The idea of an NVIS antenna is to use the proximity to ground to reflect a lot of the signal up, rather than out. If it were higher, like 1/2 wave, it would radiate horizontally, out rather than up. If you're trying to work DX, get your dipole up high. To work local stations on 40m during the day and 80m at night, keep it low.

Right and left-hand polarization have nothing to do with NVIS.

Hint: The Near Vertical Incidence Skywave needs a horizontal dipole.

Silly Hint: 40 = 10x4. The answer contains 10 and 4

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EFHW antennas often have feed point impedance as high as 2500 ohms or greater. Often a 49:1 Unun or some other type of match is used at the feed point to match to standard feed lines (50/450/600 ohm feeds)

Hint: One source stated that it’s very difficult to predict the impedance of an end-fed wire, other than to say it's high. Usually it's determined empirically.

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Halo antennas are usually horizontally polarized (the plane of the halo is horizontal, or parallel to the ground) - so they radiate in all directions, but horizontally instead of vertically polarized.

In areas where horizontal polarization is preferred, the halo is sometimes used mobile.

Silly Hint: When playing the game Halo, you can move in any direction. Omnidirectional.

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The primary purpose of antenna traps is to permit multiband operation.

Resonant LC circuits are used in parallel to isolate sections of the antenna effectively changing the antenna's "electrical length". This allows the antenna to be used for several bands rather than one static wavelength.

SILLY HINT: (Plural) Antenna traps and multiple bands

For more info see Wikipedia: Antenna (radio)_Bandwidth

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The advantage of vertical stacking of horizontally polarized Yagi antennas is that it narrows the main lobe in elevation.

The resulting narrowing of the vertical width of the main lobe results in an increase in gain, with stronger received signals and less noise.

Suggest: 'vertical stacking', vertical - phrase for elevation

For more information see Wikipedia: Yagi antenna

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An advantage of a log periodic antenna is wide bandwidth.

In this type of multi-element directional antenna, the lengths of the elements and the spacing among them are both arranged in a log-periodic manner. This allows the antenna to be operated consistently over a large range of frequencies.

Hint: A log is wide.

For more info see Wikipedia: Log periodic antenna

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A log periodic antenna is described as one in which the length and spacing of the elements increases logarithmically from one end of the boom to the other.

The advantage of this type of antenna arrangement is that it allows operation over a wide range of frequencies, rather than one static frequency band.

Memorization aid: "log" in the question = "logarithmically" in the correct answer

The word "logarithmically" appears in 2 answers; For me better to remember "log" in the question = "log - boom" in the correct answer

For more info see Wikipedia: Log periodic antenna

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“Screwdriver” antennas are vertical antennas with a usually built-in impedance matching mechanism. These antennas function by using a large motor at the base of the antenna to raise and lower a decoupler against the windings of an inductor, usually hidden underneath a plastic tubing.

The decoupler is essentially acting as a tap for the inductor. Wherever the decoupler is sitting, this will result in the different tuning of the antenna.

Lower bands, such as 80 and 160, will result in being able to see more of the coils (inductor).

With higher bands, the antenna climbs up the inductor, resulting in lower inductance.

Amateur Radio Wiki: Screwdriver Antenna

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An application for a Beverage antenna is directional receiving for low HF bands.

The Beverage antenna is not effective for transmitting but is a highly effective and relatively inexpensive antenna for the directional reception of radio signals. Its effectiveness is greatest for the lower frequency HF bands (7 MHz and lower).

It's not very portable due to its size.

Wikipedia: Beverage antenna QSL: Beverage antenna

Think Dr. Pepper is a beverage. "D"irectional "R"eceiving Helpful Hint: You Receive a Beverage

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An electrically small loop antenna (less than 1/10 wavelength in circumference) has nulls broadside to the loop, meaning it radiates the least energy in the direction perpendicular to the plane of the loop. Most of the signal is radiated in the plane of the loop. These characteristics make electrically small loop antennas useful for certain applications, particularly for direction finding, as the nulls can help in pinpointing signal direction.

Remember: Nulls are broadside to the loop!

Memory tip: The correct answer contains the word "broadside". However there are two answers that use that word, so just remember the question asks us about a "small" loop and choose the "smaller" (shorter) answer with the word broadside in it.

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Since multiband antennas are designed to be resonant at many different frequencies, they are much less resistant to signals that come in on harmonics of the frequencies for which they are tuned. This can result in interference from harmonics that may not affect a single band antenna.

Silly hint: The battle of the bands had a poor harmonica player.

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Inverted V (or Vee) antenna, can be both center and off-center fed.

Wikipedia: Inverted vee antenna

An inverted V looks like ∧, which describes the antenna geometry. Imagine two long tent poles end to end, held up only in the middle where they meet. The far ends will fall down: ⩚

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A dipole antenna mounted sufficiently above ground will exhibit a radiation pattern that’s roughly toroidal (big, fat donut), oriented co-axial with the antenna, near the resonant frequency, resulting in a radiation polarization as much vertically as horizontally.

A horizontally oriented loop antenna exhibits a radiation pattern that resembles that of a dipole that has been bent into a curve so that the toroidal pattern is still oriented co-axial to the loop but now extends around the entire loop, resulting in a combined radiation pattern outward (horizontal) omnidirectional polarization and an upward (vertical) polarization. Furthermore, as a result of the extended toroidal pattern, the relative peak vertical signal strength is lower than that of a dipole, because the extra signal strength is diverted horizontally.

Hint: 'vertical' in both Q and A

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