What is the perfect VSAT antenna?

Here at SatProf we’ve had the privilege of being associated with many antenna manufacturers over the decades.  We’ve worked with brilliant antenna design engineers and manufacturing process people, but now that our focus is on training, we can offer a fresh perspective on what design features make a difference in minimizing interference and making field technicians more productive.

So based on experience in engineering and training people to point VSAT antennas for least interference, we have compiled this list of features that we would like to see in all small, fixed VSAT antennas.  We welcome your comments!

RF PERFORMANCE

1. Of course, the antenna must meet sidelobe requirements.  If it can meet both ITU and FCC curves, so much the better.  If the pattern is specified at pointing accuracy limits (as it should be), then “pointing accuracy” must be defined in the context of a specific procedure that the field tech will actually be instructed to use – not simply “10% of beamwidth” or other arbitrary angle.

2. TX and RX efficiency should be “normal” (around 65%) and TX gain and RX G/T should be close to typical.  That way there will be no unpleasant link budget surprises, even if the link budget engineer does not have the exact specifications of the antenna.

3. The antenna should have the relevant type approval.  The type approval should not only define the guaranteed performance of the antenna, but validate the manufacturer’s production process for consistent as-built quality. The GVF Product Quality Assurance program is currently the best (and only) applicable worldwide method for VSAT antenna type approvals.

POINTING CONTROLS

4. The pointing mechanism must have a fine azimuth adjuster.   Without one, on a small antenna (1.2m or less at Ku-band) it is almost impossible to prevent adjacent satellite interference because you can’t use the beam balance method (beam centering).  This is the single most serious deficiency we see in high-volume antennas.

5. Elevation and fine azimuth adjusters should have a turns ratio of about 1 degree of beam deflection per turn.  Much more and it is too difficult to count the fractions of turns during the beam balance method.  Much less and it takes too many turns to get the antenna pointed.

6. Some backlash in the fine az and el mechanisms is acceptable but should not be more than about 1 turn of the faster (2 turns max), and it should follow a predictable hysteresis loop.

7. Lockdown shifts (i.e. the radial angle change in beam direction when the fine azimuth or elevation lock fasteners are tightened) should be a fraction of the beamwidth.  This requirement can be relaxed somewhat for el shift due to tightening fine az locks, but in that case, instructions should be provided on the back of the reflector to tighten the fine az locks before performing accurate elevation pointing.  Coarse azimuth lockdown shifts (mast clamp) can be larger, but should not be more than about 0.5 beamwidths, otherwise the field tech may lose the signal when tightening the clamp and will have to hunt for it again with the el and fine az adjusters.

8. Elevation axis should have a built-in scale that is easily readable to 1 degree precision.  It should not be obscured by thick paint!  Accuracy should be +/- 1 or 2 degrees (assuming the mast pipe is perfectly vertical).

9. A built-in polarization (feed rotation) scale is not necessary, but if there is one, it should be readable to  1 degree precision, in order to allow accurate polarization preset.  It should not be obscured by thick paint!  Accuracy should be +/- 1 or 2 degrees (assuming the mast pipe is perfectly vertical).

10. Enable polarization to be adjusted and tightened from underneath.  The field tech should not have to reach into the high RF intensity zone above the feed.  Keep in mind that polarization will be adjusted while the terminal is transmitting.

11. Mast clamps must provide extremely high friction against the mast pipe when tightened with a normal socket wrench.  The azimuth beam angle should not shift more than say 0.1 degrees after a peak wind gust, which could be over 100 km/h.  Remember, after a storm has passed through an area, there is no way to know which antennas have shifted enough to cause ASI but not enough to cause noticeable signal degradation – especially since most VSAT modems will automatically increase the transmit power to compensate for pointing error, thereby making ASI even worse!

MECHANICAL DESIGN

12. A single person should be able to assemble the antenna without damaging it.  For example, a field tech should be able to attach the az-el head to the face-down reflector, then lift the assembly and drop it over the mast pipe.  The feed support and struts should be easy to assemble without risk of cracking or bending the rim of the dish.

13. Correct assembly should be obvious even if the manual is lost.  Struts, feed support arm, and feed assembly should fit together only one way.  It should be impossible to assemble the antenna with the feed optical center not exactly at the focal point.

14. The feed and reflector should have some mechanical features to aid the field technician in checking for accurate feed horn position.  For example, notches designed to accept a measuring stick tool could be embedded in the reflector and horn rim.  This feature would allow the field tech to check for problems such as dish damage or warping, incorrect struts, excessive BUC weight causing the feed supports to sag, etc.

15. No special tools should be required to assemble or adjust the antenna.  Minimize the number of tools needed.  A 1/2″ (13 mm) socket and open-end wrench, and adjustable (crescent) wrench, and some screwdrivers should suffice.  Fastener sizes should be selected so that the nearest SAE and metric tools will both work.  Make sure that the adjustable wrench fits into tight places where it is needed, such as polarization locking screws.

16. Fasteners should all be stainless steel with anti-seizing compound provided in the kit.

17. The feed window should be super hydrophobic and protected from precipitation, snow, and ice with a shroud.

18. The main reflector should painted with white dispersive paint (so it cannot focus visible and IR energy from the sun into the feed).

19. All parts and pieces of the antenna should be ultra violet radiation resistant.

20. Provisions should be provided for cable routing conduits between the feed system and the rear of the main reflector to protect the cables from UV, weather, and creatures, as well as providing some aesthetic appeal.

21. A sighting tool and associated mounting point would be convenient, to enable the field tech to optically check line of sight angular clearance after pointing the antenna.

LABELS

22. There should be a label that defines the offset angle of the antenna and the designated surface that the offset angle is defined from.    It does the field tech no good to be told that the offset angle is X degrees without saying where the inclinometer should be placed, such as on the reflector rim, the back surface, the az-el mechanism surface, the feed support arm, or somewhere else.

23. The feed system (OMT or horn) should have a clear label or marking indicating which is the transmit polarization axis.  Inexperienced field techs have a tough time trying to figure this out just looking at the waveguide geometry, especially if there is circular or square waveguide with WR-75 flanges (which are square).

24. Provide and mark a designated surface on the feed system for the field tech to use to place an inclinometer for accurate preset of polarization.

21. The weight limit for RF electronics should be indicated on a label.  Often the selection of BUC is made in the field, or at the staging depot.

22. The antenna manufacturer name and model number should be marked on a label together with any type approval numbers.  The field tech needs this information when he/she calls the satellite operator to request permission to activate the terminal.

23. There should be a sticker on the antenna warning NOT to paint or place signs/stickers/decals on the reflector surface.

24. There should be a label to record the field technician’s name and GVF certification number, the date of installation, contact information for the service provider or maintenance organization, and other details that the end user may need.

 

Have we left anything out?  Or are we being too particular?  Please leave a comment with your thoughts!

One Response to What is the perfect VSAT antenna?

  1. Muhammad Iqtidar Iqbal says:

    Hi,

    If there is 0.1 degree error in Antenna elevation, is it acceptable? Would there be any impact on Antenna performance?

    Thanks,

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