teoría sat-codan

7/27/2019 Teora sat-codan

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This series of slides provides explanations for terms frequently used in satellite

communications.

Where applicable, the formula related to a term is included with the explanation.

k:\sattrng\slides\terms.ppt

Issue 1, page 1

Definitions of Satellite

Communications Terms


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Generally, IF signal levels are measured in dBm. For example, the power level at

the IF output of a modem may be in the range -20dBm to 0dBm (0.01mW to

1mW).

HPA output power levels are often measured in dBW. A 20W SSPA, for

example, has a maximum output power capability of +13dBW (20W). This may

also be expressed in dBm and is simply calculated by adding 30 to get +43dBm.

Issue 1, page 2

Power dBm the ratio (in dB) of power relative

to 1mW

0 dBm = 1mW

dBm = 10Log(P) or

P = 10dBm/10 where P is power in mW

dBW the ratio (in dB) of power relative

to 1W

0 dBW = 1W dBW = 10Log(P) or

P = 10dBW/10 where P is power in Watts

0 dBW = +30 dBm


3/12

An Isotropic antenna radiates the power fed to it uniformly in all directions. An

antenna with gain radiates more power in some directions than others.

The gain of an antenna is the ratio (in dB) of the power radiated in the direction

of interest to the power that would be radiated in the same direction by an

isotropic antenna. By definition, an isotropic antenna has 0 dBi gain.

For satellite dish antennas, most of the power is concentrated in the main beam

of the antenna radiation pattern. The beam width of an antenna is the width of

the main beam (in degrees) at the point where the gain has fallen by 3dB from the

peak value. Typical values are 1.5o for a 2.4m antenna and 1.0o for a 3.8m

antenna.

Some residual power is also radiated in the side lobes. Antenna gain and side

lobe performance is a function of antenna diameter and antenna geometry (i.e.

prime focus, offset fed, Gregorian etc). Side lobes are an unavoidable property of

antennas and cannot be completely suppressed. Side lobes can be minimised by

proper design and, more importantly, by correct antenna feed alignment during

installation. The manufacturer often provides alignment jigs for this purpose.

Correct feed alignment also maximises antenna gain.

The side lobe characteristics of earth station antennas is one of the main factors

determining the minimum spacing between satellites and therefore orbit and

spectrum utilisation efficiency.

Issue 1, page 3

Antenna Gain Expressed in dBi

(dBs relative to the the gain of an isotropic

antenna)

The gain figure of an antenna is only

applicable to a particulardirectionand

frequency

Typical figures: 2.4m C-Band antenna - Tx Gain: 42 dBi, Rx Gain: 38 dBi

3.8m C-Band antenna - Tx Gain: 46 dBi, Rx Gain: 42 dBi


4/12

The noise temperature of an antenna varies with antenna diameter, elevation

angle and antenna polarisation:

The larger the antenna, the lower the noise temperature.

At higher elevation angles, the noise contribution from the ground is less as the

side lobes no longer point at the ground.

Circularly polarised antennas have noise temperatures several Kelvins lower

than those of linearly polarised antennas.

Typical clear sky noise temperatures for linearly polarised antennas are shown

below (circularly polarised antennas are about 2 K to 3 K lower).

Elevation angle 2.4m 3.8m

10o 52 K 31 K

20o 46 K 25 K

30o 45 K 21 K

40o 44 K 21 K

Note that it is incorrect to say degrees Kelvin or write oK. The units of noise

temperature are simply Kelvins!

Issue 1, page 4

Antenna Noise Temperature

It is the measure of all the external noise

collected by a receiving antenna

Measured in Kelvins (K)

Noise sources include Cosmic noise (from the sun, moon, radio stars e tc)

Ground noise (from noise energy radiated from the soil - the

smaller the side lobes in the direction of the ground, the lowerthe ground noise)

Miscellaneous sources (losses, cross-pol leakages)


5/12

When considering the power radiated in a particular direction from an antenna,

the EIRP is the power that would need to be fed into an isotropic antenna (i.e. one

that transmits power uniformly in all directions) to get the same signal strength inthat direction.

By definition, an isotropic antenna has 0 dBi gain.

Issue 1, page 5

EIRP(Effective Isotropic Radiated Power)

Used to indicate the power transmitted from

an antenna

EIRP = Power + Antenna Gain

Both EIRP and Power expressed in dBW

Antenna Gain expressed in dBi


6/12

LNA noise temperatures have been steadily decreasing as advancing technology

provides better HEMT GaAs FET devices for use in LNA input stages.

This reduction in noise temperature has made the measurement of LNA noise

temperature a more complex task. Previously, Noise Figure meters were able to give

sufficiently accurate measurements, but now Hot/Cold Noise source equipment with

liquid nitrogen cooling is required.

Noise temperature is calculated on the basis that a resistor generates noise, the level

of which is dependent on its temperature (measured in Kelvins*). The noise

temperature of an LNA is the temperature a terminating resistor would have to have

if connected to the input of the LNA (assuming it does not generate any noise) to

result in the same noise power at the LNA output.

LNA noise performance is sometimes specified in terms of Noise Figure which is

measured in dB. To convert from one to the other, use the following:

NF = 10Log(1 + T/290)

T = 290 (10NF/10 - 1) NF in dB, T in Kelvins

e.g if T = 45 K, then NF = 0.63 dB

*Note: 0 K = -273oC (absolute zero)

290 K = 17oC (about room temperature - in Adelaide!)

Note that it is incorrect to say degrees Kelvin or write oK. The units of noise

temperature are simply Kelvins!

Issue 1, page 6

LNA Noise Temperature

Is measured in Kelvin (K) - the lower the

better!

Is a measure of the amount of noise generated

by the LNA

Typical performance: 30 K to 70 K

Isolated LNAs have higher noisetemperatures or cost more for the same

performance (but have better input VSWR)


7/12

The earth station G/T determines the received carrier to noise ratio. Increasing

station G/T will increase the C/N of the received carrier.

For systems without a TRF, the system noise temperature is calculated as

follows:

TSYS = TA + TLNAwhere TA is the antenna noise temperature and

TLNA is the LNA noise temperature

If the system includes a TRF, the degradation due to the loss in the TRF must be

taken into account as follows:

TSYS = TA + TTRF(10L/10

- 1) + TLNA

where L is the loss of the TRF (in dB) and

TTRF is the actual temperature of the TRF in K (typ 298 K, 25oC)

For example, if a system includes the following components:

Antenna: TA = 44 K, G = 38 dBi (for a 2.4m antenna at 40o elevation angle)

LNA: TLNA = 45 K

TRF: L = 0.1dB, TTRF = 298 K

then: TSYS = 44 + 7 + 45 = 96 K

and: G/T = 18.2 dB/K

If the TRF is omitted, then G/T = 18.5 dB/K

(i.e. the 0.1dB loss in the TRF degrades G/T by about 0.3dB)

Issue 1, page 7

G/T

(Gain To Temperature Ratio)

G/T = Antenna Gain - 10 Log(Sys Noise Temp) Antenna Gain in dBi

System Noise Temperature in K

G/T is the Figure of Merit for an earth stationand is expressed as dB/K (dB per K)

The higher the better - G/T can be raised by using

a higher gain antenna or a lower temperature LNA


8/12

When interconnecting transmission systems, it is assumed that the impedances of

the two systems are exactly matched (i.e. are the same). In practice, this is not

the case as it is impossible to manufacture equipment that has input and output

impedances which are 100% accurate.

VSWR and Return Loss are both measures of how close to ideal the actual

impedances are. Both VSWR or Return Loss can be used to describe impedance

matching at any frequency, but VSWR is usually used to measure matching at

microwave frequencies while Return Loss is commonly used to measure

matching in IF systems .

If the impedances are not matched (i.e. high VSWR or low Return Loss),

standing wave patterns are created. If the transmission path is long enough (as

can exist in IF and RF cables) the frequency response may no longer be flat and

may exhibit ripples across the band. Ideal matching results in a VSWR of 1:1 or

an infinite Return Loss (i.e. infinity dB!).

Equipment such as SSPAs and converters have input and output VSWR figures

of 1.3:1 to 1.5:1. Non-isolated LNAs may have an input VSWR as high as 2.5:1,

however this is not usually a problem as the signal path length is very short (the

LNA is directly connected to the antenna) and ripples due to the poor VSWR are

very small. Return Loss is measured in dB and typical figures are 15dB to 26dB.

To convert from VSWR to Return Loss and vice versa, use the following:

RL = 20 Log VSWR+1 VSWR = 10RL/20 + 1VSWR -1 10RL/20 - 1

Issue 1, page 8

VSWR(Voltage Standing Wave Ratio)

VSWR is a measure of the accuracy of

impedance matching at a point of connection

VSWR is expressed as a ratio e.g. 1.3:1

A perfect match is a VSWR of 1:1

VSWR is usually used at microwave

frequencies Return Loss (expressed in dB) is most

commonly used at IF frequencies


9/12

OPBO and IPBO are commonly used to determine the operating levels in a

satellite transponder TWTA.

OPBO is also used when calculating intermodulation distortion levels at the

output of SSPAs and TWTAs.

For SSPAs, the OPBO reference point is the 1dB Gain Compressed Power,

while for TWTAs, it is the saturated output power. Intermod figures are often

given in as plots of the intermod levels vs the OPBO.

The use of IPBO and OPBO for satellite transponders avoids the need to worry

about actual signal levels. For example, the satellite operator simply specifies thetransponder saturated output power and the satellite receiving antenna input flux

density (Saturation Flux Density, SFD) to cause saturation. When doing link

calculations, the flux density at the satellite is calculated and this is then used to

determine the IPBO (i.e. difference between the actual flux density and the SFD).

Using data provided by the satellite operator, the OPBO can then be calculated

which can then be used to directly calculate the actual satellite EIRP.

It should be noted that there is not necessarily a one-to-one correspondence

between the IPBO and the OPBO because the input level vs output level response

of any amplifier is non-linear near maximum output (i.e. they exhibitcompression). This effect is greater for TWTAs than SSPAs.

Issue 1, page 9

OPBO, IPBO Output Back-Off

The level of a signal at the output of an amplifier

relative to the maximum possible output level

e.g. if the maximum output level is +40dBm and the

measured output level is +34dBm, the OPBO is 6dB

Input Back-Off

The level of a signal at the input of an amplifier relative

to that level at the input that would result in themaximum possible output level

e.g. if an input level of -20dBm causes max output and

the actual input level is -25dBm, the IPBO is 5dB


10/12

When a signal is transmitted from an antenna, it spreads out and the original

power is distributed over a larger area. At a distance from the antenna, only a

fraction of the original transmitted power will be received by an antenna.

The amount of power received depends on the size of the antenna (in particular

the area of the antenna intersecting the beam) and the amount of power in that

area (measured in watts per square metre or, more commonly, dBW/m2).

Saturation Flux Density is the flux density (i.e. signal power ) required to saturate

the transponder (i.e. cause maximum transponder TWTA output power).

Saturation flux density is a parameter specified by the satellite operator and

varies across the satellite reception beam area. It is, in effect, a transpondergain parameter since it indicates the output power that will result with a given

input signal level.

The satellite operator can usually change the gain of a transponder (via its on-

board attenuators) in order to suit different system requirements (e.g. a

transponder used for SCPC signals from small antennas is usually given higher

gain than one used for TV broadcasting - this reduces the earth station EIRP

requirements from these small earth stations). The different gain settings then

result in different values of saturation flux density.

Issue 1, page 10

Saturation Flux Density Flux density is a measure of signal strength at

a point in space and is measured in Watts/m2

or dBW/m2

Saturation Flux Density

Usually applied to signals received at a satellite

It is the flux density required to saturate a

satellite transponder TWTA Flux density may also be used to calculate

output power from an earth station receiving

antenna


11/12

The polarisation of an RF wave is used in satellite systems to separate two signals at the same

frequency and allows frequency reuse in satellite systems.

Because antennas can be constructed to receive and transmit a specific polarisation, by usingLHCP and RHCP (or horizontal and vertical) polarisations, only a signal of the required

polarisation will be received (or transmitted). The signal of the opposite polarisation will be

rejected (or not transmitted) even if it is on the same frequency.

The polarisation of an RF wave in space is defined by the orientation of the electric vector (E) of

the wave. This vector - which is perpendicular to the direction of propagation - can vary in both

direction and intensity during one RF cycle T (=1/f). That is, the E vector can both rotate and

vary in intensity.

Ideally in a circularly polarised wave, the E vector does not vary in intensity as it rotates (i.e. the

tip of the E vector traces a circle). If it traces an ellipse, the ratio of the maximum and

minimum values of the ellipse (i.e. the ratio of the major and minor axes) is called the Axial

Ratio (AR). When AR=1, the wave is perfectly circularly polarised and an antenna designed to

receive a LHCP signal will not receive a RHCP signal and vice versa. When AR


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