[P17] Converting the Kathrein 2.2m to C120 LNB.

(

The huge feed cup/hub and its covering can would require considerable effort to convert to the more diminutive C120 standard. While the sheer size of the hub might seem to mask the dish it amounts to a tiny proportion of the whole dish area. Almost all of the area within any circle lies in the outer radii. (The area of a circle being Pi (3.142) x the square of the radius) Adding more radius very rapidly increases the area.

I have now decided to use exactly the same method as I did with the Salora 1.8m dish. I shall remove the modified hub, IRTE feedhorn and Inverto LNB and fit it straight onto the hefty arms of the larger dish. The bore of the much heavier Kathrein arms is exactly the same as the (much thinner) alloy tubes used on the 6' Salora. (10mm ID x 12mm OD)

So I can use the same expanding bolts inside the Kathrein arms to hold the C120 hub in place. Some stainless threaded rod and few nuts: Job done!








The (potential) downside is not being able to use the original Kathrein feedhorn. This would have been optimised for the dish's particular diameter and focal ratio. The IRTE feedhorn is fairly universal so should work well enough with the 2.2m dish. Unfortunately the Kathrein's 0.34 focal ratio lies just below the IRTE feedhorn's expected range. Minimum .38.

A serious mismatch can leak infra-red noise from around the dish's circumference. Or, much worse, can limit the clear (effective) aperture seen by the LNB. This is most likely to occur with a "fast" focal ratio like the Kathrein. Where the LNB needs to "see" a very wide angle to be able to gather the incoming signal from the vital outer radii of the dish. If the LNB cannot see the outer reaches of the dish then it will see a much smaller dish. Thereby wasting all the effort and expense that went into obtaining the bigger dish.

The adjustable Invacom feedhorn is a much better theoretical match. Having a claimed (focal ratio) range from .32 to .43. The alternative is to turn an identical feedhorn to the original Kathrein in the lathe. The main problem being to source a large enough lump of aluminium stock. Though I may have some scrap brass somewhere which might do. Albeit it is a much harder material to turn quickly than aluminium. The necessary flanges add a lot of extra diameter. Soldering brass flanges and copper tube is another option.

So-called "fast" focal ratio dishes offer theoretically more gain but are more sensitive to noise leakage and feedhorn mismatching. Unfortunately my UK supplier was out of stock of the Invacom feedhorns when I placed my order. So I ordered the IRTE instead. Adding insured postage to Denmark would almost double the price of the Invacom feedhorn! I'll have a look at eBay instead.

Click on any image for an enlargement.

)