[P14] Kathrein 2.2m Mounting.

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Having cleaned the huge dish I now have to support it to point upwards at 25 degrees local altitude. I could have dug a meter-deep hole (3'3") and poured concrete around the 4" steel pole I had previously sourced. This would have provided an immovable support. Except that this was a problem in itself. The dish could not be easily moved if it was decided that the original site was unsuitable.

Hedges and trees can easily block the dish's view of the satellite without constant trimming. Conversely, the more exposed the situation the less shelter is available from the wind. The huge area of such a large dish can be caught by a storm with devastating effect. Horizontal forces on such a large area can be enormous. The dish must point south east to capture the weak signal from the satellite. Gales and storms usually blow in from the south to south west. The dish can catch the wind like an aerofoil under certain conditions. Further adding to the forces trying to topple it.

The original mounting support (seen in the image above) was a vertical stub of thick-wall, 114mm diameter x 30cm tall, steel pipe. This is welded to a 1.9m long, 80mm box-section steel beam.

The adjustable dish mounting itself rests on the 4" pipe and is clamped to it with two large U-bolts. Fine, vertical adjustment was provided by a screwed rod and lock nuts. This arrangement had survived for several decades of bad weather attached to the massive, lattice mast. The mounting was still in perfect condition if I wanted to re-use it. All I needed was to arrange the 114mm pipe of suitable height for the mounting to rest on.

An alternative arrangement would be to discard the original mounting. I could use the original, vertical pivots where they attached the dish's inner support ring to the mounting. This would require two sloping support bars set at 65 degrees spaced (at 63cm) to match these pivots. 65 degrees is the compliment of the desired 25 degrees altitude for the satellite dish to point at the Astra 2 satellite at 28E.

These sloping support bars would need to be strongly supported themselves. Probably by an A-frame arrangement. Though Kathrein show a tetrahedral stand being available for their dishes back in the mid-1980s. Even a dish mounted on a car trailer with a tetrahedral stand!

Here is a mock-up in slotted angle to judge the scale and rigidity of a tetrahedral stand. All members are 2m in length. (6'6") This would need considerable reinforcement to cope with any wind loading on a 2.2 metre dish. In fact it would probably sag under the weight of the dish itself! I had planned to use such a stand temporarily. Just to confirm that the new dish was going to give me improved signal strength over the present 1.8m Salora dish.

A tetrahedron is a pyramid with a triangular base. It offers the stiffest natural geometry of any simple solid since all sides are (undeformable) triangles. Using the vertical pivot eyes on the dish reinforcing ring would allow such a simple supporting stand.

The 2.2m Kathrein dish is now in place on the temporary, tetrahedral stand. The balance is perfect when the dish is supported by the pivot eyes. Only the lightest touch is necessary to increase or reduce the altitude pointing angle. I was able to use a length of cord to attach the original, fine vertical adjustment screw. Nothing stronger was needed just for reception testing purposes. Not that I intend to subject it to a gale. It is well sheltered in its present position and the forecast for falling wind speeds.

Mass loading of the base would help to keep the structure safely on the ground when a gale is blowing. This has the advantage of allowing the entire arrangement to be relatively easily re-sited if required. Removal of the ballast would allow the stand to be dragged. Or even rolled around the lawn with the addition of suitable wheels. Offering the chance for a clearer view of the sky. Or better shelter from the wind. The dish plus its bare mounting frame would be too heavy for one person to to lift but could still remain reasonably mobile. It is not just a case of building for survival in a storm but to avoid any rocking in a gale. The signal acceptance angle is very small on such a large dish. Any unwanted movement would result in the dish losing the satellite.  

Click on any image for an enlargement.

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