Kathrein 2.2m dish

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Removing the 4-way DiSEqC switch from the coaxial cable between the 6' Salora dish and the receiver had helped to extend the hours of reception slightly. Certainly better than having the switch in series. With much less audio hiccuping and picture break-up. Though it could still not be called reliable and no reception was possible in the evenings.

Then came the chance to obtain a redundant 2.2 meter Kathrein satellite dish. At 7'2" in diameter it had an extra 14" of aperture over the 6' (1.8m) Salora. Which offered an extra 2-3dB of theoretical gain over the 1.8m dish. [Kathrein claim 47dB gain in the Ku-band] Every extra dB is useful where fringe reception is concerned. +3 dB represents a doubling of signal strength.

The problem was getting the Kathrein dish down from a communal TV reception mast. The large and heavy dish was sitting at least 20' off the ground. The bottom of the mast was protected by plating around the exterior to combat unofficial climbers. The passing years had further provided a tall and dense growth of prickly hawthorn bushes! Climbing and using ropes was obviously not a very practical method of getting the big dish safely down to the ground.

Having obtained permission to remove the dish I hired a large cherry picker. Thus the task of lowering the dish (in the hydraulic platform cage) was quite easily accomplished. Thanks to the quality of the original U-clamps and a spray of penetrating oil, all the nuts came undone without too much difficulty. I had feared vertigo but working at that height from the sturdy, hydraulic platform was completely without drama.

The surface of the painted aluminium dish was disfigured by several decades of bird's muck. Birds would roost in the mast above the dish and drop what came naturally onto the dish below. Weathering must have helped to clean the dish over time. However, many dark and very rough stains spoilt the appearance of the dish and probably reduced its efficiency.

Using my recent experience with the Salora GRP dish I donned old clothes and waterproof, rubber gloves. Then I began to scrub with a fibrous, foam backed, pan scourer aided by domestic scouring cream. The cream quickly turned to a fawn colour. So something was certainly happening. The use of steel wool or similarly abrasive material would have removed the paint completely. Probably making the bare aluminium dish into a very efficient solar cooker! Not good for the poor little LNB at all! Pale coloured, matt paint is vital to the longevity of the LNB at the focus of such a large dish. The greater the collective area the more heat is concentrated at the focus. 

The dish remained in the trailer, for support, while I worked on the outer areas. Though I rotated the dish to bring a new area into easy reach I could not reach the central 3' of its diameter. So the dish had to be stood on edge, on the lawn, resting against the side of the trailer.

I always ensured that the dish material itself was not resting directly onto anything which might dent or damage it. Surface accuracy must be kept as original as possible. The reinforcing cage of alloy tubing was very useful in this regard. Nicely protecting the more fragile, spun aluminium parabola itself. 

A couple of hours later, after lots of rinsing, the dish now looks a lot cleaner. The mild abrasion had thinned the original paint slightly but the overall appearance was much improved. I think there is enough paint left to protect the LNB from burning. I will check with my hand at the focus when the dish is directly facing the sun.

The folding stepladder and wheelbarrow help to show the sheer scale of this even larger dish than the last. When standing in front of it the dish it seems to tower over me. I cannot reach the top with my arm fully outstretched even on tiptoe when the dish is vertical. Not without the aid of a beer crate to stand on. There is an interesting acoustic effect when I speak while standing in front of the dish. This is maximised at the centre of curvature. Which is twice the focal distance. Any sound made at this point is reflected right back to exactly the same point by the entire surface of the dish. The focus is where parallel electromagnetic waves, from a great distance, are reflected from the dish surface and brought to a point. Much as a concave mirror can act as a burning glass or a reflecting telescope functions.

The weight of the dish is only just manageable by one (fairly fit) person. Though a helper makes it much easier to move. Lifting the thing is still undesirable except for two fit men. Rolling it on edge (preferably on a soft lawn to protect it) is by far the easiest way to move it any distance. The rear, reinforcing cage is also handy for getting a secure grip on this huge, ungainly object.

The dish must not catch the wind or the lone worker could easily lose control. I worked behind a 6' thick 10' high conifer hedge so had no problems. The dish has a strong leaning back tendency due to the reinforcing cage and all of the dish material being literally behind the rim. So standing it vertically is very unlikely to cause it to flop over, face down. At least, not without the help of the wind.

Here I must thank my new found friend for his invaluable help in organising the entire proceedings. Without his contacts and vital diplomacy I would never have heard of, nor have been able to obtain this larger dish. He even helped me to remove the dish from the mast by adding useful muscle power. I shall not name him to protect his privacy and modesty. My grateful thanks for all his help.

In the next post I shall discuss alternative mounting arrangement for the bigger dish.

Click on any image for an enlargement.

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