Schneier on Security

Clive Robinson • July 14, 2015 2:37 AM

One of the problems with TORUS antenna systems is maintenance.

Satellites orbits change and decay at different rates, every few months you have to realign the feeds, and with new sats coming into service it sometimes requires the reflector to be moved.

This takes betwen two and seven days to do by a specialist crew of techs. Thus the “up-time” compared to individual dishes is not good. The obvious solution is to install two TOROUS dishes and interleave maintainance but it appears not to have been done.

Other disadvantages are the minimum angle of beam seperation, it’s a function of the size of the feed points and their distance from the dish. It’s around 2 degree which is a little on the large side as geo sats can be less than one degree seperated.

So the standard parabolic dish is not going to be going out of service at these sites for the foreseable future.

Also it’s a passive antenna system, there are active fully electronic “phase array” antennas that can be used which allow for finer angles of seperation, but only a small number of beams. Then there are “fractal” antennas which can do some fairly amazing tricks, if you can get the maths right. The most common use for these is in mobile broadband dongles and the like.

Clive Robinson • July 14, 2015 6:13 AM

@ Wael,

Now that’s a type of antenna I didn’t know of! Live and learn!!!

You’d be suprised where they turn up, and for what purposes they can be leveraged.

There’s an interesting introduction to the subject of “introducing fractals to other scientists and engineers”,

http://classes.yale.edu/fractals/

Have a look at chaos, 1/f noise, music, poetry and the prisoners dilemma problem.

Somebody I know who is involved with certain types of engine for space vehicles has told me about how fractal solutions help deal with certain mixing issues at injector plates and the like.

Even the design of nuclear weapons to design solutions for the implosion charges etc.

As for the 1/f and chaos relations it brings fractals into the analysis of “random” which brings us into the security engineering field.

In some respects fractals, cellular automata, and state machines can be seen in the same or similar light, which raises other questions about computation etc.

Yet for all that fractals appears not to have become a field of it’s own… maybe it just needs a little more time.

Clive Robinson • July 14, 2015 7:59 AM

@ Les,

Isn’t this basically the same tech that millions use for satellite TV? Many providers use more than one satellite

It depends on how you look at it.

Technically they are quite different.

What you describe is “offset feed” to a normal parabolic dish. That is the dish is parabolic in both the vertical and horizontal directions and thus has a tight beam with high gain. What you do is you use a dish with lower gain and broader beam and position the beam between the two satellites and then “squint” the two feeds on the sides of the beam to get the two satellites.

The torous antenna is parabolic only in the vertical direction and circular in the horizontal direction. Thus it looks like a slightly flattened segment cut out of a doughnut. Thus the beam is tightly focused in the vertical plane but very broad –upto 90degrees– in the horizontal direction.

However there is a price to pay for the torus design, in that whilst the wave front is uniformly in phase from the vertical component, it is not from the horizontal component. There is only a limited amount of correction you can do at the feed point to correct this, so often the feed is designed to see a vertical slot not a circle, thus it is of much lower gain than would be possible if the phase distortion could be better dealt with.

There are now designs of dish that are kind of a half way house, in that they use different parabolic profiles in the vertical and horizontal directions which give a wider beam in the horizontal direction, but with much less in the way of phase issues, that can be corrected much better at the feed point and thus better utilize the surface of the reflector.

But all such beam broadening systems have issues with some types of polarized signals.

Clive Robinson • July 15, 2015 3:11 AM

@ Curious,

I guess HFDF that was mentioned above is similar to something I vaguely recall having read about WW2, with “huff duff” systems, ‘high-frequency direction finding’, radio direction finding.

Yes that’s the beasty.

However it has it’s problems….

A simple HF DF antenna is a shielded magnetic loop used with a vertical element, you can see them on ships. The problem is that the resulting shape is a cardoid and is only directional at the null. They have to be rotated to work and are about the upper limit on size and still have them roubust enough to survive.

The next type of DF antenna is either a “frame” or “Adcock” and these have similar “null” directionality issues, only the rotating part they use is a crossed coil commutator called a goniometer.

Basicaly these smaller than a wavelength DF antennas don’t give very accurate directions, to within 10degrees is considered good, thus they are used for search and rescue type activities where you use the direction to guide you to make a closer reading and so on untill you get within visual range (look up “Ham foxhunts” for more details).

To get bearings of a degree or better you need a physically large antenna that is either real or synthesized. Have a search for “RAF Chicksands elephant cage” to get old photos and ground plots of one the former. To synthesize you can make a “Very Long Baseline” (VLB) array of physicaly small antennas and then using “phase selection” techniques often by modern DSP systems get an accuracy that is related to the length of the VLB. Such systems are common in radio astronomy, and of recent times have become the subject of “amateur astronomer” construction.

During WWII the Germans used a synthetic system in reverse, to generate very narrow radio beams for aircraft to fly along. The trick they used was to have two HF directional antennas with a 15-20 degree wide beam width and use one to transmit “dashes” whilst the other transmitted “dots”. The antennas were sighted and pointed such the the two beams just overlapped at the edges. The result was a pilot flying across the beam would hear dots, then a continuous tone then dashes, so could turn the aircraft and fly up the beam in a snake like slithering pattern till another beam cut across with a different set of tones to signify when the bombs should be dropped. The British ended up using a much simpler and more acurate system using time based ranging called “Gee”. Later systems used the phase difference between multiple harmonic frequency related transmitters, sufficient to locste “lobster pots” in the north sea. Aircraft however used a more interesting system called VOR where the output of a transmitter was both phase and frequency modulated and fed into a phased antenna system such that the phase relationship between the AM and FM modulation gave a direct indication of your bearing from the transmitter. The latest is of course GPS which again uses time differences, but for various reasons has to user Einsteins theory of relativity to correct for the accumulated time difference on the satellites to maintain the positional accuracy.