Figure 6 is also a portion of Figure 4, illustrating that increasing the physical size of the antenna offers an advantage only at the lower frequencies if operational or other requirements establish a minimum beamwidth.

Figure 7 illustrates the frequency dependence of available signal power if directive antennas are used at both terminals. Note that available power increases with frequency.

Figure 8 illustrates the leveling off of available signal power at lower and lower frequencies as physical size of the earth terminal antenna increases with an operational or other limitation of antenna beamwidth.

Figure 9 illustrates that available signal power levels off at higher and higher frequencies as operational or other factors decrease the required or available beamwidth for an antenna of fixed physical size.

Figure 10 illustrates that a plateau in the frequency range develops if both terminals have maximum antenna size and minimum antenna beamwidth: limitations. Note that for fixed minimum beamwidth limitations the plateau shifts to lower frequencies as antenna sizes increase. Antenna sizes and beamwidths may be selected to narrow the plateau until available signal power is maximum at a discrete frequency.

Figure 11 illustrates shift of the plateau to the higher frequencies if antenna physical sizes are fixed and beamwidth limitations are reduced.

Signal Absorption in The Atmosphere:

Figure 12 is a nomogram to estimate atmospheric absorption of the signal as a function of frequency, terminal elevation and vertical reception angle. The nomogram is based on theoretical absorption in an atmosphere typical of Washington, D. C. in August. Values from this chart can be combined with chart 1 and charts 4 through 11 to estimate available signal power in the absence of rainfall. Additional theoretical and experimental work are necessary to more completely determine atmospheric absorption. This chart is a first approximation.

Figure 13 is a momogram to estimate signal absorption due to rainfall. These values should be added to those of Figure 12 to estimate total absorption during rainfall. The total absorption may be further combined with the free space available signal power from chart 1 and charts 4 through 11 to estimate available signal during rainfall. Estimation of absorption due to rainfall is complicated by variation of drop size distributions for the same rainfall rate and by turbulence which may produce a different water

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content in the air than indicated by surface measurements.

Figure

13 applies to a typical drop size distribution in steady rainfall. Vertical Angle to a "Stationary" Equatorial Satellite

Figure 14 is a diagram of vertical reception angles, measured above the ground, to an equatorial stationary satellite at 105° west longitude.

RADIO NOISE:

Figure 15 is a nomogram to estimate noise power at the receiver. If effective antenna temperature is known enter with this temperature and bandwidth. If effective, temperature is not known it can be estimated from frequency and vertical reception angle in the left hand portion of the nomogram.

Signal to Noise Ratios:

Figure 16 combines the data of the previous charts to illustrate the frequency dependence of available signals and noise in a simple satellite system. The orbit is 1000 kilometers from the earth, the earth terminal has a sea level location, the satellite has an isotropic antenna, the antenna at the earth terminal is limited to 20 meters in diameter and the minimum beamwidth is 0.2 degrees. Note the available signal starts to decrease between 5 and 6 Gc/s at all vertical angles and at the lower vertical angles starts to decreasé at even lower frequencies during heavy rainfall. The same general shape of the curve holds for a broad fixed beamwidth antenna on the satellite, e.g. 20 degree beamwidth for antennas one meter in diameter or larger. Available power will increase but frequency dependence is not altered.

Figure 17 illustrates available signal to noise in a more sophisticated satellite system using highly directive antennas in a 6000 kilometer orbit. Note that adequate signal power is extended to higher frequencies especially in absence of rainfall.

Figure 18 illustrates slightly different assumptions than those reflected in Figure 17.

Figure 19 illustrates available signal power in`an even more sophisticated satellite system using "stationary" orbit and extremely directive antennas. Note that available signal power remains adequate at even higher frequencies especially at vertical angles exceeding 5 degrees.

Figure 20 is the same as Figure 19 except the system has been further improved by the elevation of the earth terminal and its location in an area of "moderate" rainfall.

Conclusions:

(1) For all-weather unstabilized satellite communication systems,

available signal to noise will decrease as frequency is increased above about 6 Gc/s. The exact frequency is dependent upon maximum antenna size and minimum beamwidth limitations at the earth terminal.

(2) As systems become more sophisticated through stabilized satellites and ability to use narrow beam antennas the upper frequency limit increases.

(3) The upper frequency limit may extend to above 15 Gc/s for sophisticated systems if reception is not required at very low angles.

(4) Theoretical disadvantages at the higher frequencies estimated on the basis of clear channel operation may be offset by the increased likelihood of successful frequency sharing at these frequencies since:

(a) Sharper antenna directivity tends to reduce the
vertical angle at which interference or noise from the
earth will dominate the signal from the space vehicle;

(b) Sharper antenna directivity reduces the degrees in
asimuth from which interference is likely;

(c) Wider bandwidths available at the higher frequencies permit "spread spectrum" modulation techniques which promise gains in immunity to interference;

(d) Atmospheric absorption tends to reduce low angle interfering signals relative to the higher angle satellite signals.

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FREQUENCY

FIGURE 2