(A). Larger diameter elements can increase the bandwidth of a Yagi antenna.

A Yagi or Yagi-Uda antenna is composed of a driven element and several parasitic elements (a reflector and one or more directors). Changing to a larger diameter element can increase the bandwidth and SWR of the antenna.

Hint: the fatter the antenna element, the larger bandwidth, which is true for all antennas.

For more info see Wikipedia: Yagi-Uda antenna

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The approximate length of the driven element of a Yagi antenna is \(1 \over 2\) wavelength.

A Yagi or Yagi-Uda antenna is made up of a driven element and parasitic elements (a reflector and one or more directors). The driven element is (by definition!) \(1 \over 2\) wavelength. It is possible to make an antenna with a \(1 \over 4\) wavelength driven element but it would not be a yagi, since \(1 \over 2\) wavelength is part of the yagi design.

Silly Hint: The question asks about a Yagi but technically that's only half the full name of Yagi-Uda

For more info see Wikipedia: Yagi-Uda antenna

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For a three-element, single-band Yagi antenna, the director is normally the shortest parasitic element.

The 3-element Yagi or Yagi-Uda antenna is made up of a driven element (essentially a 1/2-wave dipole) and two parasitic elements - the director and the reflector. The director is typically the shortest element and is usually 95% of the length of the driven element. The reflector is then the longest element, and is typically 105% of the driven element length.

The differences in element length and their positioning contribute to the Yagi being unidirectional.

For more info see Wikipedia: Yagi-Uda antenna

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(A). In a three-element, single-band Yagi antenna, the reflector is normally the longest parasitic element.

The three element Yagi or Yagi-Uda antenna consist of a driven element (essentially a 1/2-wave dipole), and two parasitic elements - the director and the reflector. The director is the shortest element and is typically 95% of the driven element length. The reflector is constructed as the longest element and is typically 105% of the driven element length.

For more info see Wikipedia: Yagi-Uda antenna

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Gain increases as boom length is increased and directors are added to a Yagi antenna.

Both increasing the boom length and adding directors to a Yagi or Yagi-Uda antenna will increase the directivity or gain of the antenna. The extra directors serve to influence and concentrate the directivity of the signal. Boom length has the greatest overall effect on the gain of the Yagi antenna.

HINT: Increasing and adding makes GAINS

For more info see Wikipedia: Yagi-Uda antenna

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(C). The Yagi antenna is often used for radio communications on the 20 meter band because it helps reduce interference from other stations to the side or behind the antenna.

The Yagi or Yagi-Uda antenna is constructed of a driven element and additional parasitic elements (the reflector and one or more directors). The orientation of the parasitic directors and reflector make this a very effective directional antenna, with good signal gain and reduced interference from signals in directions other than the focus point of the antenna.

For more info see Wikipedia: Yagi-Uda antenna

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(C). The "front-to-back ratio" in reference to a Yagi antenna is the power radiated in the major radiation lobe compared to the power radiated in exactly the opposite direction.

One advantage of using a directional antenna such as the Yagi, is that the greater portion of the power (major lobe) is directed to the front, or the focused signal direction of the antenna. A much smaller part (minor lobe) is at the 180 degree direction. The ratio between the power in the major lobe as compared with the 180 degree lobe is the "front-to-back" ratio.

Memory Hint: front-to-back are opposite directions from each other.

Silly hint: Opposites attract.

For more info see Wikipedia: Yagi-Uda antenna, Front-to-back ratio

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The "main lobe" of a directional antenna is the direction of maximum radiated field strength from the antenna.

A directional antenna, such as the Yagi or Yagi-Uda antenna, radiates most of its energy in one focused direction. This major or "main lobe" is then much greater, with less signal loss to the sides or opposite direction.

Silly Hint: "Directive Direction". Directive is in the question and only one answer has Direction in it.

For more info see Wikipedia: Main lobe, Yagi-Uda antenna

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(A). The approximated maximum theoretical forward gain of a three element, single-band Yagi antenna is 9.7 dBi.

The unit dBi refers to "decibels with respect to an isotropic antenna." An isotropic antenna is one that ratiates equally in all possible directions. The Yagi by contrast is a directional antenna. This high value of 9.7 dBi is an indicator of the strong directional nature of this type of antenna.

For more info see Wikipedia: Yagi-Uda antenna

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All of the choices are correct.

The Yagi antenna design can be adjusted to optimize forward gain, front-to- back ratio and SWR bandwidth by any or all of the following: The physical length of the boom, The number of elements on the boom, and The spacing of each element along the boom. Therefore all of these factors should be taken into consideration when designing this antenna

For more info see Wikipedia: Yagi-Uda antenna

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(A) The purpose of a gamma match used with Yagi antennas is to match the relatively low feed-point impedance to 50 ohms.

The gamma match in respect to antenna systems refers to balancing the impedance of the load with an antenna tuner, a matching transformer or matching networks composed of inductors and capacitors.

The normal feedpoint impedance of the 1/2 wave driven element of the Yagi antenna is typically 25 ohms or less. By using gamma matching, the impedance can be brought closer to the standard coaxial cable feed line impedance of 50 ohms.

For more info see Wikipedia: Antenna (radio)_Impedance, Yagi antenna

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Yagi antennas typically have an impedance of 20–25 Ω. This would result in a standing wave ratio of 2:1 when used with a 50 Ω coax cable. There are a number of ways you can match impedances; the most common one used with mono-band Yagi antennas is a gamma match, which is essentially a short section of parallel conductor transmission line with an adjustable capacitor.

Notice that the gamma match relies on both inductance and capacitance, which eliminates one of the distractors directly, and also eliminates the "all of these choices are correct" as a possible answer.

Notice also that even though there is such a thing as multi-band Yagi antennas, gamma matches are typically used with mono-band Yagi antennas. That eliminates the final distractor.

The advantage of a gamma match is that the elements don't need to be isolated (or insulated) from the boom, which allows for simpler more sturdy antenna construction.

SILLY HINT: Gamma radiation makes the Incredible Hulk go BOOM!

For more info see Wikipedia: Antenna (radio)_Impedance, Yagi antenna

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(A). Each side of a quad antenna driven element is approximately 1/4 wavelength.

In a quad antenna, all of the elements are square shaped loops. It is easy enough to think that if the whole driven element is approximately a full wavelength then each of the four sides, makes up 1/4 of the whole. This makes each side about 1/4 wavelength long. Hint: Quad = Quarter

For more info see Wikipedia: Quad antenna

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The forward gain of a 2-element quad antenna is about the same as the forward gain of a 3-element Yagi antenna.

Similarly, a 3-element quad antenna has a higher gain than a 3-element Yagi antenna.

For more info see Wikipedia: Quad antenna, Yagi antenna

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The whole length of the reflector element of a quad antenna is about 105% of the length of the driven element (one full wavelength). So each of the four sides of the square reflector element is slightly more than 1/4 wavelength long.

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(D). The two-element delta-loop beam antenna has about the same gain as the two-element quad antenna.

The delta-loop antenna is similar to the quad antenna in that it uses loop elements that are about one wavelength long. The loops of the delta-loop are triangular rather than square. Because of the similar loop construction and spacing the two-element antennas of each type have about the same gain.

For more info see Wikipedia: Quad antenna

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(B). Each leg of a symmetrical delta-loop antenna is 1/3 wavelength.

The delta-loop antenna uses a driven element, reflector and one or more directors all of which are triangular in shape. The driven element is about 1 full wavelength long, therefore each side of one of the triangular elements is approximately 1/3 wavelength long.

Hint: The Greek letter Delta is written as Δ, which has 3 sides.

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(A). When the feed point of a quad antenna is changed from the center of either horizontal wire to the center of either vertical wire, the polarization of the radiated signal changes from horizontal to vertical.

The direction of polarization of the quad antenna corresponds directly to the feed-point location on the quad loop. A horizontal polarization results from a feed point on the top or bottom of the loop (the top and bottom sides being horizontal in orientation). A vertical polarization results from the feed point being located on either of the two vertical sides of the loop.

For more info see Wikipedia: Quad antenna

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In order for one of the elements to be used as a reflector in a two- element quad antenna to operate as a beam antenna, the reflector element must be approximately 5% longer than the driven element.

To create a quad antenna that functions as a directional beam antenna, a reflector is required to "focus" the beam in the forward direction. The reflector should be about 5% longer than the driven element. In the case of a quad antenna, the driven element is about a full wavelength, so the reflector should be in total about 105% of the full wavelength. Therefore each of the four sides of the reflector element will be slightly more than 1/4 wavelength long.

For more info see Wikipedia: Quad antenna

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3 dB corresponds to twice the gain.

By placing the two Yagi antennas in a "stacked" orientation at 1/2 wavelength apart vertically, the forward gain of the "stack" doubles.

Two antennas => twice as strong.

For more info see Wikipedia: Yagi antenna, Decibel

Note: Just follow the "3's"

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