mjb@pnet02.cts.com (Martin Brown) (01/23/88)
I was working on a space station picture in Sculpt 3D, and in particular, a
parabolic dish antenna, which led me to wonder just how the parabolic formula
is derived for a particular dish. Is it a matter of frequency? distance to
source? both? Anybody familar with this stuff? Help and examples would be
appreciated! Thanx!
- Martin Brown -
UUCP: {ihnp4!scgvaxd!cadovax rutgers!marque}!gryphon!pnet02!mjb
INET: mjb@pnet02.cts.comshf@well.UUCP (Stuart H. Ferguson) (01/27/88)
In article <2252@gryphon.CTS.COM> mjb@pnet02.cts.com (Martin Brown) writes: >I was working on a space station picture in Sculpt 3D, and in particular, a >parabolic dish antenna, which led me to wonder just how the parabolic formula >is derived for a particular dish. Is it a matter of frequency? distance to >source? both? Anybody familar with this stuff? Help and examples would be >appreciated! Thanx! > - Martin Brown - I'm no expert, but I think that many so-called "parabolic" reflectors are aproximated by a piece of sphere. For very flat dishes such as you might find on spacecraft, the aproximation works reasonably well and the spherical segments are much easier to construct than their parabolic siblings. (This is true in real life as well as Sculpt-3D :-). The position of the focus changes slightly as a function of wavelength (chromatic abberation I believe it's called), but normally the focus is one-half radius from the center of the dish. >UUCP: {ihnp4!scgvaxd!cadovax rutgers!marque}!gryphon!pnet02!mjb >INET: mjb@pnet02.cts.com This and other useless and often incorrect information can be found in the mind of ... -- Stuart Ferguson (shf@well.UUCP) Action by HAVOC (shf@Solar.Stanford.EDU)
wtm@neoucom.UUCP (Bill Mayhew) (01/28/88)
You can really make any sort of parabolic dish that you'd like. The trick of the parabola is that any energy entering parallel to the axis of the dish will be reflected to the focus of the dish. Any energy that enters not parallel to the dish's axis will be reflected someplace else. The radiation from a transmitter several thousands of miles away looks pretty much like a coherent wavefront -- for practical purposes. I know, if you wanted to be picky... So the upshot of this is that an antenna positioned at the focus of a parabolic dish can only see a distant transmitter only when the dish's axis is pointed right at that transmitter. The accuracy of placing the receiving antenna at the dish's focus is crucial to the selectivity and sensitivity of the dish. Some pragmatic aspects do govern dish size and shape. Typical earth based dishes have focus/diameter ratios around 0.3. For instance my 8 foot diameter TV dish has a 36 inch focal length (as measured along its axis. This puts the receiving antenna element just about at the lip of the dish reflector. A benefit of using a deep dish is that the metal of the dish reflector tends to block microwaves (terrestrial telephone for instance) that enter at an angle 90 degrees to the dish's axis. A shallow dish is easier to make than a deep dish. A shallow dish tends to have more susceptibility to off-axis interferrence. A shallow dish has a greater effective aperature for a given amount of metalwork. As you can see there are tade-offs of ease of assembly versus sensitivity and interferrence rejection. You can guestimate the effective field strengh from a distant transmitter using standard propagation formulae from an electromagnetic fields book.* Knowing the field strength at your locale, you can then figure out the watts/square-meter. Knowing dB power input sensitivity of your receiver you can convert to power in watts watts needed and then you'll know the required surface area of your dish. *My fields book is at work; I know I'd get clobbered if I goofed the formula, so I suggest looking it up at your local library. It is amazing that an eight foot dish is sufficient for terrestrial reception, considering that most satellite transponders put out about 5-10 watts. The satellites are in orbit approximately 23000 miles above the equator. The path to us in North America is more than 24000 miles from the satellites. Space based receiving dishes can be somewhat smaller than earth dishes since ground transmitter powers are typically several hundred watts, and very large (10-20 meter) ground transmitter dishes are used. Shallower dishes can be used in space too becase there is less likelyhood of illumination from off-axis sources. Dish size is not necessarily related to the frequcency being transmitted unless the frequency has a wavelenght close to the size of the dish. It is true that the atmosphere does tend to more readily absorb higher frequency microwaves, thus a bigger dish is required for a given path when higher frequencies are used. --Bill