ANTENNA TYPES.
DEFINATION :
An antenna is a transducer, i.e. a “converter” between two media. The antenna converts electromagnetic energy in a cable into electromagnetic radiated power in free space.Various characteristics of the antenna determine”how efficient” this conversion is performed:
ANTENNA CHARACTERISTICS.
1.To have as much energy as possible carried on as radiation in the room is of great importance, i.e. the antenna should not be capable of consuming energy itself.
2.It is important that the antenna concentrates its radiated energy as efficiently as possible in the required way.it If so, either a wide coverage range or a reduced transmitting power can be achieved, until the range is exactly suiting the one required.
The antenna converts electromagneticenergy
in a cable into electromagneticradiated power in free space
Omni directional Antenna 
Directional Antenna
Antenna Types
1. Base Station Antennas
2. Antennas for movable units
Base Station Antennas :
Base station antennas can be:
· Omnidirectional Antennas
· Directional Antennas
· Antennas with radiation being specially constructed, the latter yielding the antenna a quite specific coverage range
Antennas for movable (”mobile”) units
· ”Mobile” antennas can be split up into: Land Mobile Antennas (vehicle antennas)
· Marine Antennas (for ships)
· Portable Antennas (antennas for portable communication equipment)
· (Air Craft Antennas) .
"Mobile" antennas can be split up into:
1. Land Mobile Antennas (vehicle antennas)
2. Marine Antennas (for ships)
3. Portable Antennas (antennas for portable communication equipment) (Air Craft Antennas) .
Examples:
Base Station Antennas
Base Station Antenna
"Mobile” antennas
Land Mobile Antenna
NB Colloquially, the word ”mobile antenna” is usually used as designation for a vehicle antenna.
NB Colloquially, the word ”mobile antenna” is usually used as designation for a vehicle antenna.
Marine Antenna
Portable Antenna
Air Craft Antenna
Portable Antenna
How to describe how good an antenna is?
The four ”figures” describe how good an antenna is compared to the required performance:
· SWR = Standing wave ratio .
· D = Directivity .
· G = Gain .
· BW = Bandwidth .
SWR .
If the impedance of the antenna is different from the impedance of the cable, the antenna will reflect back some of the induced energy through the feeder cable to the transmitter, which naturally is undesirable.Normally, the impedance of the cable is 50 Ω. If Ra indicates the impedance of the antenna, the standing wave ratio is defined as:SWR = Ra/50 Ω (if Ra is more than 50 Ω)SWR = 50/Ra Ω (if Ra is less than 50 Ω)Examples:If Ra = 50 Ω is SWR = 1.0If Ra = 100 Ω is SWR = 2.0If Ra = 25 Ω is SWR = 2.0Consequently, it is of importance that the SWR is as close to 1.0 as possible thus obtaining the highest power being transmitted from the cable to the antenna.
· BW = Bandwidth .
SWR .
If the impedance of the antenna is different from the impedance of the cable, the antenna will reflect back some of the induced energy through the feeder cable to the transmitter, which naturally is undesirable.Normally, the impedance of the cable is 50 Ω. If Ra indicates the impedance of the antenna, the standing wave ratio is defined as:SWR = Ra/50 Ω (if Ra is more than 50 Ω)SWR = 50/Ra Ω (if Ra is less than 50 Ω)Examples:If Ra = 50 Ω is SWR = 1.0If Ra = 100 Ω is SWR = 2.0If Ra = 25 Ω is SWR = 2.0Consequently, it is of importance that the SWR is as close to 1.0 as possible thus obtaining the highest power being transmitted from the cable to the antenna.
Directivity D .
The directivity D is an indication of the capability of the antenna to conduct the radiated power “to a certain site”.Normally, omnidirectional or directional antennas are mentioned.
Omnidirectional:
An omnidirectional antenna with high directivity has a radiation being similar to a pancake.
Directional:
A directional antenna with high directivity has a radiation being similar to the cone of light from a projector.
Gain G .
The gain of an antenna is defined as G = η x D, where η indicates the efficiency of the antenna.Consequently, in the gain value possible loss in the antenna is comprised. The η-figure is always less than the directivity.For most antenna types the own loss is so low that G = D can be considered.
The gain of an antenna is defined as G = η x D, where η indicates the efficiency of the antenna.Consequently, in the gain value possible loss in the antenna is comprised. The η-figure is always less than the directivity.For most antenna types the own loss is so low that G = D can be considered.
Bandwidth .
The bandwidth of the antenna is the frequency range, in which it operates properly,i.e. both gain and SWR are within the more specified limits.
How to measure how good an antenna is?
The performance of an antenna can be determined from the following formula:
· SWR = Standing Wave Ratio
· G = Gain
· BW = Bandwidth
The following explains how to determine SWR, Gain og BW
Measuring SWR.
Standing Wave Ratio:
The standing wave ratio is normally measured by inserting adevice in the feeder cable of the antenna. How much powerreflected back from the antenna compared to the powertransmitted forward is measured?Such a device is called a directional coupler.Directional couplers:Directional couplers or SWR meters for frequencies lower than150 MHz are relatively cheap and quite accurate.For frequencies above 400 MHz exact meters are expensive.The most well known universal instrument is a BIRD-wattmeter.
Measuring the standing wave ratio .
Measuring gain .
The gain of an antenna is measured in relation to the gain of another further specified antenna through a comparison measurement.The latter antenna is called a reference antenna and the reference antenna is different, irrespective of weather base station antennas,mobile antennas or portable antennas are considered.
Measurement setup/procedure .Measuring BW.
The bandwidth is generally specified as the area of the lowest part of the frequency ranges, in which SWR and gain observe the specifications
¼ wave and collinear at the car roof centre.
COMMON TV ANTENNAS.
The Dipole
This is the simplest TV antenna. Variations on the dipole are the bowtie (which has wider bandwidth), the folded-dipole (which can solve an efficiency problem) and the loop (a variation on the folded dipole). All four have the same gain and the same radiation field: a torroid (doughnut shape). The gain is generally 2.15 dBi. “dBi” means “dB of improvement over an isotropic radiator”, which is an antenna that radiates equally in all directions. This sounds like a discussion of transmitting antennas, and it could be. An antenna will have the same gain when receiving as when transmitting, and also the same radiation pattern.
The dipole has positive gain because it does not radiate equally in all directions. This is a universal truth. To get more gain, an antenna must radiate in fewer directions. Imagine a spherical balloon. Now press on it from opposite sides with a finger of each hand. Push in until your fingers meet. The result looks like the torroid above. But more importantly, the balloon expanded in the other directions. A-hah! Gain! That’s the way antennas work.
Keep this balloon analogy in mind. More complicated antennas work by reducing radiation in most directions. They distort the balloon considerably, but the volume of the balloon remains constant.
Another rating system for antennas uses dBd, which means dB of improvement over a dipole antenna. To convert dBd to dBi, just add 2.15. Antenna makers specify their gains in dB. They actually mean dBd, but given the way they exaggerate their claims, dBi is usually closer to the truth.
In the US, TV antennas are always horizontal. If you rotate an antenna about the forward axis (a line from the transmitting antenna) the signal strength will vary as the cosine of the angle. In other words, when the antenna elements are vertical, no signal is received because TV signals have horizontal polarization.
Yagi Antennas .
A Yagi antenna has several elements arranged in echelon. They are connected together by a long element, called the boom. The boom carries no current. If the boom is an insulator, the antenna works the same.
The rear-most element is called the reflector. The next element is called the driven element. All the remaining elements are called directors. The directors are about 5% shorter than the driven element. The reflector is about 5% longer than the driven element. The driven element is usually a folded dipole or a loop. It is the only element connected to the cable. Yet the other elements carry almost as much current.
The Yagi is the most magical of all antennas. No attempt will be made here to explain why it works. The more directors you add, the higher the gain becomes. Gains above 20 dBi are possible. But the Yagi is a narrowband antenna, often intended for a single frequency. As frequency increases above the design frequency, the gain declines abruptly. Below the design frequency, the gain falls off more gradually. When a Yagi is to cover a band of frequencies, it must be designed for the highest frequency of the band.
An antenna has an aperture area, from which it captures all incoming radiation. The aperture of a Yagi is round and its area is proportional to the gain. As the leading elements absorb power, diffraction bends the adjacent rays in toward the antenna.
The formula for the aperture area of any TV antenna is A=Gl2/4p where l is the wavelength and G is the gain factor over an isotropic antenna (not dB).
The bandwidth of a Yagi can be increased by sizing the reflector for the lowest frequency of the band while sizing the directors for the highest. But this decreases the best gain of the antenna. (It is said that the gain-bandwidth product remains the same.) A better way to increase the bandwidth is to replace the reflector element with a corner-reflector assembly.
This boosts the performance on the lower numbered channels without hurting the high channels. Although the Yagi/Corner-Reflector might not be the best antenna, it is the most common UHF TV antenna, mainly because it can be mounted on the front of a VHF antenna without degrading the VHF antenna.
A UHF Yagi today is designed for channel 69. If you see an old Yagi, it might be intended for channel 82. In the future they will be cut for channel 51. It is not possible to tell by looking at a Yagi which era it belongs to, so be careful.
Radiation patterns :
As you can see, the 8-Bay is a very directional antenna. If miss-aimed by 5° you can lose 1 dB of signal. If the skyline is more than 5° above horizontal, you should tilt the antenna up to point at the skline.
The overhead view shows nulls at 30° and 90° to both sides. These can be used to eliminate multi-path (ghosts) or interference. You simply rotate the antenna until the offending signal is in one of the nulls.
A Yagi also has some forward nulls that can be used as ghost killers. But a Yagi/Corner-Reflector acts more like a corner reflector for most channels, and has no nulls. At channel 60 you can finally see the Yagi pattern start to emerge.
This author prefers the 8-Bay over the Yagi/Corner-Reflector because
It has high gain.
Its gain is evenly distributed over the channels.
It has nulls that can eliminate multi-path.
It has a rectangular aperture that permits efficient stacking when more than 8 bays are necessary.
But the high gain means it is hard to aim. In good-signal areas, avoid high gain antennas.
REFERENCE:
http://www.kyes.com/antenna/environment.html
http://www.bbc.co.uk/reception/analoguetv/picture.shtml
http://www.hdtvprimer.com/
¼ wave and collinear at the car roof centre.COMMON TV ANTENNAS.
The Dipole
This is the simplest TV antenna. Variations on the dipole are the bowtie (which has wider bandwidth), the folded-dipole (which can solve an efficiency problem) and the loop (a variation on the folded dipole). All four have the same gain and the same radiation field: a torroid (doughnut shape). The gain is generally 2.15 dBi. “dBi” means “dB of improvement over an isotropic radiator”, which is an antenna that radiates equally in all directions. This sounds like a discussion of transmitting antennas, and it could be. An antenna will have the same gain when receiving as when transmitting, and also the same radiation pattern.
The dipole has positive gain because it does not radiate equally in all directions. This is a universal truth. To get more gain, an antenna must radiate in fewer directions. Imagine a spherical balloon. Now press on it from opposite sides with a finger of each hand. Push in until your fingers meet. The result looks like the torroid above. But more importantly, the balloon expanded in the other directions. A-hah! Gain! That’s the way antennas work.
Keep this balloon analogy in mind. More complicated antennas work by reducing radiation in most directions. They distort the balloon considerably, but the volume of the balloon remains constant.
Another rating system for antennas uses dBd, which means dB of improvement over a dipole antenna. To convert dBd to dBi, just add 2.15. Antenna makers specify their gains in dB. They actually mean dBd, but given the way they exaggerate their claims, dBi is usually closer to the truth.
In the US, TV antennas are always horizontal. If you rotate an antenna about the forward axis (a line from the transmitting antenna) the signal strength will vary as the cosine of the angle. In other words, when the antenna elements are vertical, no signal is received because TV signals have horizontal polarization.
Yagi Antennas .
A Yagi antenna has several elements arranged in echelon. They are connected together by a long element, called the boom. The boom carries no current. If the boom is an insulator, the antenna works the same.
The rear-most element is called the reflector. The next element is called the driven element. All the remaining elements are called directors. The directors are about 5% shorter than the driven element. The reflector is about 5% longer than the driven element. The driven element is usually a folded dipole or a loop. It is the only element connected to the cable. Yet the other elements carry almost as much current.
The Yagi is the most magical of all antennas. No attempt will be made here to explain why it works. The more directors you add, the higher the gain becomes. Gains above 20 dBi are possible. But the Yagi is a narrowband antenna, often intended for a single frequency. As frequency increases above the design frequency, the gain declines abruptly. Below the design frequency, the gain falls off more gradually. When a Yagi is to cover a band of frequencies, it must be designed for the highest frequency of the band.
An antenna has an aperture area, from which it captures all incoming radiation. The aperture of a Yagi is round and its area is proportional to the gain. As the leading elements absorb power, diffraction bends the adjacent rays in toward the antenna.
The formula for the aperture area of any TV antenna is A=Gl2/4p where l is the wavelength and G is the gain factor over an isotropic antenna (not dB).
The bandwidth of a Yagi can be increased by sizing the reflector for the lowest frequency of the band while sizing the directors for the highest. But this decreases the best gain of the antenna. (It is said that the gain-bandwidth product remains the same.) A better way to increase the bandwidth is to replace the reflector element with a corner-reflector assembly.
This boosts the performance on the lower numbered channels without hurting the high channels. Although the Yagi/Corner-Reflector might not be the best antenna, it is the most common UHF TV antenna, mainly because it can be mounted on the front of a VHF antenna without degrading the VHF antenna.
A UHF Yagi today is designed for channel 69. If you see an old Yagi, it might be intended for channel 82. In the future they will be cut for channel 51. It is not possible to tell by looking at a Yagi which era it belongs to, so be careful.
Radiation patterns :
As you can see, the 8-Bay is a very directional antenna. If miss-aimed by 5° you can lose 1 dB of signal. If the skyline is more than 5° above horizontal, you should tilt the antenna up to point at the skline.
The overhead view shows nulls at 30° and 90° to both sides. These can be used to eliminate multi-path (ghosts) or interference. You simply rotate the antenna until the offending signal is in one of the nulls.
A Yagi also has some forward nulls that can be used as ghost killers. But a Yagi/Corner-Reflector acts more like a corner reflector for most channels, and has no nulls. At channel 60 you can finally see the Yagi pattern start to emerge.
This author prefers the 8-Bay over the Yagi/Corner-Reflector because
It has high gain.
Its gain is evenly distributed over the channels.
It has nulls that can eliminate multi-path.
It has a rectangular aperture that permits efficient stacking when more than 8 bays are necessary.
But the high gain means it is hard to aim. In good-signal areas, avoid high gain antennas.
REFERENCE:
http://www.kyes.com/antenna/environment.html
http://www.bbc.co.uk/reception/analoguetv/picture.shtml
http://www.hdtvprimer.com/
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