29 Questions
For optimal transmission and reception, the transmitting and receiving antennas must both be of the same ______
polarization
During transmission over long distances, the ______ of waves changes (propagation effects)
polarization
Even if polarization is not matched, a signal is usually ______
received
An antenna can be a length of wire, a metal rod, or a piece of ______
tubing
An important criterion for radiation is that the length of the conductor be approximately one-half or one-quarter ______ of the ac signal
wavelength
Horizontally Polarized Electric field is ______ to the earth
parallel
Vertically Polarized Electric field is ______ to the earth
perpendicular
Circular Polarization Electric and magnetic fields ______ as they leave the antenna
rotate
Right-hand Circular Polarization (RHCP) rotates ______
clockwise
Left-hand Circular Polarization (LHCP) rotates ______
counterclockwise
Low-frequency Antennas: spacing may be 2 or ______ in.
3
Radiation pattern and gain of a folded dipole are the same as those of a standard dipole; offer greater ______
bandwidth
When one-half wavelength is calculated, the velocity factor of twin-lead cable (0.8) must be ______
included
Example: Find final length of a folded dipole antenna cut for 100 ______.
MHz
Impedance matching antenna tuners are used when baluns and matching sections cannot do the job. They typically include a variable inductor, one or more variable capacitors, or a combination of these components. Inductor and capacitor values are adjusted until the ______ indicates that the impedances match.
SWR
An example of an Antenna Tuner is the Transmatch Circuit, which uses a coil and three capacitors to tune the antenna for optimal ______.
SWR
In the Transmatch Circuit, the SWR meter is usually connected in series with the ______.
antenna
In the context of Antennas and Waveguides, for optimal transmission and reception, the transmitting and receiving antennas must both be of the same ______.
polarization
For radiation to be effective, an important criterion is that the length of the conductor be approximately one-half or one-quarter of the AC ______.
signal
A Marconi or Ground-Plane Vertical Antenna is one-quarter wavelength ____________ antenna
vertical
An advantage of a Marconi or Ground-Plane Vertical Antenna is that it is one-half the length of a ________ antenna
dipole
The radiation pattern of a Marconi or Ground-Plane Vertical Antenna is still doughnut-shaped, but seen from above the ________
antenna
An omnidirectional antenna has an equal amount of energy in the ________ direction
horizontal
The center conductor of a Marconi or Ground-Plane Vertical Antenna is connected to the vertical radiator, and the shield is connected to earth ________
ground
An antenna with drooping radials can be adjusted using a 50Ω coaxial ________ line
transmission
A 5/8 λ Vertical Antenna is one-eighth wavelength longer than one-half wavelength and has about a 3-dB gain over a basic ________
dipole
Directivity in antennas refers to the ability to send or receive signals over a narrow horizontal ________ range
directional
A conventional half-wave dipole antenna is bidirectional, meaning it receives signals best in two directions, and unidirectional, meaning it sends or receives signals in one direction ________
only
Power gain is concentrated into a small beam, giving the effect as if the antenna had amplified the transmitted signal, this is known as ________ Gain
Beam
Study Notes
Antennas and Waveguides
- For optimal transmission and reception, the transmitting and receiving antennas must have the same polarization.
- During long-distance transmission, the polarization of waves changes due to propagation effects, but a signal can still be received even if the polarization is not matched.
The Basic Antenna
- An antenna can be a length of wire, a metal rod, or a piece of tubing.
- The length of the conductor is dependent on the frequency of operation, and an important criterion for radiation is that the length of the conductor be approximately one-half or one-quarter wavelength of the ac signal.
Common Antenna Types
- Marconi or Ground-Plane Vertical Antenna:
- One-quarter wavelength
- Omnidirectional antennas
- Radiation pattern is still doughnut-shaped, but seen from above the antenna
- Equal amount of energy in the horizontal direction
- One-half of the pattern is below the surface of the earth (vertical radiation pattern)
- Center conductor is connected to the vertical radiator, and the shield is connected to earth ground
- Earth acts as a type of electrical “mirror”
- Good electrical contact with the earth is necessary for effective operation
- A ground plane, radials, and counterpoise can be used to improve the antenna's effectiveness
- Radiation resistance is approximately one-half the impedance of the dipole (approximately 36.5Ω)
- May vary due to height above ground, length/diameter ratio, and presence of surrounding objects
- Loading Coil:
- Used to compensate for a vertical antenna that is less than one-quarter wavelength, which would decrease inductance and prevent resonance at the desired frequency
- Top Hat:
- Used to increase the effective capacitance of an antenna
- Adds conductors at the top of the antenna
- Arrangement is impractical and heavy
- 5/8 λ Vertical Antenna:
- One-eighth wavelength longer than one-half wavelength
- Gives about a 3-dB gain over a basic dipole
- Narrower vertical radiation pattern at a lower angle to the horizon
- Ideal for long-distance communication
Directivity
- Omnidirectional:
- Can send messages in any direction and receive them from any direction
- Directivity:
- Ability to send or receive signals over a narrow horizontal directional range
- Depends on physical orientation
- Eliminates interference from other signals
- Receiving antenna is pointed directly at the station
- Greater efficiency of power transmission
- Unidirectional:
- Sends or receives signals in one direction only
- Conventional Half-wave Dipole:
- Bidirectional – receives signals best in two directions
- Unidirectional – sends or receives signals in one direction only
Gain
- Power Gain:
- Concentrates power into a small beam, making it seem as if the antenna had amplified the transmitted signal
- 𝐏𝐭𝐫𝐚𝐧𝐬𝐦𝐢𝐭𝐭𝐞𝐝 𝐝𝐁 = 𝟏𝟎𝐥𝐨𝐠 𝐏𝐢𝐧
Relationship Between Directivity and Gain
- 𝐁= 𝟐𝟎𝟑 𝟏𝟎 𝐱
- B is the beam width of the antenna, in degrees
- x is the antenna power gain in decibels divided by 10 (x = dB/10)
- Assumes a symmetric major lobe
Polarization
- Horizontally Polarized:
- Electric field is parallel to the earth
- Vertically Polarized:
- Electric field is perpendicular to the earth
- Circular Polarization:
- Electric and magnetic fields rotate as they leave the antenna
- Depends on the direction of rotation
- Right-hand Circular Polarization (RHCP) - clockwise
- Left-hand Circular Polarization (LHCP) - counterclockwise
- Both transmitter and receiver must use either left- or right-hand polarization
- A vertical or horizontal antenna can receive circular polarized signals, but the signal strength is reduced
Impedance Matching
- Antenna Tuners:
- Used when baluns and matching sections cannot do the job
- A variable inductor, one or more variable capacitors, or a combination of these components
- Inductor and capacitor values are adjusted until the SWR indicates that the impedances match
- Example: Transmatch Circuit
- Uses a coil and three capacitors to tune the antenna for optimal SWR
- SWR meter is usually connected in series with the antenna
Test your knowledge on polarization of antennas and waveguides, including horizontally, vertically, and circularly polarized electric fields. Explore the concepts related to the orientation of electric and magnetic fields in relation to the earth.
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