DETERMINATION OF THE LEVEL OF EXTERNAL INTERFERENCE AT WAVELENGTHS OF 1–30 m

L. P. Serafinovich

(Presented by the scientific seminar of the Radio Engineering Faculty)

Introduction

When conducting radio observations of meteors, and in a number of other cases, a more concrete knowledge of the level of external interference is required than the data used up to now, obtained by Scott for an isotropic antenna [1]. However, it is known that the level of this interference when receiving on a directional antenna does not remain constant, but depends on the time of day and on the direction of the antenna. When working with a weakly directional antenna, for example with a half-wave dipole, the interference level also changes during the day and depends to some extent on the geographic latitude of the observation site and on the orientation of the antenna. The data of radio astronomy on cosmic radio emission obtained in recent years make it possible to approach more correctly the estimation of the level of external interference.

Sources of External Interference

a) Sources of cosmic interference

Cosmic interference consists of the radio emission of cosmic objects, the characteristics of which are given in [2,3]. Let us consider the main components of this emission.

The radio emission of cosmic relativistic electrons during their braking in the magnetic fields of the Galaxy gives rise to the spherical component of the radio emission of the Galaxy. Its intensity is distributed rather uniformly over the sphere: the ratio of the radiation intensity at the center of the Galaxy to the intensity at the poles decreases with increasing wavelength and is approximately 10 at a wavelength of 3 m and approximately 4 at a wavelength of 16.4 m. This component has no structural features and predominates substantially at wavelengths greater than 1 m in the total cosmic radio emission.

The radio emission of interstellar ionized gas is the second—thermal—component of cosmic radio emission. Gas having an electron kinetic temperature of approximately 10000° emits radio waves whose intensity depends on the frequency and on the thickness of the gas layer in the direction under consideration. It is known that the interstellar gas of the Galaxy has a strong concentration toward the center of the Galaxy and

very strong—toward its plane, which lies in the Milky Way. In connection with this, the intensity of the thermal component has a large value in the region of the center of the Galaxy and falls to a very small value at a distance from the center of \(\sim 60^\circ\) along the plane of the Galaxy and \(\sim 10^\circ\) in the direction perpendicular to it. Its role in the total cosmic radio emission is small (at meter wavelengths), since only in the direction toward the center of the Galaxy at a wavelength of \(3\) m is it equal to the spherical component, and it decreases with increasing wavelength. This component has small secondary maxima in Cygnus and in the anticenter of the Galaxy.

The radio emission of discrete sources (about 2000 are known) is of a thermal and nonthermal nature, like both components. Most powerful discrete sources are of galactic origin and are concentrated toward the plane of the Galaxy. Discrete sources of extragalactic origin form the metagalactic component of cosmic radio emission. Its intensity, averaged over the sky, amounts to from \(0.5 \div 0.7\) to \(0.25\) of the intensity of the galactic component when the wavelength changes from \(1.25\) to \(3\) m [4], i.e., it decreases sharply with increasing wavelength. The most powerful discrete sources of radio emission at meter wavelengths are Cassiopeia A and Cygnus A.

The radio emission of the Sun is to a large extent variable in character and changes on average by a factor of \(10^3\), reaching a maximum in years of maximum solar activity. The radiation flux from the “quiet” Sun decreases with increasing wavelength, amounting to \(\sim 0.08\%\) at a wavelength of \(10\) m and \(\sim 2\%\) at a wavelength of \(1.5\) m of the total cosmic radio emission. During solar bursts the radiation intensity may increase by up to \(10^6\) times at meter wavelengths (the more so, the greater the wavelength). But this lasts for an insignificant time, of the order of an hour.

b) Sources of atmospheric interference

The sources of atmospheric interference are processes occurring in the atmosphere: local, distant, and tropical thunderstorms, quiet discharges between masses of ionized air or vapor, as well as redistribution of electron density in the ionized layers of the ionosphere. The direction of arrival of atmospheric interference of distant origin changes during the day from southeastern to southwestern.

Level of external interference at wavelengths \(1\)—\(30\) m

a) When using a directional antenna.

As can be seen from the preceding, the intensity of cosmic radio emission is maximal in the direction toward the center of the Galaxy, which lies in the direction of the constellation Sagittarius and has equatorial coordinates: right ascension \(\alpha = 17\) h. \(42\) m. \(40\) s. and declination \(\delta = -28^\circ 50' 55''\). Its greatest altitude \(\beta\) above the horizon can be determined from the formula

\[ \beta = 90^\circ - \gamma + (\pm \delta), \tag{1} \]

where \(\gamma\) is the geographical latitude of the observing site.

The minimum intensity of cosmic radio emission is found in extensive “cold” regions near the galactic poles. The coordinates of the centers of these regions in the galactic system are as follows: southern—\(l \simeq 200^\circ,\ b = -40^\circ\); northern—\(l \simeq 180^\circ,\ b \simeq\)

\(\simeq 45^\circ \div 50^\circ\) (the transition from one coordinate system to another can be found in [5]).

Consequently, the maximum spread of the level of external interference for a directional antenna depends on where the antenna is pointed: toward the center or toward the “cold” region of the Galaxy.

The effect of external interference on the receiving device is characterized by the antenna temperature \(T_a\). The latter is defined as the temperature of a resistance equal to the output resistance of the antenna which, when connected to the receiver input in place of the antenna, delivers to the receiver load the same noise power as the observed source.

Let us determine the maximum and minimum temperatures of a directional antenna whose angular dimensions of the lobe at half power are \(\varphi \simeq \Theta \simeq 20^\circ\). The expression for \(T_{a\,\min}\) at frequencies \(10 \div 300\) Mc/s will be obtained from the temperature values given for the “cold” regions by I. S. Shklovskii [2]:

\[ T_{a\,\min}=\frac{2.08\cdot 10^8}{f_{\text{Mc/s}}^{2.8}}. \tag{2} \]

The graph constructed from this formula is given in Fig. 1.

To calculate the maximum antenna temperature we shall use the isophotes of radio emission obtained by Shain and Higgins for the frequency 18.3 Mc/s and by Bolton and Westfold for the frequency 100 Mc/s (the isophotes may be found in [2]). We choose the region around the center of the Galaxy. The angular dimensions of this region are \(\varphi \simeq \Theta \simeq 20^\circ\). Finding the mean brightness temperature \(T_b\) of this region (\(T_b\) characterizes the radiation intensity in the chosen direction) and taking into account that in this case \(T_a=T_b\) [3], we obtain

\[ T_{a\,\max}=\frac{3.10\cdot 10^8}{f_{\text{Mc/s}}^{2.42}}, \tag{3} \]

at the same frequencies (graph in Fig. 1).

With an increase in the angular dimensions of the lobe, \(T_{a\,\max}\) will decrease; with a decrease, it will increase.

\(T_{a\,\min}\) retains its values over a large range of variation of the angular dimensions of the lobe, since the angular dimensions of the “cold” regions are on the order of several tens of degrees. In the limit, when passing to a weakly directional antenna, its \(T_a \to T_{a\,\min}\).

The entry of the Sun into the antenna directivity pattern can significantly increase \(T_a\), depending on its activity. Pointing the antenna toward the most powerful sources also causes an increase in the antenna temperature, depending on the power of the source. For example, the source Cassiopeia A at a frequency of 40 Mc/s increases the temperature of an antenna with \(\varphi \simeq \Theta \simeq 20^\circ\) by approximately a factor of two. In addition, in the northern galactic hemisphere there is a region near \(l=0\), extending from the Galactic equator to its pole. The intensity of this region at meter wavelengths is \(1/4\) higher than the surrounding background and does not depend on frequency [6].

In reality, at frequencies below 40 Mc/s, absorption takes place in the \(F_2\) and \(D\) layers of the ionosphere. Therefore, at these frequencies there should be a weaker dependence of \(T_a\) on frequency, i.e., the curves will be flatter in the range 10–40 Mc/s.

b) When using a weakly directional antenna (half-wave horizontal vibrator).

The level of external interference of a horizontal dipole depends on the geographic latitude of the observation site, the orientation of the dipole, and the time of day. Owing to the motion of the Earth around the Sun, the times of the daily maximum and minimum do not remain constant, but occur 4 min earlier than on the preceding day. Thus, the time

Fig. 1. Dependence of temperature on frequency for the center \((T_{a\,\max})\) and the “cold” regions \((T_{a\,\min})\) of the Galaxy.

of the maximum coincides with noon approximately on December 31, whereas the time of the minimum coincides with noon approximately on September 1 [7].

The most recent and most accurate \((\simeq 1 d b)\) measurements of the radiation of the whole celestial sphere were carried out by Cottony and Jowiler [7]. For the measurements they used a half-wave horizontal dipole located at a height \(\lambda/4\) above the ground and oriented east to west. In this case the greatest altitude of the center of the Galaxy above the horizon was \(\sim 22^\circ\) (the measurements were made at latitude \(\varphi = 39^\circ\) N), exactly in the south. From the data they obtained one can find that, for a half-wave vibrator,

\[ T_{a\,\max} = \frac{6.13 \cdot 10^7}{f_{\text{MHz}}^{2.3}};\qquad T_{a\,\min} = \frac{2.82 \cdot 10^7}{f_{\text{MHz}}^{2.3}}. \tag{4} \]

The graphs are given in Fig. 2, where, for comparison, Scott’s data [1] are also given. With the dipole oriented from south to north, these values will, naturally, be smaller. Absorption by the ionosphere is not taken into account here; therefore the dependence is weaker than in the preceding case. It is not clear, however, why this dependence persists at frequencies above 40 MHz, where the dependence of \(T_a\) on \(f\) should be stronger. Bursts of radio emission from the Sun can considerably increase \(T_a\), but not for long (\(\sim 1\) hour).

We shall take the expression for \(T_{a\,\max}\) as the temperature averaged over the whole celestial sphere.

c) General interference level at frequencies 10–300 MHz.

Atmospheric interference increases the general noise level. It has been established that in the range above 10 MHz atmospheric interference sharply

decrease with frequency \((E \sim f^{-n},\) where \(n \sim 1 \div 3)\) and become smaller than cosmic ones at frequencies above \(20\) MHz. Atmospheric interference at frequencies above \(10\) MHz reaches a maximum in the summer months of the year and, in the second half of the day throughout the year, at 16–20 hours local time.

Measurements of the level of atmospheric interference by different authors differ greatly from one another (by up to several tens of db when reduced to the parameters of the same apparatus). Therefore, for estimating

Fig. 2. Dependence of temperature on frequency when receiving with a weakly directional antenna.
— Measured values
— Assumed values.

the general level of interference we shall use the maximum values of the field strength given in [7]. We shall also characterize the effect of external interference on the receiver by the antenna temperature. Formally this is quite possible, since \(E_a = E_{\text{interference}}\cdot h_0\), where \(h_0\) is the effective height of the antenna.

Using, in addition, the values \(T_{a\,\max}\) from formula (4), we obtain dependences of the antenna temperature characterizing the general maximum level of external interference when receiving with a weakly directional antenna, different for two frequency ranges (Fig. 3).

For frequencies \(10 \div 22.5\) MHz,

\[ T_{a\,\max}=\frac{1.03\cdot 10^{16}}{f_{\text{MHz}}^{8.36}}. \tag{5} \]

For frequencies \(22.5—300\) MHz,

\[ T_{a\,\max}=\frac{6.13\cdot 10^{7}}{f_{\text{MHz}}^{2.3}}. \]

Fig. 3 also gives the temperature values corresponding to the internal noise of a well-designed receiver [1]. Thus it can be seen that internal noise begins to have an effect at frequencies \(\sim 90\) MHz and becomes predominant at frequencies \(>130\) MHz. The total antenna temperature can be found by simple addition of the temperatures corresponding to the external and internal noise of the receiver.

The expressions obtained make it possible to estimate the level of external interference for directional and weakly directional antennas. However, in some cases it is necessary to know the variation of the level of external

interference over, say, a day. This requires the use of isophotes of the radio emission of the celestial sphere at the operating frequency. There are almost none of these for the entire celestial sphere, except for the isophotes of Bolton and Westfold at a frequency of 100 MHz. It is desirable to obtain such isophotes for different frequencies, taking into account the activity of the ionosphere. In addition, it is desirable to study more thoroughly the level and direction of arrival of atmospheric interference at frequencies above 10 MHz.

Vertical axis: antenna temperature \(T_a\), degrees K.
Horizontal axis: frequency, MHz.

Fig. 3. Dependence on frequency of the temperature characterizing the overall interference level when receiving with a weakly directional antenna. ——— Temperature corresponding to the internal noise of a well-designed receiver.

REFERENCES

1. Maximum Range of Operation of a Radar Station, ed. B. Vysotsky, 1947.
2. I. S. Shklovsky, Cosmic Radio Emission, Moscow, 1956.
3. A. D. Kuzmin, Radiotekhnika, 12, No. 1, 12–21, 1957.
4. G. G. Getmantsev, Astronomicheskii zhurnal, 32, No. 1, 22–28, 1955.
5. P. G. Kulikovsky, Handbook of the Amateur Astronomer, Gostekhizdat, 1953.
6. G. G. Getmantsev, Proceedings of the 5th Conference on Questions of Cosmogony, Academy of Sciences of the USSR, pp. 468–493, 1956.
7. H. V. Cottony, J. R. Johler, Proc. IRE, 40, No. 9, 1053–1060, 1952.