Noise antenna temperature
Noise antenna temperature - characteristic of the noise power of the receiving antenna. Noise temperature has nothing to do with the physical temperature of the antenna. It is given by the Nyquist formula, and is equal to the temperature of the resistor, which would have the same thermal noise power in this frequency band:
Where
Noise power, - noise temperature, - frequency band, - Boltzmann constant.
The source of noise is not the antenna itself, but noisy objects on Earth and in space. The cosmic component of noise depends on the diameter of the antenna: the larger the diameter and gain, the narrower the main lobe of the radiation pattern, respectively, the less extraneous cosmic noise the antenna amplifies along with the useful signal. The terrestrial component of the antenna’s noise temperature depends on the elevation angle - the lower the antenna “looks”, the more it receives industrial interference and noise from sources on the Earth’s surface. Therefore, the noise temperature is not a constant, but a function of the elevation angle. As a rule, it is indicated in the specification for one or more elevation values. Typical noise temperature of a parabolic antenna with a diameter of 90 cm in the Ku-range for an elevation angle of 30 degrees is 25-30K.
Noise temperature of the antenna in radio astronomy
The concept of antenna noise temperature along with the concept of antenna temperature is widely used in radio astronomy. Antenna temperature characterizes the total power of the received antenna radiation, i.e. noise power and power of the studied objects, while noise temperature is only the power of noise (interfering factors). If not a single radio source falls into the radiation pattern, then the antenna temperature is equal to noise. Thus, the useful signal depends on the difference between the antenna and the noise temperature.
As a rule, the noise temperature consists of two parts: constant and stochastic. The constant component can be compensated, but the stochastic component imposes fundamental restrictions on the sensitivity of radio telescopes. Therefore, to increase the signal-to-noise ratio when designing radio telescopes, the main attention is paid to reducing the stochastic component. For this, low-noise amplifiers are used, the receivers are cooled with liquid nitrogen or helium, and so on.
see also
References
Wikimedia Foundation. 2010.
- Shumov, Alexander Vitalievich
- Shumovka (Shumyachsky district, village)
See what “Antenna noise temperature” is in other dictionaries:
Noise temperature - effective value, serving as a measure of the power of noise in radio receivers. Sh. T. Tsh is equal to the temperature of the agreed resistance (antenna equivalent) at which the power of its thermal noise is equal to the noise power of a given ... ...
equivalent noise temperature of satellite line Recom - The noise temperature at the output of the receiving antenna of the earth station, corresponding to the power of the radio frequency noise creating the total noise observed at the output of the satellite line, with the exception of noise created by interference from satellite lines, ... ... Technical Translator Reference
Satellite line equivalent noise temperature - 1. The noise temperature at the output of the receiving antenna of the earth station, corresponding to the power of the radio frequency noise creating the total noise observed at the output of the satellite line, with the exception of noise created by interference from satellite lines, ... ... Telecommunication Dictionary
Antenna temperature - a value characterizing the power of electromagnetic radiation received by the antenna. Often used in radio astronomy. Antenna temperature has nothing to do with the physical temperature of the antenna. Like noise temperature, it ... Wikipedia
Brightness temperature - photometric quantity characterizing the radiation intensity. Often used in radio astronomy. Contents 1 In the frequency range 2 In the wavelength range ... Wikipedia
Radio antenna - Antenna of the radio telescope RT 7.5 MGTU im. Bauman. RF, Moscow region, Dmitrovsky district. The diameter of the mirror is 7.5 meters, the operating wavelength range is: 1 4 mm Antenna is a device for emitting and receiving radio waves (varieties of electromagnetic ... ... Wikipedia
Orbit - I Orbit (from lat. Orbita rut, path) circle, scope, distribution; see also Orbit (honey.), Orbits of celestial bodies, Orbits of artificial space objects. II Orbit (honey.) The orbit, the bone cavity of the Skull, in which ... ... Great Soviet Encyclopedia
GOST 24375-80: Radio communication. Terms and Definitions - Terminology GOST 24375 80: Radio communication. Terms and definitions original document: 304. Absolute instability of the frequency of a radio transmitter Instability of a frequency of a transmitter Definitions of a term from different documents: Absolute instability ... ...
RADIO TELESCOPE - a device for receiving and measuring radio emission from cosmos. objects ranging from decameter to millimeter wavelengths (within the “transparency window” of the Earth’s atmosphere for radio waves). Measurements at longer wavelengths are made from space. R.… … Physical Encyclopedia
GOST R 50788-95: Installations for direct reception of satellite television broadcasting programs. Classification. Main settings. Technical requirements. Measurement methods - Terminology GOST R 50788 95: Installations for direct reception of satellite television broadcasting programs. Classification. Main settings. Technical requirements. Measurement methods Original document: 3.1.4 Antenna device for receiving ... ... Glossary of terms of normative and technical documentation
This parameter is entered only for receiving antennas. Moreover, its value is largely decisive if the antenna is used in combination with a highly sensitive radio receiver. In this case, the antenna, in relation to the latter, acts not only as a signal generator, but also as a source of noise (passive interference). Under the influence of alternating fields of industrial electrical and radio installations, lightning discharges in the atmosphere, as well as thermal radiation from the Earth and cosmic radiation sources, an EMF will be induced in the antenna, depending on the power of all external interference and their spatial distribution relative to the antenna.
By analogy with the law relating the power of noise and bandwidth (Nyquist formula):
P W \u003d k T E Df,
where k is the Boltzmann constant;
T E - effective noise temperature, K O,
noise power in the receiving antenna will take the form:
P W \u003d k T A Df.
Here T A is the noise temperature of the antenna.
It is defined as follows:
and depends on:
KND antennas in this direction;
T I (q, j) is the distribution of the brightness temperature in space, characterizing the distribution of the intensity of external interference.
Thus, the noise temperature of the receiving antenna largely determines the location of the antenna bottom relative to the sources of noise (radiation). As a rule, the thermal radiation of the Earth and, to a large extent, of the atmosphere acts on the side lobes of the NAM. If the main lobe of the radiation path is directed towards the sources of cosmic radiation (for example, in space communication systems, ionospheric radio communications), then the noise temperature of the antenna increases significantly. In addition to the direction, the distribution of the brightness temperature also depends on the range of operating frequencies. The brightness temperature is determined by special graphs. In the general case, the intrinsic noise of an antenna is determined by the loss resistance of the antenna, the temperature of which must be considered equal to the ambient temperature. At the same time, it can be considered that if there are no powerful discrete sources of cosmic radio emission in the “field of view” of the antenna, then the noise temperature component due to cosmic noise is approximately 5 K O, due to atmospheric noise, approximately 15 K O, and due to the reception of thermal radio emission Earth along the lateral and posterior petals of the NAM - about 3 K.
7. Frequency, spatial and polarization coordination of the transmitting and receiving antennas.
Under the frequency consistency of antennas understand their ability to work in the same frequency range. If the antennas operate in different frequency ranges, then frequency consistency is not ensured. Although the EMF is induced in the receiving antenna under the influence of an electromagnetic field with a different frequency (which is the noise), the power of this signal at the input of the receiving device will be much less due to poor matching of the antenna with the feeder path.
The spatial coherence of the antennas is understood as their relative position in space, in which their radiation paths are directed towards each other and provide the most advantageous transmission of electromagnetic energy. It is understood that one antenna is transmitting and the other is receiving. Obviously, with narrow antennas, the requirement for the relative position of the antennas must be strict.
When considering the polarization consistency of antennas, it should be borne in mind that, based on the principle of reciprocity, the polarization properties of the receiving antenna are completely determined by the polarization parameters of the same antenna in transmission mode. This leads to the conclusion that if you take two identical antennas, one as the receiving one and the other as the transmitting one and arrange them identically in space, then the polarization coordination of these antennas will be achieved automatically. This allows us to formulate the following conditions for complete polarization consistency:
The ellipticity coefficients of the transmitting and receiving antennas should be equal in modulus;
The angles of inclination of the polarization ellipses of the transmitting and receiving antennas should be equal;
The directions of rotation of the field vectors should be opposite if both polarization ellipses are viewed from the side of any one antenna.
The figure shows various options for the location of polarizing ellipses of the transmitting (1) and receiving (2) antennas, provided they are polarized consistent.
To evaluate the reception efficiency of waves of any polarization, a polarization efficiency coefficient is introduced:
where K E 1 and K E 2 are the ellipticity coefficients of the antennas;
Dg - differential angle of inclination of ellipses.
In the case of complete polarization consistency, ceteris paribus, the EMF in the receiving linear antenna will induce the maximum EMF, and the output power in the aperture-type antenna will be maximum. And vice versa, choosing the polarization properties of the antenna for the polarization structure of the interference EMV, it is possible to significantly weaken its effect on the receiving antenna. If the EMF in the receiving linear antenna is equal to 0 (or in the antenna of the aperture type - output power), then we are talking about complete polarization isolation. The figure shows various options for the location of the polarization ellipses of the transmitting (1) and receiving (2) antennas, provided they are polarized mismatched.
Since the noise is more broadband than the receiving device, in the future we will assume that at the input of an ideal receiver there is a noise voltage idealized by white noise. Then the only characteristic that will be needed in the following chapters is the spectral density of this equivalent noise, recalculated to the input of the receiver. To find this characteristic, we consider the causes of noise and the quantitative characteristics of noise. First of all, we note that even if the receiving device itself were perfectly noise-free, noise voltage would occur at the input of the receiver. The reasons for the occurrence of these input noises are indicated below. Since the receiving device itself is imperfect and creates additional noise, the noise voltage at the receiver output will be determined by both input noise and intrinsic noise. If the receiver does not contain low-noise high-frequency amplifiers, then the noise voltage at the output will be determined by its own noise.
In order to quantify how much a real receiver differs from an ideal non-noise one, the concept of the noise figure of a receiver is usually introduced.
The noise coefficient of some linear four-terminal network is a number that shows how many times the signal-to-noise power ratio by
its input is greater than the corresponding signal-to-noise ratio at the output,
where is the ratio of the signal power to the noise power at the input in the passband of the four-terminal network; ratio of signal power to output noise power.
From relation (2.2.1) it can be seen that for an ideal, noise-free quadripole, the noise coefficient is equal to unity, and for any real
Imagine the receiving device in the form of series-connected four-terminal devices that have corresponding noise factors: If the loads of the four-terminal devices are matched, then for the noise figure of the receiving device it is easy to obtain the following relation:
where the gains of the four-port power.
It can be seen from the expression obtained that if the receiver has a high-frequency amplifier with a large gain, then its noise figure will be mainly determined by the intrinsic noise of this amplifier and input circuits.
However, often centimeter-range receivers at high frequency amplifiers do not have. In such receivers, the first intensely noisy element will be a mixer, the second is an intermediate frequency amplifier. The noise of the mixer consists of the intrinsic noise of the crystal detector and the noise of the local oscillator. Usually the noise properties of a mixer are usually characterized by relative noise temperature
Effective noise temperature of the mixer; absolute temperature of the elements of the receiving device; power transmission coefficient and mixer noise figure.
Then the receiver noise figure is written as
Here is the noise figure of the amplifier.
Thus, the noise figure of the receiver is mainly determined by the noise of the intermediate frequency amplifier and the noise of the mixer. There are many reasons for the appearance of noise in the amplifier. You can indicate, for example, noise sources such as thermal noise of resistances, noise arising from the shot effect in electron tubes, etc.
As follows from formula (2.2.2), the noise figure of the entire intermediate-frequency amplifier is determined mainly by the noise factors of its first stages. Therefore, when designing receiving devices, special attention is paid to the noise properties of the input circuit and the first stages of the amplifier. Without dwelling on these issues in detail, we indicate that the noise figure for centimeter-range receivers in which there is no amplification at a high frequency is usually on the order of 10–16 dB
If the receiving device has a high-frequency amplifier, which is used as a traveling wave lamp, then the noise figure of such a receiver is of the order of 3-dB.
Knowing the value of the noise figure, you can easily calculate the noise power at the output of the amplifier. From the form we get
where is the gain in the power of the receiver path to the second detector.
The noise power at the input in the bandwidth of the amplifier can be calculated according to the well-known formula
Where is the equivalent noise temperature at the inlet, expressed in absolute units; effective bandwidth k is the Boltzmann constant.
Then the noise power at the output of the amplifier will be equal to
where is the effective noise temperature of the receiver.
It should be noted that the noise figure included in these formulas is the noise figure measured at the effective input noise temperature, which may differ from the standard temperature. Then you can use the following relationship between the noise figure measured at temperature and the standard noise figure measured at temperature
The noise of the receiver limits the real sensitivity of the receiving device, and hence the ultimate range of the radar station. In addition, due to the presence of noise, additional fluctuation errors occur in measuring the coordinates of the target. In this regard, the most important task in the design of receiving devices for radar stations is to reduce the noise level.
Recently, significant successes along this path have been achieved mainly through the use of parametric and molecular amplifiers. Their own noise is comparable or less than the level of input noise.
In this case, the input noise will be called the noise that occurs before the first low-noise amplifier. For reasons of occurrence, they can be divided into two groups. The first group includes noise arising from the emission of a celestial background (cosmic noise), secondary radiation of an absorbing medium (atmospheric noise), thermal radiation of the earth, perceived by the side lobes of the antenna pattern. The second group includes noise, which
occur in the antenna and elements of the receiving path preceding the amplifier. These include noise arising after. due to the final conductivity of the surface of the metal antenna, losses in the waveguide path from the antenna to the low-noise amplifier, direct losses in the antenna switch, etc.
If we can influence the noise components of the antenna due to heated ground by lowering the side lobes by improving the design of the antenna, then a more difficult problem is to reduce the noise level of the sky received by the station from the direction of the main lobe of the antenna pattern. This noise consists of components due to scattering and absorption in the atmosphere, as well as noise coming from a space outside the ionosphere (cosmic noise). Although the question of the dependence of the noise level on the operating frequency of the station has not yet been sufficiently studied, there is information that makes it possible to judge that the level of cosmic noise is inversely proportional to the frequency. This is illustrated in fig. 2.1, borrowed from the Figure shows the frequency dependence of the effective noise temperature of ideal antennas. At higher frequencies (atmospheric noises begin to strongly affect above, which increase with increasing station operating frequency. From here, in particular, it can be seen that there is some optimal range of operating frequencies at which the antenna noise temperature is minimal. In addition, the graph below makes it possible to estimate antenna noise temperature
The noise of the elements of the receiving path to a low-noise amplifier can be easily estimated. If there is some source with equivalent noise temperature and we need to calculate the effective noise temperature at the output of a passive four-terminal with a power transfer coefficient, then we can use the following formula:
where is the absolute temperature of the passive quadrupole.
the influx of additional four-pole elements can easily be estimated by the consistent application of expressions of this type.
When using parametric or molecular amplifiers, a more convenient characteristic of the noise properties of the receiver is the effective noise temperature
Fig. 2.1. Effective noise temperatures of ideal antennas, molecular and parametric amplifiers: 1 - ideal antenna horizontally directed at the galactic center; 2 - an ideal antenna vertically directed at the galactic pole; 5 - molecular amplifier; 4 - parametric amplifiers.
It will consist of the noise temperature at the input and the temperature of the noise of the amplifier:
As a result, we find that the effective noise temperature of the receiving device can be estimated by the formula
where is the equivalent noise temperature of the antenna; absolute temperature of the waveguide path; power transmission coefficient of this path; high-frequency noise amplifier
According to fig. 2.1, we can estimate the effective noise temperature of molecular and parametric amplifiers and its dependence on the operating frequency. As can be seen from the graph, the noise temperature of the molecular amplifier turns out to be especially low (therefore, input noise begins to play a very important role in (receiving devices with such amplifiers). In this regard, reducing input noise is a serious problem. This can be done by improving the antenna design , the choice of the operating frequency of the station, the cooling of the elements, the antenna-waveguide path to a low-noise amplifier and the reduction of losses in these elements.
In the future, we will everywhere operate with the spectral density of noise, which is easy to obtain from the above relations:
Internal noise is the noise of the active resistance of the antenna loss loss Tlos (loss) and the noise of the active resistance of the feeder loss TF. Their level depends on the frequency to the extent that the active losses in the antenna and feeder depend on it.
thermal noise feeder TF
Knowing the feeder loss in dB, it is not difficult to calculate it using the formula Tf \u003d To (1 - efficiency), where To is the temperature of the medium (feeder) in gr. Kelvin. Why do we need to transfer the known feeder losses from dB to the efficiency and make a calculation. For example, with a feeder loss of 1 dB, its efficiency is 0.89. At 17 ° C, this feeder will have a noise temperature of TF \u003d 290 (1 - 0.89) \u003d 32 °.
thermal noise Tlos antenna
Its value can also be calculated from known losses in the antenna material. An antenna made of perfect material does not make noise. From the real noise to the extent that its loss resistance is part of the radiation resistance of the antenna. The choice of power points and matching devices together with R emissions. and R loss is also reduced to the INPUT antenna impedance.
Loss in dB in an antenna made of real material can be determined by the difference in antenna gain from ideal and real material. Transferring dB to the ratio of values \u200b\u200band subtracting from unity, we obtain the fraction of R losses in R emissions. or R input Multiplying the fraction of R losses by the ambient temperature in ° Kelvin, we obtain T noise R losses or T loss with an accuracy of more than sufficient for normal VHF antennas.
For example, a 50 ohm antenna made of ideal material has a gain of 13 dB, of aluminum 12.81 dB. The difference of 0.19 dB corresponds to a ratio of U or R of 0.9783. 1.0 - 0.9783 \u003d 0.0217 is the share of losses. With R in 50 ohms reduced to the input loss resistance is 0.0217 x 50 \u003d 1.085 ohms. If the temperature of the medium is 290 ° Kelvin, then T loss will be: 290 ° K x R loss / Rin. In our case, this amounts to 290 x 1,085 / 50 \u003d 6.3 ° K.
With sufficient accuracy, you can calculate easier. From the decibel table, we find the numerical value of the gain difference, subtract 1 and multiply by 290 °. In our example, 0.19 dB \u003d 1.022. In this case, Tlos will be equal to 290 (1,022-1) \u003d 6.4 °. In the table below, the Tlos calculation for the commonly available losses in pure aluminum VC antennas made in MMANA is made. Given losses in the feeder, the effective temperature Tlos at the receiver input will be equal to Tlos x feeder efficiency.
Translation table of antenna gain difference calculated for ideal material and pure aluminum in Tlos
EXTERNAL NOISE AFS
External noise is the noise received by the antenna from the noise sources of the outer space in the same way as a useful signal. Such sources are the thermal noise of the earth Tz or Tearth (earth - earth), the technogenic noise Tt and cosmic noise (sky noise) Tk or Tsky (sky - sky). It is obvious that the total external noise of the AFS will depend on the noise temperature of these sources and on the diagram and position of the antenna relative to these sources and therefore it cannot be normalized. thermal noise of the earth T earth
Strictly speaking, the noise temperature of the earth Tearth is equal to its physical temperature T, multiplied by 1 - Ф, where Ф is the reflection coefficient of the earth's surface, which in turn depends on the angle of inclination, electrical properties of the earth's surface and polarization of the antenna. But on the VHF bands, the Rayleigh condition is usually satisfied, the surface of the earth is considered rough, the reflection from it is diffuse, Φ tends to 0, and Tearth - to the physical temperature of the earth, which is usually assumed to be 290 ° K in calculations. The level of thermal noise of the earth does not depend much on frequency.
man-made noise TT
The noise of electrical devices, from household appliances, computer networks to power lines, electric vehicles and industrial. enterprises. The level can be very different, from 0 ° K in a deserted area without rail, pipeline and electrical communications within a radius of 100 km, to thousands and tens of thousands of degrees in business centers of cities and industrial zones. Or simply if a neighbor has a Chinese charger or PSU connected to the network without a noise filter. With increasing frequency, the intensity of technogenic noise decreases, but not as fast as we would like.
tsky sky noise
As can be seen on the Tsky sky map for a frequency of 136 MHz, its various regions have very different noise temperatures Tsky, from 200 ° to 3000 ° K. At a frequency of 430 MHz, the noise temperature of the same areas is on average 15 times less. The noise temperature of Tsky is variable over time, it depends on solar activity. In addition, Tsky includes the noise of the disk of the Sun, Moon, and planets, which are also unstable and very different in time.
AFS NOISE TEMPERATURE EVALUATION
The evaluation methodology is well described by DJ9BV and F6HYE in the journal “DUBUS” -3 / 1992. Translation of this article Quality assessment of an EME system can be found on the VHF portal. Translated by Nikolay Myasnikov, UA3DJG.
AFS GENERAL NOISE TEMPERATURE
The noise temperature of the antenna Ta at the input to the feeder is the arithmetic sum of the noise temperatures of internal and external noise sources. The noise temperature of the AFS at the receiver input is also the arithmetic sum of the noise temperature of the antenna Ta, taking into account its losses in the feeder and the noise temperature of the feeder Tf itself. Tafs \u003d Ta x Efficiency + TF. The specific feeder TF can be calculated in advance by its attenuation and is not involved in the calculations below, then only Ta antennas or antenna systems (stacks) are considered.
CALCULATION OF ANTENNA NOISE TEMPERATURE
There are several methods for calculating Ta. For example, one of them is given in:
In a number of cases, it turns out to be convenient to determine the noise temperature of the antenna through the scattering coefficients β i. By the scattering coefficient in transmission mode is meant the ratio of the fraction of power enclosed within a given solid angle to the total power emitted by the antenna. Typically, the total and differential scattering coefficients are distinguished. The total scattering coefficient is the ratio of the total power radiated by the antenna to the side and back lobes of the radiation pattern to the total radiated power. Naturally, the total scattering coefficient is the sum of the differential coefficients β i.
If, for example, the space surrounding the antenna is divided into three regions: 1) the region of the main lobe, .2) the region occupied by the lobes of the anterior half-space (with respect to the opening of the antenna), 3) the region of the rear half-space, then the effective noise temperature of the antenna, without accounting ohmic losses, can be determined through the scattering coefficients from the expression Ta \u003d T 1 (1 - β) + T 2 β 2 + T 3 β 3, where T 1 is the average brightness temperature of the medium within the main lobe of the diagram; T 2 - the average brightness temperature of the noise radiation received by the side lobes in the area of \u200b\u200bthe front relative to the aperture of the antenna half-space; T 3 the average brightness temperature of noise radiation in the rear half-space; β is the total antenna dispersion coefficient beyond the main lobe of the diagram; β 2, β 3 - scattering coefficients, respectively, in the front and rear hemispheres β 1 \u003d β 2 + β 3 The total noise temperature of the antenna, taking into account ohmic losses in the transmission line, is equal to: Ta у \u003d Ta η + Ty \u003d Т 1 (1 - β) η + T 2 β 2 η + T 3 β 3 η + T 0 (1 - η). Thus, the noise temperature of the antenna depends not only on the intrinsic characteristics of the antenna (β, η), but also on the temperature of the external noise radiation (T 1, T 2, T 3). Therefore, depending on the orientation of the antenna, its noise temperature will vary.
In the given method, there is no specific parameter or their complex by which you can compare the antennas with each other and make a choice. The reason is the variability of the noise temperature of external sources and its dependence on the position of the antenna relative to them. I. Goncharenko DL2KQ writes about this at his forum.
Question:
Are there any formulas for calculating Ta, G / Ta, T los. Why is only YA324 calculating this data, but MMANAGAL is not?
Answer:
The noise temperature of the antenna (aka Ta) came to us from radio astronomy. Ta is calculated as the product of the space noise density (solar flux unit, sfu) S (1S \u003d 10-22 W s / m2) and the effective aperture area of \u200b\u200bthe antenna A divided by two Boltzmann constants 2 k (where k \u003d 1.380662 10-23). Replacing the opening area through the formula connecting it with Ga (see, for example, clause 3.1.7 in the second part of “HF and VHF”), we obtain and simplify by calculating the degrees and constants we obtain: Ta \u003d SG λ² / 3.47, where: S - sfu is dimensionless, today's value (see, for example, Geophysical alerts); G - in times (not in dB); λ - in meters.
As you understand, having calculated G in the program (both maximum and current, in an arbitrary direction along the vector), it is not difficult to calculate Ta, G / Ta, Tlos. Let's do it in GAL-ANA. Why not done in MMANA-GAL? Because the free MMANA-GAL was made by us under our personal (and possibly erroneous) idea of \u200b\u200bwhat is understandable and convenient in antenna calculations. According to the mentioned opinion, the use of the temperatures of the feeder and antenna is an inconvenient thing. See for yourself: the Tlos formula includes the variable temperature of the environment To, and the Ta formula contains the variable solar-dependent solar flux unit. As a result, Tlos and Ta walk on the weather. Is it convenient to use such floating parameters? Of course, you can introduce some standard-average To and S. But this is not standardized yet, which is why in various publications who is in the forest, who is for firewood. the answer is written 24.1.2007, at 8:11
Radio amateurs have adopted the method of calculating the noise properties of the antenna as the G / T ratio, where G is the antenna gain and Ta is its noise temperature. The gain G is quite certain, and the noise level Ta is determined only for T los, the remaining components depend on unstable external noise sources and the antenna orientation relative to them, so they must be agreed upon in advance.
The orientation of the antenna or stack of them relative to the ground is accepted as the position of the antenna in horizontal polarization with an angle of inclination of the maximum relative to the horizon (elevation) 30 °
External conditions, T noise of the sky and T noise of the earth, are taken evenly distributed over the upper and lower hemispheres around the antenna. The temperature of the sky noise in the 144 MHz range is taken to be 200 °, and in the 432 MHz range 15 °. Silence of the earth on both ranges is accepted 1000 °.
The results of calculating G / T antennas in 2 x 2 stacks are presented in table VE7BQH.
CONTACT NOISES
There is still a source of noise, which programs are not aware of, and radio amateurs sometimes forget - contact noise. Contact noise is directly proportional to the magnitude of the current, the power density decreases with increasing frequency (1 / f), but under certain conditions on VHF it can reach a value that interferes even with local communications. This is the noise of variable contact points in antennas with a mechanical connection of elements, a beam, metal fasteners to each other. Threaded connection, press-in, crimp with a clamp, tight fit of the tube into the tube, RF connector - everywhere galvanic contact is not on the entire surface but at several points. Despite their many, any minor effect breaks some points of contact and forms others. Under the influence is meant displacement from the wind, resizing with temperature, the process of surface corrosion, breakdown of the RF by the voltage of the oxide film and its restoration upon receipt, "stray currents" of the mains and electrostatics, etc. As a result, when the contacts are reliable from the point of view of an electrician, the current path and antenna geometry are constantly changing. The rustling and crackling resulting from this are usually attributed to external interference. The bolted connection between the vibrator and the cable from dissimilar metals and fully possesses these disadvantages. In VC antennas, in which the vibrator and gamma matching are fastened by crimping the strip, the same reasons are possible at 145 MHz, and at 1296 MHz they will inevitably lead to instability and deterioration of the antenna parameters.
Literature (and they are also links sites where they can be downloaded):
1 - Current problems of antenna-waveguide technology Collection of articles of the USSR Academy of Sciences
2 - Handbook of a radio amateur - shortwave S. G. Bunin, L. P. Yailenko
3 - Methods for suppressing noise and interference in electronic systems G. Ott
4 - Handbook of radio relay ed. Borodich S.V.
5 - Elementary radio astronomy Kaplan
6 - Radio astronomy J. Kraus
Noise antenna temperature
Noise antenna temperature - characteristic of the noise power of the receiving antenna. Noise temperature has nothing to do with the physical temperature of the antenna. It is given by the Nyquist formula, and is equal to the temperature of the resistor, which would have the same thermal noise power in this frequency band:
Where
Noise power, - noise temperature, - frequency band, - Boltzmann constant.
The source of noise is not the antenna itself, but noisy objects on Earth and in space. The cosmic component of noise depends on the diameter of the antenna: the larger the diameter and gain, the narrower the main lobe of the radiation pattern, respectively, the less extraneous cosmic noise the antenna amplifies along with the useful signal. The terrestrial component of the antenna’s noise temperature depends on the elevation angle - the lower the antenna “looks”, the more it receives industrial interference and noise from sources on the Earth’s surface. Therefore, the noise temperature is not a constant, but a function of the elevation angle. As a rule, it is indicated in the specification for one or more elevation values. Typical noise temperature of a parabolic antenna with a diameter of 90 cm in the Ku-range for an elevation angle of 30 degrees is 25-30K.
Noise temperature of the antenna in radio astronomy
The concept of antenna noise temperature along with the concept of antenna temperature is widely used in radio astronomy. Antenna temperature characterizes the total power of the received antenna radiation, i.e. noise power and power of the studied objects, while noise temperature is only the power of noise (interfering factors). If not a single radio source falls into the radiation pattern, then the antenna temperature is equal to noise. Thus, the useful signal depends on the difference between the antenna and the noise temperature.
As a rule, the noise temperature consists of two parts: constant and stochastic. The constant component can be compensated, but the stochastic component imposes fundamental restrictions on the sensitivity of radio telescopes. Therefore, to increase the signal-to-noise ratio when designing radio telescopes, the main attention is paid to reducing the stochastic component. For this, low-noise amplifiers are used, the receivers are cooled with liquid nitrogen or helium, and so on.
see also
References
Wikimedia Foundation. 2010.
See what “Antenna noise temperature” is in other dictionaries:
The effective quantity serving as a measure of the power of noise in radio receivers. Sh. T. Tsh is equal to the temperature of the agreed resistance (antenna equivalent) at which the power of its thermal noise is equal to the noise power of a given ... ...
equivalent noise temperature of satellite line Recom - The noise temperature at the output of the receiving antenna of the earth station, corresponding to the power of the radio frequency noise creating the total noise observed at the output of the satellite line, with the exception of noise created by interference from satellite lines, ... ... Technical Translator Reference
Satellite line equivalent noise temperature - 1. The noise temperature at the output of the receiving antenna of the earth station, corresponding to the power of the radio frequency noise creating the total noise observed at the output of the satellite line, with the exception of noise created by interference from satellite lines, ... ... Telecommunication Dictionary
A value that characterizes the power of electromagnetic radiation received by the antenna. Often used in radio astronomy. Antenna temperature has nothing to do with the physical temperature of the antenna. Like noise temperature, it ... Wikipedia
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Antenna of the radio telescope RT 7.5 MGTU im. Bauman. RF, Moscow region, Dmitrovsky district. The diameter of the mirror is 7.5 meters, the operating wavelength range is: 1 4 mm Antenna is a device for emitting and receiving radio waves (varieties of electromagnetic ... ... Wikipedia
I Orbit (from lat. Orbita track, path) circle, scope, distribution; see also Orbit (honey.), Orbits of celestial bodies, Orbits of artificial space objects. II Orbit (honey.) The orbit, the bone cavity of the Skull, in which ... ... Great Soviet Encyclopedia
GOST 24375-80: Radio communication. Terms and Definitions - Terminology GOST 24375 80: Radio communication. Terms and definitions original document: 304. Absolute instability of the frequency of a radio transmitter Instability of a frequency of a transmitter Definitions of a term from different documents: Absolute instability ... ...
Device for receiving and measuring cosmic radiation. objects ranging from decameter to millimeter wavelengths (within the “transparency window” of the Earth’s atmosphere for radio waves). Measurements at longer wavelengths are made from space. R.… … Physical Encyclopedia
GOST R 50788-95: Installations for direct reception of satellite television broadcasting programs. Classification. Main settings. Technical requirements. Measurement methods - Terminology GOST R 50788 95: Installations for direct reception of satellite television broadcasting programs. Classification. Main settings. Technical requirements. Measurement methods Original document: 3.1.4 Antenna device for receiving ... ... Glossary of terms of normative and technical documentation
