to overall product cost.
The element is generally printed directly onto the product’s PCB and can be made self-resonant or externally resonated with discrete components. The actual layout is usually product specific.
Despite its cost advantages, PCB antenna styles are generally inefficient and useful only for short-range applications. A loop can be very difficult to tune and match and is also sensitive to changes in layout or substrate dielectric constant.
This can introduce consistency issues into the production process. In addition, printed styles are difficult to engineer, requiring the use of expensive equipment, including a network analyser.
An improperly designed loop will have a high SWR at the desired frequency, which can introduce instability. For these reasons loops are generally confined to low-cost transmitter devices, such as garage door openers, car alarms, etc.
The company offers several low-cost planar and chip antenna alternatives to the often-problematic ‘printed’ antenna. These tiny antennas mount directly to a product’s PCB and provide good performance for their compact size.
Some designers attempt to attenuate fundamental output power by shortening or lengthening the antenna to shift its point of resonant efficiency away from the fundamental.
This is not usually a good idea for two reasons.
First, by raising the SWR and reducing an antenna’s efficiency at your intended fundamental frequency you have potentially increased the output efficiency at a harmonic.
Second, by creating such a mismatch the RF stage may become unstable. Some Linx products allow power levels to be adjusted via programming or an external resistor. In other cases an attenuation T-pad should be used.
Finally, in the design process the antenna should be viewed as a critical component in system performance.
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