Friday, November 17, 2017

Comparing Antenna Performance

While it is possible to get 2x as marketing touts, the ThinKom (2Ku) antenna achieves about 1.5 times the spectral efficiency as a competing 2-axis antenna along typical routes. The ThinKom antenna may perform even better in the tropics, where the elevation angles may be highest.

The ThinKom antenna can operate without any performance reduction due to skew angles that may be encountered in the tropic regions, unlike a 2-axis antenna which may suffer severe degradation.

Theoretically, these same benefits await other flat-panel antennas.  Practically, no other antenna has  made public, legitimate claim to a design matching instantaneous bandwidth, efficiency, robustness, and scan loss, in the same size package.  VICTS weakness is cost, weight, form-factor; all of which electronic phased array may have advantage. 

Figure of Merit (G/T)

A link budget offers a means to analyze receiving the signal of interest in the face of noise and interference, to account for spectral efficiency.

A receiver is specified by figure of merit, or G/T.
G is the effective gain of the receive antenna.
T is the noise temperature of the receiver.

G/T provides a scalable degree of performance of the antenna subsystem.

A larger antenna aperture (effective radiated area) has a higher gain, usually leading to a higher G/T. 

Aero antennas are hobbled by the need to remain low in profile. Most aero antennas use a steerable aperture on an azimuth/elevation 2-axis gimbal. These 2-axis antennas point the aperture at the satellite, providing maximum performance regardless of elevation or azimuth.  Their inherent need for a clear "swept volume" limits the size of their aperture.

An example of a popular 2-axis antenna is the KuStream 1000.

The KuStream 1000 shows a G/T of 11.8 dB/K.

ThinKom produces a VICTS antenna, KU3030, which uses two individual 30 inch diameter apertures, one for transmit, the other for receive.  This antenna is marketed by Gogo as 2Ku.  VICTS antenna uses a stack of platters that rotate independently around a single gimbal axis.  The effective radiated aperture is a function of satellite elevation, where it peaks at zenith (90 degree elevation) and falls off roughly as the sin(elevation angle).  The fall off is referred to as scan loss.

Ideally, at 30 degrees, the drop in G/T is -3 dB; at 20 degrees is -5 dB; at 10 degrees -8 dB.

G/T analysis provided by Bill Milroy, ThinKom
Thinkom Zenith G/T is 18.2; 30 degree is 15.2 (-3 dB); 20 degree is 13.5 (-4.7 dB); 10 degree est. 10 (-8.2 dB) 

ThinKom overlays another competing antenna which they modeled at about 11.5 dB/K.  The drop-off shown at low elevation has to do with increasing noise when pointing near the Earth's surface.

All antenna perform uniformly along azimuth. 

2-axis antennas perform uniformly along elevation whereas VICTS encounters scan loss.

VICTS skew angle effects are pronounced only while the satellite elevation is low whereas 2-axis antenna skew angle effects are uniform.

Elevation/Skew Along a Great-Circle Route

Flying a great-circle route between two city pairs creates a range of azimuth, elevation, and skew angles while targeting a geostationary satellite.  The position of the satellite (effectively the equatorial longitude) influences the range of angles.  

Flying from Boston to Los Angeles follows a path that reaches across the extent of Continental United States (CONUS). 

The average elevation along the route is 42 degrees and no skew angle issues are encountered if optimally positioning the satellite at 100W.

Flying north or south from the tropic regions into mid or extreme latitudes naturally limits skew angle that are encountered with reasonably positioned satellites.  In general, skew angle effects may be a problem mostly in the tropic regions. Skew angle effects in CONUS can be encountered if the skew limits are low (e.g. 35 degrees) and the satellite is poorly situated.

Flying from Los Angeles to Sao Paulo crosses the equator.

If optimally positioned at 70W, the average elevation is 45 degrees.  Skew angle ranges up to 90 degrees.

Skew Angle

Speaking generically, a 2-axis antenna encounters severe skew effects above a threshold skew angle.  

The forward channel (receive) suffers only where there are adjacent satellites with overlapping services.  

The return channel (transmit) suffers due to a regulatory spectral mask limiting interference to adjacent satellites. 

Areas of difficulty for 2-axis antennas are shown below as the red zone "bow tie".  The base of the interference is the satellite longitude. The threshold ranges from 35 to 60 degrees skew angle, where zero skew is due N/S along the satellite longitude and +/- 90 degrees is along the equator. 

The ThinKom antenna skew angle effects are a function of satellite elevation.  The lower the satellite elevation, the more pronounced the skew effect.  There is little skew effect while the satellite is at high elevations.

For generic reference, a fabricated 30" flat panel antenna is shown in comparison, whereby the red zone "ear-muffs" are limited to the outer (lower elevation) service areas. 

The route to Sao Paulo crosses the equator, while the satellite is high in elevation.  The 2-axis antenna may suffer from skew effects, while the ThinKom antenna will not.  The 2-axis antenna may encounter outages or greatly reduced performance, while the ThinKom antenna has no reduction.

Other routes could produce satellite and skew angle with less favorable combinations. It is possible to operate the ThinKom antenna in the tropics and not have any reduction in performance due to skew angle.  

Spectral Efficiency (bps/Hz)

Both the CONUS and equatorial routes have an average elevation around 40 degrees.  What benefit is that?

The rationale for satellite communication is to deliver bits per second.  The physics of satellite communication drives a received signal plus noise plus interference. 

Spectral efficiency (SE) is a useful factor.  SE is bits per second (information rate) divided by the spectrum (Hertz) occupied.

While link budgets are complicated, ignoring noise and interference allows the received signal C/N (Carrier/Noise) to be a function of G/T.  A 4 dB improvement in G/T can be reflected as a 4 dB improvement in C/N.

DVB-S2X is a current standard for encoding that realizes the following spectral efficiency as a function of C/N.  DVB-S2X improves upon DVB-S2 by 
  • allowing for spread spectrum to serve weak signals
  • more modulation and codings (modcod) to improve granularity
  • higher performing modcods to serve the most advantaged signals

Assuming 40 degree elevation, the ThinKom antenna has about a 4 dB advantage over a competing 2-axis antenna.

For a very weak signal (-4 dB), the improvement to 0 dB drives SE from 0.3 to 0.8 (2.6x)

For a weak signal (0 dB), the improvement to 4 dB drives SE from 0.8 to 1.3 (1.6x)

For a good signal (6 dB), the improvement to 10 dB drives SE from 1.8 to 2.8 (1.5x)

For a strong signal (10 dB), the improvement to 14 dB drives SE from 2.8 to 4 (1.4x)

Alternatively, one could surmise that strong signals are accompanied by high elevation angle, and weak signals by a low elevation angle.  In that sense.

very weak (10 degree)  -2 dB equates to -1.3x to -3x
C/N -4 to -6  (0.3 to 0.1)
C/N 0 to -2 (0.8 to 0.6)

weak (20 degree)  +1 dB equates to 1.1x to 1.2x
C/N 0 to 1 (0.8 to 1)
C/N 4 to 5 (1.4 to 1.6)

good (40 degree) +4 dB equates to 1.5x to 1.6x
C/N 0 to 4 (0.8 to 1.3)
C/N 6 to 10 (1.8 to 2.8)

strong (60 degree) +5 dB equates to 1.5x to 1.7x
C/N 5 to 10 (1.6 to 2.8)
C/N 10 to 15 (2.8 to 4.2)

Taking stock of both methods, the latter seems the most representative. 
CONUS elev 30-45 degrees --> 1.4x to 1.6x
Sao Paulo elev 30-90 degrees -->1.4x to 1.7x

Every method arrives at about 1.5x benefit.

Stay tuned!

Peter Lemme

peter @
Follow me on twitter: @Satcom_Guru
Copyright 2017 All Rights Reserved

Peter Lemme has been a leader in avionics engineering for 35 years. He offers independent consulting services largely focused on avionics and L, Ku, and Ka band satellite communications to aircraft. Peter chairs the SAE-ITC AEEC Ku/Ka-band satcom subcommittee, developing ARINC 791 and 792 characteristics and contributes to the Network Infrastructure and Interfaces (NIS) subcommittee developing Project Paper 848, standard for Media Independent Secure Offboard Network.

Peter was Boeing avionics supervisor for 767 and 747-400 data link recording, data link reporting, and satellite communications. He was an FAA designated engineering representative (DER) for ACARS, satellite communications, DFDAU, DFDR, ACMS and printers. Peter was lead engineer for Thrust Management System (757, 767, 747-400), also supervisor for satellite communications for 777, and was manager of terminal-area projects (GLS, MLS, enhanced vision).

An instrument-rated private pilot, single engine land and sea, Peter has enjoyed perspectives from both operating and designing airplanes. Hundreds of hours of flight test analysis and thousands of hours in simulators have given him an appreciation for the many aspects that drive aviation; whether tandem complexity, policy, human, or technical; and the difficulties and challenges to achieving success.

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