Antennas and Feed Lines Flashcards
Feed lines: characteristic impedance and attenuation; standing wave ratio (SWR) calculation, measurement, and effects; antenna feed point matching
Which of the following factors determine the characteristic impedance of a parallel conductor feed line?
A. The distance between the centers of the conductors and the radius of the conductors
B. The distance between the centers of the conductors and the length of the line
C. The radius of the conductors and the frequency of the signal
D. The frequency of the signal and the length of the line
A. The distance between the centers of the conductors and the radius of the conductors
Neither of the following have anything to do with the characteristic impedance of a parallel conductor antenna feed line:
The length of the line
The frequency of the signal
By eliminating all the answers containing these two things, the only answer remaining is the correct one.
What is the relationship between high standing wave ratio (SWR) and transmission line loss?
A. There is no relationship between transmission line loss and SWR
B. High SWR increases loss in a lossy transmission line
C. High SWR makes it difficult to measure transmission line loss
D. High SWR reduces the relative effect of transmission line loss
B. High SWR increases loss in a lossy transmission line
Simply put, an SWR reading measures match between transmitter, feedline, and antenna. If it is high ( an inefficient impedance match ) power is attenuated by reflecting it back, and it is lost in the feedline and amp finals.
https://en.wikipedia.org/wiki/Standing_wave_ratio
https://en.wikipedia.org/wiki/Feed_line
Note that lossy is a key word in the answer. As in “line loss is the lossy-est loss to lose”
What is the nominal characteristic impedance of “window line” transmission line?
A. 50 ohms
B. 75 ohms
C. 100 ohms
D. 450 ohms
D. 450 ohms
Window line (also known as ladder line) generally has a 450 ohm impedance, however there are 300 ohm (twin lead, from older analog TV installations) and 600 ohm (open wire ladder line) variants. https://en.wikipedia.org/wiki/Twin-lead#Ladder_line
(A ladder is how you get to the highest place (pick the highest number))
What causes reflected power at an antenna’s feed point?
A. Operating an antenna at its resonant frequency
B. Using more transmitter power than the antenna can handle
C. A difference between feed line impedance and antenna feed point impedance
D. Feeding the antenna with unbalanced feed line
C. A difference between feed line impedance and antenna feed point impedance
Impedances of the feed-line and antenna feed-point should be matched to prevent power reflection as a standing wave (the greater the difference in impedance, the greater the reflected energy). Matching impedances optimizes the system for more complete signal power.
easy remember point to point in answer
How does the attenuation of coaxial cable change with increasing frequency?
A. Attenuation is independent of frequency
B. Attenuation increases
C. Attenuation decreases
D. Attenuation follows Marconi’s Law of Attenuation
B. Attenuation increases
Attenuation is another word for loss, where some of the energy is converted to heat. All cables have some amount of loss, generally measured in loss in dB per unit length (e.g. feet or meters).
The higher the frequency, the higher the attenuation and loss.
Think about how heat works: it’s molecules that are jiggling. The faster they jiggle, the higher the temperature. The higher the frequency, the more often electrons are bumping into molecules and setting them in motion.
A useful analogy is to think about people walking in a corridor. When there are few people they’re much less likely to bump into each other. The more crowded it is, the more likely people are to bump into each other.
Memory hint: Attenuation increases when frequency increases
In what units is RF feed line loss usually expressed?
A. Ohms per 1,000 feet
B. Decibels per 1,000 feet
C. Ohms per 100 feet
D. Decibels per 100 feet
D. Decibels per 100 feet
The values for RF feed line losses are usually expressed in dB per 100 ft.
The amount of signal loss through a substance is referred to as its attenuation. The attenuation of various feed lines, such as coaxial cables standardized in units of dB per 100 ft (or metric dB/meter). It is important to note that attenuation is also frequency dependant, and so the dB per 100 feet will often be expressed along with a standard frequency, such as for coax at 750 MHz.
What must be done to prevent standing waves on a feed line connected to an antenna?
A. The antenna feed point must be at DC ground potential
B. The feed line must be an odd number of electrical quarter wavelengths long
C. The feed line must be an even number of physical half wavelengths long
D. The antenna feed point impedance must be matched to the characteristic impedance of the feed line
D. The antenna feed point impedance must be matched to the characteristic impedance of the feed line
When the impedances are not matched, the standing waves may be reflected back which will raise the feed line standing wave ratio (SWR). These reflections should be eliminated by matching impedances to maximize power output and reduce the SWR.
If the SWR on an antenna feed line is 5:1, and a matching network at the transmitter end of the feed line is adjusted to present a 1:1 SWR to the transmitter, what is the resulting SWR on the feed line?
A. 1:1
B. 5:1
C. Between 1:1 and 5:1 depending on the characteristic impedance of the line
D. Between 1:1 and 5:1 depending on the reflected power at the transmitter
B. 5:1
It won’t help to adjust the transmitter end of the feed line, you need to adjust the antenna end of the line.
Sticking a matching network adjusted to 1:1 at the transmitter end of things will leave you with an unchanged standing wave ratio of 5:1.
Hint: The answer is in the question: the Standing Wave Ratio (SWR) of the feed line is 5 to 1
What standing wave ratio results from connecting a 50-ohm feed line to a 200-ohm resistive load?
A. 4:1
B. 1:4
C. 2:1
D. 1:2
A. 4:1
Remember, SWR is always 1:1 or greater. Thus, you can eliminate the distractors 1:2 and 1:4, which are both less than 1:1.
The standing wave ratio that will result from the connection of a 50-ohm feed line to a non-reactive load having a 200-ohm impedance is 4:1.
To calculate the standing wave ratio (SWR) in a case where the load is non-reactive, simply divide the greater impedance by the lesser impedance, thereby giving a value greater than one.
What standing wave ratio results from connecting a 50-ohm feed line to a 10-ohm resistive load?
A. 2:1
B. 1:2
C. 1:5
D. 5:1
D. 5:1
In cases where the load is non-reactive, the SWR may be calculated by simply dividing the greater impedance value divided by the lesser impedance value (whichever fraction will give a result greater than 1).
What is the effect of transmission line loss on SWR measured at the input to the line?
A. Higher loss reduces SWR measured at the input to the line
B. Higher loss increases SWR measured at the input to the line
C. Higher loss increases the accuracy of SWR measured at the input to the line
D. Transmission line loss does not affect the SWR measurement
A. Higher loss reduces SWR measured at the input to the line
The higher the transmission line loss, the more the SWR will read artificially low.
SWR is defined as the ratio of forward power to reflected power.
Higher transmission line losses mean a larger portion of the reflected power will be absorbed, thus leading to an artificially lower SWR reading.
What is a characteristic of a random-wire HF antenna connected directly to the transmitter?
A. It must be longer than 1 wavelength
B. Station equipment may carry significant RF current
C. It produces only vertically polarized radiation
D. It is more effective on the lower HF bands than on the higher bands
B. Station equipment may carry significant RF current
One disadvantage of a directly fed random-wire antenna is that you may experience RF burns when touching metal objects in your station.
The simple single wire length of the random-wire antenna acts as both feed line and antenna and is directly connected to the transmitter. Because of the proximity of the antenna to your equipment, RF energy can “feed back” to your equipment and create RF hot spots. Proper grounding of the antenna and equipment cases is important to reduce burn risk.
For more info see Wikipedia: Random-wire antenna
Which of the following is a common way to adjust the feed point impedance of an elevated quarter-wave ground-plane vertical antenna to be approximately 50 ohms?
A. Slope the radials upward
B. Slope the radials downward
C. Lengthen the radials beyond one wavelength
D. Coil the radials
B. Slope the radials downward
With a minimal impact to antenna performance, angling the radials downward instead of placing them horizontally can increase the feed point impedance. W5ALT states that sloping downward at approximately 45 degrees below horizontal is sufficient to obtain feed point impedance of about 50 ohms:
Silly Hint: Ground -> Down
Which of the following best describes the radiation pattern of a quarter-wave ground-plane vertical antenna?
A. Bi-directional in azimuth
B. Isotropic
C. Hemispherical
D. Omnidirectional in azimuth
D. Omnidirectional in azimuth
A quarter-wave ground plane is sort of like a vertical dipole, it radiates horizontally in all directions. So, that leaves out bi-directional.
Isotropic is a theoretical antenna, a point source that radiates in all directions. A quarter-wave ground-plane radiates very little “up” and “down” - so it’s certainly not an isotropic radiator.
Hemisperical also theoretical and would be like isotropic/spherical, but with no transmission on one side of a bisecting plane. If the bisection were horizontal, it would have equal horizontal and vertical transmission above (or below) that midline. A quarter-wave ground-plane is again disqualified because it radiates very little “up” (or “down”).
What is the radiation pattern of a dipole antenna in free space in a plane containing the conductor?
A. It is a figure-eight at right angles to the antenna
B. It is a figure-eight off both ends of the antenna
C. It is a circle (equal radiation in all directions)
D. It has a pair of lobes on one side of the antenna and a single lobe on the other side
A. It is a figure-eight at right angles to the antenna
A standard dipole antenna radiates in a doughnut pattern in 3-D, but when examined in a polar diagram will resemble a figure-8 at a right angle to the antenna axis.
“The radiation pattern of a half-wave dipole antenna [is] that the direction of maximum sensitivity or radiation is at right angles to the axis of the RF antenna.”
Dipole Radiation Pattern & Polar Diagram
Silly Hint: The correct answer is the only option containing the word “right”.
How does antenna height affect the azimuthal radiation pattern of a horizontal dipole HF antenna at elevation angles higher than about 45 degrees?
A. If the antenna is too high, the pattern becomes unpredictable
B. Antenna height has no effect on the pattern
C. If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional
D. If the antenna is less than 1/2 wavelength high, radiation off the ends of the wire is eliminated
C. If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional
When a horizontal dipole antenna is close to the ground, the signals reflected from the ground cause more of the signal to be directed back at high vertical angles. This changes the “figure 8” pattern to a more omnidirectional “doughnut”.
For more info see Wikipedia: Dipole antenna
Hint: Look for answer with Azimuthal
Where should the radial wires of a ground-mounted vertical antenna system be placed?
A. As high as possible above the ground
B. Parallel to the antenna element
C. On the surface or buried a few inches below the ground
D. At the center of the antenna
C. On the surface or buried a few inches below the ground
By placing the radial wires at or just below ground level, you can create an artificial ground screen to act as a more effective ground plane. Depending on your type of installation and the ground conductivity you may need to install 8, 16, 32 (multiples of 2) or more 1/4 wavelength or longer radial wires to form this “ground screen”.
How does the feed point impedance of a horizontal 1/2 wave dipole antenna change as the antenna height is reduced to 1/10 wavelength above ground?
A. It steadily increases
B. It steadily decreases
C. It peaks at about 1/8 wavelength above ground
D. It is unaffected by the height above ground
B. It steadily decreases
Placing the antenna at least 1/4 wave above ground is optimal for this type of antenna. In addition to decreasing the feed-point impedance of the antenna, lowering the antenna below 1/4 wavelength above the ground will also greatly alter the radiation pattern of the antenna.
Memory tip: Decreasing the height, decreases the impedance.
How does the feed point impedance of a 1/2 wave dipole change as the feed point is moved from the center toward the ends?
A. It steadily increases
B. It steadily decreases
C. It peaks at about 1/8 wavelength from the end
D. It is unaffected by the location of the feed point
A. It steadily increases
The center of a 1/2 wave dipole antenna is usually the best place to mount the feed-point. The impedance is lowest at this point at about 72 ohms, which is close to matching the feed line impedance of 75-ohm coaxial cable. By moving the feed point toward the ends of the antenna, the impedance increases steadily and can reach a level of several thousand ohms!
Silly hint: As you move from Center to Ends you move up in the alphabet.
Which of the following is an advantage of using a horizontally polarized as compared to a vertically polarized HF antenna?
A. Lower ground losses
B. Lower feed point impedance
C. Shorter radials
D. Lower radiation resistance
A. Lower ground losses
By polarizing the antenna horizontally, currents are induced along the surface of the ground, which has lower reflection losses. This horizontal polarization also has the advantage in that waves reflected from the ground will recombine with the non-reflected signal wave pattern of the antenna and form a stronger signal.
What is the approximate length for a 1/2 wave dipole antenna cut for 14.250 MHz?
A. 8 feet
B. 16 feet
C. 24 feet
D. 33 feet
D. 33 feet
What is the approximate length for a 1/2 wave dipole antenna cut for 3.550 MHz?
A. 42 feet
B. 84 feet
C. 132 feet
D. 263 feet
C. 132 feet
What is the approximate length for a 1/4 wave monopole antenna cut for 28.5 MHz?
A. 8 feet
B. 11 feet
C. 16 feet
D. 21 feet
A. 8 feet
Which of the following would increase the bandwidth of a Yagi antenna?
A. Larger-diameter elements
B. Closer element spacing
C. Loading coils in series with the element
D. Tapered-diameter elements
A. Larger-diameter elements
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.
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
B. 1/2 wavelength
Silly Hint: The question asks about a Yagi but technically that’s only half the full name of Yagi-Uda
How do the lengths of a three-element Yagi reflector and director compare to that of the driven element?
A. The reflector is longer, and the director is shorter
B. The reflector is shorter, and the director is longer
C. They are all the same length
D. Relative length depends on the frequency of operation
A. The reflector is longer, and the director is shorter
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.
How does antenna gain in dBi compare to gain stated in dBd for the same antenna?
A. Gain in dBi is 2.15 dB lower
B. Gain in dBi is 2.15 dB higher
C. Gain in dBd is 1.25 dBd lower
D. Gain in dBd is 1.25 dBd higher
B. Gain in dBi is 2.15 dB higher
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
What is the primary effect of increasing boom length and adding directors to a Yagi antenna?
A. Gain increases
B. Beamwidth increases
C. Front-to-back ratio decreases
D. Resonant frequency is lower
A. Gain increases
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
What does “front-to-back ratio” mean in reference to a Yagi antenna?
A. The number of directors versus the number of reflectors
B. The relative position of the driven element with respect to the reflectors and directors
C. The power radiated in the major lobe compared to that in the opposite direction
D. The ratio of forward gain to dipole gain
C. The power radiated in the major lobe compared to that in 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.
What is meant by the “main lobe” of a directive antenna?
A. The magnitude of the maximum vertical angle of radiation
B. The point of maximum current in a radiating antenna element
C. The maximum voltage standing wave point on a radiating element
D. The direction of maximum radiated field strength from the antenna
D. 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.
In free space, how does the gain of two three-element, horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart typically compare to the gain of a single three-element Yagi?
A. Approximately 1.5 dB higher
B. Approximately 3 dB higher
C. Approximately 6 dB higher
D. Approximately 9 dB higher
B. Approximately 3 dB higher
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”
Which of the following can be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth of a Yagi antenna?
A. The physical length of the boom
B. The number of elements on the boom
C. The spacing of each element along the boom
D. All these choices are correct
D. All these 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.
What is a beta or hairpin match?
A. A shorted transmission line stub placed at the feed point of a Yagi antenna to provide impedance matching
B. A 1/4 wavelength section of 75-ohm coax in series with the feed point of a Yagi to provide impedance matching
C. A series capacitor selected to cancel the inductive reactance of a folded dipole antenna
D. A section of 300-ohm twin-lead transmission line used to match a folded dipole antenna
A. A shorted transmission line stub placed at the feed point of a Yagi antenna to provide impedance matching
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.
Which of the following is a characteristic of using a gamma match with a Yagi antenna?
A. It does not require the driven element to be insulated from the boom
B. It does not require any inductors or capacitors
C. It is useful for matching multiband antennas
D. All these choices are correct
A. It does not require the driven element to be insulated from the boom
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!
Which of the following antenna types will be most effective as a near vertical incidence skywave (NVIS) antenna for short-skip communications on 40 meters during the day?
A. A horizontal dipole placed between 1/10 and 1/4 wavelength above the ground
B. A vertical antenna placed between 1/4 and 1/2 wavelength above the ground
C. A horizontal dipole placed at approximately 1/2 wavelength above the ground
D. A vertical dipole placed at approximately 1/2 wavelength above the ground
A. A horizontal dipole placed between 1/10 and 1/4 wavelength above the ground
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
What is the feed point impedance of an end-fed half-wave antenna?
A. Very low
B. Approximately 50 ohms
C. Approximately 300 ohms
D. Very high
D. Very high
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.
In which direction is the maximum radiation from a VHF/UHF “halo” antenna?
A. Broadside to the plane of the halo
B. Opposite the feed point
C. Omnidirectional in the plane of the halo
D. On the same side as the feed point
C. Omnidirectional in the plane of the halo
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.
What is the primary function of antenna traps?
A. To enable multiband operation
B. To notch spurious frequencies
C. To provide balanced feed point impedance
D. To prevent out-of-band operation
A. To enable 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
What is an advantage of vertically stacking horizontally polarized Yagi antennas?
A. It allows quick selection of vertical or horizontal polarization
B. It allows simultaneous vertical and horizontal polarization
C. It narrows the main lobe in azimuth
D. It narrows the main lobe in elevation
D. 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
Which of the following is an advantage of a log-periodic antenna?
A. Wide bandwidth
B. Higher gain per element than a Yagi antenna
C. Harmonic suppression
D. Polarization diversity
A. 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.
Which of the following describes a log-periodic antenna?
A. Element length and spacing vary logarithmically along the boom
B. Impedance varies periodically as a function of frequency
C. Gain varies logarithmically as a function of frequency
D. SWR varies periodically as a function of boom length
A. Element length and spacing vary logarithmically along the boom
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
How does a “screwdriver” mobile antenna adjust its feed point impedance?
A. By varying its body capacitance
B. By varying the base loading inductance
C. By extending and retracting the whip
D. By deploying a capacitance hat
B. By varying the base loading inductance
“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.
What is the primary use of a Beverage antenna?
A. Directional receiving for MF and low HF bands
B. Directional transmitting for low HF bands
C. Portable direction finding at higher HF frequencies
D. Portable direction finding at lower HF frequencies
A. Directional receiving for MF and 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.
Think Dr. Pepper is a beverage. “D”irectional “R”eceiving Helpful Hint: You Receive a Beverage
In which direction or directions does an electrically small loop (less than 1/10 wavelength in circumference) have nulls in its radiation pattern?
A. In the plane of the loop
B. Broadside to the loop
C. Broadside and in the plane of the loop
D. Electrically small loops are omnidirectional
B. Broadside to the loop
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.
Which of the following is a disadvantage of multiband antennas?
A. They present low impedance on all design frequencies
B. They must be used with an antenna tuner
C. They must be fed with open wire line
D. They have poor harmonic rejection
D. They have poor harmonic rejection
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.
What is the common name of a dipole with a single central support?
A. Inverted V
B. Inverted L
C. Sloper
D. Lazy H
A. Inverted V
