(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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A Three-Element yagi has exactly what it sounds like, three elements. The reflector, driven element, and director. The driven element is simply a dipole, nothing more nothing less. The dipole becomes resonant at about 1/2 Wavelength of the desired frequency.

Next is the reflector. This is the longest element on a 3-E Yagi. The reflectors purpose is to add an inductive element to the antenna. To make this element inductive, we need to make it larger than the resonant frequency length. The reflector is usually, as you guessed it from the question, 5% larger. A way to remember that the reflector is the longest element is "reflector" contains an "L" like "longer".

Lastly, the Director element. This is the shortest element on the antenna. It introduces a capacitive element to the antenna. This is done by making part of the antenna shorter than the desired resonant frequency. As you may have guessed again, approximately 5% shorter than the resonant frequency.

Mnemonic:
Reflection/reflector takes Longer
Directing/director takes Shorter time.

Another way to remember which of the two "longer/shorter" answers is correct, is to draw an imaginary outline around the yagi antenna. It looks like a trapezoid, with the narrow side directed towards the destination. Less energy is radiated out the wide side (the back), it is instead reflected back towards the director and the destination.

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gain describes how well the antenna converts input power into radio waves headed in a specified direction —wikipedia

• dBi - compared to an isotropic antenna
• dBd - compared to a reference dipole

A reference dipole has 2.15 dB higher gain than an isotropic antenna. This makes sense, because an isotropic antenna radiates equally in all directions, whereas a dipole concentrates the radiation along an axis.

The question, however, isn't whether the reference dipole has a higher gain than the isotropic antenna (it does), but whether a given antenna will have a higher gain number when compared to the reference dipole (dBd) or an isotropic antenna (dBi).

If you have an antenna that is 1dBd, it means it has a gain 1dB above the gain of the reference dipole antenna, which in turn has a gain 2.15dB above an isotropic radiator.

• 1 dBd = (1 + 2.15) dBi = 3.15 dBi

Logarithmic scales like decibels are convenient because multiplication of values correspond to addition in logarithmic scales. This means that you can immediately disregard the distractors talking about square roots and reciprocals.

Mnemonics:

• The letter “i” (isotropic) is “higher” in the alphabet than “d” (dipole).
• Gain in dBi is high-er (twice as high)(even though that's not really accurate in dB)

Silly hint: The correct answer contains the "extra letter" after db:

dbI hIgher
dbi lower
dbd higher
dbd lower

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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 "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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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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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 hairpin match is a relatively short coil, sometimes just a one-turn coil, that's used to raise the impedance of the feed point of an antenna. It's not a piece of coax. It's not a capacitor. It's not a section of 300 ohm twinlead. It's just a coil that goes across the feed point. Just remember that a beta or hairpin match goes at the feed point.

SILLY HINT: Remember to feed your stubby beta fish.

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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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