By

Joseph P. Campbell, HNC Marine Nav. Sys. Eng.

Contents 1. History and Development

2. A folded ½ Wave Dipole

3. The Yagi-Uda Array: Elementary Principle

4. Effects of Additional Parasitic-Elements on the Driven-Element’s Electrical Characteristics

5. Side-Lobes and Computer Simulation

6. Parasitic-Element Spacing and Aerial Gain

7. Yagi-Uda Arrays and Noise Temperature

continued...

contents…continued

8. Stacking and Baying Yagi-Uda Array Aerials

9. Stacking-and-Baying: The Advantages

10. The Yagi-Uda Array: Its Applications

11. Yagi-Uda Array Aerial Paradigms 1 – 8

12. Synopsis

13. References 1 - 6

14. About The Author

History and Development During the early 1920’s Professors Yagi and Uda, based at a university in Japan, set about researching and developing a directional aerial for the High Frequency (HF), Very High Frequency (VHF), Ultra High Frequency (UHF) and the longer wavelengths of the Super High Frequency (SHF) microwave band –centimetric-waves*longer than about 5cm approx... photo. 1

Professor Hidetsugu Yagi

* Centimetric-waves are waves whose wavelengths are most appropriately measured in centimetres

A folded ½ wave dipole

...The folded ½-wavelength dipole is the elementary aerial that most forms of practical commercially made Yagi-Uda Arrays, “Yagi’ aerials”, are based on, except for ones designed for receiving High Frequency (HF) band signals and some two-element VHF-designs. This folded dipole will have a feed-point impedance of about 300Ω if not used in a Yagi’ aerial arrangement, see photo. 2...

photo. 2 ½ wavelength folded dipole

The Yagi-Uda Array: Elementary Principle ...A half-wave dipole has minimal

directional properties. Indeed when it’s vertically polarized it can be considered to be a practical omnidirectional antenna.

In order to increase its directionality (and therefore “gain”) we can place a parasitic-element behind it of a specified greater length than this `dipole, and at a specified distance from it with the same polarization. We now have a basic Yagi’ aerial comprising two elements as shown in fig. 1... fig. 1

Addition of a “director” element ... If we just make do with a half-

wave dipole and a director - element we won’t usually have enough directionality for most of the applications concerning radio frequency (RF) and microwave -wireless reception and transmission. In order to mitigate the problem it’s possible to add a further parasitic-element, but this time, in front of our half-wave dipole; the `dipole being our “driven-element” i.e. the point where a feeder cable is connected to our Yagi’ aerial. See fig. 2...

fig. 2

Reflector and 1st Director-Element: Proportional Size to the Driven-Element

fig. 3

Yagi’ Array Aerials: Further Principles

...The parasitic elements of the Yagi’ aerial 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’s found that the amplitude and phase of the current that’s induced in the parasitic-elements is dependent upon their length and the spacing between them and the dipole or driven-element as it’s known...

Yagi’ Array Aerials: Further Principles – Part 2

...Using a parasitic-element it’s not possible to have complete control over both the amplitude and phase of the currents in all the elements. This means that it’s not possible to obtain complete cancellation in one direction. Nevertheless, it’s still possible to obtain a high level of gain in addition to having a high degree of cancellation in another in order to provide a good front-to-back ratio and front-to-rear ratio...

Yagi’ Array Aerials: Further Principles – Part 3

...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 Yagi’ aerial to radiate more power away from it. An element that does this is called a reflector...

Yagi’ Array Aerials: Further Principles – Part 4

...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 a parasitic-element in the direction of the aerial’s remaining ones. An element which does this is called a director...

Yagi’ Array Aerials: Further Principles – Part 5

...A parasitic-director element can be made capacitive by tuning it above its resonant frequency value i.e 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 aerial, increasing the gain and reducing the beamwidth. The addition of further directors makes no noticeable difference above a certain number for a Yagi’ aerial designed for a particular radio-frequency (RF) or SHF sub-band...

Effects of additional parasitic elements on the driven element’s electrical characteristics:

...With the addition of a reflector parasitic element the impedance (Z) at the cable feedpoint of the ½ wave-dipole acting as the “driven-element” decreases. Adding a further parasitic element, this time in front of the driven-element and known as a “director”, decreases this impedance further. In fact, the greater the number of parasitic-directors placed in front of the driven-element, the smaller the value of Z at the Yagi’ aerial’s feed-point...

Effects of additional parasitic elements...continued

...This phenomenon has implications concerning the correct Z-matching to the 50Ω or 75Ω coaxial feeder- cable that the aerial at issue is connected to. In order to circumvent this problem the driven-element is a “folded” ½-wavelength dipole. Such a `dipole on its own has a feedpoint Z of about 300Ω; the addition of parasitic-elements brings this Z value down to approximately the value of the coaxial-feeder that’s appropriate for the receiving or transmission-system in question. The spacing distance between the driven-element and the parasitic- elements will also affect the value of impedance at the ½-wave’ dipole’s feed-point ...

Half-Wave Dipole, 2-Element and 3-Element Yagi-Uda Arrays (all horizontally polarized); directional characteristics, i.e. “radiation plots”:

fig. 4 Half-wave dipole (horizontally polarized)

fig. 6 3-Element Yagi’ aerial...

fig. 5 2-Element Yagi’ aerial

Side-Lobes and Computer Simulation

... Depending on the spacing of the parasitic-reflector and director elements a Yagi’ aerial will display side lobes to a greater or lesser extent. The spacing distance will also determine the “front-to-back ratio” and therefore the size and number of rear-lobes present at the aerial...

fig. 7 A radiation plot that has been output from a computer aerial design and simulation software application for a 2 meter (VHF amateur RF allocation) 9-element Yagi-Uda Array’ with a gain of 16.3 dBi

Sidelobes and Computer Simulation...continued

A home-made –“home-brewed” in amateur-wireless parlance – Yagi’ aerial will nearly always display a greater number of side-lobes – and ones that are more pronounced – than an aerial for the same frequency allocation which is commercially made, photo. 3 – top of chimney stack...

photo. 3

Sidelobes and Computer Simulation...continued (2)

...manufacturers have access to state-of-the-art aerial design simulation PC software applications that outputs the optimal parasitic element spacings for a Yagi’ aerial given the required RF or microwave-wireless bandwidth and gain characteristics that are input into the computer in question concerning a particular band in addition to the diameter of the driven and parasitic- elements*...

*There’s a restriction imposed by OFCOM on diameter, concerning

transmitting uses, subject to the maximum range permitted for the particular RF or microwave-wireless system at issue.

Parasitic Element Spacing and aerial gain

...As I’ve outlined, the spacing of the parasitic-elements determines the number, size and position of the side lobes of a Yagi’ aerial. Thus parasitic-element spacing is a major factor in determining the aerial’s gain at a given frequency within its bandwidth as well as, of course, the number of ’elements comprising it and the impedance value at the Yagi’ aerial’s driven-element feedpoint, fig. 8...

fig. 8 A graph showing the plot of the power transmitted by a Yagi-Uda’ Array aerial for a given spacing value of parasitic director element expressed in fractions of a wavelength above and below the full-wavelength (λ) of a given Yagi’ aerial’s resonant frequency.

Parasitic Element Spacing and Aerial Gain...continued

... Because of the reciprocity theory concerning a given aerial used for transmitting or receiving, or both, the radiation plot of it will be identical for both types of operation and so, therefore, fig. 9 will have a very similar graphical form to fig. 8...

. fig. 9 A graphical plot of received current in microamperes (μA) against parasitic element spacing expressed as fractions of a wavelength above and below the full-wavelength (λ) of the resonant frequency of the given Yagi-Uda’ aerial featured in fig. 10

Yagi-Uda Arrays and Noise Temperature

...As illustrated in fig. 10, the greater the size of the side and rear-lobes of the Yagi’ aerial in question, the greater the pickup of unwanted electromagnetic- noise and therefore the greater the value of noise temperature present in it...

fig. 10 The effect of side-lobes and rear-lobes on the noise temperature value present in a Yagi’ aerial.

Yagi-Uda Arrays and Noise Temperature...continued

...One of the objects in avoiding a large number of side-lobes and of the ones left, minimizing their size at the design stage, is to avoid receiving more free-space propagated electromagnetic-noise than is absolutely necessary. Such noise generators include the atmosphere, solar and galactic sources e.g. the milky way; the galaxy where our solar system resides, see fig. 11 ...

Natural Sources (Sky Noise)

• Cosmic noise – emitted from star and inter- stellar matter

– Decreases with frequency – negligible above 1GHz

– Certain parts of the sky have “hot sources” - avoid

• Sun (T ≅ 12000 f-0.75 K)

– Point antennas away from it

• Moon

– Black-body radiator: 200 to 300 K if the aerial is pointed at the moon

• Propagation medium

– e.g. rain, oxygen, water vapour – microwave emissions detectable from water vapour

fig. 11 factors contributing to the noise-temperature value (K=Kelvin) at a receiving aerial

Stacking & Baying Yagi-Uda Arrays

... A method of increasing the directionality over that present with one Yagi’ aerial is to use two or more Yagi’ aerials mounted vertically above one another and separated by a specified number of wavelengths (λs) that’s in accordance with proven aerial-theory. This technique is known as “stacking”. For a practical example of two stacked Yagis’, see slide 29...

Stacking & Baying: The Advantages

...If you stack two identically polarized Yagi’ aerials correctly, an therefore in accordance with proven aerial theory, you’ll decrease the vertical beamwidth available when compared to only using one Yagi’ aerial i.e. you’ll have a more directional aerial arrangement in the vertical plane – see fig. 12. For a practical example, see slide 29.

If you bay two identically polarised Yagis correctly you’ll decrease the horizontal-beamwidth available i.e. you’ll increase the directionality in the horizontal plane; see fig. 13.

Stacking and baying, and combined stacking and baying (“stacking-and-baying”), are both forms of “spatial-diversity” receiving and/or transmitting aerial arrangements...

Stacking & Baying: The Advantages – Part 2

...With both techniques you must use an electrical circuit known as a “combining-unit” which combines the output of the two or more aerials of interest in phase before sending it down a coaxial feeder-cable to an RF or microwave receiver or, if this arrangement is used for transmitting , feeds the RF or microwave-energy into the stacked and/or bayed configuration in phase...

Stacking & Baying: The Advantages – Part 3

... It’s possible to have both a stacked and bayed arrangement using a minimum of four Yagi’ aerials. If you both stack and bay identically polarized Yagis then you’ll decrease the beamwidth value in both the vertical and horizontal planes in comparison to that available when using only one Yagi’ aerial if it’s identical to the ones used in the multiple array under discussion. One combining unit is used with a stacked-and-bayed Yagi’ aerial arrangement...

Stacked and Bayed Yagi-Uda Array Aerials – Schematic Diagrams

fig. 12

fig. 13

Fig. 12 shows a stacked arrangement of two Yagi’ aerials whilst fig. 13 shows two Yagi’ aerials in a bayed-arrangement. It’s possible, for some applications, to use a greater number of Yagi’ aerials in a stacked or bayed formation. Additionally , it’s possible to have a stacked-and-bayed arrangement of four or more Yagis or, indeed, some other forms of aerial e.g. parabolic-dish designs.

Stacking Yagi-Uda Arrays – Practical Application Example

photo. 4

...Photo. 4 shows a stacked-arrangement of two twelve-element Yagi-Uda Arrays, vertically polarized, and dimensioned for a UHF telemetry frequency allocation somewhere between 300 MHz and 420 MHz. The telemetry signals they’re transmitting are various operational parameters which concern the functioning of a remote water-utility covered service reservoir that’s located outside the town of Moffat in Dumfries & Galloway, Scotland; …continued

Stacking Yagi-Uda Arrays – Practical Application...continued

...the receiving site for these telemetry transmissions being located some distance away. Note the solar panels (photovoltaics), at the base of the stacked- aerials support stalk, that are used to generate DC electrical power for this transmission/receiving arrangement.

You can also mount two identical Yagis side-by-side and spaced by the optimal number of λs apart; this technique being known as “baying”...

The Yagi-Uda Array Aerial: Its applications

1. VHF and UHF broadcast-television and VHF radio reception.

2. low power UHF studio-transmitter links (STLs) that are licenced by OFCOM for use with Restricted Service Licence (RSL) temporary radio broadcasters such as those organised by college, university, voluntary or religious groups.

3. VHF and UHF-wireless telemetry systems used in public utilities concerning electricity, gas and water supply installations.

The Yagi-Uda Array Aerial Its applications – continued

4. emergency embassy HF radio-communications systems using sky-wave propagation (i.e. via ionospheric reflection) that are used in the event of their Low Earth Orbit (LEO) satellite-communications (SATCOMs), using microwaves, failing.

5. military HF, VHF and UHF wireless-telemetry and other communications systems.

continued...

The Yagi-Uda Array Aerial Its applications – continued (2)

6. amateur-wireless operations covering the radio frequency band starting at about 12 MHz (HF) and going up to the Super High Frequency (SHF) microwave amateur-band allocations below about 6.2 GHz...

The Yagi-Uda Array Aerial Its applications – continued (3) 7. as part of a stacked-and-bayed spatial-diversity

receiving arrangement comprising four Yagi-Uda Arrays for the reception of low field-strength SHF microwave-wireless transmissions, sometimes known as “centimetric-waves”, that lie above 3.2GHz –equating to a wavelength of about 9.38 cm – but below about 6.2 GHz – equating to wavelengths longer than about 4.84 cm. In addition to providing increased signal gain, combined stacking and baying of identical Yagi-Udas’ provides an increased bandwidth and combats multipath-propagation (“multipath”) effectively in comparison to using only one Yagi-Uda’ aerial...

The Yagi-Uda Array Aerial Its applications – continued (4)

...These microwave emissions can suffer from multi-path propagation difficulties and so more than one Yagi-Uda’ Array aerial maybe needed in order to provide more directionality and so remedy this problem – see slides 24, 25, 26, 27, 28 and 29 further back...

UHF Broadcast Television Channel-Numbers and how they equate to aerial-group letters and colours (fig.14):

fig. 14

UHF Band IV and Band V TV Broadcast Channel-Numbers: Band IV

Band V

fig. 15

fig. 16

Figure 15 shows a chart equating UHF Band IV channel numbers to their respective carrier frequencies; vision-carrier frequency only, if considered in the context of PAL analogue TV channels. Figure 16 shows the UHF Band V channel numbers and the carrier frequencies they equate to (the former PAL TV statement applies here, also). Thus, a Yagi’ aerial suitable for receiving Group A (colour-code: RED) channels will have elements many centimetres longer than Group C/D (‘code: GREEN) Yagi’ aerials; aerial group C/D covering reception of the highest-’Band V frequencies.

…Figure 17 shows a graph displaying curves

that show UHF Band VI and V Yagi’ aerial-

group gain in dBd (decibels-relative-to-a-

dipole’s gain) versus UHF channel-

frequency. This graph shows why it’s not

good practice to always install a wide-band

Yagi’ aerial (Group W: colour-code BLACK)

to receive any arrangement of UHF

television-channel groups i.e. in order to

ensure maximum signal-level across a

broadcast TV receiver’s aerial-circuit you

should select a Yagi’ aerial that just covers

the channel-frequency numbers that are

required to be received…

Aerial-Group Gain versus Channel-Frequency

fig. 17

Aerial-Group Gain versus channel-frequency …continued

…Additionally, “blanket-use” of Group W aerials will unnecessarily increase the noise-temperature across a TV receiver’s aerial circuit which, if the signal-strength levels are low, will manifest itself as a fuzzy – or fuzzier – TV picture than needs to be necessary in the case of the remaining analogue PAL TV services being received; you’ll usually get away with it in the case of DTT reception, but it’s not adviseable, as in rare cases it could put the received digital-TV picture over the “digital-cliff”*!...

* The bandwidth of a Yagi-Uda Array aerial is inversely-proportional to its gain – i.e. B α 1/G

Yagi-Uda Array Aerial Paradigms Photo. 5 shows a conventional Yagi’ aerial for installation in high signal-strength areas and used for UHF band VI or V broadcast-television reception; either for Digital Terrestrial Television (DTT) or the remaining analogue television services:

Photo. 6 shows a higher gain form of Yagi’ aerial, once known as an “X-Beam” Yagi’, again used for UHF broadcast-television reception but in low field-strength areas and still in production today. It’s pictured horizontally polarized...

photo. 5 photo. 6

Yagi-Uda’ Aerial Paradigms – Part (2)

fig. 12 Fig.12: a two-element array for the digital radio age; it’s dimensioned for the reception of VHF Band III Digital Audio Broadcasting (DAB) transmissions, located on the part of the VHF spectrum spanning 217.5 – 230 MHz in high to moderate signal strength areas. The length of the driven element, and therefore the director element, will be shorter than those of a two element Yagi’ aerial for VHF Band II (analogue FM radio) reception.

fig. 13 Fig.13: A three-element array for DAB reception in moderate signal strength areas...

Yagi-Uda’ Aerial Paradigms...Part (3)

fig. 14 Fig.14: a four-element array for DAB VHF Band III radio reception in low signal-strength areas i.e. those areas just outside the official coverage area of the transmitter concerned, or screened by foliage*.

fig. 15 Fig.15: a six-element array for DAB radio-reception in lower signal strength areas i.e. a significant distance away from the periphery of a specified transmitter’s coverage area**...

* foliage attenuates RF at VHF and above noticeably. This effect increases with the shortening of wavelength and therefore with increasing frequency (λ α 1/f). Wet foliage has an even greater attenuation and signal-reflection effect leading to higher field-strength multi-path propagation.

** a broadcast transmitter’s coverage area is defined in terms of a minimum electric field-strength value that’s to be received if a particular area is to be described as being within its service area. The sub unit of `field-strength used here is the μV/m.

Yagi-Uda’ Aerial Paradigms...Part (4)

photo. 7

Photo. 7 shows, top, a 10-element Yagi-Uda Array Aerial for high field-strength UHF Band V DTT areas and originally used for PAL analogue TV reception and located at my address in Wirral, UK. Below it, on the same stalk, is a VHF Band II 3-element Yagi’ Aerial for moderate field-strength reception of BBC national analogue FM radio services from the Holme Moss radio transmitter nr Leeds, West Yorkshire, which serves most of Northern England.

Yagi-Uda’ Aerial Paradigms...Part (5)

photo. 8

Photo. 8 shows a very similar arrangement to that shown in photo. 7, the difference being in the fact that the UHF Band V Yagi’ aerial mounted at the top of the stalk is of a higher gain than that at my address; it has another 7- elements making up this Yagi-Uda Array – a total of seventeen elements in all. The aerial installer was obviously inferior as he should have fitted a 10-element array as this address is located in the same road as mine...continued

Yagi-Uda’ Aerial Paradigms...Part (6)

...at the time this UHF Band V aerial was installed, the Winter Hill main television `Band V transmitter was carrying PAL analogue TV which reached The Leas in Thingwall, Wirral, at high field strength i.e. the additional aerial gain over my 10-element Yagi wasn’t necessary. However, with the advent of DTT radiated at a considerably lower power, this gain just wouldn’t go amiss; it’s, therefore, now an appropriate gain figure though by accident rather than design!

Yagi-Uda’ Aerial Paradigms...Part (7) Photo. 9 shows a Yagi’ aerial for an amateur-wireless frequency allocation below about 6 GHz on the SHF part of the microwave- spectrum – note the very short parasitic elements:

Photo. 10 shows an 11-element Yagi’ aerial for the amateur-wireless 144 MHz frequency allocation on VHF; also known as the 2-metres band in amateur-wireless operators parlance...

photo. 9 photo. 10

Yagi-Uda’ Aerial Paradigms...Part (8)

Photo. 11 shows, from top of the stalk: VHF 2m (144 MHz band), 4m (70 MHz band) and 6m (50 MHz band) Yagi’ aerials:

Fig. 16 shows a 3-element Yagi’ aerial for the 20-metre wavelength short-wave radio amateur-band, equating to a 15 MHz HF frequency allocation, with “traps”...

photo. 11 fig. 16

Synopsis

1. The Yagi-Uda Array aerial, “Yagi’ aerial”, is a directional aerial,

also known as a “beam-aerial”, based on the ½ wavelength dipole in a folded-form for practical commercially made designs, other than those for the HF band and some two-element VHF designs.

2. The more parasitic-director-elements a Yagi’ aerial has, the greater the directionality, i.e. the gain, that the aerial under examination possesses. However, above a certain number of elements for a given frequency band, there are diminishing returns in terms of increasing the received signal-level input into an RF or microwave- wireless receiver connected to it. The highest Yagi’ aerial gain designs that are practical can be engineered for the longer- wavelength SHF frequencies; that’s longer than 4.84 cm in wavelength – that is, below 6.2 GHz in frequency...continued

Synopsis...continued

3. Yagi’ aerials have applications in a variety of telecommunications and communication-systems in general, for public, commercial, industrial, governmental and military users.

4. Using the techniques of stacking and baying it’s possible to create very high gain, i.e. highly directional, aerial arrangements on the longer wavelength SHF-microwave frequencies (longer than 4.84 cm which equates to a frequency of 6.2 GHz) and so negating the need for an expensive parabolic-dish aerial which, for some applications, may provide an unnecessarily higher gain value…

References

1. “Radio and RF Engineering Pocket Book – Third Edition” – authors Steve Winder BA, MSc, CEng, MIET, MIEEE & Joe Carr first pub. 1994, third edition 2002 by Newnes – an imprint of Elsevier Science:

2. “Newnes Telecommunications Pocket Book – Third Edition” – author: Steve Winder BA, MSc, CEng, MIET, MIEEE, pub. 2001. by Newnes – an imprint of Butterworth Heinemann:

References (2) 3. “Satellite Communications – Link Design (Part 2)” a

presentation by Dr. Leila Z. Ribeiro and downloaded via the Internet from the website of the George Mason University in PDF form:

4. “Radcom” – Various editions of the Radio Society of Great Britain’s journal including the “Antennas” feature of the June 2008 edition: author Peter Dodd, G3LDO and the “In Practice” technical feature of the November 2009 edition written by Dr. Ian White, GM3SEK:

References (3)

5. The Reception Advice area of the website for the British Broadcasting Corporation. This area of the BBC’s website provides, amongst other technical topics, basic reception advice for broadcast receiving-systems using the Yagi’ aerial i.e. those receiving UHF Band IV and Band V Digital Terrestrial Television (DTT) and analogue channels, Digital Audio Broadcasting (DAB) radio on VHF Band III, and the analogue FM radio channels, located on VHF Band II:

References (4)

6. The website of the Christian Restricted Service Licenced (RSL) broadcasting organisation known as Flame Christian & Community radio - formerly “Flame FM” (see ref. 7 also):

7. An informal visit to the Flame FM RSL radio station based at the Wirral Christian Centre in the year 2004. This station transmitted for a number of two week periods:

Reference (5)

8. The Confederation of Aerial Industries – For information on domestic and commercial TV and radio-broadcast reception aerial-installation standards:

9. The public website of OFCOM – the independent regulator and competition authority for the UK’s communications

industries:

Reference (6) 10. Maxview – broadcast-reception aerial

manufacturers:

11. Triax – broadcast-reception aerial

manufacturers:

About The Author

Joseph P. Campbell is 39 years-old and is qualified with a BTEC Higher National Certificate (HNC) in marine navigational systems engineering from Liverpool Community College. His HNC was awarded to him in the year 1997. This branch of engineering involves the use of tele-communications and communications systems in general. He’s a qualified radio-amateur and a member of the RSGB; his OFCOM call-sign being G7OKR. He has in the past been active on the 20, 40 and 60 metres short-wave amateur bands (HF). Additionally, he occasionally builds small electronic project kits that require soldering skills to complete. His email address is g7okr@hotmail.co.uk .

Joseph P. Campbell

© 2010 Joseph P. Campbell, HNC Marine Nav. Eng.

Revised 17th July 2011