Horn antenna The horn antenna is used in the transmission and reception of RF microwave signals, and the antenna is normally used in conjunction with waveguide feeds. The horn antenna gains its name from its appearance. The waveguide can be considered to open out or to be flared, launching the signal towards the receiving antenna. Horn antennas are often used as gain standards, and as feeds for parabolic or 'dish' antennas, as well as being used as RF antennas in their own right. One particular use of horn antennas themselves is for short range radar systems, such as those used for automotive speed enforcement. When used as part of a parabolic reflector, the horn is orientated towards the reflector surface, and is able to give a reasonably even illumination of the surface without allowing radiation to miss the reflector. In this way it is able to maximise the efficiency of the overall antenna. The use of the horn antenna also minimizes the spurious responses of the parabolic reflector antenna to signals that are not in the main lobe.
Horn antenna used for RF microwave applications
Basic horn antenna concept
The horn antenna may be considered as an RF transformer or impedance match between the waveguide feeder and free space which has an impedance of 377 ohms. By having a tapered or having a flared end to the waveguide the horn antenna is formed and this enables the impedance to be matched. Although the waveguide will radiate without a horn antenna, this provides a far more efficient match. In addition to the improved match provided by the horn antenna, it also helps suppress signals travelling via unwanted modes in the waveguide from being radiated. However the main advantage of the horn antenna is that it provides a significant level of directivity and gain. For greater levels of gain the horn antenna should have a large aperture. Also to achieve the maximum gain for a given aperture size, the taper should be long so that the
phase of the wave-front is as nearly constant as possible across the aperture. However there comes a point where to provide even small increases in gain, the increase in length becomes too large to make it sensible. Thus gain levels are a balance between aperture size and length. However gain levels for a horn antenna may be up to 20 dB in some instances.
Horn antenna types
There are two basic types of horn antenna: pyramid and conical. The pyramid ones, as the name suggests are rectangular whereas the corrugated ones are usually circular. The corrugated horn provides a pattern that is nearly symmetrical, with the E and H plane beamwidths being nearly the same. Additionally it is possible to control the side lobes better with a conical or corrugated horn antenna.
Summary
The horn antenna is a particularly useful form of antenna for use with RF microwave applications and waveguide feeder. Although it is not used below RF microwave frequencies because waveguides are not used at low frequencies as a result of the sizes needed, the horn antenna is nevertheless a very useful form of RF antenna design for use at high frequencies.
The Yagi antenna Yagi antenna tutorial includes: • Yagi antenna • Yagi antenna gain • Yagi impedance & matching The Yagi or Yagi-Uda RF antenna or aerial is one of the most successful RF antenna designs for directive applications. It is used in a wide variety of applications where an RF antenna design with gain and directivity is required. It has become particularly popular for television reception, but it is used in very many other applications where an RF antenna design is needed that has gain. The full name for the antenna is the Yagi-Uda antenna. It was derives it name from its two Japanese inventors Yagi and his student Uda. The RF antenna design concept was first outlined in a paper that Yagi himself presented in 1928. Since then its use has grown rapidly to the stage where today a television antenna is synonymous with an RF antenna having a central boom with lots of elements attached.
The Yagi antenna The Yagi RF antenna design has a dipole as the main radiating or driven element. Further "parasitic" elements are added which are not directly connected to the driven element. Instead they pick up power from the dipole and re-radiate it such a manner that it affects the properties of the RF antenna as a whole.
Basic concept of a Yagi antenna
The parasitic elements of the Yagi antenna operate by re-radiating their signals in a slightly different phase to that of the driven element. In this way the signal is reinforced in some directions and cancelled out in others. It is found that the amplitude and phase of the current that is induced in the parasitic elements is dependent upon their length and the spacing between them and the dipole or driven element. Using a parasitic element it is not possible to have complete control over both the amplitude and phase of the currents in all the elements. This means that it is not possible to obtain complete cancellation in one direction. Nevertheless it is still possible to obtain a high degree of reinforcement in one direction and have a high level of gain, and also have a high degree of cancellation in another to provide a good front to back ratio. To obtain the required phase shift an element can be made either inductive or capacitive. If the parasitic element is made inductive it is found that the induced currents are in such a phase that they reflect the power away from the parasitic element. This causes the RF antenna to radiate more power away from it. An element that does this is called a reflector. It can be made inductive by tuning it below resonance. This can be done by physically adding some inductance to the element in the form of a coil, or more commonly by making it longer than the resonant length. Generally it is made about 5% longer than the driven element. If the parasitic element is made capacitive it will be found that the induced currents are in such a phase that they direct the power radiated by the whole antenna in the direction of the parasitic
element. An element which does this is called a director. It can be made capacitive tuning it above resonance. This can be done by physically adding some capacitance to the element in the form of a capacitor, or more commonly by making it about 5% shorter than the driven element. It is found that the addition of further directors increases the directivity of the antenna, increasing the gain and reducing the beamwidth. The addition of further reflectors makes no noticeable difference. The antenna exhibits a directional pattern consisting of a main forward lobe and a number of spurious side lobes. The main one of these is the reverse lobe caused by radiation in the direction of the reflector. The antenna can be optimised to either reduce this or produce the maximum level of forward gain. Unfortunately the two do not coincide exactly and a compromise on the performance has to be made depending upon the application.
Polar diagram of the Yagi antenna
The Yagi antenna is a particularly useful form of RF antenna design. It is widely used in applications where an RF antenna design is required to provide gain and directivity. In this way the optimum transmission and reception conditions can be obtained.
Yagi Antenna Gain - notes, details and tables of Yagi antenna gain.
Yagi antenna tutorial includes: • Yagi antenna • Yagi antenna gain • Yagi impedance & matching One of the chief reasons for using a Yagi antenna is the gain it provides. This gain is of great importance, because it enables all the transmitted power to be directed into the area where it is required, or when used for reception, it enables the maximum signal to be received from the same area. Gain for reception and transmission are equal when a passive antenna is used - i.e. one without any active elements.
Yagi gain / beamwidth considerations It is found that as the Yagi gain increases, so the beam-width decreases. Antennas with a very high level of gain are very directive. Therefore high gain and narrow beam-width sometimes have to be balanced to provide the optimum performance for a given application
Yagi gain vs beam-width
Yagi gain considerations A number of features of the Yagi design affect the overall gain:
Number of elements in the Yagi: One of the main factors affecting the Yagi gain, is the number of elements in the design. Typically a reflector is the first element added in any yagi design as this gives the most additional gain. Directors are then added. Element spacing: The spacing can have an impact on the Yagi gain, although not as much as the number of elements. Typically a wide-spaced beam, i.e. one with a wide spacing between the elements gives more gain than one that is more compact. The most critical element positions are the reflector and first director, as their spacing governs that of any other elements that may be added. Antenna length: When computing he optimal positions for the various elements it has been shown that in a multi-element Yagi array, the gain is generally proportional to the length of the array. There is certain amount of latitude in the element positions.
The gain of a Yagi antenna is governed mainly by the number of elements in the particular RF antenna. However the spacing between the elements also has an effect. As the overall performance of the RF antenna has so many inter-related variables, many early designs were not able to realise their full performance. Today computer programmes are used to optimise RF antenna designs before they are even manufactured and as a result the performance of antennas has been improved.
Yagi gain vs number of elements Although there is variation between different designs and the way antennas are constructed, it is possible to place some very approximate figures for anticipated gain against the number of elements in the design.
Number of elements
Approx anticipated gain dB over dipole
2
5
3
7.5
4
8.5
5
9.5
6
10.5
7
11.5
It should be noted that these figures are only very approximate. As an additional rule of thumb, once there are around four or five directors, each additional director adds around an extra 1dB of gain for directors up to about 15 or so directors. The figure falls with the increasing number of directors.
Yagi Front to Back ratio One of the figures associated with the Yagi gain is what is termed the front to back ratio, F/B. This is simply a ratio of the signal level in the forward direction to the reverse direction. This is normally expressed in dB. Front to back ratio = Signal in forward direction / signal in reverse direction
Yagi front to back ratio Front to back ratio = F / B
The front to back ratio is important in circumstances where interference or coverage in the reverse direction needs to be minimised. Unfortunately the conditions within the antenna mean that optimisation has to be undertaken for either front to back ratio, or maximum forward gain. Conditions for both features do not coincide, but the front to back ratio can normally be maximised for a small degradation of the forward gain.
Yagi Feed Impedance - notes and details of the essentials of Yagi impedance matching, what governs it and the ways of Yagi matching.
Yagi antenna tutorial includes: • Yagi antenna • Yagi antenna gain • Yagi impedance & matching As with any other type of antenna, ensuring that a good match between the feeder and the antenna itself are crucial to ensure the performance of the antenna can be optimised. The impedance of the driven element is greatly affected by the parasitic elements and therefore, arrangements needed to be incorporated into the basic design to ensure that a good match is obtained.
Feed impedance of Yagi driven element It is possible to vary the feed impedance of a Yagi antenna over a wide range. Although the impedance of the dipole itself would be 73 ohms in free space, this is altered considerably by the proximity of the parasitic elements. The spacing, their length and a variety of other factors all affect the feed impedance presented by the dipole to the feeder. In fact altering the element spacing has a greater effect on the impedance than it does the gain, and accordingly setting the required spacing can be used as one design technique to fine tune the required feed impedance. Nevertheless the proximity of the parasitic elements usually reduces the impedance below the 50 ohm level normally required. It is found that for element spacing distances less than 0.2 wavelengths the impedance falls rapidly away.
Yagi matching techniques To overcome this, a variety of techniques can be used. Each one has its own advantages and disadvantages, both in terms of performance and mechanical suitability. No one solution is suitable for all applications. The solutions below are some of the main solutions used and applicable to many types of antenna. There also not the only ones:
Balun: A balun is an impedance matching transformer and can be used to match a great variety of impedance ratios, provided the impedance is known when the balun is designed. Folded dipole: One method which can effectively be implemented to increase the feed impedance is to use a folder dipole. In its basic form it raises the impedance four fold, although by changing various parameters it is possible t raise the impedance by different factors. Delta match: This method of Yagi impedance matching involves "fanning out" the feed connection to the driven element. Gamma match: The gamma match solution to Yagi matching involves connecting the out of the coax braid to the centre of the driven element, and the centre via a capacitor to a point away from the centre, dependent upon the impedance increase required.
Balun for Yagi matching
The balun is a very straightforward method of providing impedance matching. 4:1 baluns are widely available for applications including matching folded dipoles to 75Ω coax. Baluns like these are just RF transformers. They should have as wide a frequency range as possible, but like any wound components they have a limited bandwidth. However if designed for use with a specific Yagi antenna, this should not be a problem. One of the problems with a balun is the cost - they tend to be more costly than some other forms of Yagi impedance matching. They may also be power limited for a given size.
Folded dipole
The folded dipole is a standard approach to increasing the Yagi impedance. It is widely used on Yagi antennas including the television and broadcast FM antennas. The simple folded dipole provides an increase in impedance by a factor of four. Under free space conditions, the dipole impedance on its own is raised from 75Ω for a standard dipole to 300Ω for the folded dipole.
Simple folded dipole antenna
Note on folded dipole:
The folded dipole is a from of dipole that has a higher impedance than the standard half wave dipole - in the standard version it has four times the impedance. However different ratios can be obtained by changing the mechanical attributes. Click for a Folded dipole tutorial
Another advantage of using a folded dipole for Yagi impedance matching is that the folded dipole has a flatter impedance versus frequency characteristic than the simple dipole. This enables it and hence the Yagi to operate over a wider frequency range. While a standard folded dipole using the same thickness conductor for the top and bottom conductors within the folded dipole will give a fourfold increase in impedance, by varying the thickness of both, it is possible to change the impedance multiplication factor to considerably different values. Delta match
The delta match for of Yagi matching is one of the more straightforward solutions. It involves fanning out the ends of the balanced feeder to join the continuous radiating antenna driven element at a point to provide the required match.
Delta match for dipole - often used for Yagi impedance matching
Both the side length and point of connection need to be adjusted to optimise the match. One of the drawbacks for using the Delta match for providing Yagi impedance matching is that it is unable to provide any removal of reactive impedance elements. As a result a stub may be used. Gamma match
The gamma match is often used for providing Yagi impedance matching. It is relatively simple to implement.
Gamma match for dipole - often used for Yagi impedance matching
As seen in the diagram, the outer of the coax feeder is connected to the centre of the driven element of the Yagi antenna where the voltage is zero. As a result of the fact that
the voltage is zero, the driven element may also be connected directly to a metal boom at this point without any loss of performance. The inner conductor of the coax is then taken to a point further out on the driven element - it is taken to a tap point to provide the correct match. Any inductance is tuned out using the series capacitor. When adjusting the RF antenna design, both the variable capacitor and the point at which the arm contacts the driven element are adjusted. Once a value has been ascertained for the variable capacitor, its value can be measured and a fixed component inserted if required.
Dipole antenna - overview, summary, tutorial about the basics of the dipole antenna or dipole aerial that is widely used on its own and as the basis for other RF antenna designs.
Dipole antenna tutorial includes: • Dipole antenna • Dipole length calculation
• Dipole feed impedance • Folded dipole antenna The dipole antenna or dipole aerial is one of the most important and commonly used types of RF antenna. It is widely used on its own, and it is also incorporated into many other RF antenna designs where it forms the radiating or driven element for the antenna. The dipole is a simple antenna to construct and use, and many of the calculations are quite straightforward. However like all other antennas, the in-depth calculations are considerably more complicated.
Dipole antenna basics As the name suggests the dipole antenna consists of two terminals or "poles" into which radio frequency current flows. This current and the associated voltage causes and electromagnetic or radio signal to be radiated. Being more specific, a dipole is generally taken to be an antenna that consists of a resonant length of conductor cut to enable it to be connected to the feeder. For resonance the conductor is an odd number of half wavelengths long. In most cases a single half wavelength is used, although three, five, . . . . wavelength antennas are equally valid.
The basic half wave dipole antenna
The current distribution along a dipole is roughly sinusoidal. It falls to zero at the end and is at a maximum in the middle. Conversely the voltage is low at the middle and rises to a maximum at the ends. It is generally fed at the centre, at the point where the current is at a maximum and the voltage a minimum. This provides a low impedance feed point which is convenient to handle. High voltage feed points are far less convenient and more difficult to use. When multiple half wavelength dipoles are used, they are similarly normally fed in the centre. Here again the voltage is at a minimum and the current at a maximum. Theoretically any of the current maximum nodes could be used.
Three half wavelength wave dipole antenna
Dipole polar diagram The polar diagram of a half wave dipole antenna that the direction of maximum sensitivity or radiation is at right angles to the axis of the RF antenna. The radiation falls to zero along the axis of the RF antenna as might be expected.
Polar diagram of a half wave dipole in free space
If the length of the dipole antenna is changed then the radiation pattern is altered. As the length of the antenna is extended it can be seen that the familiar figure of eight pattern changes to give main lobes and a few side lobes. The main lobes move progressively towards the axis of the antenna as the length increases. The dipole antenna is a particularly important form of RF antenna which is very widely used for radio transmitting and receiving applications. The dipole is often used on its own as an RF antenna, but it also forms the essential element in many other types of RF antenna. As such it is the possibly the most important form of RF antenna.
Dipole antenna length calculation & formula
- notes and details about the dipole antenna length calculation & formula.
Dipole antenna tutorial includes: • • • •
Dipole antenna Dipole length calculation Dipole feed impedance Folded dipole antenna
The length of a dipole is the main determining factor for the operating frequency of the dipole antenna. Typically a dipole is a half wavelength long, or a multiple of half wavelengths. However the dipole length is not exactly the same as the wavelength in free space - it is slightly shorter.
Dipole length variation from free space length Although the antenna may be an electrical half wavelength, or multiple of half wavelengths, it is not exactly the same length as the wavelength for a signal travelling in free space. There are a number of reasons for this and it means that an antenna will be slightly shorter than the length calculated for a wave travelling in free space. For a half wave dipole the length for a wave travelling in free space is calculated and this is multiplied by a factor "A". Typically it is between 0.96 and 0.98 and is mainly dependent upon the ratio of the length of the antenna to the thickness of the wire or tube used as the element. Its value can be approximated from the graph:
Multiplication factor "A" used for calculating the length of a dipole
Dipole length formula It is quite easy to use In order to calculate the length of a half wave dipole the simple formulae given below can be used: Length (metres) = 150 x A / frequency in MHz Length (inches) = 5905 x A / frequency in MHz
Using these formulae it is possible to calculate the length of a half wave dipole. Even though calculated lengths are normally quite repeatable it is always best to make any prototype antenna slightly longer than the calculations might indicate. This needs to be done because changes in the thickness of wire being used etc may alter the length slightly and it is better to make it slightly too long than too short so that it can be trimmed so that it resonates on the right frequency. It is best to trim the antenna length in small steps because the wire or tube cannot be replaced very easily once it has been removed. Computer simulation programmes are normally able to calculate the length of a dipole very accurately, provided that all the variables and elements that affect the operation of the dipole can be entered accurately so that the simulation is realistic and therefore accurate. The major problem is normally being able to enter the real-life environmental data accurately to enable a realistic simulation to be undertaken.
Dipole antenna feed impedance - Notes and overview about the feed impedance of a dipole antenna - what affects it, how it may be determined, & other key details.
Dipole antenna tutorial includes: • • • •
Dipole antenna Dipole length calculation Dipole feed impedance Folded dipole antenna
The feed impedance of a dipole antenna is of particular importance. To ensure the optimum transfer of energy from the feeder, or source / load, the feed impedance of the dipole should be the same as that of the source or load. By matching the feed impedance of the dipole to the source or load, the antenna is able to operate to its maximum efficiency.
Dipole feed impedance basics The feed impedance of a dipole is determined by the ratio of the voltage and the current at the feed point. A simple Ohms Law calculation will enable the impedance to be determined. Although a dipole can be fed at any point, it is typically fed at the current maximum and voltage minimum point. This gives a low impedance which is normally more manageable.
Most dipoles tend to be multiples of half wavelengths long. It is therefore possible to feed the dipole at any one of these voltage minimum or current maximum points which occur at a point that is a quarter wavelength from the end, and then at half wavelength intervals.
Three half wavelength wave dipole antenna showing feed point points λ/4 from either end could also be used
The vast majority of dipole antennas are half wavelengths long. Therefore they are centre fed the point of the voltage minimum and current maximum.
The basic half wave dipole antenna with centre feed point
The dipole feed impedance is made up from two constituents:
Loss resistance: The loss resistance results from the resistive or Ohmic losses within the radiating element, i.e. the dipole. In many cases the dipole loss resistance is ignored as it may be low. To ensure that it is low, sufficiently thick cable or piping should be used, and the metal should have a low resistance. Skin effects may also need to be considered. Radiation resistance: The radiation resistance is the element of the dipole antenna impedance that results from the power being "dissipated" as an electromagnetic wave. The aim of any antenna is to "dissipate" as much power in this way as possible.
As with any RF antenna, the feed impedance of a dipole antenna is dependent upon a variety of factors including the length, the feed position, the environment and the like. A half wave centre fed dipole antenna in free space has an impedance 73.13 ohms making it ideal to feed with 75 ohm feeder.
Factors that alter the dipole feed impedance The feed impedance of a dipole can be changed by a variety of factors, the proximity of other objects having a marked effect. The ground has a major effect. If the dipole antenna forms the radiating element for a more complicated form of RF antenna, then elements of the RF antenna will have an effect. Often the effect is to lower the impedance, and when used in some antennas the feed impedance of the dipole element may fall to ten ohms or less, and methods need to be used to ensure a good match is maintained with the feeder.
Folded dipole antenna - notes and summary about the folded dipole antenna, folded dipole impedance, unequal conductor folded dipoles, and multi-wire folded dipoles.
Dipole antenna tutorial includes: • • • •
Dipole antenna Dipole length calculation Dipole feed impedance Folded dipole antenna
The standard dipole is widely used in its basic form. However under a number of circumstances a modification of the basic dipole, known as a folded dipole provides a number of advantages. The folded dipole is widely used, not only on its own, but also as the driven element in other antenna formats such as the Yagi antenna.
Folded dipole basics In its basic form the folded dipole consists of a basic dipole with an added conductor connecting the two ends together to make a complete loop of wire or other conductor. As the ends appear to be folded back, the antenna is called a folded dipole. The basic format for the dipole is shown below. As can be seen from this it is a balanced antenna, like the standard dipole, although it can be fed with unbalanced feeder provided that a balan of some form is used to transform from an unbalanced to balanced feed structure.
Simple half-wave folded dipole antenna
One of the main reasons for using the folded dipole is the increase in feed impedance that it provides. If the conductors in the main dipole and the second or "fold" conductor are the same diameter, then it is found that there is a fourfold increase in the feed impedance. In free space, this gives an increase in feed impedance from 73Ω to around 300Ω ohms. Additionally the RF antenna has a wider bandwidth.
Folded dipole impedance rationale In a standard dipole the currents flowing along the conductors are in phase and as a result there is no cancellation of the fields and radiation occurs. When the second conductor is added this can be considered as an extension to the standard dipole with the ends folded back to meet each other. As a result the currents in the new section flow in the same direction as those in the original dipole. The currents along both the half-waves are therefore in phase and the antenna will radiate with the same radiation patterns etc as a simple half-wave dipole. The impedance increase can be deduced from the fact that the power supplied to a folded dipole is evenly shared between the two sections which make up the antenna. This means that when compared to a standard dipole the current in each conductor is reduced to a half. As the same power is applied, the impedance has to be raised by a factor of four to retain balance in the equation Watts = I^2 x R.
Folded dipole advantages There are a number of advantages or reasons for using a folded dipole:
Increase in impedance: When higher impedance feeders need to be used, or when the impedance of the dipole is reduced by factors such as parasitic elements, a folded dipole provides a significant increase in impedance level that enables the antenna to be matched more easily to the feeder available. Wide bandwidth: The folded dipole has a flatter frequency response - this enables it to be used over a wider bandwidth.
Unequal conductor folded dipoles It is possible to implement different impedance ratios to the standard 4:1 that are normally implement using a folded dipole. Simply by varying the effective diameter of the two conductors: top and bottom, different ratios can be obtained.
Folded dipole with unequal conductor diameters
In order to determine the impedance step up ratio provided by the folded dipole, the following formula can be used:
Where: d1 is the conductor diameter for the feed arm of the dipole d2 is the conductor diameter for the non-fed arm of the dipole S is the distance between the conductors r is the step up ratio
When determining the length of a folded dipole using thick conductors, it should be remembered that there is a shortening effect associated with their use as opposed to normal wire or thin conductors.
Folded dipole applications Folded dipoles are sometimes used on their own, but they must be fed with a high impedance feeder, typically 300 ohms. However they find more uses when a dipole is incorporated in another RF antenna design with other elements nearby. This has the effect of reducing the dipole impedance. To ensure that it can be fed conveniently, a folded dipole may be used to raise the impedance again to a suitable value.
