Antennas and Feed Lines Flashcards
What is a beam antenna?
A. An antenna built from aluminum I-beams
B. An omnidirectional antenna invented by Clarence Beam
C. An antenna that concentrates signals in one direction
D. An antenna that reverses the phase of received signals
C. An antenna that concentrates signals in one direction
The term “beam antenna” is just another name for adirectional antenna; it’s an antenna that concentrates signals in one direction. You can think of it as “beaming” the signals in a certain direction.
This term may be in common usage, but using it to identify an antenna is not a good practice. More accurate alternatives are “high gain”, “directional”, or “electrically large” antenna.
Which of the following describes a type of antenna loading?
A. Electrically lengthening by inserting inductors in radiating elements
B. Inserting a resistor in the radiating portion of the antenna to make it resonant
C. Installing a spring in the base of a mobile vertical antenna to make it more flexible
D. Strengthening the radiating elements of a beam antenna to better resist wind damage
A. Electrically lengthening by inserting inductors in radiating elements
Inductors in series make an antenna appear electrically longer. So you’d insert an inductor into the radiating portion of the antenna to make it appear electrically longer.
Adding a resistor will reduce current flow, but it wouldn’t affect the resonant frequency.
The spring at the base of an antenna would absorb the effects of collisions with other objects, but absorbing collisions has nothing to do with loading. It might make the antenna slightly longer, especially at higher frequencies, but the distractor doesn’t say anything about that.
Resisting wind effects has to do with what can be known in civil engineering as “wind loading” - but that’s not what they’re referring to when they talk about loading.
Which of the following describes a simple dipole oriented parallel to Earth’s surface?
A. A ground-wave antenna
B. A horizontally polarized antenna
C. A travelling-wave antenna
D. A vertically polarized antenna
B. A horizontally polarized antenna
The orientation of the conductor of an antenna relative to the earth’s surface determines its “polarization”. If the polarization of the sending station’s antenna does not match the polarization of the receiving station’s antenna significant loss in signal can be the result.
If the antenna is vertical (perpendicular to the ground), as most antennas are thought to be, then it is “vertically polarized” and if it is horizontal (parallel to the ground), then it is “horizontally polarized”.
What is a disadvantage of the short, flexible antenna supplied with most handheld radio transceivers, compared to a full-sized quarter-wave antenna?
A. It has low efficiency
B. It transmits only circularly polarized signals
C. It is mechanically fragile
D. All these choices are correct
A. It has low efficiency
Smaller antennas use electrical components to maintain resonance on the target frequencies, but having less surface area they don’t absorb (or emit) as much power. Therefore they do not transmit or receive as effectively as a regular full-sized antenna.
As a general rule of thumb, the shorter the antenna on a given band the worse the performance will be and the longer the better. Of course, other factors such as the resonance of the antenna on the frequencies used can also affect this!
The only reasons to use rubber duck type antennas are that they take up less space and are usually more durable than longer antennas, along with generally being cheaper. This makes it easier to keep an HT on your belt compared to a possibly much longer antenna. These sorts of tradeoffs are more typically worthwhile for commercial users such as security guards who will be near a repeater or other HT users almost all the time, so they aren’t nearly as concerned as much about gain as most amateur operators.
Which of the following increases the resonant frequency of a dipole antenna?
A. Lengthening it
B. Inserting coils in series with radiating wires
C. Shortening it
D. Adding capacitive loading to the ends of the radiating wires
C. Shortening it
Antenna length is inversely related to frequency. The higher the frequency, the SHORTER the wavelength. Cannot be longer!
Another way to remember it: Antenna length is directly related to the wavelength. Recalling that wavelength and frequency are inversely related, we must shorten the antenna length.
Also note that coils are inductors, and adding inductors to an antenna is a way to electrically lengthen the antenna, therefore adding a coil and lengthening the antenna both achieve the same result. Since there is no ‘All of the above’ answer, you can immediately eliminate those two answers.
Which of the following types of antenna offers the greatest gain?
A. 5/8 wave vertical
B. Isotropic
C. J pole
D. Yagi
D. Yagi
What is a disadvantage of using a handheld VHF transceiver with a flexible antenna inside a vehicle?
A. Signal strength is reduced due to the shielding effect of the vehicle
B. The bandwidth of the antenna will decrease, increasing SWR
C. The SWR might decrease, decreasing the signal strength
D. All these choices are correct
A. Signal strength is reduced due to the shielding effect of the vehicle
What is the approximate length, in inches, of a quarter-wavelength vertical antenna for 146 MHz?
A. 112
B. 50
C. 19
D. 12
C. 19
146 MHz is in the 2 meter band.
300 / 146≈2m
2 meters is almost 80 inches.
1/4 × 80in = 20in.
19 inchesis the closest answer.
OR
300 / 146MHz = 2.0547m
2.0547m×39.37in/m=80.89in
Then to determine size of antenna from the question,80.89×14[λ]=20.22inrounded to nearest whole number is20in.
OR
The speed of light may be expressed as11.8GHz×inches. To get the length of the wave divide by the frequency in GHz.
11.8GHz×in0.146GHz≈80in
80×14[λ]=20in
Since the question asked for quarter wave we must divide by four. For more information Google: “speed of light in gigahertz inches”.
What is the approximate length, in inches, of a half-wavelength 6 meter dipole antenna?
A. 6
B. 50
C. 112
D. 236
C. 112
In which direction does a half-wave dipole antenna radiate the strongest signal?
A. Equally in all directions
B. Off the ends of the antenna
C. In the direction of the feed line
D. Broadside to the antenna
D. Broadside to the antenna
What is antenna gain?
A. The additional power that is added to the transmitter power
B. The additional power that is required in the antenna when transmitting on a higher frequency
C. The increase in signal strength in a specified direction compared to a reference antenna
D. The increase in impedance on receive or transmit compared to a reference antenna
C. The increase in signal strength in a specified direction compared to a reference antenna
What is an advantage of a 5/8 wavelength whip antenna for VHF or UHF mobile service?
A. It has more gain than a 1/4-wavelength antenna
B. It radiates at a very high angle
C. It eliminates distortion caused by reflected signals
D. It has 10 times the power gain of a 1/4 wavelength whip
A. It has more gain than a 1/4-wavelength antenna
What is a benefit of low SWR?
A. Reduced television interference
B. Reduced signal loss
C. Less antenna wear
D. All these choices are correct
B. Reduced signal loss
What is the most common impedance of coaxial cables used in amateur radio?
A. 8 ohms
B. 50 ohms
C. 600 ohms
D. 12 ohms
B. 50 ohms
Nearly all amateur radio transmitters are designed to use coaxial feed lines with a 50-ohm impedance. The coaxial cable used for television, CATV, cable and satellite installations is all 75 ohm. At first glance, the cable looks the same, but a close examination of the cable type printed on the jacket will allow you to reference the specifications on a data sheet.
Why is coaxial cable the most common feed line for amateur radio antenna systems?
A. It is easy to use and requires few special installation considerations
B. It has less loss than any other type of feed line
C. It can handle more power than any other type of feed line
D. It is less expensive than any other type of feed line
A. It is easy to use and requires few special installation considerations
