That sinking feeling:

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Thursday:  The Invacom ADF-120 has finally turned up in the post! There were no instructions enclosed for tuning to the dish's F/D. Fortunately I had already found these online. So I know were the ballpark lies. 5.5mm throat extension beyond the scalar rings should be about right for the 0.34 Kathrein dish. I will keep an open mind rather than allowing this figure to dictate the results.  I have yet to produce a suitable clamp for the Invacom. Which lacks the IRTE's excellent, built-in collar and skew-clamping ring. It might be possible to use the Invacom's LNB fixing flange with a suitable clamping ring. I have yet to compare the two feedhorns side by side.

The Invacom is a nicely turned bit of kit. With a very high surface finish compared with the much rougher, cast IRTE feedhorn. Though the weather will probably discolour the Invacom quite quickly. Not that it matters in either case.

I took the advice of the satellitesuperstore.uk website [though I didn't buy it from them due to the much higher price and £15 minimum European postage.] They suggested adding some petroleum jelly to the fine adjustment threads to reduce the risk of binding. The clear, light grease is visible in the image alongside. The packaging shows the nominal range of F/D capacity. If they weren't going to enclose tuning instruction why not print a graph on the side of the box?

Now back to the pole hole digging! I seemed to have chosen a particularly warm and sunny time of year to dig. There was a transition around 90cm where I could no longer reach the bottom of the hole with my gloved hand. Not even when lying flat on the grass alongside. Up until that point it was easiest to scoop up the loose soil with my gloved hand. Then I ran out of arm length! The high cost of hole borers and specialist pole hole digging implements was quite ridiculous for making just a single hole. None of our collection of  gardening implements helped much with removing the sticky clay. It either couldn't reach or wouldn't scoop up the soil in the narrow confines of the hole. While the heavily offset, trenching spade simply allowed the clay to roll off the tip each time I tried to pull it back up!

I finally resorted to a salad serving spoon from the cutlery drawer of the local charity shop to lift the loose stuff. Then I hit a stone which lay perfectly across the bottom of the hole in sticky clay. Careful scraping around the stone with my salad server allow me to hook a small crowbar under the stone. I was then able to roll the stone up the side of the hole until I could lift it clear.

After that I began digging with a 6' length of Dexion, slotted angle. It made a handy tool in the deep, narrow hole provided I wore hide working gloves to protect my hands. The clay became steadily damper and stickier until I was able to make quicker progress with the clay sticking well to the angle iron. I passed one meter then 110cm. This was my target and allowed 10cm of large gravel to line the bottom of the hole for nominal drainage. Not that the clay was likely to allow much water movement. First I tamped the bottom of the hole well with a fence post and then worked on the layer of gravel.

Finally I was able to lower the very heavy, 2.67m x 114mm, galvanized pipe into the hole. The bottom flange settled nicely onto the gravel but the heavy pipe was still used as a final tamper for bit more compaction. No point in having the pole slowly sink on me over time. My reckoning suggests that, on the mounting, the bottom of the dish will be slightly less than 60cm or 2' above the ground. Enough to get our rechargeable electric mower under the lip without difficulty.

Tomorrow I shall have to buy some bags of ready-mixed sand and gravel and a bag of  cement. I had deliberately kept the hole neatly rectangular and not much bigger than the welded flange on the bottom of the pipe. So it won't take a huge quantity of concrete.

BUT: Some dish manufacturers suggest a cubic metre of concrete for a dish this size. Given that my dish is very well protected by a very dense 10' high conifer hedge, on the windward side and trees and dense shrubs behind perhaps I shouldn't worry too much? The dish has shown no interest in moving on its lightweight trapezium stand so far despite some gales.

Let's think about this before it is too late. Is the deep, narrow hole (and resulting concrete block) likely to be a real weakness in actual practice? The concrete greatly increases the area of hard material pressing against the relatively weak soil compared with the bare 4" pipe. The larger the block the larger the surface area of the concrete. This considerably lowers the surface pressure per square inch/foot/cm/meter. Since the surface area of the block increases on all four sides. Hole/block depth remains the same in all cases. The lower the surface pressure between soil and concrete block the less the soil can compress.

A larger block of concrete obviously has a very much higher overall weight than a smaller one. Increased weight will greatly resist the block and pole/mounting/dish assembly from tilting. Or even lifting bodily out of the ground in a storm. There will also be a considerable increase in resistance from the surface roughness of the concrete where it is cast closely against the equally rough walls of soil.

So increasing the hole size offers quite a number of obvious advantages at relatively low cost. Except in time required for the excavation itself and a little time more for mixing the concrete. The material itself is fairly low in cost. My own time is free since I am a volunteer working on my own project at home. So there are no travelling costs and meals are normally provided anyway. At least so far. Impatience with the lack of TV might prompt industrial action!

I remember setting a tall 4" steel pole in the lawn on a previous unrelated project. I did not use concrete but tried hard rammed stones and gravel instead. The pole quivered at the slightest touch though it remained upright over time. I even filled the pole with sand to try and stop it shaking. This had no obvious effect.

Am I willing to risk the dish shaking in a wind beyond the tight tolerances for pointing common to such large dishes? Decisions-decisions! Making the hole square in cross section, instead of the present rectangular, would double the width. Thereby greatly increasing the mass and the concrete block's surface area. Unless I hit a large stone the work is not too arduous. Certainly no harder than I've managed so far.

Perhaps the limitations of using a 4" pipe will be the determining factor in the amount of flexure under high wind loads? Filling the pipe with concrete [later] would be much stiffer than sand. While telescoping the lower half of the 4" pipe with a much larger pipe, then filling the intervening space with concrete, would help enormously. Except that I don't have any 8" or larger, steel pipe handy.

I could buy a concrete drainage pipe and slip that over the 4" one while it is down in the hole. Not an easy task given the likely weight of the concrete drainage pipe and the intended height of the 4" pole above the ground! Though this idea would certainly lift the flexure point well above the concrete block/ground surface. To considerably shorten the beam subject to flexure.

The concrete drainage pipe need not reach the bottom of the hole. It's purpose is only to shorten the 4" pipe's unsupported length. So I could half fill the hole. Let the concrete go off just enough to support the drainage pipe. Then continue filling the hole and interstitial space between the two pipes to achieve a uniform mass of concrete. Or, I could support the drainage pipe somehow without pausing the fill. Though I don't think this is really necessary.

Perhaps I am just being too paranoid about pole flexure? It's just that we do get quite a lot of storms with high winds. The Gilbertini 1.2m needed a stack of concrete posts as ballast to hold its massive cast iron stand down. The Kathrein at 2.2m is almost twice the diameter and vastly larger [4x!] in potential, wind-catching area. That is four times the wind loading. I had better star digging again even if I don't add a larger transition pipe. This is all beginning to seem like an awful lot of work for only a few hours of British TV around lunchtime!

One sweaty hour later and the hole has increased to 40cm square by 110cm deep. [Instead of the former 20 x 30 x 110cm deep] I shall undercut the bottom of the hole into a truncated pyramid. The so-called elephants foot foundation block. The pyramidal form resists lifting and toppling due to wind loads and greatly improves resistance to frost heave. The larger base also resists sinking on softer ground. [My damp clay] Much better than a simple cylinder. And, very much better than a typical (amateur dug) conical-shaped excavation with the point at the bottom!

The concrete has been put off for the moment. I am still unsure whether the post hole is big enough.

Well, I have decided to go for a simple pole in a 40cm square hole 100cm deep. I have enough sand and gravel mix to fill a hole that big. Now I just need the cement. I have rammed the hardcore again to be sure.

Weds 18/6/14: Concrete completed in windless, hot sunshine and 70F. I mixed the concrete in a wheelbarrow using a rake and 3.5:1 støbemix to cement. Total expenditure 200DKK [about £20] and I still have one and a half bags of cement and some sand/gravel mix left. I have enclosed the casting and lower pole in polythene to retain the moisture while the concrete goes off. Though, after today, the temperature is due to drop and become more cloudy.

I have been monitoring the temperature of the spun alloy, Kathrein dish to see if the thinned surface paint (from cleaning) affects solar temperature gain. Quite unexpectedly, it remains quite cool. There is also no sense of warmth, at all, at the focus while placing my hand over the feedhorn with the sun full on the dish. A good result.Though the risk of frying the LNB is much lower on a prime focus dish compared with an offset one. The latter usually have a plastic cap over the concealed feedhorn where heat might be concentrated. The prime focus dish has a naked, metal feedhorn with little or nothing to absorb heat.

At day four since I poured the concrete I am still waiting for it to gather more strength before fitting the heavy dish on the steel pole. Despite the polythene skirt being constantly covered in droplets internally the concrete surface is always dry when I lift the plastic to check. I have been adding water each time and shading the base from the summer sunshine to keep the concrete cool.

Click on any image for an enlargement.

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