Amateur transmitting CB antenna
In some countries (including Russia) hams except LW and УSW the ranges of selected small area in the LW range (135,7...137.8 kHz). After experimenting in this range taken by the team RU6LWZ (log talked about them in the June issue this year), the interest of Russian кLW ham radio has increased significantly. Many would like to start experimenting on this range, but its development was largely constrained by the lack of widely accessible information about what is needed for this technique. The proposed article devoted, perhaps, the most fundamental aspect LW technology - transmission antennas.
Currently the main task that needs to be addressed for a wide mastering LW Russian radio Amateurs, is to increase the number of transmission Amateur LW stations. Indeed, before taking signals, it is essential that they existed. If the LW from Amateur stations very strong and at large distances to the transmitter, for the start of the experiments on LW it is highly desirable that the source was relatively close. Particularly acute this problem before Amateurs Asian part of our the vast country. Somewhat easier to reside in the European part Russia. In Western Europe a lot of Amateur radio operators transmitting on long the waves, the signals of which can be taken at distances up to one or two thousand kilometers when in Telegraph and up to several thousand kilometers when QRSS (slow Telegraph signal processing on the computer).
The main problem that must be solved to any Amateur radio operator begins work in LW range, this is a construction of the transmitting antenna. It is well known that on LW the antenna has a considerable influence on the success of the work, but on LW, perhaps, it the effect even more. The transmitter at frequencies of about 136 kHz to produce relatively easy. It is not much different from the transmitter LW range. But the antenna is different! The properties of the antenna fundamentally depend on the ratio of the wavelength and the size of the antenna and the wavelength corresponding to Amateur band 136 kHz, about 2.2 km, which is more than ten times exceeds the maximum length of the waves, previously used by radio Amateurs.
LW antennas differ substantially from those generally used in SW. Direct copying LW LW antennas on impossible, as obtained absolutely antenna available for radio Amateurs sizes. In addition, LW is usually no the opportunity to offer specific Amateur radio transmission design the antennas. It is largely determined by local conditions, and designing the antenna to the radio, usually have myself. Even though it not difficult, since the LW is not the diversity of the types of antennas that can be observed on LW, but still designing LW antenna requires some understanding of the what are her options as they affect the operation of the antenna, and what determines how to improve the transmitting complex, consisting of transmitter and the antennas.
All this prompted the author to write this article, which describes the main the principles of creating an Amateur transmitting LW antennas. Of course, most of set out in article material can be found in the professional literature, but specially for radio Amateurs of this presentation have not yet been. It is not surprising since LW range has become available for ham radio recently. The author tried to avoid the complicated theory, limited only quality outlining also the most simple formulas that are needed for intelligent design of the antenna. The main attention was paid to the principal difference in the construction of LW and LW antennas. How did it readers to judge.
A characteristic feature of LW antennas are their sizes are much smaller than a quarter of the wavelength. This is true even for professional LW stations, and for Amateur and even more so. Indeed, the usual quarter-wave LW the pin for the range of 136 kHz should have a height of 500 m, as the TV the TV tower!
The second important point that should be considered in the design and manufacture transmitting LW antenna is the polarization of the radiated wave antenna should be only vertical. This is due to the properties of the earth: such low frequencies it is close to a perfect conductor, and the height of any real LW antenna is much smaller than the wavelength. Effectively radiate horizontal electric field will fail for the simple reason that the earth just a "short-circuit" this field. If to speak more strictly, the reason is that, as is known from electrodynamics, the electric field vector on the surface a perfect conductor is always perpendicular to the surface.
Of course, the earth is not a perfect conductor, and the height of the antenna, though small, is equal to zero. Therefore the use on LW low (compared with wavelength) transmitting antennas with horizontal polarization (for example, horizontal dipole) is very interesting and requires experimentation. But to recommend such transmitting antennas ham, only begins work on LW, does not. The experiments require a solid experience, and to compare the experimental antenna must with something known.
Due to the fact that the dimensions of any real LW antenna is much less than a quarter the wavelength of the transmitting antenna LW can be divided into two broad classes - electrical and magnetic.
The magnetic antenna is enclosed frame, most often rectangular in shape, located necessarily in the vertical plane (vertical polarization!) and having dimensions at least of the order of tens of meters. Some ham radio operators in Western Europe and the US are experimenting with such transfer antennas, and they are able to radiate power, not much lower than in the case of electric antennas of comparable size. But this is still experimental class of transmitting antennas.
The main type of transmitting antenna for LW is much shorter vertical the radiator is fed relative to the earth. The latter means that the second pole to connect the generator is grounded. Many such antennas have a lot wires arranged horizontally. But we emphasize that the actual emitter is only the vertical part of the antenna, and all horizontal conductors only serve to create in the vertical wire as possible greater and more evenly distributed current.
Some types of transmission LW antennas is shown schematically in Fig. 1.
In Fig. 1 and shows the antenna in the form of vertical wires without capacitive load; Fig. 1,b - vertical antenna with a capacitive load, in the form of "umbrella", which might be part of the guy wires that support the mast; in Fig. 1,a three-beam T-antenna in Fig. 1,d - beam G-antenna with inclined capacitive load; in Fig. 1 ,d - beam T-antenna with inclined capacitive load; in Fig. 1,e - beam T-antenna with an inclined vertical part, in Fig. 1 ,Zh - the antenna is "inclined".
Possible configurations of antennas is not limited to one shown in Fig. 1. Possible, for example, multi-beam G-antenna. The number of conductors that make up the "umbrella" (Fig. 1,b), not necessarily equal to four. The vertical portion may also to consist of several parallel or divergent "fan" of wires, etc. It is also clear that the LW antenna in many cases, you can use LW antenna by changing its method of feeding. For example, LW dipole successfully serve in the quality of the T-antenna, if you connect both wires of the feeder together and to feed them relative to the earth.
Note that none of these antennas fed by a coaxial cable. They are all like "antennas with open wire feeder, though this "feeder" is actually emitter. Ham, who has not experienced the problems associated with interference to the TV at work on LW, may very skeptical about this diet transmitting antenna. Especially when it still further would be recommended to use as grounding water pipes. The author hastens to calm him down: LW interference television is usually much less of a problem than when it operates on SW. Let's use an example from practice. The wire from the antenna to the transmitter power of about 50 W at a height of several inches above the top cover TV. On it lay a neon light, which shone brightly in pressing the key. And thus interference to television reception absolutely was not observed! Maybe not always, the situation is so favorable, but apparently, TVs are not sensitive to electromagnetic fields such low frequencies.
Since the height of the LW antenna is always much less than a quarter wavelength, reactive part of the input impedance vertical electric radiator always has a capacitive character and is very high in comparison with the active component input resistance. To the current in the antenna has reached a significant size, the capacitive part of the input impedance of the antenna should be compensated inductance, reactance which is equal in absolute value reactive impedance of the capacity of the antenna. Thus, the use of the extension coil on LW is absolutely mandatory (in Fig. 1 coil not shown). An extension coil is connected in series with the antenna.
In order to estimate the inductance of extension of the coil, you must know the capacity of the antenna, which is a very important parameter transmitting LW antenna. The larger the capacity of the antenna, the less you need the inductance of the extension coil. Accordingly, the larger the capacity of the antenna, the less will be useless loss of transmitter power due to ohmic (active) extension resistance of the coil. And power losses in lengthening the coil is very essential when working on LW.
In addition, when the larger capacity of the antenna decreases the voltage on it, which for LW even at relatively low-powered transmitter reaches of units, and even tens of kilovolts. Reducing the voltage on the antenna simplifies the problem of isolation. There are a number of reasons, which we will discuss later in the discussion so called "loss of the environment", which should strive to make as the large capacity of the antenna. The increase of the total capacitance of the antenna (along with obtaining a more uniform current distribution in the vertical part) is the reason in transmitting LW antennas are trying to make a horizontal part as much as possible and often several parallel wires (multibeam G - and T-shaped antenna).
Capacity LW antenna with acceptable accuracy Amateur radio practices can be estimated by a simple rule: each meter wire antenna (vertical and horizontal parts) gives about 6 pF capacitance of the antenna. If multiple the wires are parallel to each other, with a small distance between them the total capacitance is reduced. Therefore, in the manufacture of G - or T-shaped the multibeam antenna with a horizontal part should, if possible, to maintain the distance between the wires is not less than 2...3 m has no More meaning and less distance leads to a decrease in capacity per each meter of wire.
The reactance of the capacitance of the antenna can be found on his well-known formula XC = 1/(2πfС). Since the reactance of an extension coil must be in absolute value, from communication reactance and inductance XL = 2πfL you can find the inductance. For practical purposes convenient formulas, which are obtained if we substitute the value of the frequency f = 136 kHz and convert units of measure: Cs = 1170000/C, XL = 0,85 L, L = XL/0.85, where substituted resistance in ohms, capacitance in PF, and the inductance in microhenry.
Quite rough for approximate calculations we can assume that at a frequency of 136 kHz the reactance of the capacitance of 1000 pF is 1000 Ohms and proportionally increases with decreasing capacity compared to 1000 pF. Respectively for the inductance of each of microgenre gives about 1 Ohm. These figures are easy to remember. Greater accuracy of the calculations are very often not needed, as calculated the values still have to be clarified experimentally. The influence of others the antenna of items to consider theoretically extremely difficult!
To imagine the order of the parameters of the antenna in a typical Amateur radio conditions, get an estimate for this example. Let be a G - or T-shaped antenna with single-beam horizontal part of the length of 80 m, located at a height of 20 m. the Length of the vertical part will be 20 m, the total wire length of 100 m. The capacity of such antenna will be about 600 PF, i.e. the reactive part of the input resistance of about 2000 Ohms. For compensation of reactive resistance the capacitance of the antenna will require lengthening the coil with an inductance of a few more 2000 µh.
The question may arise, why not find the inductance of the extension coil, knowing the capacitance of the antenna and using the formula for conventional oscillating circuit? Of course, you. But the calculation using reactive resistance allows to evaluate, for example, the voltage at the antenna at a given current and resistance loss of extension of the coil with a known q-factor, in the example, it is immediately clear that the voltage on the antenna will be about 2000 V per amp of current in the antenna. Since the active part of the input impedance of the antenna much less reactive, the voltage on the antenna at approximately volts equal to the current of the antenna in amperes, multiplied by the reactance of the antenna in omwh. The resistance losses of the coil, its reactance and q-factor related by a simple formula: Rкат = XL/q When q is Q = 200 resistance losses will be 2000/200 = 10 Ohms.
The second critical parameter LW antenna is its effective height. Not taking into account while the dependence of the effective height from the construction details antenna, we note two extreme cases. Effective height single vertical wire without capacitive load at the top is equal to the half of its geometric height. For G - or T-shaped antenna with horizontal capacity parts of a lot more than the capacity of the vertical part of effective height approaching the height of the suspension of the horizontal part of the antenna above the ground.
Just note that we must strive to make the existing height of the antenna as more, at least 10... 15 meters, and preferably 30 to 50. But, perhaps, 50 m - this is the maximum that can be achieved in Amateur conditions. Something like this get the current height of the G - or T-shaped antenna with a large horizontal part suspended between two 16-storey houses.
Why effective height of the antenna is so important? The thing is that when the antenna dimensions are much smaller than the wavelength, the field strength is taken correspondent, is directly proportional to the product of (let us denote it as (A) power the current in the antenna on the existing height of the antenna, measured in metrapark. Than more effective height of your antenna, the stronger your signal. Power, emitted by the transmitting station of the radio isotopic laboratory (LC to be confused with power output the transmitter!) linked to this piece by the relation (for frequency 136 kHz): radio isotopic laboratory = 0.00033A2.
To navigate in the resulting values, consider an example. Let the effective antenna height is 20 m. the current in the antenna when the output the transmitter power of 100 W is usually within 1...3A. Let it was a 2 A. Then A = 40 tetramer and the radiated power is 0.5 watts.
The example shows that the efficiency of the Amateur transmitting LW antennas is very small, because is emitted only 0.5% of power output by the transmitter. And it's still very good! Often the efficiency is less than 0.1%. And only when you use a "giant" ( Amateur radio standards) antenna efficiency can reach several tens of percent. An example is the antenna of the first Russian far DX-xpedi-tion, conducted by staff RU6LWZ when I used the mast with a height of over 100 m.
Low efficiency Amateur transmitting LW antennas leads to the fact that the power radiation is usually measured in tenths and hundredths fractions of a watt, rarely reaching the units of watts. Nevertheless, with such scanty radiated powers fans, using special types of work (primarily QRSS - slow Telegraph), conduct communication at distances of thousands, or even 10... 15 thousand kilometers! The usual Telegraph with possible ties to several hundred, and sometimes, when good passing, a special receiving antennas and low noise level, one to two thousand kilometers.
We see that the situation with transmission LW antennas is quite different from that we're used to on SW. If LW is usually an efficiency close to 100% (except except that 160-meter band, and even then not always), then LW he is small. If at LW we try to focus the radiation in one direction and operate the concept of gain, LW radiation is almost always circular and no gain to speak of. If LW we aim to get the sloping angles of radiation, LW radiation angle is almost always the same. If on LW antenna is usually fed via a coaxial cable and we aim to get good SWR, LW antenna always powered directly and the concept of the CWS loses its meaning. The only thing we have to "fight" when working on LW, it radiated power, or, equivalently, the maximum number of meromero in the antenna.
Let us now consider in more detail the dependence of the effective height of the antenna from its the geometric dimensions and construction details for the most common types of antennas. As already indicated, the current simple vertical height wire bus capacitive load at the top (Fig. 1,a) is just equal to half geometric height of the antenna. Similarly, the effective height of the antenna "italics the beam (Fig. 1,) is equal to half the height of the top point of the antenna. If the antenna has horizontal capacitive load (for example, Fig. 1), the effective height hд such antenna is determined by the ratio of vertical tanks and St horizontal SG parts, and geometric height h suspension the horizontal part. It can be found by the formula hд = h(1-0,5/(SG/St+1))
Capacity horizontal and vertical parts of the antenna can be, as for all the antenna is defined by the rule "6 pF per meter of wire". From the formula it is seen, what if the SG is much more ne, effective height approaching hд geometric altitude p. require Special consideration cases inclined the "vertical" part (Fig. 1 ,e) and an inclined capacitive load (Fig. 1,6, g d). If the "vertical part" inclined, and capacitive load almost horizontal (Fig. 1,e), almost nothing has changed, only a few increases SV due to longer wires, but the formula remains the same.
If the T-antenna vertical portion connects quite exactly in the middle inclined capacitive load (Fig. 1,d), the formula also works only in as h is necessary to take the height above the earth connection point of the vertical part to the horizontal. In this antenna the vertical component of the electric field, created by two shoulders capacitive load, cancel each other out But in The l-shaped antenna (Fig. 1,g), or in an "umbrella" antenna (Fig. 1,6), such compensation does not occur. Therefore the formula becomes somewhat different: hд = 0.5 h( 1 + a - A2/(SG/St+ 1)), where a = h1/h is the ratio of the heights of the upper and lower the ends of the capacitive load.
We emphasize that for the cases shown in Fig. 1,b and Fig. 1 ,g is undesirable to lower the lower end of the capacitive load to the ground. This will lead to lower effective height of 0.5 h. If there is no opportunity to raise these points (for example, there is only one mast), better wire constituting the capacitive load, continue to the ground insulating cord (you can use wire, breaking it in two or three places insulators).
If the anchor point of the antenna is determined by the "local environment", and ham no desire to do the calculations, you can use this simple rule: we must strive to ensure that the maximum number of wires it was located as high as possible (and, as will be apparent from further away from trees, walls, etc.). Oh and effective height is what happens!
Having dealt with the first factor "basic parameter" - works effective height for a current in the antenna, consider what determines the second the cofactor is the electric current in the antenna, and how to make more. Of course, current depends on transmitter power. But not only. Still it depends on the active part the input resistance R, which, in turn, is the sum of maprotiline losses of RP and maprotiline radiation Rизл, as shown in the equivalent circuit Fig. 2.
The radiation resistance (in ohms) at a frequency of 136 kHz is determined by the formula Rизл = 0,00033hд2 and for Amateur radio antennas is usually not more than few tenths of an Ohm. In most cases, resistance losses far more radiation resistance. Actually, that's why turns out low efficiency, equal Rизл /(Rизл + RP). Under these conditions the current in the antenna based predominantly on the loss resistance and radiation resistance for a current almost not affected.
Such ratio of the loss resistance and radiation resistance is the reason the radical differences between LW and LW antennas. On LW, where the current in the antenna is mainly determined by the radiation resistance, it doesn't matter itself the magnitude of this current. The antenna can "eat up" or "to eat voltage, current will be different, and the radiated power is the same. On LW the situation is fundamentally different. The current in the antenna is determined by the resistance losses and radiated power is proportional to the square of the current. So you must strive to make the strength of the current as much as possible for what we should do less resistance loss
If the resistance losses in the antenna RP is known, then for a known output the transmitter power P is easy to find current I in the antenna: I =v (P/RP).
The resistance loss is the sum of the ohmic resistance of the wire antenna, the active part of the resistance of the extension coil, ground impedance, and the so-called resistance losses to the environment (enviroment loss). Last associated with the energy losses due to the current induced in the surrounding area (houses, trees, etc.).
The resistance of the copper wire antenna with a diameter of 2 mm is usually very little and it can not be ignored. The exception may be a case where the horizontal the antenna part (capacitive load) very long (hundreds of meters) and is made in a single thin wire. Other components of the loss resistance is much more.
Resistance to loss of extension of the coil substantially, especially when a low quality factor. The quality factor is the ratio of the reactive (inductive) the resistance of the coil at this frequency to resistance losses. Last add up the losses in the magnetic circuit, the armature and the wire. In transmitting LW the antennas do not use a coil with a magnetic core, which is associated with large currents, in which it is difficult to avoid its saturation. Dielectric loss of frame is usually small, nevertheless a fair recommendation: the less material goes to the frame, the better. Of course, it is desirable to use high quality the dielectric
But RF current flows mainly along the surface of the wire (skin effect) and therefore resistance is significantly greater than the DC or audio frequencies. In many books you can find the formula for the specific (Ohm/m) resistance of copper wire considering skin effect: Rуд = (0,084/d)vf where d is the wire diameter in mm; f is the frequency in MHz. Apparently, it is possible to count specific the resistance wire coil according to this formula, multiply by the length of the wire and to obtain the resistance losses in the coil. Unfortunately, except for the skin-effect is and the proximity effect, resulting in that the resistance wire in coil is significantly more resistance straight wire. Because effect of turns on each other, the current is not flowing evenly across the surface wire, and mostly on the part of the surface facing the inside of the coil. Consequently, the smaller the effective surface resistance.
According to the results of research conducted by the author, due to the proximity effect the resistance wire single layer coil increases to 1 + 4,9(d/a)2 times, where d - the diameter of the wire; and a step of winding. If the winding step to make a small (winding turn to turn), the coil inductance of one coil will become more turns need less reduced and the length of the wire. But will significantly increase the proximity effect. If you take the big step of winding, resistance increase for the account of the proximity effect will be less, but will have to wind more turns and will become more wire length. It turns out that there is an optimum, which is observed in the winding step is about two times larger than the diameter of the wire. In other words, the gap between turns should be approximately equal to the diameter of the wire.
Does the resistance losses in the coil from the wire diameter? No matter how surprisingly, almost none. With a larger diameter wire will increase the length winding, and if you make a multilayer coil, it will increase the proximity effect. Accordingly it is necessary to do more turns. If all this details to analyze mathematically, the rather unexpected result: the quality factor of the coil (and thus the resistance losses at a given inductance) depends mainly on the diameter of the coil form! And the quality factor is directly proportional to the diameter. And wire diameter the quality factor is almost independent. To avoid confusion, note that this is true only in the case where the wire diameter is substantially greater than the thickness of the skin layer. Frequency 136 kHz is performed for copper wire with a diameter of 0.5 mm and more (it usually happens).
Thus, to obtain small losses need to do a coil of large diameter. Some value is the ratio of the diameter of the frame and the length of the winding. Found that the quality factor of the coil is maximum when the diameter of the frame 2...2,5 times the length of the winding. In these conditions for a very rough estimate (or rather usually not necessary) at a frequency of 136 kHz with solid copper wire, optimal the ratio of the winding pitch and wire diameter, and the diameter of the frame and length winding q-factor of a single-layer coil can be put equal to the diameter of the frame in millimetres.
Let us return to the example above, where the reactance of the coil should be of the order of 2000 Ohms, active - 10 Ohms, and the q - 200. Diameter frame should take about 200 mm greater diameter of the cage will have to choose for less resistance losses in the coil. We see that extension of the transmitting coil LW antenna has to make a very large dimensions. Therefore, the coil is usually not inserted into the transmitter, and place separately.
However, there is one ability to significantly reduce the dimensions of the coil under the same losses or to reduce losses in the former dimensions. It is necessary to wind the coil is not solid copper wire, and special Litz wire transmitters. He consists of a large number (several hundred) very thin, isolated from each other copper conductors. On top of the conductors is usually a braid of silk. Applying liandrat, it is necessary to pay special attention to ensure that each (!!!) the wire was soldered to the connection points of the coil. Unfortunately, the author does not aware of any theory that allows to calculate the quality factor of the coil from liandrat, from experience we know that with the same dimensions, the quality factor of the coil from liandrat approximately twice as much as when winding a continuous copper wire.
The resistance losses of the extension coil is an important part of overall of the loss resistance of the antenna. But if you make the coil is large enough, but still acceptable diameter (in millimeters 200...400), the main contribution to the total loss will give the grounding resistance and the resistance losses of the environment. Their it is usually difficult to separate, and often it is the total resistance is called resistance of the earth.
Note first that the RF grounding resistance not coincide with the earthing resistance at low frequencies. So if there "electrical" ground with a known resistance, it is, of course, you can and should use it, but its impedance at the frequency of 136 kHz will be much more than at the industrial frequency of 50 Hz.
Unfortunately, to calculate the losses in the ground hams usually impossible. The formula used by professionals, not applicable for such a small comparison with the wavelength of Amateur radio antennas. And unlike professional, Amateur antennas are usually located among the houses, trees and other objects, which significantly affects the losses in the antenna. Hams usually do not make special grounding, and use water pipes, and so p. It also complicates the calculation. Thus, you'll be limited to only stating that typically the resistance losses in the ground along with resistance losses of the environment is on the order of 30 to 100 Ω, and recommendations for reducing the magnitude of these losses.
As already mentioned, it is necessary to maximize the current in the antenna. Less than resistance losses, the more he. To reduce the resistance loss grounding in Amateur practice, it is necessary to connect everything possibly buried in the ground and located on the earth's surface of the metal. It can be water pipes, different metal constructions etc. Just do not use gas pipes! This is unacceptable for reasons fire safety!
In professional practice to reduce losses in the earth grounding is performed in the so-called "metallization of the earth" under the antenna. It's buried on the system small depth or lying on the surface of the earth wires. Area metallization should, if possible, to cover the entire surface under the horizontal part of the antenna beyond the projection of the antenna on the plane earth at a distance of the order of the height of the antenna. If the horizontal part (capacitive loading) no, the radius of the metallization should be about the height of the antenna. Quite the metallization in the form of a true circle, the radius means just the typical size. It is possible to make the radius of the metallization more, but to double it no longer makes much sense.
Again, in professional practice, the distance between the individual wires system "metallization of the earth" choose the order of 1 meter and are sometimes even used solid metal sheets. Unlikely in Amateur practice possible. Therefore, even if there is some kind of grounding system, the distance between the wires will most likely be more. How much, depends on the capabilities of Amateur radio. Of course, with a more "rare" metallization of land loss in the ground increase.
Metallization of the earth can greatly raise the efficiency of the transmitting LW antenna for by significantly reducing losses. But if ham is not possible make metallization of the ground under the antenna (which often happens), it is not necessary to despair! Most Western European radio Amateurs operate successfully, using the existing ground water. Therefore actually it turns out that the grounding resistance in ham radio so much, much more resistance grounding professional LW antennas, where resistance losses in the ground is often of the order of 1 Ohm, even for relatively small low-power antennas LW stations. And antennas LW broadcast stations when in the ground buried tens or even hundreds of tons (!!!) metal, and even less - tenths and sometimes hundredths of ohms.
Accordingly, the efficiency in this case is very close to 100 percent. But this radio Amateurs count is usually not necessary, except that succeed, when case, to use the LW antenna.
But not only the quality of the grounding system are determined by the losses in the antenna. If the conductors of the antenna are close to houses, trees, etc., arise additional losses of RF energy used for heating these surrounding objects. Actually, this is the loss of the environment. It is necessary that the antenna cables, under high RF potential was located, if possible, on least 1...3 m from the surrounding objects. But if such a wire long and runs parallel to "interfere", the distance you have to choose another more.
The situation is illustrated in figure. 3.
Loss in the case of Fig. 3,and substantially less than the case of Fig. 3,b. Vertical wire in the latter case will be bridging in the wall home significant RF currents, leading to useless power loss transmitter, spending it on the heating wall. Such a situation must be avoided.
But what to do if you include wall-vertical wire antenna impossible? In this case makes sense to modify the antenna as shown in Fig. 3,V. And although the current in the vertical wire will be virtually the same as in the case of Fig. 3 a,but the RF potential with respect to ground on it is small (it is big only after lengthening coils). Accordingly, the reduced and the influence of the walls of the house. The coil, however, have to make a few more inductance, as the capacity of the antenna connected to the coil, capacity will be only horizontal wire. In this case, it is inconvenient to highly customize button a coil. The solution is simple - most of inductance to place the "top" and near the sender to include a small variometer, only for fine adjustment the antenna into resonance. The voltage on the wire, passing close to the wall, will slightly increase, but it will not be as significant as in the case of Fig. 3,b.
A similar situation is shown in Fig. 3,g where the transmitter is located on top floor of a multistory building. It would seem that the antenna is not vertical, but it actually is. Simply it is the role of the ground wire, for example, water pipes. They are located in the vicinity of the walls, but so as the RF potential on them a little, like on the vertical part of the antenna Fig. 3, the influence of weak walls. So the antenna will work perfectly satisfactorily.
The examples show that especially large loss environment be in the case when the near surroundings are part of the antenna, bearing a high potential. Of course, reducing the voltage across the antenna, also as the decrease of the voltage at the antenna parts, minimize losses to the environment. It explains the previously made observation that the increase in total capacity antenna increases the efficiency of the antenna. Indeed, the increase in the capacity of the antenna leads to the decrease of tension on it and, hence, a reduction in losses to the environment. At the same power transmitter in the vertical part of the antenna will be able to obtain higher currents and, as a consequence, the emitted signal will grow.
Of course, the illustrations and commentaries do not exhaust all situations that can happen in the practical implementation of the antenna. But the author hopes, what they illustrate a General approach to the design of LW antenna with a minimum in the loss of the environment. Well, in each case ham will have to think, experiment and make decisions independently.
Finally - a few words about the connection of the antenna to the transmitter. From set out clear that the input impedance of the antenna after tuning into resonance coil in most cases will not be equal to 50 or 75 Ohms. However, this is not necessary, the coaxial cable. Just in the transmitter to provide the ability to adjust the output resistance. This is easiest to do with transformer push-pull circuit of the output stage of the transmitter. The secondary winding of the transformer in this case must be done with taps and to install the switch. It appears that a number of termination resistors 5, 7, 10, 15, 20, 30, 50, 70, 100, 150, 200 Ohm is sufficient for any Amateur radio antennas, including as "very bad" and "very good". Ideal standard the disk toggle on 11 positions.
To fine tune the antenna to resonance, it is highly desirable to have a Vario from LW or CB transmitter. The author uses the variometer from mediumwave unit radio SCR-5, having a maximum inductance of about 700 µh. Of course, this is not sufficient, and in series with the included Vario additionally, a fairly large coil constant inductance and Vario serves only to adjust.
In the described embodiment, the connection of the antenna to the transmitter configuration is to select the position of the switch, providing the maximum current in the antenna, and adjusting the inductance of the extension coil. After each switching output impedance of the transmitter it is necessary to adjust the inductance (Vario) to produce resonance, achieving the maximum current in the antenna.
There are other possible embodiments of the output circuit of the transmitter, and other methods settings, but their discussion would lead us too far from the main topic of the article. So to conclude my presentation, wish the reader of successful experiments, and to meetings on long waves!
Author: Alexander Yurkov (RA9MB)