Indoor television antennas work in conditions drastically different from the operating conditions of the external antenna.
It is caused by the structure of the electromagnetic field VHF indoors. There is a field typically has a clear interference in nature, being a field of standing waves, which can be explained by the presence of reflections from the walls and various objects in the room. The intensity of the electromagnetic field in a room can dramatically change when you move to 1-1.5 m. For the various frequency components of a television signal distribution spehtra maxima and minima of the electromagnetic field may be different, which leads to a distortion of the television signal frequency.
In most cases, the electromagnetic field in the room is a set of horizontally and vertically polarized fields, sometimes vertically polarized field prevails. Vertically polarized field with horizontal polarization radiation field of the television station appear due to the excitation of various metal constructions in the walls of buildings, lighting circuits, and so on. D. Therefore, often receiving a vibrator in the room has to be installed inclined and even vertical.
The weakening of the strength of the electromagnetic field inside the building (with respect to the field strength outside the building) can vary from 2 to 30-40 times, due to the partial absorption of electromagnetic energy in the walls of the building.
Indoor antenna sensitive to external noise sources which are located in the same building (elevator motors, electric bells, and so on. D.).
Quality, an indoor antenna can be obtained by installing the antenna in the most unexpected places — on the cabinet, on the ceiling, on a window sill, and, in various positions, which only allowed the construction of the antenna.
From the above it can be concluded that the proper installation of an indoor antenna requires some skill, and above all patience.
It can be argued that in Moscow, within a radius of 6-7 km from the television station, reception is possible in the indoor antenna, except in certain cases: the first floors of buildings, concrete structure of the building with windows facing in the direction opposite to the telecentre, and so on. Etc. .
Unlike external indoor antenna may not be aligned with the cable as the latter tend to be very short and misalignment can cause the appearance of multiloop.
ROOM TELEVISION KTTA telescopic antenna.
At present, our industry produces television room KTTA telescopic antenna, which is split half-wave vibrator telescopic design. «Whiskers» vibrator enshrined in the plastic base, each consist of four members each other handsets.
Connect the antenna to the TV coaxial cable is made of RK-1 or a symmetrical two-wire feeder with a characteristic impedance of about 75 ohms.
The antenna can be used on any of the five television channels, wherein the transition from the channel to cocoa produced by changing the length of the vibrator. The exact length to which must be separated «whiskers» vibrator for each channel, it is difficult to specify, as it depends on the position of the vibrator with respect to the walls of various subjects, the angle between the «mustache».
In order to simplify and reduce the cost of the antenna is not provided with a balun. The absence of a balun is not a significant disadvantage of the antenna, because in most cases itself a vibrator mounted in the room loses symmetry due to unequal halves vibrator locations with respect to the walls of the room, and so on of various items. D.
The diagram in the horizontal plane has the form "Eight"And directional coefficient is unity. The bandwidth of the antenna is about 10 mc at the unevenness of 1 dB.
Thus, from the viewpoint of antenna television reception has good electrical data, enabling to obtain a good image. However, due to the specific TV reception indoors to implement fully the advantages of the antenna is difficult. In this respect the crucial successful siting of the antenna.
A major shortcoming of the antenna KTTA a significant length of its «whiskers», whereby the use of such an antenna in a small room is rather difficult.
Presently produced various television telescopic antenna basically similar in their electrical and structural data, and only slightly differ in some structural elements.
INDOOR ANTENNA TYPE
«Shortened linear oscillators.»
It is very interesting to construct such an indoor antenna, which, having a half-wave dipole efficiency, would have a much smaller linear dimensions.
The shortened shaker loss induced by external electromagnetic field an electromotive force that occurs due to decrease effective height is compensated by increasing the voltage in the matching transformer. Last recounts a large load resistance in the low resistance equal to the active component of the input impedance of a short dipole.
The reactive component of the input impedance should be compensated. As a result, the voltage at the input of the receiver becomes approximately equal to the voltage that would develop a half-wave dipole. Bandwidth configured and coherent shortened vibrator has a half-wave dipole bandwidth. The shorter the vibrator, the narrower band. Therefore, make shorter vibrator fails.
The following describes how to perform the first television channel indoor antenna such as «truncated linear vibrator.»
The total length of the vibrator is 1.5 m, t. E. It is almost half the length of a half-wave dipole. Transformation-matching device, consisting of two identical coils L1 and L2 and the coil L3, the resistance compensates the reactive component of the vibrator, the load impedance transforms and makes a transition to the dipole on the coaxial cable. The coils L1 and L2, and the vibrator tube of brass, steel or duralumin are mounted on a common getinaksovoy board. 16 mm diameter tubes. The length of each tube 770 mm (each tube can be collapsible).
Coils L1 and L2 have eight turns of wire PEL-one winding pitch of 0.8 to 1 mm. The coils L1, L2 are wound on a common ribbed carcass. Connecting the coils L1 and L2 to the tube of the vibrator is produced by means of short lengths of copper wire with a diameter of 1-1.5 mm.
Particular attention should be paid to the correct manufacture of the coil L3. Solenoid L3 is composed of two symmetrical coils of cable RK-1, which collapsed in the direction of the arrows. Upper round 1 of the cable is a continuation of the decline. The end of the coil is connected to the harness braid collected by the end of the coil L2 and the living — by the end of the coil L1. The lower coil 2 is also made of cable RK-1, but its implementation is only used metal braiding. The veins on both sides are cut flush and do not connect. One end of the lower coil is soldered to the sheath of the upper coil. The other end of the lower coil is collected in a harness, and is soldered to the point a, t. E. To the same point where the upper coil is soldered lived.
Thus, the actual search coil L1 is formed of metal braided cable RC-1, but one of the two symmetrical windings used simultaneously as an extension cable reduction.
After assembly, the coil is wound with threads so that the turns evenly and tightly laid one on another. It should be carefully monitored so that the metal braid in the places where they are not soldered, do not touch each other.
Textolite board with a vibrator secured to the rack height of 25 to 30 cm, has a stable stand. The antenna can be placed on the TV, on the windowsill, a cupboard, hang on the wall, and so on. D.
Final adjustment of the antenna is made of the test pattern change in a small range of inductances L1 and L2. It should be remembered that the inductance must remain the same.
The head can be enclosed in a box of Micarta or polystyrene. The directivity of the antenna described in comparison with the half-wave dipoles of 0.8-0.9 voltage. Polar Pattern — «eight». The bandwidth of the antenna provides a sufficiently good image quality.
ROOM ring antenna.
Indoor ring antenna is quite difficult to manufacture at home, so give them a detailed description does not make sense.
Single ring antenna, designed for receiving a television program, is bent into a ring uncut linear vibrator, short lumped capacitance connected between the outer ends of the vibrator. This capacity n is an element antenna tuning into resonance. Reduced antenna cable runs RC-1, which is connected to the antenna through delta transformer representing a two-wire line located in the plane of the ring and convergent in a direction from the periphery to the center of the ring. Cable entry reduction is carried out at zero potential.
In horizontal ring antenna has a radiation pattern in the horizontal plane, similar in shape to a circle, which reduces the noise immunity antenna and difficult struggle with multi-loop. With a vertical arrangement of the ring antenna has a radiation pattern in the horizontal plane, close to the «eight», the direction of the zero reception is aligned perpendicular to the plane of the ring. But in this arrangement the ring antenna responds only to vertically polarized field (when taken with the horizontal direction).
The bandwidth of the antenna is 4 mc of the voltage decline by 3 dB. Antenna gain relative to a half-wave vibrator is 0.45 voltage.
The double ring antenna design for a little more complicated than a single ring antenna, but at the same time, it is more compact.
Double-ring antenna characterized by the same disadvantages as the single ring antenna.
Antenna that is built into the drawer of the TV.
Most efficiently perform such an antenna in the form of a small planar antenna frame or vibratory type. The following describes one embodiment of a simple built-in antenna, which is a customized cutting small vibrator. Imagine this customized vibrator in the form of equivalent series oscillating circuit, where Ek — electromotive force induced in the vibrator electromagnetic field, Ra — the active component of the input impedance of the vibrator, is numerically equal to its resistance to radiation, Ha — reactive component of the input impedance of the vibrator, X l — inductive resistance include compensation for the reactive component of the input impedance of the vibrator, Zp-load resistance.
The vibrator shorter than a half wavelength, the higher Xa and hence the higher the characteristic impedance of the equivalent oscillating circuit. On the other hand, the shorter the vibrator, the lower the Ra, which is the equivalent loss resistance of the oscillatory circuit. Both of these factors lead to an increase in the unloaded Q of tuned short vibrator. If in order to obtain high efficiency vibrator configured to agree with the cable for this purpose include a matching transformer between the terminals a — b and the load terminals, the insertion into the equivalent oscillating circuit losses will be equal to its own losses in the circuit. If the total length of the vibrator is smaller, the resulting bandwidth will be so narrow that such an antenna can not be used for receiving television transmissions, even in the same channel.
How then increased bandwidth vibrator small length, to use it as a built-in antenna?
This is achieved by a sharp increase in the transverse dimensions of the vibrator, but it leads to an increase in the overall dimensions of the vibrator. To avoid this, you must change the appropriate mood vibrator equivalent oscillating circuit large attenuation is much greater than his own. This can be achieved by connecting to the terminals a — b without much resistance load matching transformer, t. E. Placing a vibrator mode almost complete mismatch with the cable. This reduces efficiency, but expanding the bandwidth of the antenna while working as a 75-ohm and 300-ohm input of the TV.
Built on this principle without a built-in antenna covers the restructuring of the five television channels.
Built-in antenna is as follows. On textolite disc or fiber thickness of 2-3 mm and a diameter of 450 mm is pasted vibrator copper or aluminum foil. Adjusting the antenna to resonate at the central frequency band from 48.5 to 100 Mc is made the same two inductors L1 and L2, twelve turns of PEL-1 1.0. The winding diameter of 10mm, the winding pitch of 1 mm. The cable that connects the antenna to the input of the receiver is connected to the points a — b. Cable length of 30-40 cm.
The antenna is located inside the TV box directly under the top cover of the coils down. The gap between the antenna and the cover disc is 5-6 mm. Attaching the antenna to the lid is made with a metal or dielectric pin which passes through a hole in the center of the disc. On the same pin as axis, the disc is rotated, which enables to position the best antenna having the directivity diagram of the «eight». Attachment point center of the disc to the top cover should be chosen so as to drive out on the 3-4 mm in size TV box into the slot height of 4-5 mm. The gap is necessary for this purpose, specially cut in the back of the TV. With this disk can be rotated without removing the back cover. The relative position of the disk and the gap height should be such that its smooth side of the disc (ie. E. Side facing the top cover of the TV), without canting, slightly rubbed on the upper edge of the slot. This will make the drive more stable and ensure the fixation of his position after orientation.
Built-in antenna can be connected to both 300-ohm input of the receiver using a symmetric brand CATV cable with impedance of 300 ohms, and a 75-ohm input of the receiver using the cable RC-1 or symmetrical feeder with 75 ohm. Cheaper still be connected to the built-in antenna 300-ohm input of the receiver, as the efficiency of the antenna operating at the edges of the band gets higher, and the frequency response of the antenna within each TV channel is a more uniform.
Here are the main electrical parameters of the built-in antenna, measured at the antenna is connected to the 300-ohm input of the receiver.
The unevenness of the frequency characteristics in the frequency band of the third television channel (76-84 Mc) does not exceed 1 dB in the frequency band of any other television channel frequency response unevenness less. Most low efficiency equal to 0.11 (with respect to the half-wave dipole) antenna has a first television channel.
Sufficient contrast when applying for a multi-channel built-in antenna can be obtained within a radius of about 5-6 km away from the television station (in relation to the ITC), if the receiver has a sufficiently high sensitivity, equal to 50 100 mV. When the receiver sensitivity equal to 500-1000 mV, you can use the built-in antenna for a radius of 2-3 km from the television station.