Yagi-Uda (Beam) Antenna
Gary A. Thiele KD8ZWS (Ex W8RBW)
Co-author of Antenna Theory & Design John Wiley & Sons, 1981, 1998, 2013 Yagi-Uda (Beam) Antennas
Outline
• Preliminary Remarks • Part I Brief history of the Yagi-Uda (Beam) • Part II Brief personal notes • Part III A few antenna basics • Part IV A technical discussion
Note: No equations!
Book Facts
• 3rd edition currently in print, 3rd printing pending • 3rd edition to be published in Korean, Portuguese and maybe Chinese • Over 1500 equations (but none in this talk!) • 822 pages, 16 chapters • 3rd edition pirated on the internet
Part I A Brief History
• Objective: to present some historical facts and early uses of the Yagi-Uda antenna. Part I A Brief History
• The Yagi-Uda antenna was invented in 1926 in Sendai, Japan by Asst. Prof. Shintaro Uda in colaboration with his mentor Prof. Yagi.
Shintaro Uda History – cont.
• Uda initially was tasked to design an oscillator to radiate at 4.4m wavelenth ( ~68 Mhz.) • For the antenna he used a resonant loop. Then he placed a parasitic loop nearby and observed more directive radiation.
History – cont.
• Subsequently he replaced the loops with metal rods observing that more rods (now know as directors) increased the field intensity in the desired direction. • He then undertook a systematic investigation to determine the effect on antenna directivity of changes in length, spacing and geometric arrangement of the parasitic elements. • The 8 director 68 MHz Yagi-Uda on the next slide is a result of his investigation.
History – cont.
• Uda and Yagi jointly published the results of their investigation in 1926 in Japan. There followed a series of papers in the Journal of the IEE Japan (in Japanese). • U.S. engineers learned of the new antenna at a Tokyo conference in Nov. 1926. This was followed by a classic IEEE journal publication in the U.S. in May 1927 by Yagi and Uda. • Prof. Yagi visited the U.S. in 1928 giving a number of talks about the antenna further publicizing it. History – cont.
• The Yagi-Uda antenna was widely used in WWII radar sets, both by Japan and the Allies. • Recall, kilowatts of power then could not be generated at centimeter wavelengths. Hence, the use of the Yagi-Uda at meter wavelengths. • When Japan captured Singapore and the British radar sets, they were at first interested in the Yagi-Uda antennas, not realizing they were invented in Japan in 1926!
History – cont.
• What was needed in WWII was a sufficient power source at centimeter wavelengths to enable the development of airborne radar. • In 1941 the British developed a cavity magnetron, capable of many kilowatts of power at centimeter wavelengths, and shared it in great secrecy with the U.S. • Centimeter wavelength radar was a major contributor to the defeat of the U-boat threat.
History – cont.
• After the war, the Yagi-Uda (Beam antenna) became a Ham favorite and various perturbations were developed for TV reception. Part II Some Personal Notes
Objective: To provide some insights into the Japanese culture, and to show the pride the Japanese take in the fact that the Yagi-Uda was invented in their country. Japan 1978
• A senior colleague and I are in Japan for an antenna conference in Sendai. • We had a number of Japanese friends who had spent one or two years at the Antenna Laboratory at The Ohio State University when I was a graduate student 1963-1967. • Our friends are treating us to an overnight at the Hitachi Corp. Guest House. Mountains near Tokyo 1978 Hitachi Corp. Guest House Pre Dinner Table Dinner Table Saying Goodbye
Shintaro Uda
Historical Recognition
• The Yagi-Uda antenna was named an IEEE Milestone in 1995. Part III A Few Antenna Basics
Objective: To define a few basic terms used in Part IV.
• Wavelength • Electromagnetic Field • Radiation Patterns • E-Plane and H-Plane • Gain Wavelength
180o 360o 0o 90o
270o
The Electromagnetic Field Received by an Antenna
The electric and magnetic fields are at right angles to each other and at right angles to the direction of propagation along the z-axis to the right.
E x Figure: The spatial behavior of the electric (solid) and magnetic (dashed) fields of a linearly z (vertical) polarized wave for a fixed instant of time.
E and H in Time domian time phase H picture y S T U T Z M A N & T H I E L E
Radiation Patterns
A radiation pattern (antenna pattern) is a graphical representation of the radiation (far-field) properties of an antenna.
The radiation fields from a transmitting antenna vary inversely with distance, e.g., 1/r. The variation with observation angles (θ, ), however, depends on the antenna only.
The transmit and receive patterns of an antenna are identical by reciprocity.
Reciprocity allows the calculation or measurement of an antenna pattern in either the transmit or receive case, whichever is more convenient.
S T U T Z M A N & T H I E L E
Pattern Characteristicsz
Main lobe Major lobe (unidirectional) First null beamwidth (FNBW) Half-power beamwidth (HPBW)
0.5 Minor lobes Side lobe
y
Minor Back lobe (in this example) lobes Minor lobes x 3-D Picture S T U T Z M A N & T H I E L E Polar Radiation Patterns – cont.
(c) Decibel pattern (b) Relative field pattern
S T U T Z M A N & T H I E L E Gain The concept of (maximum) gain: “how much does an antenna concentrate radiation in one direction at the expense of other directions (if antenna ohmic losses are included).”
Note:
1. Gain here does not include losses arising from impedance and polarization mismatches.
2. Manufacturers usually include the impedance mismatch loss in the gain. For a well-matched antenna, this inclusion results in the published gain being slightly reduced.
S T U T Z M A N & T H I E L E
Part IV A Technical Discussion
• Objective: To show the influence of the reflector and director elements, and their effect on the wave velocity. Also, to explain how the Yagi-Uda radiates.
2 Ways to understand the Yagi-Uda
• Element Phasing
• Wave approach Yagi-Uda (Beam) Antennas Spatial Phase Delay (Ordinary Endfire, v=c )
o o o → Forward
90o 0o -90o
⋮ λ/4 ⋮ λ/4 ⋮
The Wave Approach
Wave Launcher Guided Wave Surface
Wave Launcher: Reflector and Driven Element
Guided Wave Surface: Director Elements How does a Yagi-Uda radiate?
When an electromagntic wave encounters a discontinuity, something will happen.
In the case of the Yagi-Uda two things happen at the “end” of the guided wave surface: radiation and reflection.
Radiation does not occur along the directors, only at the end where there is a discontinuity.
That is, The Yagi-Uda is not a leaky wave antenna. 27 element currents Why is there usually only one reflector element?
Yagi-Uda Antenna - effect of reflector element
Optimum reflector spacing SR (for maximum directivity) is between 0.15 and 0.25 wavelengths as Fig. 5-35 shows. Note that the gain above an isolated dipole is more than 2.5 dBd, whereas if a flat plate were used, instead of a simple wire-like element, the gain would be 3 dBd. Thus, a single wire-like reflector element is almost as effective as a flat plate in enhancing the gain of a dipole! 3
Figure 5-35 Measured 2 gain [21] in dBd of a dipole and reflector element for different SR 1 spacings SR. Reflector Driver
0.05 0.10 0.15 0.20 0.25 0.30 0.35
Spacing SR () of reflector behind driven element S T U T Z M A N & T H I E L E
Why was a folded dipole used for the driven element in the previous slide?
Answer: The mutual coupling of the driven element with the nearby parasitic elements reduces the feed point impedance, sometimes dramatically. The self impedance of a folded dipole is 4 times the self impedance of straight wire half-wave dipole, which helps to overcome the effects of mutual coupling. Yagi-Uda Antennas - design
0.50 0.42λ Long Yagi
1 Dir, 1 Refl. SD 0.2λ 0.49 Reflector 2.2λ Long Yagi
10 Dir, 1 Refl. SD 0.2λ 0.48
0.47 Directors 0.8λ Long Yagi 3 Dir, 1 Refl, SD 0.2λ 0.46 1.2λ Long Yagi
4 Dir, 1 Refl, SD 0.25λ 3.2λ Long Yagi
0.45 15 Dir, 1 Refl, SD 0.2λ 4.2λ Long Yagi 13 Dir, 1 Refl, S 0.308λ 0.44 D
0.43 Length L Spacing SD 0.42
0.41 Diam d D1 D2 D3 . 0.40 . . D4 0.39
0.38 0.001 0.002 0.003 0.004 0.006 0.008 0.01 0.02 0.03
Figure 5-37 Design curves for Yagi’s in Table 5-2.
S T U T Z M A N & T H I E L E Yagi-Uda Antennas - correction for support boom
0.030
0.025
0.020
0.015 Figure 5-38 Graph showing effect of 0.010 supporting un-insulated metal boom on the length 0.005 of Yagi parasitic elements.
0.002 0.006 0.01 0.02 0.03 0.04
S T U T Z M A N & T H I E L E Question
Why can we electrically connect the metallic parasitic elements to a metallic boom? Half wavelength
V (+) Current
Voltage (-) TV Channel 12 Yagi Antenna Design
A 12-element Yagi for TV channel 12 at 205.25 Mhz is to be designed using 1-cm-diameter elements insulated from a metallic boom [23]. The boom length is to be 2.2. Table 5-4 indicates that 0.2 spacing is required. The wavelength at 205.25 Mhz is 1.46 m. Thus, the spacing between all elements is 29.2 cm. To obtain the element lengths, the following four steps are followed: Etc, etc
Result for L,L0.483λ 0.4375 λ case “B”: RD1
S T U T Z M A N & T H I E L E Yagi-Uda Antennas - design
0.50 0.42λ Long Yagi
1 Dir, 1 Refl. SD 0.2λ 0.49 Reflector 2.2λ Long Yagi B case 10 Dir, 1 Refl. SD 0.2λ 0.48
0.47 Directors 0.8λ Long Yagi 3 Dir, 1 Refl, SD 0.2λ 0.46 1.2λ Long Yagi
4 Dir, 1 Refl, SD 0.25λ 3.2λ Long Yagi
0.45 15 Dir, 1 Refl, SD 0.2λ 4.2λ Long Yagi 13 Dir, 1 Refl, S 0.308λ 0.44 D
0.43 Length L Spacing SD 0.42
0.41 Diam d D1 D2 D3 . 0.40 . . D4 0.39
0.38 0.001 0.002 0.003 0.004 0.006 0.008 0.01 0.02 0.03
Figure 5-37 Design curves for Yagi’s in Table 5-2.
S T U T Z M A N & T H I E L E TV Channel 12 Yagi Antenna Design - cont.
x y
1.0 1.0
0.75 0.75
0.5 0.5
0.25 0.25
z z
(a) E-plane. (b) H-plane.
Figure 5-41 Calculated field patterns for Example 5-1. S T U T Z M A N & T H I E L E Why not use 40 or 50 director elements?
Yagi-Uda Antennas - effect of director elements
While it is usually not worth while to use more than one refector element, adding director elements does increase the gain of the Yagi antenna. However, the “law of diminishing returns” does apply as Fig. 5-36 shows. In many cases, if more than about 12 dBi of gain is desired, it is better to consider “stacking” or arraying Yagi’s to achieve additional gain. 13 12 11 10 9 8 Figure 5-36 Gain of a typical 7 Yagi-Uda antenna versus the 6 total number of elements. The 5 element Spacings S = S = 4 R D 3 0.15. The conductor diameters 2 are 0.0025. (From Green [20].) 1
1 2 3 4 5 6 7 8 9 10 11
S T U T Z M A N & T H I E L E
An 8-Element Yagi-Uda Antenna Illustrating the Effect of Director Length on Pattern and Bandwidth
Conclusion for the 8 element array
• At a director length of 0.435 wavelengths the back lobe is only down 6 dB. That is, the front to back ratio is but 6 dB, which is unacceptable.
• Further, at a director length of 0.430 wavelengths, the wave has been slowed down too much and the pattern has deteriorated . It has deteriorated even more at 0.435 wavelengths.
• The sensitivity to director length is indicative of the narrow band characteristic of the Yagi-Uda. Thank You ! Questions? The End