Jan 09 2010

Bandpass System

By admin

Bandpass System

Television antenna

Simple/indoor

Very common "rabbit ears" set-top antenna of older model

See also: Yagi antenna

An aerial or rooftop antenna generally consists of multiple conductive elements that are arranged such that it is a directional antenna. The length of the elements is about one half of the signal wavelength. Therefore, the length of each element corresponds to a certain frequency. The longer elements for picking up VHF frequencies are in the "back" of the antenna, relative to the devices directionality, and the much shorter; UHF elements are in the "front", and the antenna works best when "pointing" to the source of the signal to be received.

The smallest elements in this design, located in the "front", are UHF director elements, which are usually identical and give the antenna it's directionality, as well as improving gain. The longest elements, located in the "back" of the antenna form a VHF phased array. Other long elements may be UHF reflectors Another common aerial antenna element is the corner reflector, a type of UHF reflector which increases gain and directionality for UHF frequencies.

The longer the beam length of the antenna, the more directors it has, and the more accurate the tuning of those directors, the higher its gain will be, although for the commonly used Yagi antenna, this is not a linear relationship. Gain is the amount of signal the aerial "collects" and this is inversely proportional to its acceptance angle. The thickness of the rods on a Yagi antenna and the frequency bandwidth (receivable frequencies) are inversely proportional. Larger rods will allow wider bandwidth acceptance. This renders it optimal to have smaller conductor elements; however, they must be large enough for physical strength against the weather.

Since rooftop antennas are directional, two or more can be set up and connected to one receiver. Rooftop antennas are also sometimes placed in attics.

Types of Outdoor Antennas

Small Multi-directional: The smallest of all outdoor television antennas. They are designed to receive equal amounts of signal from all directions. These generally receive signals up to a maximum of thirty miles away from the transmitting station, greatly depending on the type. But, things such as large buildings are thick woods may greatly affect signal. They come in many different styles, ranging from small dishes to small metal bars, some can even mount on existing satellite dishes.

Medium Multi-directional: A step up from the small multi-directional, these also receive signals from all directions. These usually require an amplifier in situations when long cable lengths are between the television receiver and the antenna. Styles are generally similar to small multi-directionals, but slightly larger.

Large Multi-directional: These are the largest of all multi-directional outdoor television antennas. Styles include large "nets" or dishes, but can also greatly vary. Depending on the type, signal reception usually ranges from 30 to up to 70 miles.

Small Directional: The smallest of all directional antennas, these antennas are multi-element antennas, typically placed on rooftops. This style of antenna receives signals generally equal to that of large multi-directionals. One advantage that small directionals hold, however, is that they can significantly reduce "ghosting" affects of television picture.

Medium Directional: These antennas are the ones most often seen on suburban rooftops. Usually consisting of many elements, and slightly larger than the small directionals, these antennas are ideal for receiving television signals in suburban areas. Signal usually ranges from 30 to 60 miles away from the broadcasting station.

Large Directional: The largest of all common outdoor television antennas, these antennas are designed to receive the weakest available stations in an area. Larger than the medium directional, this type of antenna consists of many elements and is usually used in rural areas, where reception is difficult. When used in conjunction with an amplifier, these antennas can usually pick up stations from 60 up to and over 100 miles, depending on the type.

The use of outdoor antennas with an amplifier can improve signal on weak channels.

Setup

A short antenna pole next to a house; this setup would only work well for receiving signals on that side of the house as they would not go through stone, especially.

An antenna pole setup in a chimney, reaching 35 feet (10.7 meters) off the ground

See also: radio masts and towers

Aerial antennas are commonly placed on rooftops, and sometimes in attics. Placing an antenna indoors is generally considered to significantly attenuate its performance. Directional antennas must face the direction of the transmitter they are receiving; generally pointing in the general direction of the city of a station works well enough. In the analog television days; a signal picked up by an antenna facing the wrong direction would often produce ghosting in the image. This would also happen if multiple antennas connected to the same receiver were picking up the same station, especially if the lengths of the cables connecting them to the splitter/merger were different lengths or the antennas were too close together.

A tripod is often used to attach an aerial to a roof as well as to elevate a few feet off the roof, or a pole is installed next to the house or building and attached to the building to achieve roughly the same elevation. This is generally sufficient in most areas. In some places; however, such as a deep valley or near other taller structures, the antenna may need to be placed significantly higher. This is done by placing it on a lattice tower or mast.

Generally, the higher the antenna is placed, the better it will perform. The higher the gain of the antenna, the farther signals it will receive, but it will not work well through buildings, the ground, or dense forest. There are dangers inherent to high or complex antenna setups, of course, such as falling or the structure itself being destroyed by the weather. There are also varying local ordinances which restrict and limit such things as the height of a structure without obtaining permits. For example, the Telecommunications Act of 1996 states that "An antenna that is designed to receive local television broadcast signals" is covered by the rule, which is that any homeowner may install one. However, it also states that "Masts higher than 12 feet above the roofline may be subject to local permitting requirements."

Attic installation

Sometimes it is desired not to put an antenna on the roof; in these cases, aerial antennas designed for outdoor use are mounted in the attic. Putting an antenna indoors significantly decrease its performance; however, within short distances of the transmitter, this setup will likely work to a satisfactory level. One layer of asphalt shingles, roof felt, and a plywood roof deck are considered to result in an approximate 50% decrease in the performance of the antenna.

Multiple antennas

Two aerials setup on a roof. Spaced horizontally and vertically

Since more powerful antenna aerials are highly directional, it is common to use two antennas pointing in different directions connected to one television/receiver to cut out the need for a rotator. To do this successfully without the antennas interfering with each other, they must be spaced vertically so that the spacing between the booms is half the wavelength of the lowest frequency to be received in meters (Distance=/2). The wavelength of 54 MHz (Channel 2) is 5.5 meters ( x f = c) so the antennas must be a minimum of 2.25 meters, or ~89 inches apart. It is also important that the cables connecting the antennas to the signal splitter/merger be exactly the same length, to prevent phasing issues (such as ghosting with analog reception). That is, the antennas might both pick up the same station and the one with the shorter cable length will get to the splitter, then receiver, quicker than the signal picked up by the other antenna. The receiver will get the same signal twice at two different times. If both antennas pick up the same station, there may be phasing issues even with the same length of downlead cable. This may also cause severe ghosting. Bandpass filters of "signal traps" may help to reduce this problem.

For horizontal placement of multiple antennas as is common in a limited vertical space such as an attic, the distance between them should be one full wavelength of the lowest frequency to be received at their closest point.

Often when multiple antennas are used, one is for the full range of stations and the other is a single channel antenna designed to pick up a specific frequency. This is either to pick up a weaker signal or a single station from a different direction and/or strength.

Safety

There are a few safety considerations involved with TV antennas. One is that they are large conductors and attract lightning; acting as a large lightning rod. The use of a lightning arrestor is common to protect against this. A large grounding rod should be connected to both the antenna and the mast/pole.

There is inherent danger in any activity taking place on the rooftop of a house, such as installing or adjusting a television antenna. An antenna rotator is a simple solution to the problem of having to adjust the antenna.

When a mast is used, especially one of significant height, it should be guyed with galvanized cable with no insulators (as they are not necessary). It should be able to aptly handle whatever weather conditions are common to the area and should be spaced so that it will not interfere with power lines in operation or should it fall.

References

^ FCC Table of Frequency Allocations. 47C.F.R. 2.106 (2008).

^ http://www.dennysantennaservice.com/1496626.html

^ http://www.kyes.com/antenna/rabbitear.html

^ http://www.electronics-tutorials.com/antennas/antenna-basics.htm

^ http://www.terrestrial-digital.com/faqs.html#faq5

^ http://www.tvantenna.com/support/tutorials/atticantennas.html

^ http://www.tvantenna.com/support/tutorials/combining.html

^ http://www.fcc.gov/mb/facts/otard.html

^ http://www.tvantenna.com/support/tutorials/atticantennas.html

^ http://www.terrestrial-digital.com/faqs.html#faq9

^ http://www.tvantenna.com/support/tutorials/combining.html

^ http://www.nojolt.com/lightning_protection_for_tv_antennas.shtml

External links

Article on the basic theory of TV aerials and their use

http://www.antennaweb.org/aw/welcome.aspx

http://www.siber-sonic.com/FM_reception/rooftop.html

Categories: Antennas (radio) | Radio electronics | Radio frequency antenna types | Radio frequency propagation | Radio technology
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