T-9
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Subelement T9
Antennas and feed lines
A horizontally polarized antenna describes a simple dipole mounted so the conductor is parallel to the Earth’s surface. The electric field is perpendicular to the Earth is true of a vertical antennas. The disadvantage of the “rubber duck” antenna supplied with most handheld radio transceivers is it does not transmit or receive as effectively as a full-sized antenna. A good reason not to use a “rubber duck” antenna inside your car is the signals are significantly weaker than when it is outside of the vehicle. 19 inches is the approximate length of a quarter-wavelength vertical antenna for 146 MHz. VHF or UHF mobile antennas often mounted in the center of the vehicle roof because it normally provides the most uniform radiation pattern. A properly mounted 5/8 wavelength antenna for VHF or UHF mobile service offers a lower angle of radiation and more gain than a 1/4 wavelength antenna and usually provides improved coverage.
An antenna that concentrates signals in one direction is a beam antenna. The quad, Yagi, and dish are directional antennas. Broadside to the antenna is the direction radiation strongest from a half-wave dipole antenna in free space. Shorten a dipole antenna to make it resonant on a higher frequency. 112 inches is the approximate length of a 6-meter 1/2-wavelength wire dipole antenna. Inserting an inductor in the radiating portion of the antenna to make it electrically longer describes a type of loading. The gain of an antenna is the increase in signal strength in a specified direction when compared to a reference antenna.
To allow the efficient transfer of power and reduce losses it is important to have a low SWR in an antenna system that uses coaxial cable feed line. 50 ohms is the impedance of the most commonly used coaxial cable in typical amateur radio installations. Coaxial cable is easy to use and requires few special installation considerations. As the frequency of a signal passing through coaxial cable is increased it also increases the loss. The electrical difference between the smaller RG-58 and larger RG-8 coax cables is RG-8 cable has less loss at a given frequency. Air-insulated hard line coax cable has the lowest loss at VHF and UHF frequency. A loose connection in an antenna or a feed line will cause erratic changes in SWR readings. An antenna tuner matches the antenna system impedance to the transceiver’s output impedance.
A Type N connector is most suitable for frequencies above 400 MHz. PL-259 type coax connectors are commonly used at HF frequencies. Coax connectors exposed to the weather should be sealed against water to prevent an increase in feed line loss.
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SUBELEMENT T9
Antennas and feed lines
[2 Exam Questions – 2 Groups]
What is a beam antenna?
- An antenna that concentrates signals in one direction
Which of the following describes a type of antenna loading?
- Inserting an inductor in the radiating portion of the antenna to make it electrically longer
Which of the following describes a simple dipole oriented parallel to the Earth’s surface?
- A horizontally polarized antenna
What is a disadvantage of the “rubber duck” antenna supplied with most handheld radio transceivers when compared to a full-sized quarter-wave antenna?
- It does not transmit or receive as effectively
How would you change a dipole antenna to make it resonant on a higher frequency?
- Shorten it
What type of antennas are the quad, Yagi, and dish?
- Directional antennas
What is a disadvantage of using a handheld VHF transceiver, with its integral antenna, inside a vehicle?
- Signals might not propagate well due to the shielding effect of the vehicle
What is the approximate length, in inches, of a quarter-wavelength vertical antenna for 146 MHz?
- 19
What is the approximate length, in inches, of a half-wavelength 6 meter dipole antenna?
- 112
In which direction does a half-wave dipole antenna radiate the strongest signal?
- Broadside to the antenna
What is the gain of an antenna?
- The increase in signal strength in a specified direction compared to a reference antenna
What is an advantage of using a properly mounted 5/8 wavelength antenna for VHF or UHF mobile service?
- It has a lower radiation angle and more gain than a 1/4 wavelength antenna
T9B01 (B)
Why is it important to have low SWR when using coaxial cable feed line?
- To reduce signal loss
What is the impedance of most coaxial cables used in amateur radio installations?
- 50 ohms
Why is coaxial cable the most common feed line selected for amateur radio antenna systems?
- It is easy to use and requires few special installation considerations
What is the major function of an antenna tuner (antenna coupler)?
- It matches the antenna system impedance to the transceiver’s output impedance
In general, what happens as the frequency of a signal passing through coaxial cable is increased?
- The loss increases
Which of the following connectors is most suitable for frequencies above 400 MHz?
- A Type N connector
Which of the following is true of PL-259 type coax connectors?
- They are commonly used at HF frequencies
Why should coax connectors exposed to the weather be sealed against water intrusion?
- To prevent an increase in feed line loss
What can cause erratic changes in SWR readings?
- A loose connection in an antenna or a feed line
What is the electrical difference between RG-58 and RG-8 coaxial cable?
- RG-8 cable has less loss at a given frequency
Which of the following types of feed line has the lowest loss at VHF and UHF?
- Air-insulated hard line
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