Friday, February 13, 2015

Ka-band Airborne Antenna Figure of Merit (G/T)

Estimated Receive Performance
Circular and Square 24"x24" ESA
30"x6" rectangular horn array
Figure of Merit, or G/T, of an airborne antenna is a key scaler of the forward channel.

Downlink PSD is limited by regulatory or coordination considerations.

The actual dBW contour, whether spot beam or wide beam, whether beam peak or beam edge, is only a factor in capacity and economy.
see Forward Channel Downlink Regulations for a discussion on this point.
Symbol rate (or transponder bandwidth) is another key scaler, and one parameter most benefiting from high throughput satellites (HTS).

Receive performance is generally driven most by two antenna parameters: the gain (or aperture) and the level of noise inherent in the receiving system.

The noise temperature of the sky can be associated with elevation angle.

I have added a temperature rise starting at 30 degrees elevation through 10 degrees elevation.

I have assigned three different noise temperatures to the three antenna types.

The assumed temperatures are fabricated.

I assume the circular array offers the lowest temp.

There are some antennas steered using mechanical means and others using phase-shifting.

I am modeling the mechanically steered horn array at a lower temp than the electronically steered square aperture on the assumption the phase-shifting will be noisier.

A gimbaled horn array is mechanically steered so that the aperture is square to the illuminating satellite by design.

The gain of a horn array can be approximated by the gain of an equivalent reflector.

The beam pattern of a horn array can be approximated by a simple reflector of similar aperture (extent).

Side lobe can create additional sensitivities to neighboring satellites.

Tapering the array layout may provide some benefits in reduced side lobes, but also increase the main bore site beamwidth slightly.

Noise temperature is applied to the book gain to arrive at a figure of merit.

Skew effects do not change the gain of the antenna.

Skew effects do not change the noise temperature, albeit under some circumstances a high-axial beam pattern can become unexpectedly sensitivity to low elevation noise.

Skew effects to effect the level of interference, in that a broadening beamwidth along the GSO introduces adjacent satellite interference (ASI).

Interference effects are contingent on an interfering (overlapping) neighbor, which is not always the case.

Particular antenna design features may create higher noise temperatures depending on the implementation.

It is not possible to predict G/T with absolute accuracy.

Relative performance can be evaluated with arbitrary G/T.

The following table shows some idealized calculations for three receive antennas

  1. 24"x24" square electronically steerable array (ESA)
  2. 30"x6" gimbaled horn array
  3. 24"x24" circular ESA

The yellow shaded areas with beamwidths (HPBW) between 3 and 6 degrees are assumed to have strong coupling to two adjacent satellites.

The red shaded areas with beamwidths greater than 6 degrees are assumed to have strong coupling to four adjacent satellites.

I have heard modem suppliers suggest a 10 dB beamwidth is more revealing than 3 dB (HPBW).

Peter Lemme

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Check out these related topics

Link Budget Background, Definitions, and Assumptions
Rain Fade
Skew Angle and Effective Aperture of Airborne Antennas
Spot beams Vs. Wide beams Ku band

Forward Channel Considerations
Transponder Downlink Contours
Forward Channel Downlink Regulations
Ku-band Airborne Antenna Figure of Merit (G/T)

Return Channel Link Budget
Ku-band Return Channel Maximum PSD
Ka-band Return Channel Maximum PSD