Matching the feedhorn to a faster dish II?

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Despite the rather negative summary above I was still mulling over the purchase of an Invacom adjustable feedhorn. This was really a last gasp chance to improve signal quality when almost all else had failed. While I was probably clutching at straws I wanted to be able to tick off the remaining possibilities. Otherwise I would never know if I had missed some obvious trick to extend 28E UKSB reception beyond the present paltry few hours.

The tubular throat pipe of the Invacom ADF-120 feedhorn is threaded to allow the matching scalar rings to be moved ahead of or behind the central throat. The more the protrusion of the throat, beyond the scalar rings, the wider the feedhorn "sees."

With a "fast" F/D ratio dish the feedhorn must be even more carefully matched than with "slower" parabolas. The feedhorn must "see" wide enough to capture the vital signal from the very edge of the dish. Where ironically by far the most dish area is situated. Otherwise the feedhorn has effectively reduced the diameter of the dish. If, however, the feedhorn is adjusted to see beyond the edge of the dish it picks up more noise. Which may be just as bad as a weaker but cleaner signal from the (effectively) smaller dish.

Here is the graph showing the required protrusion of the ADF-120 throat beyond the scalar rings. The F/D (Focal Ratio) of the dish is on the vertical axis.

The Kathrein 2.2m dish has an F-ratio of 0.34. [75/220cm] Carrying the 0.34 line across the ADF-120 graph to the falling curve suggests ~5.4mm protrusion. At least it does on the ADF-120. Will the F/D 0.38 IRTE feedhorn respond in exactly the same way?  The only way to find out is to try:


 There are two ways of extending the throat protrusion. The scalar rings could be cut back in the lathe. Or the waveguide throat can be extended with an add-on ring. The first is rather permanent if it fails and leaves the bare edges of the scalar rings open to corrosion unless painted. A press-on throat extension ring is a much simpler matter and has no permanent effect on the feedhorn.

So I made an aluminium extension ring in the lathe to fit on the central throat of the IRTE feedhorn. Making it fairly tight would ensure it would not fall off in its naturally downward facing condition. The situation was slightly complicated by the clear plastic cap on the feedhorn throat. This thin cap is intended to keep insects out. Particularly spiders which might fill the entire length of the waveguide feedhorn and LNB bore with cobwebs. I decided to leave the cap in place and made the bore of the ring the same as the IRTE feedhorn throat.

First I made the "socket" to fit nicely on the feedhorn throat cap. Then I gradually shortened the ring until I measured just over 5 mm to the face of the plastic cap from the front edge of the ring when in place. I turned the outside of the ring to match the diameter of the feedhorn throat.

Now all I need to do is test the modified feedhorn on a real signal. I doubt it will make a dramatic improvement in signal quality but it is easily adjusted for more overhang (protrusion) and instantly removable if it proves to be worthless.

And here is the extension in place on the feedhorn. The internal step at the ring/feedhorn interface is an illusion due to the plastic cap. The bores are carefully matched and fit closely together to avoid any gap.

So much for theory. The results of fitting the extension ring to the IRTE feedhorn were awful! Signal S/Q dropped instantly from 82/56% [bare] to 72/30%. [ring fitted] That was on the 28E Pan-Euro beam. Ouch!

There was the usual No Signal, with or without the extension ring, on the 28E UK Spot Beam after 14.00 pm. [Which was when I first switched on.]

The only hope for improvement, over the bare IRTE feedhorn, is to shorten the extension ring in small steps. Testing at each protrusion length until it has no extra extension left. The IRTE feedhorn has a slight protrusion of the throat pipe. I may have completely overdone things by making the ring extension far too deep/long. I had already turned a mandrel to hold the ring in the lathe so shortening it in (say) millimetre steps between testing will be easy enough.

I will have to wait until tomorrow to have a UK SB signal to test my shortening results. The difference between the F/D minimum of 0.38 (IRTE feedhorn) and the Kathrein dish's 0.34 will probably need only a tiny ring extension. Tomorrow will tell.

After further thought I realised that it is the extension beyond the front surface of the scalar rings which matters. I had used the plastic throat cap as my baseline. Where the IRTE feedhorn throat already has some protrusion. Next step is to cut the ring down in stages to 5.5mm above the scalar rings to match the [F/D 0.34] tuned Invacom.

Click on any image for an enlargement.

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Final Summary?

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Only a much larger dish at this geographical location will improve the signal. Moving the same dish to another geographical location might offer far greater benefits. I must presume that my dish is lying on, or close to, the null line which passes N-S through mid Denmark.

My own results suggest I am in much the same problem situation as my fellow TV fringe enthusiast. Who lives only a few miles away. He has exactly the same 2.2m dish and enjoys(?) very similar reception only over a limited time of the day. Though again with the same slight  but unreliable variations at odd hours on odd days as I have myself experienced.

Such is the narrowness of the signal acceptance angle on a large dish that it may not be able to receive both 2E and 2F at the same time. The spread between the two transmitting satellites places them either side of optimum axial gain at the focus of the dish. One, or the other, or both satellites become effectively offset. Just as if the dish were poorly aligned.

The Astra 2E/F UK Spot Beam has succeeded in denying an adequate signal even to those fringe TV enthusiasts willing to invest a huge amount of time (and potentially a great deal of money) just to receive the same UK programmes as before. It was once relatively easy to receive UK TV from the previous Astra 2D also at 28E. Often with only slightly oversize dishes and some care in set-up. I myself had perfect reception around the clock in all weathers from a 120cm Gilbertini offset dish.

Many would consider even this a very large dish for many domestic situations. With the recent change in satellites I first tried a 1.8m (6') dish and now the relatively huge Kathrein 2.2m dish at 7'4") Still without more than partial and very time restricted reception around late morning until just after lunch. Exactly when the UK TV channels are full of absolute dross. The evenings being the best chance of any programme quality (at all) are now denied to myself and many others.

The denial of a 'home' TV service to so many British ex-pats has largely succeeded. The TV companies can continue to maximise their income from their license-subsidised production empires. Simply by selling to strictly limited, geographically-organised, commercial cable and retransmission scrambled satellite services. To sell the programmes onto their already jaded viewers sandwiched between their own commercials for further profit. And all, at greatly increased levels of irritation and expense to the end consumer.

Or the film producers can drive yet more potential viewers elsewhere. To obtain their entertainment by other means than satellite TV. The Internet has become the entertainment medium of choice for many. Sadly there is still no fast, universal optical fibre service over much of Europe.  So streaming films in HD is difficult to impossible due to a lack of bandwidth. I have a 50/50Mps service but have never tried to stream a film. Just getting a decent HD video experience from YouTube can often be a struggle!

No doubt the pirates and conmen will profit enormously from the changes in UK satellite TV reception in Europe. Where there is a market somebody will always step forward to supply it. UK TV denial could be looked at as a form of prohibition and we all know what happened then!

DVDs and Bluray disks continue to decline in sales as Hollywood continues their downward decline into totally absurd, drug, money and celeb fuelled, mediocrity. Their products selling at outrageous profit per sales unit. With unduly restrictive legal barriers to the re-use of something already purchased and presumably owned. We truly live in an era of false accounting and virtually total lack of real creativity. Monopolies rule by playing with monopoly money at the expense of the poor consumer.

My own UK TV journey has finally come to an end. There is nothing left other than the huge investment in one, or even two, very much larger dishes. Nothing less will squeeze enough out of the present, deliberately weakened signal to make watching UK satellite TV worthwhile. The cost alone makes this quite impossible. Having even larger dishes in our rural garden would increase the risk from the increasingly vicious storms due to global warming. The loss of visual amenity in housing such vast structures, in what is supposed to be a normal garden, makes it even less desirable! 

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UK TV RIP!

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I have obtained a TV which I could read at some distance in bright sunlight. It was propped up in the shade of my car boot with the Fortec Star Passion HD receiver alongside. After initial tests with a twin coax video/sound cable I swapped to a short HDMI betwen the two. The picture was easily bright and sharp enough to read from the dish at some distance. I set the receiver menu to Signal Strength and Quality display. While the readings might not be scientifically accurate they are easily repeatable within the same system. No more is required for testing purposes.

First I confirmed that the Kathrein 2.2m dish was indeed aligned perfectly on 28E UKSB as tightly as possible. No movement of the dish (in any direction) increased the Signal or Quality. In fact the readings fell. So there was no more to be had here. The wind was light enough to be having no effect.

I could then move on to maximising LNB focus and skew. The brass feedhorn was already set at the calculated focal distance of 75cm. I peered through the small hole in the middle of the dish to ensure the feedhorn was centred and pointing straight at the centre of the dish. No change in Strength or Quality was noted with increased accuracy of alignment.

Quality dropped with all changes in feedhorn focusing so I left that alone too. Skew was fairly insensitive but I found the mid point of the angular limits and tightened the LNB clamping screws. 

I then noted the signal Strength and Quality readings on the screen and removed the home-made brass feedhorn to fit the cast alloy IRTE. Quality immediately increased by 2%. So the IRTE (despite a slight theoretical F/D mismatch) was obviously the better choice. I tried changing the focus and skew with the IRTE but they remained best at the same, brass feedhorn settings.

The HD100 cable had softened slightly in the unusual 76F heat and bright sunshine. So I stretched it out as straight as possible. Though this had no visible effect on the signal readings there is a theoretical chance of signal being lost with too many tight turns at such high frequencies. No point in leaving anything to doubt.

The Inverto Black Pro C120 LNB is highly thought of on the satellite forums. Unless I have been unlucky and received a poor example there is not likely to be any serious improvement by fitting any other LNB.

The IRTE feedhorn could be swapped for an Invacom Universal. Whether this would be worth the extra expense is difficult to say. The gain is likely to be only very marginal. 

The Fortec Star Passion HD receiver fails to lock below about 53% signal Quality. I have tried a borrowed Kathrein receiver and found it almost the same for sensitivity and ability to lock onto weak signals. There may be slight gains to be had from a change of receiver but I doubt it would be worth the expense of actually importing one.

Which only leaves the Kathrein 2.2m dish. Though old, it is a professional quality, spun aluminium dish in undamaged condition. Unlike some plastic dishes with embedded, open mesh reflectors the solid aluminium dish cannot lose signal by changing between C and Ku band. Thanks to its rear, skeleton reinforcing frame the Kathrein is unlikely to lose its shape. So it should have retained the surface accuracy with which it was originally made.

I recorded my maximum signal levels across the range of UK TV channels and transponders between 13.00 and 13.15pm. By 14.00 signal quality had already dropped by around 5% in many cases. While signal Strength had strangely increased by 2-3%. No changes were made to the system when 14.15pm marked the local death of most of the BBC channels. With many of the ITV channels falling out at 14.30pm. Only a few stragglers remained with solid reception until I finally lost patience with hiccuping sound and lack of pictures at 14.45pm.

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Recabling.

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I have just obtained some very high quality Webro HD100 coaxial cable from a fellow fringe reception enthusiast.

The idea was to test the effect of brand new, copper shielded cable against (at least a decade old and supposedly ageing) aluminium shielded cable of roughly the same overall length.

Originally, I was using two long lengths of the old, white, coax cable. With a straight connector joining them in the middle to reach the Kathrein dish behind the house.

Only half the length is usually required at the usual dish site in front of the house. No DiSEqC switch was involved in either test case. 

I am (almost) afraid to report that swapping the old, series connected pair for a single run of 33 metres copper shielded HD100, with freshly fitted plugs on bright metal, had absolutely no effect whatsoever. None at all.

NO discernible change in measured signal level, quality or reception. 

Why does this remind me of HiFi cables and snake oil? :-))

The Webro is black and much stiffer than the old, white, aluminium-screened coax.

 I borrowed this spec list and image for the HD100 from an eBay vendor:

- CLASS A+ COMPLIANT
- TRI SCREENED
- SOLID FOAM DIELECTRIC
- SOLID COPPER CONDUCTOR
- CAI APPROVED
- ROHS COMPLIANT
- LSNH TO: IEC 61034-1, IEC 60332-1, IEC 60332-3 CAT C, IEC 754-1

Physical Characterises:-

Conductor: CU 1.0 +/- 0.02mm
Dialectic Thickness: 1.81mm
Dialectic Material: Gas injected physical foam
Dialectic Diameter: 4.65 +/- 0.15MM
Dialectic Colour: Natural
1st Foil: CU 115%
Braid: CU 0.12+/-0.008x 16x5
Braid coverage: 55%
2nd Foil: AL-PET 115%
Sheath thickness: 0.66mm
Sheath: LSNH
Overall Diameter: 6.70 +/- 0.15
CAI Approval number: CAI 0068 A

Electrical Characteristics:-

Rated Voltage: 30V
Capacitance: 75
470 – 1000 MHz: >75
1000 – 2150 MHz: >65

Next day testing had BBC channels dying by 3.30pm with most ITV, Channels 4, 5, etc. hiccuping and unwatchable beyond 15.45pm.  I checked the length of my pair of series white coax cables and they were about 20 meters (max) altogether. So the HD100 has a 13 metre disadvantage but no straight connector in the middle. It probably balances out. I was careful to unravel the coils of the HD100 where they had come off the drum. I didn't want to undermine its performance by having too many tight bends.  

Today I fixed some cords to the top, bottom left and right  to the skeleton frame of the dish. I could then sit comfortably at my tiny TV and make small adjustment to the dish pointing like a puppet master. Sadly this had no beneficial effect at all. The dish was at its best with the ropes relaxed.

The only changes I haven't tried  yet (while monitoring the signal Quality) are skew and focus adjustments. On the tiny CRT TV the figures are simply too small and fuzzy to read easily in bright light with any accuracy. So ideally I need a better monitor for testing these parameters. It's a shame one can't use redundant computer monitors but they only work on serial cables.

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Rain fade!!!

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Another attempt to fine tune the dish in pouring rain! All Spot Beam channels available at 11am CET. There seemed to be no match between the sheer weight of the rain falling on the car and signal drop-out. I had the doors tightly shut with both receivers taking it in turns on the passenger seat. The tiny CRT TV offers a picture too small (and fuzzy) to be certain of the S&Q readings. I brought out the car sunscreens to hide the reflected glare from the 2.2 metre dish behind me. Then I found something tall enough to prop the TV up where I could see it more easily without annoying reflections. You would not believe the hours I have wasted peering at this 4" TV over the years while dish aligning. When a small flat screen TV would have helped enormously.

Then it occurred to me that I could use the drop-out glitches to fine tune the disk pointing further. Except that it would not stop pouring down! Every time I determined to get out of the car to fiddle with the dish the rain would redouble its efforts! During one brief pause, in still steady rain, I looped a chord between the edge of the dish and the angle iron stand. Then I wound a stick through it to allow me fine aziumuth adjustment. (tourniquet style) This did not help. I could neither see nor hear the TV in the car through the pouring rain. Returning to the car to watch a channel or flick round the signal would soon go off again.

Relative dish size from 1.2m up to 3m.

It was all getting a bit silly and OCD by now. So I used an easing in the rain to escape indoors to gather my thoughts in more wasted blogging electrons. There is now a big puddle outside the car door where I have been getting in and out. I may have to move the car without damaging the cables snaking across the parking area.

I imagine the rain and rain-bearing clouds up in the sky have far more effect than what is falling locally. This would explain the mismatch between signal strength and falling rain. I had solid reception during the heaviest rain. Only for the signal to fade when the rain eased off again. The rain stopped briefly but I had no signal on several transponders which were fine only moments before. I obviously still have far too little overhead on the signal strength and quality to ensure a reliable reception lock on the receivers. The difference in signal level between all the different channels is quite ridiculous! Worst, by far, is ITV2+1.

As I sit here scribbling away the sun has come out and it has finally stopped raining. Is it just trying to tease me into trying again? Off we go again!

By now many channels had dropped out with only ITV3 & 4 still just watchable.  

13.15 and there were no Spot Beam channels left at all.  I don't think there is much of a margin left to tweak with. Millimetre movements of the edge of the dish do make a difference. Though only when there is a signal to play with.

Is it worth all the effort to dig a huge hole and cast a steel pole in solid concrete? I am really not sure at this moment.  Yes it will give me a stable base and allow much finer adjustment but for how many hours of TV per day? Most UK daytime programming is absolutely unforgivable dross! With very rare exceptions like cycle racing.

14.15 Switched on again to check the latest situation. Many of the UK SB channels were now watchable! It is windier than earlier but much brighter. With the heavy clouds clearing to blue sky in the south.

Will continue to monitor at intervals this afternoon.  BTW: No DiSEqC switch is being used now. Just 20 metres of coax with a straight coupler in the middle. Using the Fortec Star Passion HD receiver. I am now informed that the coupler causes a 2dB signal loss. Whoops! That is much the same as reducing dish size by a foot or more in terms of signal gain!

15.15 No check.

16.15 Still some channels watchable. Mostly ITV with only one BBC channel locking at intervals.

17.15 Not one UK SB channel receivable now.

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[P22] Testing-testing.

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Due to other chores I only had the system set up by 14.30pm again. Getting the dish pointed correctly from the new position was rather more difficult than anticipated. Despite having a decent compass I seemed to have found the wrong satellite again. I checked the dish's altitude while listening to the SatFinder meter's irritating whistle. It should whistle on zero gain if it is 28E.

Only when I moved the dish by a large angle in azimuth was I finally able to pick up 28E. Then there was more "fine" dish movements before I could get the UK Spot Beam. Certain transponders were stronger than others. It was now 15.00. BBC channels were again weaker than ITV. Channel 4 was variable between channels. (Presumably broadcast on different transponders.)

As the signals began to fade at 15.30 I tried swapping receivers with a Kathrein which was kindly loaned to me for comparison. I cannot be sure but I think the Fortec Star Passion HD is just marginally better at locking onto fringe signals. Though I really could not be certain I was comparing like with like. The channel lists were so different with lots of duplicates on both receivers. No scientific accuracy is claimed nor meant by anything you will read here.

By 16.00pm there were only  a few Spot Beam channels still working. Most of the others registered no signal or nothing at all. A couple of ITV channels hung on longer than the rest but hiccuped and froze at times. I gave up at 4.30pm with only Pan Euro beam channels left. It seems the new dish and turned brass feedhorn are not gaining enough to extend reception by much over the Salora 1.8m. But again I cannot be certain that present conditions are the same as before. It was raining almost continuously again today from a solid overcast. "Rain fade" is well known in fringe reception conditions. All a far cry from previously perfect  reception on a 120cm dish under all conditions on 28D year round.

I know from my early experience with the 6' Salora dish that fine azimuth adjustment is absolutely vital to fringe reception. A quarter of a turn on the Salora mounting's azimuth adjustment screws was the difference between receiving the UK Spot Beam channels or nothing at all. At the moment I am literally dragging the slotted angle stand around on the wet lawn to rotate the 2.2m Kathrein dish.

Fine adjustment is not remotely what I would call it. So the potential exists for far better fine tuning in azimuth. I already have fine vertical adjustment in altitude. Due to the flexibility of the slotted angle stand I can flex the dish from side to side and rock it up and down. My wife tells me the dish was fluttering quite noticeably in the wind the other day.

There are now two ridiculously large dishes set up in my garden as well as several smaller ones. Still with no more than marginal UK TV reception over a very limited period each day. One must assume that I am living close to the supposed null line. Which is theorised to run north-south over the middle of Denmark. Though the exact location is still open to rumour due to the lack of feedback by other big dish abusers. Some people even imagine that a positive report online will result in the satellite company tweaking their beam pattern to spoil the fun.

It is certainly odd that the many ex-patriots, who are not resident in the UK, have lost their national TV service. Enjoying their home TV was the unbreakable anchor chain which kept them feeling at home. Wherever they happened to be living at the time. Many enjoy their annual holidays on the Continent and have paid their TV license in full. The advancing tide of copyright greed has spoilt it for countless ordinary people living or holidaying "abroad".

So much for the pretence of equality and equal rights within the EU. Gutless and riddled with corruption from top to bottom! Except when the bananas need to be straight. Don't forget to vote in the present EU elections! A banana near you may need the help of the endless gravy train riders to the life of endless luxury, blatant fraud and countless wasted opportunities to reform.    

My own investment so far has not been financially crippling thanks to purchasing both dishes at favourable secondhand prices. [Complete with their respective altaz mountings.] It might still be possible to sell both of them to fellow fringe reception enthusiasts in less demanding areas of Denmark. But only if I completely lose patience with the poor reception so far. I could turn the dishes into giant bird baths complete with fountains, garden ponds (again with fountains) or even domed roofs for small, round, decorative garden sheds. Aluminium is toxic to some wildlife so the Kathrein won't make an ideal fish pond.

The Kathrein 2.2m prime focus dish beside the now redundant offset 1.2 Gilbertini. The Salora 1.8m dish lies between the two in size and is indicated to scale by the shaded circle. [From left to right: 4', 6' and 7'4" in  old money]

Those who consider a 1.2 metre grey dish as too large for their garden have absolutely no concept of how a white one, almost twice the size, looks from the front! From the side and rear they look much less significant. Being round they are not traditional rural furniture in appearance. A caravan is absolutely huge in comparison but nobody gives a caravan a second look. Not even a snow white one. A round white dish can be spotted from a great distance!

Luckily the larger dish is now behind the house out of sight. (For the moment.) The smaller, Salora might as well be taken down now. I wonder if the dishes will show up on this year's free, aerial imagery and mapping service provided by the Fyn Councils? Perhaps I should cover the big one with a green tarpaulin tent in case people imagine they are seeing UFOs! Fortunately Google Earth imagery of Denmark is often so absurdly poor and so many years out of date, that nobody will ever notice anything unusual!

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[P21] A matching feedhorn in brass.

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I was worried about the potential mismatch of the IRTE feedhorn with the faster Kathrein dish. So I decided to turn a matching feedhorn from a lump of scrap brass.

I was afraid that I was not illuminating the entire dish when using the IRTE feedhorn. This would have the same effect as using a smaller dish.   

Buying an Invacom feedhorn from the UK would be foolishly  expensive by the time P&P was added. A German dealer is much cheaper but is currently closed for a holiday.

So I have copied the original Kathrein feedhorn throat dimensions in brass. I will make the rear end longer to fit the new feed boss which I purchased for the Salora. I shall be copying the two IRTE feedhorn flanges at the rear. This will allow me to clamp the LNB to the feedhorn and to use the IRTE skew clamping ring.

The image above shows early progress with the feedhorn end just roughed out on the end of a 100mm long x 55mm Ø bar.  A time consuming task in hard brass using plunge, face cuts. I just put the lathe in back gear and engaged the longitudinal feed to cut down on chatter. The outside diameter of the single scalar ring is 50mm. Inside diameter 44mm. The waveguide bore is 19mm right through. (As is the LNB) Guided by the cast IRTE feedhorn I decided to made the throat tube slightly too thick. Though I doubt it makes much difference. I can always make it thinner if I discover this is an important factor.

The brass feedhorn is now finished. Shown with the Kathrein and IRTE feedhorns and prime focus boss.

The brass feedhorn is resting on the adjustable skew, clamping ring.

The boss is difficult to fit onto the feedhorn unless the groove just behind the feedhorn throat is made wide enough. The bent ears catch in the sides of the groove.  No problem on the IRTE. Where there is a large space and small neck behind the feedhorn.

I had to cut the original groove much wider. It was lucky I had not removed the feedhorn from the lathe chuck before I checked that the boss would fit. So removing the boss from the dish in the pouring rain (for the umpteenth time) was not a wasted effort.   

The new feedhorn now clamped to the triangular, IRTE, prime focus feed boss.

I was delayed for hours by the lack of proper indexing on the Inverto LNB clamping holes.

No four holes match any other four as the LNB is rotated relative to the feedhorn! I had indexed them correctly and then had to file the original holes oval to allow the screws to fit the LNB's spacing!

Another problem was to maintain concentricity of the LNB with the feedhorn bore. Any mismatch will cause an eccentric step in the waveguide. Hardly desirable given the care I took in making the brass feedhorn.

Another view of the Inverto Black Pro C120 prime focus LNB attached to the new brass feedhorn and IRTE boss.

I just hope it helps to drag a few more dB out of the bigger dish. 

I still don't know why Inverto use such a weird shape for the body just above the F-plug thread. It is impossible to fit a rubber boot!

I will probably have to fit a rain shield just above the LNB.





I wish I could report that the new feedhorn worked miracles. But alas no. It was 14.30pm [CET] by the time I was set up and ready to test reception. Then it started raining harder than ever. I sat in the car flicking around the channels on 28E with the remote. The receiver and TV were sitting on the passengers seat and the doors shut to keep the rain out. Pan European beam: Solid reception. Spot beam? Nothing. Not one channel!

When it finally stopped raining (briefly) I checked the feedhorn was set at the correct focal distance. [75cm] Then I taped a small round make-up mirror to the face of the new feedhorn and squinted though the peep-hole in the centre of the Kathrein dish. The feedhorn was way off from aiming at the centre of the dish. Lots of adjustments later I had the reflection of the peep-hole centred in the mirror. I then rotated the feedhorn to ensure the mirror was square to the feedhorn. It wasn't! Grrr! Trying to free the thin mirror from its plastic case would probably break it. So I did not try.

By moving the trailer from its usual parking place I was finally able to move the dish backwards by just over two metres. This should give me a clearer view over the tall hedge without resorting to loppers and high ladders in the pouring rain.

Tomorrow promises yet more showers but I shall persist at a much earlier time in the hope of pinning down the Spot Beam channels while they are still strong enough to get the dish properly aligned. Then I can try swapping feedhorns [with the IRTE] to see if either of them is superior to the other.  

Click on any image for an enlargement.

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[P20] Another hurdle to overcome!

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It often happens that an obstacle presents itself in front of our satellite dish. A roof, hedge, wall or a tree are the usual examples. The entire surface of the dish must see clear sky in the direction of the desired satellite. (assuming for the moment that the dish is fixed and only one LNB is fitted) Luckily each TV satellite sits almost perfectly still in the sky as seen from the ground. It doesn't matter if there are mountains or skyscrapers blocking your view in every other direction. All that matters is that the dish can see the required satellite without obstruction.  

By a happy coincidence Astra2 at 28E has a local altitude of almost exactly 25 degrees for my site. 25 degrees is a handy altitude angle to have. Because you can divide the distance to the object by two to get the allowed height of that obstacle. Well, near enough. Tan 25 = 0.46 rather than 0.5.  So halving the distance to check the allowable hurdle height is slightly too generous. Tan 27 degrees is nearer to a true half. So for 25 degrees satellite altitude I should really use 0.46 x distance to the potential signal blocker. Not quite so liberal as using a half x distance if it is really critical.

The simplest way to check whether the dish is seeing the satellite without any blockage is to draw a line on a piece of paper at the local altitude angle of your intended satellite.  Now mark centimetres along the bottom and draw vertical lines from your marks. Measuring up to the sloping line at any given distance point will give you the allowed height of any obstacle in the signal path to your dish. You can use the markings for any scale that suits you. Just remember to use exactly the same scale for height up to the sloping (altitude angle) line as your horizontal markers.

And, don't try and mix your units! Professionals have built bridges which did not meet in the middle because they mixed their metric units on one side with their imperial measurements on the other! No doubt the same problem has occurred with tunnels where two nation's contractors mixed their different units of measurement.

The bottom of the dish is the critical point from which to check that it can see the satellite without obstruction. It is here that any "shadow" on the signal path will be most likely to occur. Particularly with large dishes. Fortunately, many dishes are a mounted a certain distance above the ground. The height from the ground to the bottom rim can be added to the allowed height of the obstacle.

A simple example using my own site: Taking 25 degrees as the satellite's local  altitude I measure 5 metres to the hedge in front of my dish. Halving this figure gives me 2.5 metres allowed height. [Actually 2.4m] But the lower rim of my dish is a metre above level ground. So I can have a hedge 3.5 metres maximum height in front of my dish at 5 metres distance. Any higher than this and the hedge will block some signal arriving at my dish. Which would be very undesirable for fringe reception where every dB of clean signal is vital.

My own front hedge is 3 metres high and looks taller. From the dish it looks as if it might well block the signal from reaching the bottom of the dish. Those new spring shoots will have to be clipped back very soon! Raising the dish is another possibility but rarely practical.

Now I know that I must have the bottom of my 2.2m dish at least a metre from the ground too. Or I will need a lower hedge. Which would not offer the same degree of shelter from the prevailing wind.Though I suppose I could cut a lower step or notch out of the hedge in front of the dish in the direction of the satellite. Though this might make the dish far more obvious to observers beyond the hedge. Each situation is different, of course, but what the neighbours can't see won't hurt them.

Hedges can be measured with a long stick or pole and then the stick itself measured when it is lying flat. Insulating tape can be applied as a marker if the stick is much longer than the obstacle is tall. Then stand the stick up again and check from a reasonable distance that the tape coincides with the top of the obstacle. Now measure the stick or pole from the bottom up to the insulating tape to find the correct height. Much easier than trying to force a floppy tape measure to stand upright! Snake charmers excluded, of course.

Conversely, you can mark a pole at the correct allowable height for a known distance measured from the dish. Then stand the pole up using something like a stepladder or braces for support. Now use a small mirror at the bottom of the dish to check that the reflection of the hurdle beyond the pole marker is lower. Or you can grovel on the ground with your eye at the level of the bottom of the dish if you prefer.

If the hurdle is lower than your marker on your pole then the bottom of the dish is not in the signal shadow of the hurdle. If it is quite close to the marker and the hurdle can grow then clip the hurdle or move the dish. Hedges and trees have a nasty habit of growing faster than you would ever believe! So siting a dish with a fast growing hurdle in line with the satellite will almost certainly end in tears. With no useful signal sooner than you think! How much concrete did you put around your dish's support pipe? Well now you have to do it all over again with a new pipe. Or buy an expensive ground stand to allow some mobility.

If you can't use a measuring pole or stick because of the landscape in front of the dish or the dish is high on a building then life is rather more difficult. Taller and more distant objects are much more difficult to measure without a stick or measuring pole. Not everybody owns a clinometer but you can measure your own shadow on the ground. (or that of a stick) Then measure the length of the tree's shadow straight afterwards. Leave it too long and the sun's own altitude angle will have changed. Giving your calculation an inaccurate result.

Knowing the ratio of your own height to the length of your shadow you can use the same ratio to calculate the height of the tree (or other obstacle) from its shadow. The same goes for a building. This simple trick needs sunshine of course and is much easier with a reasonable solar altitude. It probably won't work well at noon at the equator nor at the Poles.

You can also use a known object in a photograph taken from some distance to measure an obstacle at the same distance in the same picture. Measure the object size in the image and then the obstacle itself. Use ratios again to determine the true height of the obstacle. Once you know the height of the object you can use a map to measure the distance between your dish and the distant object. There are measuring tools on Google Earth which can be very useful. Now add any extra height of the dish compared with the distant hurdle to your allowable hurdle height at that distance.

There are other methods to check whether obstacles will block your signal but I have already talked the subject to death. You could check which two days of the year the sun is at the satellite's altitude. Though it might be quite difficult to find this out in the exact direction of the satellite. There are plenty of free solar altitude calculators online. The problem is using them to find when the sun is at the correct altitude at the satellite's correct azimuth for your location.  

If an obstacle casts a shadow on your dish then something must be done about it. Owners of buildings, hedges and trees are highly unlikely to be sympathetic to your satellite reception needs. All you can do is raise the dish, move it back from the obstacle, remove the obstacle (if you can) or move to another location. All these possibilities have potential domestic, neighbourly, physical, visual and technical  limitations as well as almost inevitable expense.  

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[P19] Partial success.

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There was no problem receiving the UK Spot Beam from first switch on but dish pointing was ultra-sensitive. A couple of millimetres of movement on the rim of the dish was enough to throw it off the satellite. Though the Pan-European channels hardly noticed as I pushed the dish about. Reception died early today. Around 1pm I was getting nothing but No Signal notices on all the Spot Beam channels.

I tried moving the LNB nearer the dish but managed to lose all 28E channels. Traced to a poor F-plug on the straight connector between two long lengths of coax. The ground on the screening had lost contact because the poor quality aluminium braid and screen had vanished. 

I moved the feed back to the Salora dish and confirmed the lack of reception. It's odd how reception times vary so much.

What have I learnt today?

That the huge dish needs to be massively supported to avoid the slightest unwanted movement.
My test stand is only just good enough in still conditions.

That fine azimuth adjustment is as absolutely necessary as the fine vertical adjustment already provided.

That the Satfinder meter has very limited use when swamped by the signal but can still be relied on to confirm rough centring.

That correct cable preparation is vital to a good F-plug connection.

Copper screened coax cable is better than aluminium.

I shall now go ahead and make a decision on siting and which form of support to use.

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[P18] First light!

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I'm sure there must be a proper expression for first 'radio' reception but we'll pretend that it is a telescope and call it "first light."

Having fitted the dish to the tetrahedral stand and aligned the Inverto LNB and IRTE feedhorn I moved the dish onto 28E at about 18.45pm CET. This would be long after any chance of reception on the 1.8m Salora dish.

I was quickly able to get solid signals from the Pan-European beam. No surprises there. Sadly I could not lock onto a single Spot Beam channel. The Satfinder meter wailed at all settings of the control knob but was still handy to home in  optimum azimuth and altitude on 28E. Flexing the dish proved that there was nothing to gain from any further adjustment. The wail died.

Things to do tomorrow:

Move the LNB closer to the dish to check for improvement either side of theoretical focal distance. The feedhorn is presently 75.5cm from the dish when F/D should be 75cm. Easily managed by adjusting a few nuts on the feed arms holding the feed boss.

Move the dish back to give it a better chance of seeing over the eastern hedge. The angle of elevation remains the same. So a line to the satellite from the bottom of the dish clears higher obstacles as the distance from the obstruction increases.

Test for Spot Beam pointing accuracy when these channels are strongest around lunch time. This should be slightly east of the Pan Euro beam. Naturally I shall be testing long before maximum to see what is possible. The whole point of the bigger dish is to extend the useful hours of reception.

Put some weights on the frame to avoid tipping in unexpected wind gusts from odd directions.

I don't have enough coax cable to reach the receiver indoors from its present site north of the house. So I am using a receiver out by the dish with a small CRT monitor. If I take down the Salora 1.8 the temporary stand is much too low to allow the 2.2 to see over the front hedge. Raising the 2.2 higher on the stand would make it more unstable by raising the centre of gravity. I need a solid stand for the 2.2 to go where the Salora presently sits. Or slightly further back to allow the front hedge to be taller. So it hides the dish better in the winter without immediate obscuration by new growth in the spring.

Click on any image for an enlargement.

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

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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.

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[P16] Kathrein Original LNB.

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The feedhorn projecting from the protective, feed hub, bowl. The dark green stain is probably lichen.
The three, stainless steel, socket head screws clamp the skew adjustment ring. Loosening these screws would allow the fitter to rotate the LNB to maximise the signal. They would then be tightened again to hold the LNB in perfect alignment. Squareness of the LNB to the dish's axis is vital.

The complete LNB and feedhorn removed from the protective hub. The rusty plates are simple clip on protection for the hidden electronic circuitry. No doubt they were once lightly plated. Even within the protective feed container the assembly would be subject to condensation with changing temperatures.

The frequency range is stated to be 10.95-11.7 GHz.









The rear of the bare feedhorn and skew adjustment clamping ring after removal of the LNB assembly.

The thin red line is just a tightly stretched cord for measuring the depth of the dish.

The bare, protective feed hub still attached to the support arms.

Only two of the arm fixing screws are hollow to allow coaxial cable to run inside the lower, feed support arms.

Other Kathrein dishes had a heavy, aluminium, three-eared, centre boss instead of the cup and protective can.

Note the much smaller bores of the arm fixing screws compared with my own dish. Presumably from an earlier period when smaller coaxial cable was the standard.  













The protective, aluminium, feed cover is held in place on the bowl-shaped, feed boss with three socket head screws. It is made of aluminium with a coarse spray paint coating.

It is 22cm deep x 17.5cm in diameter.

The feedhorn and captive skew adjustment ring after decades of exposure to the weather.

There is no protection in the open end of the feedhorn to stop spiders entering. One spider was still living inside and there were lots of spider webs.

Modern LNBs usually have some sort of plastic screen to stop insect ingress.

The front diameter of the feedhorn which fits the large hole in the feed [bowl shaped ] boss is 63mm. The rear diameter is 60mm Ø. The skew clamping flange is 69.5mm Ø.

One of the hollow screws which clamp the feed support arms to the dish. The spanner size is 19mm. Once removed (with great difficulty!) a powdery white deposit was coating the threads. The rear screws are housed in a local dimple in the dish.

Inside the Kathrein LNB block machined from massive limps of solid aluminium alloy. The Vertical and Horizontal polarisation antennae are the small blades inside the machined housing. Modern LNBs use one wave guide leading to a pair of antennae at right angles to each other.

Another view inside the massive LNB showing part of the signal wave guide. All fixing screws are stainless steel.

The bare protective feed bowl/boss or hub from the inside.

Here the hub has been removed to show the female threads which hold the arm fixing screws. 17mm spanner/wrench flats are provided on the arms to resist torsion when fastening or loosening these fixing screws. The arms themselves are thick wall, large diameter aluminium alloy tube. I recommend that you grease the screw threads afterwards if you do manage to dismantle the hub or dish fixing screws.

I had a real struggle with one badly galled arm fixing thread. It was vital to be able to remove the cup/hub to allow conversion to a modern C120 LNB hub. Just as I did with the Salora dish.

The Kathrein hub/cup is for another standard than C120 and would be quite difficult to convert.

Click on any image for an enlargement. 

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[P15] Kathrein 2.2m Dish parameters.

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The Kathrein dish measures 220cm in diameter [D ] from edge to edge of the active concave surface.

The depth [d] to the centre of the dish, measured from a tightly stretched cord, is 40cm.

The front edge of the feedhorn throat, of the original LNB, is 75cm from the dish surface.

Theoretical focus is D^2 / 16d = 220 x 220 / 16 x 40 = 48400 / 640 = 75.6cm

Focal ratio = 75.6 / 220 cm = .34

Here is a fascinating 1986 Kathrein promotional video on YouTube:

[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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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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