Sun through antenna John A Kennewell

[ An earlier version of this material was presented as a paper at ICAP 89, the Sixth International Conference on Antennas and Propagation, held at Warwick University UK, 1989. ]


No long distance communication system is free from environmental disturbances of one kind or another. High frequency communication depends on the vagaries of the ionosphere. VHF and UHF transionospheric systems experience problems due to rapid changes in the ionospheric electron density. SHF and EHF links are subject to tropospheric attenuation, and all space-Earth links are subject to potential disruption when the Sun enters the beam of the downlink receiving antenna. This latter well known phenomenon (sometimes referred to as a "sun-outage") is commonly associated with wideband downlinks from geosynchronous satellites, but at times of high solar activity any electromagnetic receiving system pointed near the Sun may experience disruptive effects.

This note reviews the nature of solar radio emissions and provides appropriate conversion algorithms between solar radio astronomical terminology and useful communication parameters. Algorithms for the computation of the duration and severity of solar disturbances are provided, with graphical examples showing the effect of various parameters.


2.1 General

The Sun, by nature of its high temperature, is a wideband radio emitter. Its size also makes it a source to be reckoned with when low noise electromagnetic receiving systems are pointed toward it. Futhermore, the variability of its output makes it difficult to predict its exact effect on a given system.

It is convenient to divide the solar radio emission into three separate components. These are referred to as the background component, the slowly varying component and the burst component. Each of these can be identified with separate sources on the Sun.

Figure 1 is a photograph of the Sun taken in the light of the hydrogen-alpha spectral line. This shows the solar chromosphere, the "inner atmosphere" of the Sun. It is from this region, together with the lower part of the corona (the Sun's "outer atmosphere"), that radio emissions in the 1 to 20 GHz range originate. The featureless areas of the solar disc (or the regions above them) may be regarded as the source of the background component. The light white areas, termed plage, are several thousand degrees hotter than the background areas, and these are the primary source of the slowly varying component. Plage areas and sunspot groups occur in the same regions of the Sun, both phenomena being due to the emergence of large magnetic fields from lower depths. The intensely bright area is a solar flare, an explosive process whereby magnetic energy is converted into large quantities (up to 1025 Joule) of electromagnetic energy. It is these events that are responsible for the burst component.

H-alpha Sun
Figure 1 - The solar chromosphere - the source of solar microwave emission
[ Learmonth Solar Observatory image ]

When measuring the whole disc radio emission of the Sun, radio astronomers typically use the Solar Flux Unit or SFU which equals 10-22 W m-2 Hz-1. Figure 2 shows the typical range of flux densities appropriate to each of the three components.

Solar Radio Spectra
Figure 2 - Solar Radio Spectra

2.2 The Background Component

The background or quiet component of the solar emission is by definition the component that remains when all variable components have been eliminated. It is thus a constant at a given frequency and it is estimated to an accuracy of 10% within the range 1 to 20 GHz by the following second order polynomial: At the higher frequencies within this range the emission is uniformly distributed across an area only a few percent larger than the visible disc of the Sun. However, at the lower end of the range, the radio Sun is rather larger than the optical Sun, and a limb brightening effect is noticeable (figure 3). This is in contrast to limb darkening that occurs at visible wavelengths. The variation in radio output across the solar disc is only significant when the antenna beamwidth is less than the angle subtended by the Sun (approximately 0.5 degrees).

Solar Radio Size
Figure 3 - Spatial Variation of Solar Microwave
Emission ( after Kundu [ref 4] )

2.3 The Slowly Varying Component

The slowly varying component shows a variation on two quite different time scales. The first, of approximately 27 to 28 days, is the rotation period of the Sun at mid-latitudes (the equatorial rotation rate is substantially greater than the polar rate) where plage is most predominant. This component is due to the uneven longitudinal distribution of plage. As an area of intense and extensive plage rotates onto the visible disc (as seen from Earth) this component will increase. A small plage area may grow and decay within a day. A large area may persist for several solar rotations. This process will also produce random fluctuations with component periods less than a rotation. The second component varies in accompaniment with the sunspot cycle in an approximately 11 year cycle (figure 4).

Slowly Varying Component
Figure 4 - The smoothed variation of the slowly varying component
with time. Individual values within a month can exceed 300 SFU
(Algonquin Radio Observatory, after SGD [ref 5] ).

The slowly varying component is essentially produced by specific regions on the Sun, and thus shows a significant spatial variation over the disc. Examples several days apart are shown in figure 5. Again, this spatial variation is only important when the antenna beamwidth is a fraction of the angle subtended by the Sun.

Slowly Varying Component
Figure 5 - Spatial variation of slowly varying component

The slowly varying component has a maximum value at a frequency of around 3GHz and falls off at higher and lower frequencies. At 1 Ghz and 30 Ghz the value has fallen to about 25% of the value at the peak frequency. The flux density at any frequency f(GHz) can be estimated to within 10% by the following expression:

where F10 is the current value of the solar radio emission at a wavelength of 10.7cm (2800 MHz). This value is measured and made available from the Penticton Radio Observatory in Canada. It is also available on a daily basis from various space environmental services, and is broadcast on some standard frequency and time services (eg WWV, WWVH). The function ln( ) is the natural logarithm. Over one sunspot cycle S may range from 0 to above 250 SFU. Note that F10 is in fact the value of Sq + Sv at 2.8 GHz.

2.4 The Burst Component

This component is a transient emission associated with explosive energy releases in the solar chromosphere and corona. At metre wavelengths it may increase the total solar output by six orders of magnitude, the increase lasting anywhere from seconds to hours. The frequency at which these bursts occur varies throughout the sunspot cycle, with very low probabilities when F10 is at a minimum, to several times a day at times of activity maximum. The intensity of the bursts also shows an ill defined average correlation with burst frequency, although large isolated bursts have been known to occur irrespective of other general solar activity. In the frequency range of 1 to 20 GHz, the average maximum flux density for the largest bursts typically varies from 100,000 to 10,000 SFU respectively. An exception to this was noted in December 2006 when two L-band bursts to around one million SFU were recorded.

No empirical expression is possible for the burst component Sb. Some space weather agencies may attempt probability forecasts for interested parties. A moderate duration high intensity burst is shown in figure 6.

Solar radio burst
Figure 6 - Moderate intensity solar radio burst
[ Learmonth Solar Observatory ]

2.5 Summary

The total solar radio emission is specified by S = Sq + Sv + Sb. However, in most applications Sb is taken as zero. A non-zero burst component is usually readily identifiable by the presence of a large and fluctuating noise level.

The nature of the solar signal is identical to the thermal (Gaussian) noise generated in receiver circuitry. The power collected by a receiving antenna is given by:

where Ae is the effective collecting area of the antenna.

This expression is appropriate where the total solar emission is being collected - that is, an antenna beamwidth greater than the solar diameter with the Sun totally within the beam. For large antennas on the higher frequencies, the beamwidth may be less than the solar disc (figure 7). In these cases the antenna essentially assumes the temperature of that part of the Sun to which it points. Under the assumption of a uniform brightness, this temperature is given in terms of the integrated flux density S by:

where λ is the wavelength, k=1.38x10-23 J K-1 is Boltzmann's constant and Ωs = 6.8x10-5 steradians is the mean solid angle subtended by the Sun. At sunspot minimum, Tsun ~10,000K at 20 GHz. The assumption of uniform surface temperature is valid at sunspot minimum, but becomes increasingly non-valid, especially around 3 GHz and lower frequencies, as F10 rises above 100 SFU. It is more valid at frequencies of 10 GHz and above.

Antenna beamwidth & solar diameter
Figure 7 - Antenna beamwidth equals mean solar diameter


3.1 The Nature of the Problem

The potential for solar interference with a satellite downlink circuit will occur whenever the Sun passes into the beam of the ground station receiving antenna. For a geosynchroonous orbit this will occur around the times of the equinoxes. What we are interested in are the times and durations of these occurrences, together with the magnitude of the receiving system temperature increase that will occur. From this, the degradation in the system performance is readily determined.

The first problem then is one of determining the position of both the satellite and the Sun. The former, being relative stationary, is readily accomplished. The latter, due to the changing length of our year, orbital variations, and our awkward calendar, is a little more difficult. Once these positions are obtained we then need to check for relative coincidences between the two as a function of time.

The apparent motion of the Sun across the sky renders the normal elevation-azimuth coordinate system unsatisfactory for use in solving our problem. Much better is the equatorial system of declination and hour angle. The Sun in essence then moves in hour angle during a single day, with a much smaller annual motion in declination. The equatorial system is based about an axis projected through the poles to infinity. Hour angle is measured from local meridian (with east negative) about this axis (in essence celestial longitude), and declination is measured in a plane containing this axis. Zero declination is defined at the projection of the equator and angles north of this are positive (figure 8).

Equatorial Coordinates
Figure 8 - Equatorial Coordinates

At the equinoxes the solar motion is 0.4 degrees per day in declination, and thus even with an infinitely small antenna beam, the Sun will still be in the beam for at least one day per equinox (figure 9).

Antenna solar transits
Figure 9 - Transits of the Sun through an antenna
beam on successive days around equinox.

3.2 Satellite Locational Algorithms

For a truly geostationary orbit, the satellite inclination is zero, and no apparent motion is seen from the ground. If we further allow the assumption of a spherical Earth, the elevation and azimuth of the satellite from a fixed ground location (latitude L and longitudinal difference from the satellite G) are readily given by standard spherical trigonometrical equations.

The great circle angle T between the satellite and ground station:

Azimuth A: Elevation e: where the Earth radius R=6378km and the satellite orbital radius (R+h) = 42164km.

For completeness, the slant range is given by:

The elevation and azimuth are converted to hour angle (HA) and declination (δ) through the relations: Calculations with an oblate Earth model show that the errors incurred from the spherical approximation amount to around 0.1 degree in azimuth and elevation and 10km in slant range (at mid-latitudes). This angular error is one fifth of the solar diameter and less than many station keeping regimes. In military satellites, which are often given a considerable non-zero inclination, the errors involved will be unacceptable, but for most commercial purposes, the approximation is valid.

3.3 Solar Locational Algorithms

Simple formulae for the solar declination and hour angle are specified in the Astronomical Almanac (1). The argument of these relations is a modified Julian day number, including a fraction representing the time of day. This particular day number is the number of days from the start of day zero in the year 2000. This can be computed quite readily using a modification of Sinnot (2) to a single line FORTRAN algorithm by Fliegel and Van Flandern. The result is expressed most simply by four lines of BASIC code:

	N=N-730516.5+D+367*Y+INT(275*M/9)         ... eqns 7

where Y is the four digit year (eg 1989), M is the month number (1-12), and D is the day and part day (ie time within the day as a day fraction) of that month. N may then be used as input to the following relations (1), which are all expressed using degrees (not radians!):

	Mean solar longitude L
	Mean anomaly G
	L and G must be reduced modulo 360 before substitution in:
	Ecliptic longitude Le
	  Le = L + 1.915*sin(G) + 0.020*sin(2*G)
	Obliquity of the ecliptic e
	  e = 23.439-0.0000004*N
	Right ascension (same quadrant as Le ) RA
	  RA = atan( cos e tan Le )
	Declination dec
	  dec = asin( sin e sin Le )
	The Equation of Time EoT
	  EoT(minutes) = 4 ( L - RA )              ... eqns 8

The equation of time is the time that the Sun is running fast or slow. That is, local noon will occur at 12.00 - E/60 hours local time. From this the Hour Angle of the Sun when observed at longitude G0 (degrees east) is given by:

The term within the parentheses is in hours, which is the more usual unit for hour angle. However, the use of an angular measure is more appropriate in this instance. UT is the Universal Time.

If required the apparent angular diameter of the Sun in degrees is given by:

where the term in parentheses is the normalised Sun-Earth distance.

The accuracy of the declination is 0.01 degree and the hour angle 0.03 degree between the years 1950 and 2050. The hour angle accuracy corresponds to a timing accuracy of around 10 seconds.

3.4 Range of Coincidence

To find the days on which an effect may occur, the equations (7) through (8) may be cycled until the solar declination is within a specified value of the satellite declination. The specified value is generally taken as half the sum of the antenna beamwidth (θ = 1.2 λ / de where de is the effective linear aperture) plus the solar diameter (equation 10 for the higher microwave frequencies - slightly larger (figure 3) at 1 GHz). On each of these affected days the equation of time at noon may be used to find the range of affected times (equation 9). One iteration to find a corrected EoT will be more than accurate enough for any application. The greatest inaccuracy lies in selecting the appropriate antenna contour (eg 3 dB beamwidth) for the computation. This can only be accurately done after computation of the maximum solar effect on the given system.

Dates of solar RFI
Figure 10 - Dates of maximum solar interference to
satellite signals from geostationary orbit.


4.1 The Central Beam - Depth

The maximum effect will occur if and when the Sun falls centrally within the antenna response pattern. The appropriate downlink equation can then be written (neglecting tropospheric attenuation, etc): where T is the system noise temperature. It suffices to note that the net effect is to raise the effective noise temperature by an amount: and thus the decrease in the carrier to noise ratio will be given by: Note that this is independent of operating frequency when expressed as shown (ie as a function of the system collecting area to temperature). The more commonly used G/T ratio can be computed from: Table 1 shows the dB decrease in C/N for typical A/T ratios (in dB metre squared per Kelvin) for the range of solar fluxes typically encountered from 1 to 20 GHz. Figure 10 shows the same data in terms of G/T for selected frequencies over a wider range of solar fluxes which could occur during bursts.

The above formula (eq 12) only holds when the antenna beamwidth exceeds the solar angular diameter. For a narrow beam system, the appropriate relationship is:

where Tsun is given by equation 4. The effect of going to equation 13 vice equation 12 is essentially a lowering of the magnitude of the solar increase. Note that it is preferable to increase A rather decrease T if a minimal solar effect is desired.

TABLE 1 - C/N decrease (dB) due to solar radio noise

 A/T                        Solar Radio Noise (SFU)                        
dBm2/K  50  100  150  200  250  300  350  400  450  500  550  600  650  700

-51.0  0.0  0.0  0.0  0.0  0.1  0.1  0.1  0.1  0.1  0.1  0.1  0.1  0.2  0.2 
-48.0  0.0  0.0  0.1  0.1  0.1  0.1  0.2  0.2  0.2  0.2  0.3  0.3  0.3  0.3 
-45.0  0.0  0.1  0.1  0.2  0.2  0.3  0.3  0.4  0.4  0.5  0.5  0.6  0.6  0.6 
-42.0  0.1  0.2  0.3  0.4  0.5  0.6  0.6  0.7  0.8  0.9  1.0  1.1  1.1  1.2 
-39.0  0.2  0.4  0.6  0.7  0.9  1.1  1.2  1.4  1.5  1.6  1.8  1.9  2.0  2.1 
-36.0  0.4  0.7  1.0  1.3  1.6  1.9  2.1  2.4  2.6  2.8  3.0  3.2  3.4  3.6 
-33.0  0.7  1.3  1.9  2.4  2.8  3.2  3.6  3.9  4.2  4.5  4.8  5.0  5.3  5.5 
-30.0  1.3  2.4  3.2  3.9  4.5  5.0  5.5  5.9  6.3  6.6  7.0  7.3  7.6  7.8 
-27.0  2.4  3.9  5.0  5.9  6.6  7.3  7.8  8.3  8.8  9.2  9.5  9.9 10.2 10.5 
-24.0  3.9  5.9  7.3  8.3  9.1  9.8 10.5 11.0 11.5 11.9 12.3 12.6 13.0 13.3 
-21.0  5.9  8.3  9.8 11.0 11.9 12.6 13.2 13.8 14.3 14.7 15.1 15.5 15.8 16.2 
-18.0  8.3 11.0 12.6 13.8 14.7 15.5 16.1 16.7 17.2 17.7 18.1 18.4 18.8 19.1 
-15.0 11.0 13.8 15.5 16.7 17.7 18.4 19.1 19.7 20.2 20.6 21.0 21.4 21.8 22.1 
-12.0 13.8 16.7 18.4 19.7 20.6 21.4 22.1 22.6 23.2 23.6 24.0 24.4 24.7 25.1 
 -9.0 16.7 19.6 21.4 22.6 23.6 24.4 25.1 25.6 26.1 26.6 27.0 27.4 27.7 28.1 
 -6.0 19.6 22.6 24.4 25.6 26.6 27.4 28.0 28.6 29.1 29.6 30.0 30.4 30.7 31.1 
 -3.0 22.6 25.6 27.4 28.6 29.6 30.4 31.0 31.6 32.1 32.6 33.0 33.4 33.7 34.0 
  0.0 25.6 28.6 30.4 31.6 32.6 33.4 34.0 34.6 35.1 35.6 36.0 36.4 36.7 37.0 

Solar RFI magnitude
Figure 11 - Showing C/N decrease as a function of solar noise output
for three cases: (1) L-band mobile with G/T = -16 dB/K, (2) C-band 3.5m
aperture with 60 K temp, and (3) Ku band 2.2m aperture with 250 K temp.

4.2 Antenna Polarisation

Solar flux values are reported for the total solar radio output at a particular frequency. This emission is mostly randomly polarised, although both left and right hand circular polarisation burst components are seen around active solar regions with large magnetic fields. However, the non-burst components are generally random.

If a receiving antenna has circular polarisation, either left or right hand, it will receive the full solar flux. If it uses linear polarisation, however, it will only collect half the reported flux value, and thus S/2 should be used in all equations instead of the full flux density S.

4.3 The Peripheral Beam - Duration

The first order calculation of the duration of the solar effect might use the antenna 3 dB beamwidth. For a central traverse of the Sun through the antenna, the duration of the effect is then equal to the sum of the beamwidth (θ) and the solar diameter (Πs) times the rate of movement of the Sun in hour angle (4 minutes per degree). Table 2 shows this duration. For an off centre traverse, the time is given by: where the output of the inverse cosine function must be expressed in degrees, and where Δδ is the declination difference between the Sun and the antenna beam.

If the solar noise increase is significantly larger than the downlink C/N margin, then the 3 dB beamwidth may not be appropriate. An acceptable contour must then be chosen with regard to the fraction of the solar disc covered by this contour at each moment of the traverse. Inaccuracies in antenna pointing and errors in the knowledge of the exact solar output add to the complexity of this type of calculation.


Antenna                 Receiving                       
Diam                    Frequency (GHz)                 
 (m)     2    4    6    8   10   12   14   16   18   20 
 0.5   86.2 44.3 30.3 23.3 19.1 16.3 14.3 12.8 11.7 10.7 
 1.5   30.3 16.3 11.7  9.3  7.9  7.0  6.3  5.8  5.4  5.1 
 2.5   19.1 10.7  7.9  6.5  5.7  5.1  4.7  4.4  4.2  4.0 
 3.5   14.3  8.3  6.3  5.3  4.7  4.3  4.1  3.8  3.7  3.5 
 4.5   11.7  7.0  5.4  4.7  4.2  3.9  3.7  3.5  3.4  3.3 
 5.5   10.0  6.2  4.9  4.2  3.9  3.6  3.4  3.3  3.2  3.1 
 6.5    8.8  5.6  4.5  4.0  3.6  3.4  3.3  3.1  3.1  3.0 
 7.5    7.9  5.1  4.2  3.7  3.5  3.3  3.1  3.0  3.0  2.9 
 8.5    7.3  4.8  4.0  3.6  3.3  3.2  3.0  3.0  2.9  2.8 
 9.5    6.8  4.5  3.8  3.4  3.2  3.1  3.0  2.9  2.8  2.8 
10.5    6.3  4.3  3.7  3.3  3.1  3.0  2.9  2.8  2.8  2.7 

4.4 Summary

For a given collecting aperture and noise temperature, the effect of the Sun on a system increases with frequency (because of the increasing solar output). Small aperture systems are relatively unaffected by normal solar noise, although at times of high solar activity, all satellite downlinks are susceptible, and then small apertures will be affected for a longer duration. At Ka Band a relatively small aperture (eg 2m) has a beamwidth less than the solar diameter and the total solar radio energy collected then starts to decrease - in most cases, the link margin for tropospheric attenuation (precipitation) will cope with the solar traverse. In summary, it may be concluded that moderate apertures at mid-microwave experience the most significant reduction in carrier to noise ratio (with respect to the duration-depth product).

The time extent of Sun-outages and their predictability makes it relatively easy to reduce the limited impact that they cause. Alternate satellite paths, if available, obviate the problem in most cases. The one exception may prove to be mobile L-band downlinks. Very limited aperture and low system noise together with limited link margins could make these systems quite susceptible to the frequent solar bursts that occur at times of high solar activity.


Any high gain low noise system pointing near the Sun will be subject to interference. The USA Federal Aviation Authority receives advice on solar bursts for the aviation emergency channels of 121.5 and 243 MHz. Sun synchronous polar orbiting satellites are particular liable to experience several consecutive days of passing in front of the Sun. Military early warning radar is susceptible to 'solar jamming'. Historically, solar radio emissions were first recognised as such through jamming of the British coastal home chain radars during World War II.


In the inverse of the problem just discussed, a known solar radio output on a specific frequency may be used to measure the G/T of a microwave receiving system.

The sun is allowed to drift through the antenna beam, and the increase in noise power is determined. This is most accurately accomplished through the use of an attenuator in the RF line to reduce the peak solar noise back down to the level of the normal system noise. This noise power increase β(dB) may then be used to determine the system G/T through the expression:

The only precaution to be taken in this measurement is to make sure that any burst component is absent. Two successive measurements, equal within experimental error will suffice to establish this condition. IPS Radio and Space Services in Australia can provide near real-time values of solar flux and advise on burst activity. Remember that if the antenna under test is linearly polarised then the value of solar flux S should be reduced by 50%.


  1. Anawalt RA and A Boksenberg, The Astronomical Almanac 1989, USGPO (Washington) & HMSO (London), 1988
  2. Sinnot RW, "Taming Our Chaotic Calendar", Sky and Telescope, v70(#5), pp454-455, 1984
  3. Smerd SF, Annals of the IGY, v34, 1964
  4. Kundu MR, Solar Radio Astronomy, Wiley (New York), 1965
  5. US Department of Commerce, Solar Geophysical Data, various issues, Boulder Colorado, 1988

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