Antennas and Feed Lines Flashcards

Question

What is the approximate length of the driven element of a Yagi antenna? A. 1/4 wavelength B. 1/2 wavelength C. 3/4 wavelength D. 1 wavelength

Answer 1

B. 1/2 wavelength ## Footnote Silly Hint: The question asks about a Yagi but technically that's only half the full name of Yagi-Uda

Answer 2

A. The reflector is longer, and the director is shorter ## Footnote 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.

Answer 3

B. Gain in dBi is 2.15 dB higher ## Footnote 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

Answer 4

A. Gain increases ## Footnote 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

Answer 5

C. The power radiated in the major lobe compared to that in the opposite direction ## Footnote 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.

Answer 6

D. The direction of maximum radiated field strength from the antenna ## Footnote 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.

Answer 7

B. Approximately 3 dB higher ## Footnote 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. Note: Just follow the "3's"

Answer 8

D. All these choices are correct ## Footnote 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.

Answer 9

A. A shorted transmission line stub placed at the feed point of a Yagi antenna to provide impedance matching ## Footnote 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.

Answer 10

A. It does not require the driven element to be insulated from the boom ## Footnote 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!

Answer 11

A. A horizontal dipole placed between 1/10 and 1/4 wavelength above the ground ## Footnote 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

Answer 12

D. Very high ## Footnote 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.

Answer 13

C. Omnidirectional in the plane of the halo ## Footnote 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.

Answer 14

A. To enable multiband operation ## Footnote 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

Answer 15

D. It narrows the main lobe in elevation ## Footnote 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

Answer 16

A. Wide bandwidth ## Footnote 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.

Answer 17

A. Element length and spacing vary logarithmically along the boom ## Footnote 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

Answer 18

B. By varying the base loading inductance ## Footnote “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.

Answer 19

A. Directional receiving for MF and low HF bands ## Footnote 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. Think Dr. Pepper is a beverage. "D"irectional "R"eceiving Helpful Hint: You Receive a Beverage

Answer 20

B. Broadside to the loop ## Footnote 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.

Answer 21

D. They have poor harmonic rejection ## Footnote 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.

Answer 22

A. Inverted V

Antennas and Feed Lines Flashcards

Feed lines: characteristic impedance and attenuation; standing wave ratio (SWR) calculation, measurement, and effects; antenna feed point matching