* Limited use ( read, only ).
On this page, only part of Chapter 3, "Television receiving antennas. By Seymour D.Uslan", pages 51 - 54.
Chapter 3, pages 31 - 54, in one file: Television_Ant_1948_NY.zip (first scan, format - .jpg, total - 25 files, 3,837 kb).
Other Types of Television Antennas
So far in the analysis of television receiving antennas we have dealt primarily with simple and folded dipole antennas, with and without reflectors, because they are the types that are most universally used. There are, however, numerous other types of antennas used for the reception of television signals. Some of them are no more than slight variations of the antennas already analyzed. Many of the design principles and the theory involved have already been discussed in general form as applicable to all antennas. These other types of antennas are usually designed for one or more of the following reasons: better signal pickup, omnidirectional signal response, and better broadband response.
Some of these antennas may be higher priced than those previously discussed. In other words, if more time and equipment is put into the design of television antennas, the price will be high compared with other antennas. Thus the television receiver buyer, or even the television manufacturer buying antennas, is often faced with limitations on how much he can economically afford to spend on antennas. Therefore, most antenna manufacturing companies refer to the electrical advantages or superiority of their product in
respect to their price. They are trying to be as fair as possible to the public in establishing what they believe to be an honest account of what they have put into the antenna in the form of engineering design as well as material things.
In Fig. 3-22
is illustrated an antenna known as the Di-Fan. This
antenna offers broadband characteristics
Courtesy Andrew Co.
Fig,. 3-22.—A special type of antenna known as the Di-Fan. It offers broadband characteristics over the two television bands and the f-m band.
over the complete range of the two television bands and hence also includes the f-m broadcast band of 88 to 108 mc. The nominal impedance of this type of antenna is about 300 ohms and it is therefore suitable for use with a 300-ohm transmission line.
The antenna illustrated in Fig. 3-23 is known as the double-V. This type of antenna, when cut to the
Courtesy Technical Appliance Corp.
Fig. 3-23.—The double-V antenna has a broadband characteristic.
proper
frequency in either television band, is said to give a fairly
good frequency response over the entire band. When
designed to be used with the
TELEVISION RECEIVING ANTENNAS Page52
low-frequency television band it can also be used for the f-m band.
The stacked dipole, the so-called H-type antenna, is shown in Fig. 3-24. This type of antenna essentially consists of two center-fed dipoles mounted one above the other with their center points connected as shown. The transmission line is attached to the parts connecting these center points. This antenna, like a single dipole, is bidirectional. Mounting one dipole over the other allows for greater signal pickup in a broader horizontal plane. In other words, this system provides greater gain than the simple dipole. Since
Courtesy Technical Appliance Corp.
Fig. 3-24.—The stacked dipole, or H-type antenna, is bi-directional and provides greater gain than does a simple dipole.
higher gain is obtainable, this antenna is recommended by the manufacturer to be used in localities where the signal strength is low.
The stacked dipole of Fig. 3-24 is put to further use by employing two reflectors with the unit. This is shown in the antenna drawing of Fig. 3-25.
Courtesy Technical Appliance Corp.
Fig. 3-25.—The double-doublet antenna essentially consists of a simple dipole and reflector arrangement mounted above another one. The reflector elements are added to help prevent ghosts and to make the antenna unidirectional.
This antenna is called the double-doublet by its manufacturer and essentially consists of one simple dipole and reflector arrangement mounted above the other. The dipole elements are, however, connected together at their center points as in the H-type antenna. The reflector elements are added to help prevent ghosts and to make the antenna unidirectional. The over-all gain of this antenna is somewhat greater than the H-type because of the addition of the reflector ele-ments. It is recommended by the manufacturer that this antenna be used in areas where there are a num-ber of reflected signals and also in noisy areas where the signal strength is low.
There are antenna systems appearing on the market that appear to be a dipole and reflector arrangement, but the two elements are spaced somewhat closer together than the usual dipole-reflector arrange-ment. In most cases these antenna systems use the element with the transmission line attached as a director and not a reflector. Director is the name given to an antenna parasitic element that is oriented in the direction of the signal to be received. The exact spac-ing between the dipole and director is determined by the physical makeup of the antenna system and surrounding territory. In Fig. 3-26 a dipole and director
Courtesy Shur-Antenna-Mount, Inc.
Fig. 3-26.—The distance between the dipole and director is variable so that best use of the director can be attained when the antenna is installed.
system is illustrated; the distance between the two elements is variable so that the maximum use of the director can be attained wherever the antenna is installed. The director element is made slightly smaller in length than the dipole element.
The input impedance of many such systems is quite low, and special impedance matching sections very often must be used in conjunction with the transmission line so that the maximum amount of energy transfer from the antenna to the receiver can be attained. Most manufacturers incorporate the necessary equipment for correct impedance matching, or include information as to how it may be accomplished.
TELEVISION RECEIVING ANTENNAS Page53
The signal pickup can be further increased by an antenna system employing both director and reflector elements in conjunction with a dipole.
A drawing
of such a system is illustrated in Fig. 3-27, where the dipole
is the center element.
Courtesy Consolidated Television Corp.
Fig. 3-27.—The director is placed in the path of the incoming signal and the reflector is placed behind the dipole. Increased signal pickup and great directivity are provided by the use of this system.
Although not readily evident in
this drawing, the director element is slightly shorter than
the dipole and the reflector element slightly longer than the dipole.
The director is placed in the direction of the television signal. This
type of antenna system, in addition to providing increased signal
pickup, increases the directivity as compared to the dipole reflector and
dipole-director antennas. That is, this antenna is
unidirectional and the reflector element also helps eliminate ghosts.
There are a number of television antennas on the market today that are so designed that they contain two separate dipole elements, one for each television band. The element designed for the high-frequency band (channels 7-13) is smaller than the element designed for the low-frequency band (channels 1-6). In most cases the elements are cut for the center frequency of the band in question.
Fig. 3-28 illustrates one such antenna. Upon first inspection, this antenna appears as though it consists of two folded dipoles. This is not the case. Each element is a simple dipole that is folded back on itself in a U shape and shorted at the ends. The Q of each element is made smaller by this method,
Courtesy L. S. Brack Mfg. Corp.
Fig. 3-28.—Each element is a simple dipole, folded back on itself and shorted at the ends. The longer element is for the low-frequency television band and the shorter element for the high-frequency band.
system. The longer antenna is for the low-frequency television band and the shorter antenna for the high-frequency band. To prevent interaction between antenna elements, they are separated by special sections called "dividers." These dividers are the black portions of the vertical rods connecting the two antenna elements as seen in Fig. 3-28. The antenna elements and dividers are so designed that by connecting the transmission line above the dividers, good reception on both television bands will be achieved.
All the antennas discussed so far were either bi-directional or unidirectional. There are some an-tennas however, that are omnidirectional in that they will respond to signals from all directions. Such an
|
Fig. 3-29.—The total effective field pattern of two crossed dipoles is virtually circular in shape. |
TELEVISION RECEIVING ANTENNAS Page54
antenna is, of course, only suitable in regions where there is very little chance for reflected signals and thus little possibility of ghosts. Omnidirectional antennas have been used a great deal with f-m receivers with very good results. These antennas essentially consist of two dipole elements both of the same size, at right angles to each other. Their lengths are cut to the center of either band, or whatever other frequency may be desired. Their center-fed points are usually individually connected to transmission lines, which in turn are combined in a single line for whatever the impedance desired.
TWO OR MORE RECEIVERS ON ONE ANTENNA
You must know the relative signal strength in your area so you can tell if 2, 3, or 4 receivers can be connected to one antenna, as shown above. For two sets, the available signal voltage is cut 2 to 1 ; for three, it is cut 3 to 1, and for four sets, it is 4 to 1. Use carbon resistors, as wire wound may unbalance the system. Place switches and resistors in a control box. All leads must be short and direct. If a set is removed from the system, throw switch to dummy load.