Wednesday, October 22, 2008

INTRODUCTION


A Yagi-Uda antenna is a widely used antenna design due to its high forward gain capability, low cost and ease of construction, it is commonly used as a roof top television receiver. It consists of a number of metal rods called elements arranged on a central support beam. The elements are a dipole - which is the only driven element - arranged with a number of parasitic elements, of which there are two types;

A reflector

One or more directors

The function of the parasitic elements is to improve the radiation pattern in the forward direction. The reflector is placed behind the dipole and is slightly longer, it provides 3dB of additional forward gain, but having more than one reflector has
little benefit. The directors are placed in front and slightly shorter in length than the dipole provide an additional 3dB of forward gain each.
The parasitic elements provide forward gain by redistributing the energy of the EM signal generated by the dipole, since they are not driven they can only
redistribute energy in one direction at the expense of other directions.

For a particular operating frequency a typical design would be;

Element Length (l) Separation (l)
Reflector > 0.50 0.15 – 0.25
Driver 0.45 1 -
Directors 0.40 – 0.45 0.30 – 0.40

    A polar plot of the gain verses orientation (radiation pattern) is useful when characterizing antennas. Some important features that appear on
plot are;

    Forward gain – expressed in dB relative to an isotropic source or a standard dipole (in direction of maximum gain) represents the improvement in signal level to a reference antenna.

     Front to back ratio – ratio of signal level in the forward direction to the signal level in the back direction (rotated 180o) also expressed in dB.

     Beamwidth – angle between directions where the power is half the value at direction of maximum gain, hence -3dB. It gives a measure of the directivity of the antenna.

     Side lobes – these are unwanted peaks in the gain at angles other than in the forward 1 Since an EM wave has a reduced velocity in the conductor a factor of 0.95 is usually applied, so in practice the driver is generally 0.475l long.
Richard Laugesen Page 1 3/11/2001 direction, they reduce the amount of useful
energy contained in the forward direction. Other characteristics that do not appear on the polar plot but which are equally important are;

 
    Bandwidth – the range of frequency over which the antenna exhibits acceptable characteristics.

 
    Radiative Resistance – want the impedance ofthe antenna to match the impedance of the transmission cable used to drive it otherwise
signal loss and high voltages in the cable may occur. It is difficult to optimize all of these characteristics simultaneously so the aim when designing an antenna depends on the requirements of the situation in which the antenna will be used. Optimization is achieved by simulating the radiation pattern of the antenna while varying the
lengths and separations of the elements.


HISTORY


   

   The Yagi-Uda antenna was invented in 1926 by Shintaro Uda of Tohoku Imperial University, Sendai, Japan, with the collaboration of Hidetsugu Yagi, also of Tohoku Imperial University.


   Yagi published the first English-language reference on the antenna in a 1928 survey article on short wave research in Japan and it came to be associated with his name. 

   However, Yagi always acknowledged Uda's principal contribution to the design, and the proper name for the antenna is, as above, the Yagi-Uda antenna (or array).

   The Yagi was first widely used during World War II for airborne radar sets, because of its simplicity and directionality.

   The Japanese military authorities first became aware of this technology after the Battle of Singapore when they captured the notes of a British radar technician that mentioned "yagi antenna". Japanese intelligence officers did not even recognise that Yagi was a Japanese name in this context.

   Despite its being invented in Japan, many Japanese radar engineers were unaware of the design until very late in the war, due to internal fighting between the Army and Navy. A horizontally polarized array can be seen under the left leading edge of Grumman F4F, F6F, TBF Avenger carrier based Navy aircraft.

   Yagi-Uda antennas are widely used by amateur radio operators worldwide for communication on frequencies from shortwave, through VHF/UHF, and into microwave bands. Hams often homebrew this type of antenna, and have provided many technical papers and software to the engineering community.

HOW DOES A YAGI-UDA ANTENNA WORK?


   The objective of the design is to make a "travelling wave" structure with currents in the elements all contributing to the far field in the forward direction. The contributions are designed to add up in phase in the forward direction, and to cancel in the reverse direction. The director elements are cut shorter than the driving element, which is itself a little shorter than a half wavelength at the design frequency. The reflector is cut to be about a half wavelength and it is longer than the driving element, and spaced closer than are the directors. The directors present a capacitative impedance, acting like two lengths of open circuit transmission line each a little shorter than a quarter wavelength to a hypothetical generator at the centre formed from the "induced emf" set up by the impinging fields. See the SMITH chart . Similarly, the reflector presents an inductive impedance to a hypothetical emf generator at its centre. The effects of the spacings and the current progressive phase shifts mean that the contributions of the current in the various elements to the radiated fields all add up in phase. 


   For a closely spaced driving element and parasitic element, isolated from each other as far a electrical conduction currents are concerned, the currents are oppositely directed as can be seen in the discussion on folded dipoles with the folds cut off. As the spacing is increased, the currents remain oppositely directed until when the spacing is a half-wavelength, the contributions to the far field add up in phase in the "endfire direction", as can be seen from the discussion on array antennas. 

   If the director elements are cut a little short, their self-impedance is capacitative and they have to be spaced a little closer than a half-wavelength in order to maintain equality of phase in the radiation contribution with the wave arriving from the previous director. The currents in successive elements thus roughly have the pattern 

....up down up down up down ...... 

but will all be very nearly equal in magnitude to each other. There is also some progressive phase shift as the wave advances, caused by the fact that the directors are cut short (capacitative). 

   The field pattern on the yagi directors therefore advances as a travelling wave in the forward direction, with wavelength approximately equal to three director spacings. This can be seen in the table at the top of this page; at 30MHz the wavelength (lambda) is 10 metres so for a 15 element Yagi array, the length given as 47 metres is nearly five wavelengths, or 15 elements divided by 3.

   So the travelling wave structure supports a non-attenuating wave in the forward direction, and the currents in the directors are all approximately the same size, although with a progressive phase delay. It is for this reason that, for moderate numbers of elements, the forward gain is proportional to the number of elements. 

   The reflector has an induced current in it that contributes a wave in the backwards direction that just cancels the backward wave from the driven element. Only a little power is radiated backwards. The net power radiated by the reflector current has to go somewhere, so it appears as a contribution in the forward direction. The length and the spacing of the reflector have a strong influence on the residual backward radiation from the Yagi-Uda. Typically the reflector will be spaced by 1/8 to 1/4 of a wavelength, and the directors by about 1/3 wavelength each. 

   The array factor gain of a Yagi-Uda is therefore limited to the number of elements, and the element gain is that of a dipole of length about half a wavelength, which is 1.66. 

   Therefore the maximum gain we can reasonably expect from the Yagi-Uda is 1.66 times the number of elements, over isotropic, (or just a factor [equal to the number of elements] over the gain of a single half-wave dipole).

THE ELEMENTS OF A YAGI



THE DRIVEN ELEMENT

The driven element of a Yagi is the feed point where the feed line is attached from the transmitter to the Yagi to perform the transfer of power from the transmitter to the antenna.
A dipole driven element will be "resonant" when its electrical length is 1/2 of the wavelength of the frequency applied to its feed point.
The feed point in the picture above is on the center of the driven element.

THE DIRECTOR

The director/s is the shortest of the parasitic elements and this end of the Yagi is aimed at the receiving station. It is resonant slightly higher in frequency than the driven element, and its length will be about 5% shorter, progressively than the driven element. The director/s length/s can vary, depending upon the director spacing, the number of directors used in the antenna, the desired pattern, pattern bandwidth and element diameter. The number of directors that can be used are determined by the physical size (length) of the supporting boom needed by your design.

The director/s are used to provide the antenna with directional pattern and gain.
The amount of gain is directly proportional to the length of the antenna array and not by the number of directors used. The spacing of the directors can range from .1 wavelength to .5 wavelength or more and will depend largely upon the design specifications of the antenna.

THE REFLECTOR

The reflector is the element that is placed at the rear of the driven element (The dipole). It's resonant frequency is lower, and its length is approximately 5% longer than the driven element. It's length will vary depending on the spacing and the element diameter. The spacing of the reflector will be between .1 wavelength and .25 wavelength. It's spacing will depend upon the gain, bandwidth, F/B ratio, and sidelobe pattern requirements of the final antenna design.

BANDWIDTH AND IMPEDANCE

The impedance of an element is its value of pure resistance at the feed point plus any reactance (capacitive or inductive) that is present at that feed point. Of primary importance here is the impedance of the driven element, the point on the antenna where the transfer of rf from the feedline takes place.

Maximum energy transfer of rf at the design frequency occurs when the impedance of the feed point is equal to the impedance of the feedline. In most antenna designs, the feedline impedance will be 50 ohms, but usually the feed point impedance of the Yagi is rarely 50 ohms. In most cases it can vary from approximately 40 ohms to around 10 ohms, depending upon the number of elements, their spacing and the antenna's pattern bandwidth. If the feedline impedance does not equal the feed point impedance, the driven element cannot transfer the rf energy effectively from the transmitter, thus reflecting it back to the feedline resulting in a Standing Wave Ratio. Because of this, impedance matching devices are highly recommended for getting the best antenna performance.
The impedance bandwidth of the driven element is the range of frequencies above and below the center design frequency of the antenna that the driven element's feed point will accept maximum power (rf), from the feedline.
The design goal is to have the reactance at the center design frequency of the Yagi = (0),,, (j + 0).

The impedance matching device will now operate at it's optimum bandwidth. Wide element spacing, large element diameter, wide pattern bandwidth, and low "Q" matching systems will all add to a wider impedance bandwidth.


ABOUT ANTENNA PATTERNS

The antenna's radiation pattern or polar plot as it is sometimes called plays a major role in the overall performance of the Yagi antenna.

The directional gain, front-to-back ratio, beamwidth, and unwanted (or wanted) sidelobes combine to form the overall radiation pattern. The antenna's radiation pattern bandwidth is the range of frequencies above and below the design frequency in which the pattern remains consistent.

The amount of variation from the antenna's design specification goals that can be tolerated is subjective, and limits put into the design are mainly a matter of choice of the designer. "In other words.....trade offs".

Equal spaced, equal length directors may give higher gain at a particular frequency, but the bandwidth is more narrow and larger sidelobe levels are created.
Wide spacing will increase the bandwidth, but the sidelobes become large.
By varying both the spacing and director lengths the pattern and the pattern bandwidth may be more controlled.
More directors within a given boom length won't increase the gain by any great amount, but will give you better control of the antenna's pattern over a wider range of frequencies in the band of design.
If you reduce the length of each succeeding director by a set factor (%), AND increase the spacing of each succeeding director by another factor, a very clean pattern with good pattern bandwidth can be obtained.
The TRADE OFF......will be a small loss in the optimum forward gain (10% to 15%).
In a nutshell......when you make a change to one part of the antenna, this changes the performance of another part.....all changes interact with each other and the final performance!

GAIN vs FRONT-TO-BACK RATIO

With highest forward gain design, the main lobe becomes narrower in both the elevation and azimuth planes, and a backlobe is always present. When you design "out" the backlobe, the pattern gets wider and the forward gain goes down. In some cases, the sidelobes become quite large.

CONCLUSION

Advantages and Disadvantages

    Antenna Systems can make or break an wireless system installation. The engineers at Professional Wireless Systems can assist you in determining the best antenna and cable foryourneeds. Below is a list of common antennas. Each antenna has it's own advantages and disadvantages. It's not really possible to say that any one antenna is best or worst. That determination can only be made on a case by case basis for each installation. 
Other Accessories such as multicouplers, line amplifiers, passive combiner / splitters and filters are also part of the antenna system. Let PWS provide you with "Custom Solutions" for your installation. Entire installation packages are available on a custom designed basis. 


Advantages

    The Yagi-Uda Antenna is a widely used antenna design due to its high gain capability, low cost and ease of construction. It consists of a dipole arranged with various parasitic elements.

example:

6 to 9 dbi gain 
(depending on the number of elements) 
Very wide bandwidth 
(450 to 975 MHz for the model shown) 
Typically 50° to 70° beam width 

The "Log Periodic Dipole Array" is ideally suited for use with multiple receiver installations covering a wide band of UHF frequencies and where directivity, long range or back end rejection of interference is desired. Compare this antenna to a choir microphone. 

6 to 10+ dbi gain 
(depending on the number of elements) 
Narrow bandwidth 
(506 to 536 MHz for the model shown.) 
Typically 40° to 70° beam width 

This antenna is ideally suited to installations in which the range of frequencies in use is fairly small. This antenna provides long range (from the front) and high rejection (from the rear). The tight RF bandwidth and narrow beamwidth of this antenna make it ideal for custom applications with high demand requirements. Compare this antenna to a shotgun microphone with a tight acoustic filter.




To get more advantages about Yagi Uda antenna.

Optimization of the Yagi-Uda Antenna can be achieved by simulating the radiation patterns for various lengths of the elemnets and the spacing between them. Other factors that effect the radiation pattern are:

For an antenna with a length of 6 wavelengths or more the overall gain is independant of the director spacing. 

The reflector size and spacing have negligable effect on the forward gain and large effects on the backward gain and input impedance. 

The size and spacing of the directors has a large effect on the forward gain, backward gain and input impedance. 

More than one reflector provides little improvement on the directivity of the antenna. 

The addition of more directors will increase the gain of the antenna although after the addition of approximately 5 directors the advantages of adding more directors decreases significantly. 

The use of a folded dipole will increase the input impedance of the driven element. This is an advantage as the Yagi design generally has a low input impedance and the antenna impedance needs to match the transmission line impedance. 


Disadvantages

Generally, receiver of yagi uda antenna having some problem in receiving the signal. The function of these elements is to enhance the radiation pattern in the source direction. The reflector will be 5% longer than the driven element (ie diploe)and the directors will be 5% shorter. Parameter limits are:

-Driven Element only produce about 0.45-0.49 wavelengths. 

-Directors also can provide only 0.4-0.45 wavelengths. 

-Separation between Directors only have 0.3-0.4 wavelengths. 

-Radiation of directors is 0.15-0.25 wavelengths. 

-Separation between driven element an parasitics only 0.15-0.25 wavelengths.