Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

What is the practical frequency limit for (larger) dish antenna?

Status
Not open for further replies.

Darktrax

Full Member level 5
Joined
Nov 28, 2009
Messages
287
Helped
103
Reputation
206
Reaction score
100
Trophy points
1,323
Location
UK
Activity points
5,268
Assume fine engineering, with suitable shaft encoders good to about 300 milli-arc seconds (22 or 24bits for a circle), and servo-positioning systems that will point to (estimated) 0.02 degrees, or maybe better. The dish would be (say) dual-shaped Cassegrain, with surfaces measured and aligned using photogrammetric kit that can show 0.1mm deviations.

The surface(s) would be well into the diffraction limit, better than 1/20th wavelength. To keep the losses from surface RMS low, the profiles are within 0.3mm, and the actual surface is smooth.
Come to that - simply assume the physical stuff is good enough!

The feed horns are built as profiled corrugated, long enough to equalize the E-field and H-field patterns over the illumination angles of interest, made for low sidelobes, and the circular polarized axial ratio for the whole kit including polarizer performance would be in the range 0.55dB to 0.7dB

Of course, as the dish size increases, so does the gain, so one may simply use smaller dishes. This game is somewhat offset by atmospheric, water absorption and other loss conditions, but the quest for more gain comes from wanting to use ever higher data rates, which spreads the spectrum, sometimes to more than 500MHz. Some will use 3 separate modulated channels. The profligate expenditure of bandwidth also lowers the signal level, hence let's have more gain!

When the frequency gets to (say) about 30GHz, and the dish size is maybe 6.8m, or 7.6m, or 9.0m the beam-width of the radiation pattern becomes tiny.

As example, using a (quite good) beam-width estimator -3dB width (deg) =68.577*299792458 / (<Freq GHz>*1000000000*<Diameter-in-metres>),
we get only 0.09 degrees for a 7.6m dish at 30GHz. Dual-shaped optics have half-power beam widths a bit smaller, but we go with this for now.

I am not sure that the intangible radiated RF signal beam shape equipotential surface in the space above the dish can be as refined and accurate as a precision machined bit of mechanicals!
Such a beam is needle-like pointy, and coming from device bigger than the width of many living rooms and even the motor mounts are maybe 6m high. Even a tenth of a degree off boresight sees half the signal power drop away. Yes-yes, one can do tracking in all sorts of ways, but there is surely a practical limit!
(Did I mention that the target is a low-earth orbit satellite)?

What I face is assertions that "This is done routinely, the world over, all the time", followed by requests for estimated of design performance of systems that I think may simply be unrealistic!
Is this really true? When I search the Internet, I have to say that I do not think this stuff is really there unless very specialized. Big radio telescopes use smaller sections of the surface to illuminate at higher frequencies.

One can find (one) 9.0m product that claims Ka-Band, but here we fall to some industry practice of loosely referring to K-Band around (18-22)GHz downlinks as "Ka-Band" because usually it comes with the uplink at (27.5-31)GHz. Ka-Band is properly (26.5-40)GHz

So - hoping to find some experts out there who know better, if anyone is sure that frequencies like 30GHz should be routinely offered at dishes larger than 3m or 4m, I am happy to listen. My every instinct says otherwise, but all I get is that I am "stuck in the past", and that "these days" this sort of thing is taken for granted. Am I so wrong?
 

machining tolerances eventually come into play. and even things like partial heating in the sunshine and gravity drooping can distort a big antenna
 

machining tolerances eventually come into play. and even things like partial heating in the sunshine and gravity drooping can distort a big antenna

Thanks for reply (biff44)

There is at least one extraordinarily precise, (and wildly expensive!!) large antenna out there (9m) that has mechanicals it can point to 0.0048 degrees when on the move.
I don't know how good it is for thermal expansion. Pointing droop is driven out by the servos in all products, and gravity distortion group is taken care of by carbon fibre design.

Even so, for all that, my point was that the RF radiation pattern - a set of field strength values in empty space .. is not the same as a precision machined surface.
For me, the fact that it gets so narrow and pointy that (say) a 5m dish at 30GHz is at half power (ie. -3dB) for only 0.07degrees away from the main axis makes the whole notion that dishes at these frequencies can be larger. My guess is that about 3m is near a practical, realistic limit.

But - I was hoping to find out I was wrong!
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top