[P4] New support arms needed?.

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The dish is 180cm wide (D) and 28cm deep(d) measured down to the centre from a string stretched tightly across the diameter.

The original feed ring height is 68cm to the underside. (i.e. The face pointing down at the dish)

The formula for focal distance F =  D^2/16d

180 x 180 = 32400   16 x 28 = 448 So F = 32400/448 = 72.32 cm

Calculated Focal length from direct dish measurements = 72.32cm

So there is a physical discrepancy of 4.32cm somewhere!

The focal ratio of the dish is F/D = 72.32/180 = 0.4. This figure is used to adjust C120 LNB feedhorns if necessary. Though, by coincidence I have ordered a fixed horn suitable for F/D 0.4 dishes.

The plastic spider screen on the original Ku-band LNB (with built in feedhorn) lay flush with the underside of the plastic ring. Due to its rather amateur method of attachment one cannot automatically assume this feedhorn position was correct. Nor that the ring and arms were even designed for LNBs as we know them today. The dish is quite possibly from the 1980s. So may have been designed for C-band LNBs of the time. Or even some other unknown type of feed.

What we can assume is that the support arms are unchanged from new. Only the manufacturer could have formed the "plug" ends of the arms. The other ends have captive screws which are very unlikely to be anything but original. They are rusted solid.

 I have now decided to ignore the original layout. The calculated focal distance is 72.32cm. This is where the feedhorn mouth should sit (+/- a little) To allow some leeway I have cut a spacer tube of 72cm to support the feedhorn and boss. I shall use this arrangement to measure for new, light alloy arms.  The dish will be laid flat on its back for this exercise. Note how far the true focal distance extends beyond the original feed support ring!

My first arms will be made from scrap 12mm aerial tubing. Of which I have collected quite a lot from large, redundant (German TV) aerials. These were once very commonplace in TV-starved Denmark. Limited to a few hours of monopoly, Danish TV. Most of the houses in Denmark had these very large aerials on the roof.

Or mounted on high lattice towers where the landscape did not allow a clear line of sight from the rooftop. Many still have them. The lattice towers, of many different heights, still stand around in the landscape. That the Danes were willing to go to the trouble and expense to put up these towers and aerials just to receive terrestrial German TV must speak volumes about the domestic, Danish transmissions.

Sadly most of my alloy tubing is in need of straightening. Once I have confirmed best focus for the new LNB and its feedhorn I can make some much smarter arms with new tubing. Assuming, of course, that I can receive the satellite I am aiming for. Otherwise, no matter how enjoyable, all my work, and modest expense, will have been for nothing!

Click on any image for an enlargement.

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