Published: October 26, 2006 - 12:32 pm But this is not always the case. The ferrite antenna, for example, is used for shortwave reception, and the ability of the telescopic antenna to pick up radio signals is often inadequate.
reception conditions in the interior of the tall buildings and those close to the surface of the earth sometimes allow a good short wave reception, but for the most part the results are disappointing and in cases even outright bad. The purpose of this booklet is to give you some knowledge about the various types of outdoor antennas helping him make a decision about what to do to improve reception short wave. Is very important to select the appropriate type of antenna, and in view of the wide range of local situations and different possibilities to install an antenna, we have a selection of the most popular types, described below. Here
first some general observations. Shortwave signals arriving at your antenna probably have traveled thousands of miles. In your journey to your antenna, the waves are reflected by the ionized layers of the atmosphere and the surface of our planet. What remains of the signal at the end of the trip is only a fraction of the original power. Therefore, the signals need appropriate treatment. The loss of signal in the antenna and feed line must be limited as possible. Location
The real situation of the antenna is crucial for the quality of reception, especially in the presence of local interference sources such as overhead power lines, vehicles, appliances or fluorescent lamps. In most cases, will seek to install the antenna as high as possible, preferably no more than five feet above the top edge of the roof if the noise from passing vehicles on the street may cause undesirable interference .
Also choose a place as far as possible from sources of interference and avoid running the antenna cable short and parallel to overhead power lines or large metal objects such as gutters. If I had a portable radio receiver, it can help you find the best position for the antenna. Different tune shortwave bands and observe the interference levels at different points along which it intends to install the antenna. If you are afraid to be a real problem with the site makes use of the properties of specific antenna types are listed later in this booklet. Height
The higher the antenna is installed, the better the result. Similarly, the clearer you are, the better the reception. Try to find a place where eventually the shielding caused by trees, buildings, etc., Is reduced to a minimum. An antenna lying on a wet roof will not find more than a relatively low altitude over this "land" artificial and performance will be much lower than expected. So: give more air to your antenna! Length
A short antenna lengths unable to collect sufficient signal, but an excessively long antenna clearly show directional properties, so the proverb of "more is better" only applies to some extent. The total length of the outside of the antenna shall not exceed 25 meters. When the distance to cover is necessarily greater, the collation of an insulator may be the solution. Materials
The antenna wire must possess a high tensile strength to withstand high winds and also have good electrical conductivity. In the case of shortwave recommended copper wire or tinned copper 1 millimeter in diameter. This thread may be solid or stranded, bare or insulated. If not available copper wire, can be used in aluminum or galvanized steel provided to increase its diameter so that this reasonable conductivity.
Inside, the entry can be twisted and insulated cable, since it is flexible and prevents breakage and rubs deteriorate if any metal objects in the room. Isolation
The isolation of the antenna serves to protect the radio frequency signals so that they do not escape to the ground. Therefore, you need a good RF isolation. Well-known types of glazed porcelain insulators and pyrex. If you have a choice, select your type with ribs, because they have a long way to leakage, when the surface tends to become conductive due to the accumulation of dust, as well as in wet weather. If you can not choose, you can always put a couple of insulators, one after another to get the same effect.
insulators can also be fiberglass, Teflon, polypropylene and PVC. Efforts will be made with a smooth surface to limit the accumulation of dust and dirt.
If a tree is used as a support for one end of the antenna, install an insulator to keep it out of the foliage.
the case where the antenna has to finish at or near the place where they pass through the wall or window frame can be used insulators insulating spacers, sleeves entry.
If you have not mentioned materials, other use for this purpose can available locally: TV insulators, glass jars, a cover of pen or a piece of plastic tube to isolate the perforation.
antenna Falling Inside the interference level is usually higher than outside, then try the input cable as short as possible.
The use of braided copper wire in isolation and your connection to the antenna should preferably be soldered after twisting together the ends of wires in a length of 2.5 inches minimum. If you practice no soldering, place the jack inside the house to prevent rust quickly, cleaning the threads well before crimping them together or use a terminal block or a clamp to secure the connection mechanically. You can also use a plug-socket type of banana or a similar one to that effect. In general, moving the antenna is often used in the window frame. This, however, is not recommended if it is a metal frame. In this case it is easiest to drill a small hole in the wall at a suitable place as close to the receiver. If you do not have a proper inlet hose, use a piece of plastic tubing for lining inside the hole and spread 2 inches on each side. Once you have determined the place where the antenna come into the house try keep the input cable away from any wiring.
Connecting the antenna to the receiver
For good electrical contact, screw on the end of a plug cable appropriate input (banana plug) that fits into the input jack on the receiver antenna.
In some cases you can have different kinds of pins. If so, it is best to consult the manual for your receiver to ensure the kind of plug that is required for entry to the AM antenna. If your receiver does not have an external antenna connection, it is usually possible to place an additional. A common method is to connect the antenna to the receiver's telescopic antenna through a small trimmer capacitor or 20 or 30 pF (picofarads). The more practice in this case is called crocodile pin.
But perhaps the best way is to place a single coiled cable (1 m in length around the telescopic antenna (see Figure 1). This provides an inductive coupling between the exterior and telescopic antennas, resulting in an increased sensitivity of the latter.
Today, many portable shortwave receivers are equipped with highly sensitive circuits. It may be noted that these overloaded if the antenna is extended artificially. But, and this is another warning that should be taken into account is that If the position of the telescoping antenna built to avoid the collection of signals, for example by being too far from the window, here should be considered an artificial lengthening of the antenna.
If the ferrite antenna is not idle when tuning shortwave reception can be greatly improved if the receiver is turned, or is placed in different positions. Adaptation
When the antenna is not adapted to the input circuit of the receiver antenna are introduced because the signal loss is reflected in the input jack. For optimal transfer of signals between the antenna and receiver, antenna and impedance power line should be approximately equal to the antenna input impedance of the receiver. However, in most cases the receiver input impedance is not specified and must be found by trial and error the optimum signal transfer. Nevertheless, a certain lack of adaptation is permissible and receiver input impedances are not very critical.
However, an optimal adaptation should be a great help, as you will discover if necessary to build a unit of adaptation as the antenna tuner mentioned below. Local interference
most important interference is heard on the loudspeaker enters the receiver through the antenna. Generally speaking, interference produced sparks coming from household appliances such as vacuum cleaners, sewing machines, hair dryers, coffee grinders, etc.
heavier domestic appliances such as centrifuges and washing machines are usually equipped with interference suppressors and would be impractical to provide them also smaller electrical tools and appliances mentioned above.
interference suppressors are connected to the supports of the carbon brushes of electric machines and equipped with a capacitor to mitigate the sizzle.
interference from fluorescent lamps can be removed conveniently shielding auxiliary equipment. If the mains power pole has a separate ground (third wire), it must be connected to the shield.
Another potential interference generator is the television receiver. It is equipped with a horizontal deflection circuit which tends to produce many harmonics, which are in turn captured and radiated by the TV antenna. As it is not so easy unscreened TV receiver properly, maintain a short-wave antenna as far away as possible from the TV mast if this device produces spurious radiation.
if you experience excessive local interference, demagnetize also the antenna down to prevent capture this noise. Interference
broadcasting
mutual interference between broadcasting stations on shortwave is due to the large number of transmitters operating in these bands. The most irritating of all is probably the "heterodyne", a note pulse that is generated in the receiver when tuned simultaneously carry two stations. The heterodyne can be removed effectively with a so-called "notch filter", but most appliances do not usually have it.
To limit the effect of mutual interference between radio stations, modern receivers are equipped of a bandwidth somewhat smaller and some even have a variable bandwidth in order to adapt the device to the changing conditions of reception.
Note: "bandwidth" is not the same as "widening the band." The latter indicates that the shortwave band, which essentially extends from 11 to 50 meters for international broadcasting, is subdivided into various frequency ranges on the scale. The more margin, the better the band widening and the easier the tuning. In contrast, the bandwidth of the spectrum indicates that the RF received is processed in the receiver. In the old receivers, the bandwidth can be as much of 16 kHz (which indicates a maximum range of 8 kHz audio) but in the modern is usually 5 to 10 kHz. The possibility of radio interference is greater increase bandwidth. Shielding
may be useful or even necessary, part of the antenna shielding against interference pickup. Since the input is most often over interference fields is possible, if necessary, complete this part of the antenna with coaxial cable.
The application of coaxial cable has both advantages and disadvantages. If the antenna is connected to the inner conductor, and in turn the outer copper braid connected to a properly grounded, the entry will not capture any interference.
The coaxial cable application can extend it as we wish, without mentioning the gutters and house wiring and insulators mounted without spacers or bushings sleeves.
However, a disadvantage is that the coaxial cable introduces losses reception. To reduce them is desirable to use a type of low loss RF and sound quality and restrict the length of the coaxial input to a minimum. Ground
If shielded cable is used for input, or be desirable to increase antenna performance by a good ground, you might want to add a ground cable appropriate to their receiving equipment.
A ground is usually done by inserting a tube usually bare copper in the soil. Perhaps it is a fast, but not always the best. It ensures a good grounding when there is good contact between copper and the soil and often get better if it is buried a copper strip of about 2 to 4 inches wide and 1.5 to 3 meters long enough deep into the soil. It is appropriate that surrounds the copper with a material that resists moisture, before filling the hole. In public facilities, coal is often used for this purpose. A layer of about 3 inches thick enough. If it is kept moist by placing the entire system below the water downspout or if they get wet from time to time, we have an excellent grounding. To connect the receiver be used a thick piece of bare copper wire.
Lightning protection Remove the receiver antenna jack is the best protection against lightning. Automatic protection against lightning by a lightning rod is not always safe because the unit can become conductive as soon as the shock to the antenna exceeds a predetermined value that can be as high as 70 volts.
surge protectors are available commercially and are frequently used in auto-radios. Surge protectors, size quick action of a button, for transistor circuits, are on sale and must be connected as indicated in the manufacturer's instructions for use and installation.
Clearly not be any protection against a direct or nearby lightning, when the intense electric and magnetic fields that accompany it are able to damage or destroy your computer via inductive.
Different types of short-wave antenna
As good as the advice given above, it is often difficult to apply in practice. Fortunately, however, there are several antenna types to choose from, depending on the conditions and requirements to meet. We address several of them and explain their specific characteristics.
internal antenna
This is the least recommended for the simple reason that it is much less sensitive than any other outdoor types. If the receiver were located in a wooden house with a non-metallic roof, an indoor antenna can provide satisfactory results. However, in the houses built with bricks, all short-wave signal reaches them is already weakened considerably. And in modern apartment buildings, built mostly with large quantities of reinforced concrete, box rebar are virtually eliminates the reception.
However, many listeners have no alternative but to an internal antenna and, therefore, to benefit them are some tips to install.
If you live in an apartment building, built with reinforced concrete should aim your antenna parallel to the window, since this is the place where the signal can penetrate into the room. Espiralizado a thread, well insulated and as high as possible is the best method to solve the problem of capturing the signal.
rod antenna
Figure 2 shows a possible implementation of this type of antenna. Consists of a rod of about 4 or 5 feet long, mounted on an insulating quality, which in turn is fixed to the frame window. The power cable to the receiver is kept as short as possible. In the absence of a rod or if not practicable, it can be replaced by a wire stretched along a wall, a short distance therefrom.
The upper end of the antenna must be rounded or, preferably, provided with a metal disk or a sphere of 1 cm or more. The higher this "terminal capability", the better the results. If possible place the rod antenna in an open space, increase performance significantly. It
replace the rod antenna by an antenna resonant call, which is distinguished by increased sensitivity in frequency band only.
Figure 3 shows the system, which consists of a vertical rod whose length is about a quarter of the wavelength is tuned. Immediately below is the insulator, a 4-spoke system, placed at right angles to each other and horizontally. This forms an artificial ground plane. The radios have the same dimensions as the vertical radiator. Connecting a coaxial cable to the receiver between the radiator feed point and the intersection of the spokes. The inner conductor of coaxial cable connects the antenna input of the receiver, while the braid shield plug is inserted into the ground. This type is known as antenna horizontal polarization. Is sensitive to signals coming from all directions.
For best results, the lengths of all elements must be a 3% shorter than a quarter of the wavelength at which the system is tuned. To find the correct length in the case that only know the frequency of the station, proceed as follows: first calculate the wavelength (in meters) using the formula:
wavelength (m) = 300
-
----------- freq. (MHz)
In case you want the antenna for 15.30 MHz (15,300 kHz), the wavelength would be 19.61 meters. 1 / 4 of it is 4.9 meters. The exact length of each antenna element is calculated with 3% less than 4.9 meters, which gives 4.75 meters or 15'17 "(l meter = 3.28 feet).
antenna polarization horizontal can be powered by a 50 ohm cable.
optimal alignment is obtained between the feeder cable and antenna when the spokes are bent downward, until they form angles of 120 degrees to the vertical radiator.
The outer shield of coaxial cable is not normally connected to ground.
active antenna This is a short rod at its point of supply has an electronic circuit comprising a coupling unit and an antenna amplifier. This circuit is powered by the mains or via a battery. The active antenna is planned especially for small housing occupants, or others who have little space, and give results comparable to those of a normal antenna outside. It can be installed on the roof, in a window frame or some other suitable place.
recommend selecting an active antenna of a known and trusted brand. A cheap copy may disappoint because usually weak and often generates interference signals.
The wire antenna This antenna is the most popular general application because it is easy to build, no critical dimensions and provides satisfactory results in short-wave (Figure 4).
antenna basically consists of a horizontal part and a down conductor, connected to one end of the thread. It is practical to the antenna and lowering the same piece of thread. The total length of wire to the outside shall be between 9 and 25 meters.
The antenna performance can be increased when combined with a coupling unit, whose scheme is shown in Figure 5.
consists of a coil, made of enameled copper wire, with a total of 15 coils are wound around a plastic form 2.5 to 3 centimeters in diameter and with points in the 12th, 9th, 5th, 4th and 3rd loop.
jacks are connected to a switch 7 position and the scheme can be seen that this allows us to short-circuiting of the coil. Two capacitors, with maximum capacities between 360 and 470 pF, complete this unit.
It connects to the antenna, while its outputs go to the outlets of the receiver antenna and ground. A separate ground unit is optional.
This antenna tuner will allow you to get an optimal signal transfer from the antenna to the receiver. Experimenting can find the best positions for the switch and the values \u200b\u200bof the capacitors. If the receiver is equipped with an antenna trimmer, however, there is no need for any tuner.
A variant of the wire antenna that is worthwhile for those wanting to experience, is the Beverage antenna. This is a non-resonant wire antenna showing a directional effect along the axis of the cable for the signals coming from the direction which is oriented.
The longer horizontal part of the antenna directivity better. For good results, the antenna must be at least 2 wavelengths. At its end is connected to a good ground through a carbon resistance of 500 ohms. For best results the receiver should also be grounded. Beverage antenna should be placed at a height between 3 and l0 meters.
The receiver's power line is a simple cable connected as shown in Figure 4. Dipole antennas
The dipole is a resonant antenna with a clear preference for signals that reach across it. There are two versions: the single dipole, shown in Figure 6 and the folded dipole in Figure 7.
Both are 6% shorter than half the wavelength at which they are resonant. For the band means l9 meters wire length of 9.3 meters between the insulator terminals. Dipole antennas are fed at its center. The sensitivity of such antennas is approximately two times better than a normal wire antenna, which is also true for sectors of 90 degrees of width centered at right angles to the axis of the antenna wire. For the dipole
recommend a simple 75-ohm balanced feeder. As it is not available everywhere, two substitutes are recommended: coaxial wire of 70 or 75 ohms if the antenna is mainly used for frequencies below 20 MHz or lamp cord with plastic insulation (consisting of two insulated wires extending in parallel). It connected as shown in Figure 6.
For folded dipole utilizing a balance of 240 to 300 ohm (Dual conductor, ribbon, tubular line). This line is commonly used for TV antennas. Must be carried to the receiver using separation insulators. The folded dipole consists of two conductors, held parallel at a distance of 5 cm by insulating spacers. The lower conductor is interrupted in the middle, where it attaches to the center insulator. A folded dipole is relatively easy to assemble. Use a thread, which will tend you from the center insulator to an insulator terminal, then the next terminal insulator and from the center insulator.
is now easy to adapt the antenna to desired length then you have placed the spacers in place. After that you can connect the power supply.
If you use a normal insulator at the center, place the food around it and secure it with duct tape before connecting to the ends of the antenna.
Welds are preferable. Apply a few strokes of paint to protect joints from corrosion. If using coaxial cable and power cable, the cable goes in the center and the backstop are connected to the antenna. You have to remove the protective mesh in a length of 3 inches and then rolled by hand until forming a single cable. Then remove the protective plastic on the cable going into 1 inch center and connect the antenna as shown. Do not forget to cover the wire with electrical tape or masking tape to avoid possible interference.
coaxial cable comes with different impedances, for example 50, 75 and 125 ohms. Use the cable with the proper impedance for each antenna.
Dipole Antenna Installation
To obtain the correct direction of the antenna with respect to the position of the station you want is to find the direction in which the signals arrive. If this station is over 3,000 kilometers needed a globe and a piece of rope for this purpose. A sites for the transmitter and receiver with a taut rope and read the direction in which the signals arrive. Place the antenna at right angles to this direction.
As noted above, the address is not very critical, so you have to take account of local circumstances. Deviations up to 30 degrees from the optimal direction are certainly eligible. The dipoles have lower sensitivity in the direction of the axis of the wire. This property can be exploited to delete the persistent interference from another station. In this case, point its antenna to the radio interference and thus interference is deleted, at least partially. Outside
resonance, the dipole will behave as a good type of outdoor antenna, especially at higher frequencies. However, you may have the desire to combine a series of dipoles with a feeder, in order to achieve the desired directivity at various wavelengths.
In this case, the multiband dipole, illustrated in Figure 8, is one possibility. It has three simple dipole, cut for different frequency bands and all connected to the same center insulator. The impedance of this system is about 75 ohms. If the antenna is for a home receiver, whose input impedance is unknown, a 125-ohm coaxial cable can give the best results.
300 Cable Homemade ohms
In those areas where it is difficult to obtain a 300-ohm twin lead, in this case can make this cable yourself since it is not difficult. To do this choose a bare copper wire or enamelled and buy a piece that is two times the length of the feeder. Now you can make an excellent 300-ohm cable using insulating small plastic spacers that keep the mutual distance of the wires to six times the diameter. The distance is measured between the centers of the wires.
If this cable is properly constructed so that wires can not touch or in case of strong winds, you have done you a power supply that exceeds normal rates with polyethylene insulation.
And finally ...
This booklet is not intended to give a full account of the short-wave antennas. We just tried to highlight those types that are simple construction and which provide very acceptable results and emphasize a good overview.
hope that you devote enough time to experiment so you can find the antenna that best suits their local circumstances.
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