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What are ALL the potential signal loss mechanisms between the output of the earth station antenna and the input of the satellite receiver. With a bit of explanation for each if possible please.

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polarization loss

If the polarization of the two antennas are not identical, and they rarely are, then there's some loss. For linear polarization on each end:

$$ P_r \propto \cos(\theta) $$

Where:

  • $P_r$ is the received power, and
  • $\theta$ is the angle difference between the antennas.

If each antenna is circularly polarized in the same sense, then there is no loss, regardless of angle.

If each antenna is circularly polarized in opposite senses, then loss is theoretically infinite.

If one antenna is circularly polarized (either sense -- it doesn't matter) and the other is linear, then there is a fixed 3 dB loss regardless of the rotation of the antennas.

While polarization loss can theoretically be infinite, in practice "infinite" means around -30 dB since no real antenna is exactly linearly or circularly polarized, and reflections off other objects (the ground, the satellite's solar panels, etc) mean at least some of the received energy is in a different polarization.

Faraday rotation can alter the polarization of the signal as it passes through the atmosphere, introducing polarization loss.

directional loss

Antenna gains are usually given in their direction of maximum gain. This is only valid for calculations if the antennas are aligned perfectly, which is rarely the case. The difference from the maximum, on-axis gain and the actual gain could be considered a "loss" in the calculation of a link budget.

The value of this loss depends on the particular antenna's pattern.

multipath interference

Although this isn't as much of a problem as it can be for terrestrial communication, there's at least the ground to consider. For highly directional antennas the ground reflections can be negligible unless the satellite is very near the horizon. For less directional antennas a simple but effective model for this loss is the two-ray ground reflection model.

Depending on the geometry of the satellite, there can potentially be significant reflections on that end, also.

Depending on how the geometry works out, this can be a gain, not a loss.

atmospheric absorption

The atmosphere absorbs electromagnetic radiation. The absorption is frequency dependent. It also depends on the position of the satellite: when the satellite is overhead the path must go though less atmosphere than when the satellite is at the horizon. The atmosphere changes with the weather, too.

Fortunately, satellite communication frequencies are selected in regions of the electromagnetic spectrum where absorption is usually negligible.

atmospheric reflection

The atmosphere can also reflect electromagnetic energy as well as absorb it.

At VHF and higher frequencies, the reflection is very slight and diffuse. This can be exploited for terrestrial communications and is called troposcatter.

At HF, reflection from the atmosphere can be very significant. When used for terrestrial communications, this is called skywave propagation.

If you wish to calculate these losses for purposes of space communication, you can research these propagation modes, keeping in mind what is a "loss" for terrestrial communication is energy that made it past the atmosphere, which is good for space communication.

Space weather can exacerbate these issues, sometimes severely.

inverse square law

Radio communications, just like light and all other forms of electromagnetic radiation, are subject to the inverse square law:

$$ P_r \propto {1 \over r^2} $$

where $r$ is the distance between the receiver and transmitter. In other words, for each doubling of the distance, power is divided by 4.

other

Of course, we can't enumerate every possible source of loss. Possibly significant:

  • birds landing on the antenna
  • airplanes flying in the way
  • ice on the antenna
  • heavy clouds
  • dust storms
  • trees
  • wind messing with antenna alignment
  • space weather and its many effects on the atmosphere
  • supervolcanos

summary

Path losses for practical space communications are almost entirely a function of the inverse square law, antenna gain, and antenna alignment.

Multipath interference can be a problem sometimes, if the antennas are not sufficiently directional to be isolated from nearby reflective surfaces (notably the ground).

The other sources of loss are small under normal conditions and complex to calculate. For amateur needs it's sufficient to add a few dB of margin in the link budget to account for these things. People investing millions of dollars in launching satellites perform extensive studies of space weather and other stochastic factors and calculate an acceptable margin based on a specified probability of failure.

further reading

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