Easy “stressed” parabolic antenna for HRPT reception.

A "stressed" antenna is a good alternative to a solid parabolic reflector. The main advantages are ease of construction, low weight and wind resistance.

It's' known from the theory of resistance of materials that if a deforming force directed along the radius perpendicular to the axis of rotation is applied to the free end of a beam clamped on one side, then the equation of the elastic line of the generator is a parabola. But on practice, not all parabola types can be realised this way. Main criterias are the parabola's F/D (focal distance to diameter) ratio and the length of its generatix. Calculations show that that the parabola deviation error increases with decreasing F/D ratio. The deviation error is also linearly increaing with the generatix length.

Image 1: Relation of paraboloid error from F/D and generatix length.

Going from that, if our dish reflector will have a error in its form we will get a de-focused point and there will be some attenuation. You can estimate the attenuation with the graph below.

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Building the dish.

The reflector's "backbone" is made out of 24 aluminum tubes 6mm in diameter and 1m long. The centerpiece was made out of a plastic disc, with a diameter of 200mm and 25mm thickness. The holes for the rods should be 35mm deep.

The tubes have 2mm holes, into which a 1.5mm cable was threaded (6.1m used for this reflector). This should give you a 2m parabola. The cable is threaded with a lanyard,

which allows you to fine-tune the tension. Additional axisymmetry of the mirror is supported by 24 wires made of a thin 0.6 mm cable. Their attachment point to the central support tube (12mm in diameter) is shown below:

A woven 2x2mm mesh was used, 0.3mm thick. The mesh is attached to the inside of the reflector with a thin copper wire. The ends of the net are wrapped around the main cable and also stitched with a thin wire. Initially for the calculations F/D of 0.5 was assumed, so the main central tube, together with a threaded screw at the end, was in the end a bit longer than 1m. A 1694MHz -> 137MHz downconverter by Radek Vaclavik (OK2XDX) was installed at the focal point with a LHCP helical feed.

Final dish calibration consists of adjusting the axisymmetric supports, main tensioning cable and checking the focal point. After all calibrations we have a 1.95m dish at just 5 kilos! Compared the signal level with METEOSAT-9 (1695MHz) with this dish and a manufactured solid reflector 1.85m dish. When receiving NOAA satellites, the homemade dish showed improvements in signal level, and decoding was successful from 15-20 degrees elevation.

To calculate the lengths of the supports generatrix and other parameters, Parabolics and DISH-RUS

Good luck and nice weather! OlegG dx73@ukr.net

Originally from: http://meteosputnik.ru/item250