QUICKYAGI

Yagi Designer / Analyzer / Optimizer

YAGI DESIGN BASICS

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The yagi antenna has been with us for more than half a century now, but

only with the recent advent of the personal computer has its real potential

begun to be explored by radio amateurs. Hopefully, this software package

will contribute to continued exploration.

The following text contains some basic yagi design concepts aimed toward

the non-technical user who may need assistance in this area.

I ELEMENTS

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The yagi antenna's basic design is a "resonant" fed dipole (the fed dipole

will be referred to from here on as the driven element ), with one or more

parasitic elements. These parasitic elements are called the "reflector"

and the "director." A dipole will be "resonant" when its electrical length

is 1/2 of the wavelength of the frequency applied to its feedpoint.

A: THE DIRECTOR

The director is the shorter of the parasitic elements. It is resonant some-

what higher in frequency than the driven element, and its length will be

about 5% shorter. The director length can vary by a considerable amount,

depending upon the director spacing, the number of directors used in the

array, the desired pattern or pattern bandwidth, and element diameter. The

number of directors that may be used are limited only by the physical size

of the array. The parasitic director is used primarily to achieve direct-

ional gain. The amount of gain is directly proportional to the length of

the array and not by the number of directors used. The spacing of directors

can range from .1 wavelength to .5 wavelength or greater and will depend

largly upon the design criteria of the array.

B: THE REFLECTOR

The reflector is a parasitic element that is placed to the rear of the

driven element. Its resonant frequency is lower, and its length is approx.

5% longer than the driven element. Its length will vary with the spacing

and element diameter. The spacing of the reflector will be between .1 wave-

length and .25 wavelength. Spacing will depend upon the gain, bandwidth,

F/B ratio, and sidelobe level requirements of the array design.

II BANDWIDTH

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A: IMPEDANCE

The impedance of an element is its value of pure resistance at a feedpoint

plus any reactance, either capacitive or inductive, that is present at that

feedpoint. Our primary concern here is the impedance of the driven element,

as this is where the transfer of rf energy from the feedline takes place.

Maximum transfer of rf energy occurs when the impedance of the feedpoint is

equal to the impedance of the feedline. In most cases, feedline impedance is

50 ohms, but rarely is the feedpoint impedance of a yagi 50 ohms. In most

cases it will vary from approximately 40 ohms to around 10 ohms, depending

upon the number of elements and the pattern bandwidth. If the feedline

impedance does not closely match the feedpoint impedance, the driven element

cannot effectively absorb the transmitted energy, thus reflecting it back to

the feedline (SWR). For this reason, impedance matching devices are highly

recommended for optimum performance.

The impedance bandwidth is the range of frequencies above and below the

center design frequency that the driven element's feedpoint will effectively

accept power from the feedline. It is desirable to have the reactance at

the center design frequency of the yagi be nil (j +- 0). This will allow

the impedance matching device to operate at its optimum bandwidth. Also wide

element spacing, large element diameter, wide pattern bandwidth, and low "Q"

matching systems will all contribute to a wider impedance bandwidth.

B: PATTERN

The radiation pattern plays a major role in the performance of the yagi

antenna. The directional gain, front-to-back ratio, beamwidth, and unwanted

sidelobes combine to describe the radiation pattern. The radiation pattern

bandwidth is the range of frequencies above and below the design frequency

in which the radiation pattern remains consistent. The degree of non-

consistency that can be tolerated is subjective, and limits such as minimum

front-to-back ratio and sidelobe levels are mainly a matter of choice.

Equal spaced / equal length directors may give higher gain at a particular

frequency, but the bandwith is narrow and unacceptable sidelobe levels are

common, and while wide spacing will increase the bandwidth, the sidelobes

become quite large.

By varying both the spacing and director lengths (many successful combin-

ations are possible) the pattern and the pattern bandwidth may be controlled.

More directors within a given boomlength will not increase the gain by any

large measure, but will allow better control of the pattern over a wider

frequency range.

By reducing the length of each succedent director by a set factor, while in-

creasing the spacing of each succedent director by another factor, a very

clean pattern with a good pattern bandwidth can be obtained. The trade off

will be a small reduction in the optimum forward gain (10% to 15%).

III GAIN vs FRONT-TO-BACK RATIO

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The subject of gain vs front-to-back ratio can be related to the adage about

"having your cake and eating it too," which is to say that both cannot exist

at the same time. At the point of highest forward gain the main lobe becomes

narrower in both the elevation and azimuth planes, and a backlobe is always

present. When this backlobe is suppressed, the pattern becomes wider and the

forward gain decreases. In some cases, the sidelobes become quite large.

IV FEEDING THE YAGI

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There are a variety of ways to feed the yagi, but they may be condensed into

two basic categories; the balanced feed and unbalanced feed.

The balanced feed system may give a broader impedance bandwidth, but the main

problem is that the driven element must in most cases be split in the center

and insulated from the boom. Construction considerations aside, it is the

better of the feed systems. Meeting the requirements of a balanced matching

system is usually the main problem, but there are many methods available.

One method is to not split the driven element and use a "T" match, which can

be described as two gamma matches on each side of the center of the element,

fed with a 1:1 balun at the center. The main drawback is that it's difficult

to adjust.

Another method (for low impedance feedpoints) uses a split element insulated

from the boom, and is fed with a "down-step 4:1 balun" made by combining two

1/4 wavelength sections of coaxial feedline in parallel, attaching an equal

length of insulated wire to the outside of these sections, and connecting it

to the center conductors at the feedpoint end and to the shields at the feed-

line end. The impedance of this type of "balun" should be at or near the mid-

point value between the feedpoint impedance and the feedline impedance. For

example, two 75 ohm sections paralleled will equal 37.5 ohms and will match a

25 ohm feedpoint to a 50 ohm feedline with a 1.0 to 1 SWR.

The most common method in use by hams today is the gamma match. It will

provide an easy and sure method of matching to the feedpoint without any

loss of bandwidth. Run QYUTIL.EXE for gamma match construction details.

Further information on antenna design and feed systems may be found in The

Radio Amateurs Handbook, The ARRL Antenna Handbook, Dr. J.L. Lawson's Yagi

Antenna Design (ARRL), or Bill Orr's Radio Engineer's Handbook, to name

only a few.

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