26-11-2010, 11:51 AM
Antennas.ppt (Size: 1.01 MB / Downloads: 87)
Antennas
A good antenna works
A bad antenna is a waste of time & money
Antenna systems can be very inexpensive and simple
They can also be very, very expensive
Antenna Considerations
The space available for an antenna
The proximity to neighbours
The operating frequencies you will use
The output power
Money
Antenna Types
High Frequency
1.6 - 30 Mhz + 50 Mhz
160 - 6 metres
An antenna’s size/length depends on the frequency
It’s functionality largely depends on the height above ground, as well as the polarity and it’s configuration
Types of Antennas
Simple wire
Dipole
Folded dipole
Trap dipole
Offset or Windom antenna
Phased dipoles
Vertical or horizontal (both)
Beverage wave antenna
Types of Antennas
Metal
Vertical
Yagi
Trap Yagi
Phased arrays
Loops
Vertical or Horizontal
Horns for super ultra high frequencies
Mobile antennas
Antenna Polarization
Vertical or horizontal
Electrical vs Magnetic radiation
(Diagram)
Vertical waves travel @ 90◦ to the earths surface
Horizontal waves travel parallel to the earth’s surface
Usually wire antennas are horizontal but an inverted ‘V’ dipole has a vertical component
Yagi type antennas can be either vertical or horizontal
Circular antennas can be both
Usually, horizontally polarized antennas hear less noise
Isotropic Antenna
The isotropic antenna is a hypothetical point source.
It does not exist in reality but is considered as an important starting point considering different
antennas from the theoretical to the practical
The pattern is a Cardioid - a donut shape or a sphere
Polarization - Practical
Antennas radiating a vertical polarization are best received by an antenna of like polarization
Cross polarization reduces reception by as much as 30 db
Bouncing DX signals probably have both polarizations
Designing antenna polarization usually depends on the frequency being used - at 70 cm in th eUHF band the elements are very short so either polarization is possible. Usually vertical is used as repeaters are vertically polarized.
Resonance
Antenna length is dependant on frequency
The lower the frequency the longer the antenna elements
Examples
80 metres 3.750 Mhz 124 ft
40 7.055 66
10 28.5 16.4
6 52 9
2 145 3.2
Isotropic Source
An isotropic antenna is a: hypothetical point source
What is the antenna radiation pattern for an isotropic radiator? A sphere
Polarization of an antenna is determined by: the electric field
What does horizontal wave polarization mean? The electric lines of force of a radio wave are parallel to the earth's surface
What does vertical wave polarization mean? The electric lines of force of a radio wave are perpendicular to the earth's surface
What electromagnetic wave polarization does a Yagi antenna have when its elements are parallel to the earth's surface? Horizontal
What electromagnetic wave polarization does a half-wavelength antenna have when it is perpendicular to the earth's surface? Vertical
VHF signals from a mobile station using a vertical whip antenna will normally be best received using a: vertical ground-plane antenna
A dipole antenna will emit a vertically polarized wave if it is: Parallel with the ground mounted vertically
If an electromagnetic wave leaves an antenna vertically polarized, it will arrive at the receiving antenna, by ground wave: vertically polarized
Compared with a horizontal antenna, a vertical antenna will receive a vertically polarized radio wave: at greater strength
Wavelength vs Physical Length
The speed of a radio wave: is the same as the speed of light
The velocity of propagation of radio frequency energy in free space is: 300 000 kilometres per second
If an antenna is made longer, what happens to its resonant frequency? It decreases
If an antenna is made shorter, what happens to its resonant frequency? It increases
The resonant frequency of an antenna may be increased by: shortening the radiating element
To lower the resonant frequency of an antenna, the operator should: lengthen it
Adding a series inductance to an antenna would:
decrease the resonant frequency
The wavelength for a frequency of 25 MHz is:
12 metres (39.4 ft)
The wavelength corresponding to a frequency of 2 MHz is: 150 m (492 ft)
At the end of suspended antenna wire, insulators are used. These act to: limit the electrical length of the antenna
One solution to multi-band operation with a shortened radiator is the "trap dipole" or trap vertical. These "traps" are actually: a coil and capacitor in parallel
Gain, Directivity, etc.
What is meant by antenna gain? The numerical ratio relating the radiated signal strength of an antenna to that of another antenna
The gain of an antenna, especially on VHF and above, is quoted in dBi. The "i" in this expression stands for: isotropic
Approximately how much gain does a half-wave dipole have over an isotropic radiator? 2.1 dB
What is a parasitic beam antenna? An antenna where some elements obtain their radio energy by induction or radiation from a driven element
If a slightly shorter parasitic element is placed 0.1 wavelength away from an HF dipole antenna, what effect will this have on the antenna's radiation pattern? A major lobe will develop in the horizontal plane, toward the parasitic element
If a slightly longer parasitic element is placed 0.1 wavelength away from an HF dipole antenna, what effect will this have on the antenna's radiation pattern? A major lobe will develop in the horizontal plane, away from the parasitic element, toward the dipole