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So much for the plans of mice and men. It was late afternoon before I was able to work on the feed again. By which time, of course, there were no SB channels available.
The IRTE complicates the issue (considerably) by having a concave curve to the forward edges of the scalar rings. The Invacom which I was trying to emulate is flat across the face of the scalar rings. There is obviously more to feedhorn deign than throwing a few rings around the waveguide throat. The original Kathrein feedhorn has only one scalar ring and is supposed to be matched to the dish. Yet the (supposedly mismatched) IRTE bettered my brass copy [with the missing C120 LNB fixing added] by 2% points of Q.
The IRTE complicates the issue (considerably) by having a concave curve to the forward edges of the scalar rings. The Invacom which I was trying to emulate is flat across the face of the scalar rings. There is obviously more to feedhorn deign than throwing a few rings around the waveguide throat. The original Kathrein feedhorn has only one scalar ring and is supposed to be matched to the dish. Yet the (supposedly mismatched) IRTE bettered my brass copy [with the missing C120 LNB fixing added] by 2% points of Q.
First I switched the Fortec Star Passion HD receiver to Sky News on Astra 28E and checked the bare feedhorn S/Q @ 78/75% for a baseline. I refitted the ring and I instantly had 79/82% !! 7% points increase in Q is certainly worth having! What went so wrong yesterday?
Then began the task of turning the ring down to make it shorter to see what would happen. I started with a 5.5mm extension and reduced it to 4mm. 79/85%!! That's more like it! Then I reduced the extension to 3mm. S/Q dropped to 79/82%. Whoops! A bit too much!
I tried pulling the ring out a little to compensate but S/Q would not budge from 79/82%. Mind the gap!?!
So I was forced to turn another ring with more extension. This went very quickly this time because I had the old ring to measure. The throat dimensions were again matched to the IRTE feedhorn.
I made the extension too long expecting a much worse result: With a measured 5mm extension beyond the plastic cap I now had S/Q of 79/84%. Good enough! That's 9% points of Q over the bare feedhorn! I removed the new extension ring again to check. The bare feedhorn was still showing 78/75%. The new (5mm extension) ring is shown fitted to the IRTE feeedhorn above. The darker ring is just a reflection of the extension ring in the clear plastic cap.
I seem to have come full circle. Allowing for the thickness of the plastic cap the extension now matches the Invacom tuning literature at 5.5mm for 0.34 F/D. Should I shorten the ring slightly to try and get that 1% Q back again? I am now haunted by the thought that the plastic cap is insulating the extension ring when it ought to be earthed to the body of the feedhorn to form the entire waveguide from one lump of metal. I could always add the plastic cap to the extension ring if I knew what adhesive to use.
I made the extension too long expecting a much worse result: With a measured 5mm extension beyond the plastic cap I now had S/Q of 79/84%. Good enough! That's 9% points of Q over the bare feedhorn! I removed the new extension ring again to check. The bare feedhorn was still showing 78/75%. The new (5mm extension) ring is shown fitted to the IRTE feeedhorn above. The darker ring is just a reflection of the extension ring in the clear plastic cap.
I seem to have come full circle. Allowing for the thickness of the plastic cap the extension now matches the Invacom tuning literature at 5.5mm for 0.34 F/D. Should I shorten the ring slightly to try and get that 1% Q back again? I am now haunted by the thought that the plastic cap is insulating the extension ring when it ought to be earthed to the body of the feedhorn to form the entire waveguide from one lump of metal. I could always add the plastic cap to the extension ring if I knew what adhesive to use.
I tried other channels but still no UK Spot Beam on 28E at this time. [19.00pm CET]
Will the considerable "improvement" in Q on Sky News spread itself across the UK SB when there is a signal tomorrow? Who knows until I try? Why were my results so poor yesterday after I fitted the extension ring? I have absolutely no idea at the moment! Was I using a different frequency yesterday? Is the ring altering the feed [LNB/feedhorn] frequency response? Read tomorrow's exciting episode! When it (probably) all goes horribly wrong again! ;)
Tomorrow was cancelled. At least it was for the UK Spot Beam. Not one single channel showed a signal! I removed the ring thinking this was the cause but this had no effect. The Pan Euro beam was fine but with strange ups and downs in signal Quality. All depending on whether the extension ring was in place, or not. The presumption must be that the extension ring spoils the (supposedly flat) frequency response of the IRTE feedhorn. I have now ordered an Invacom ADF-120 feedhorn to see if it helps to get a better F/D match.I shall have to devise a clamp for the feedhorn neck to adapt it to the central feed boss.
Realigning the dish brought back Astra 28E UKSB for the usual short reception period around lunchtime. The feedhorn extension ring helped.
Now I have to decide whether it is worth the effort to dig a deep hole and fix a pole in concrete. I have the pole but I have the dish hanging temporarily on an angle iron trapezium. Will a firm pole and the original mounting provide any extra signal? It would certainly ensure the dish didn't move. The large dish is extremely sensitive to pointing. The problem with poles is that they are fixed. Access is still required behind the dish for adjustment. Though pushing the dish back as far as possible would be beneficial for other reasons: It takes up slightly less space in the parking area and looks slightly smaller with distance. Height is another important consideration.
Tuesday: My new Invacom feedhorn has now been with DHL Germany for five whole days without moving! [According to their own Track and Trace web page] I paid extra for premium airmail too! Thank you DHL for falling over yourselves to get the item to me! You can look forwards to my negative comments on Trust Pilot. Wednesday: DHL updates to its T&T say the package arrived in Denmark 3 days ago. Though it took them three days to update this information. Still no word whether it is out for delivery today
Wednesday: It seems the package might have been handled by Deutsche Post for delivery via Post Danmark but who knows? DHL's T&T web page can't even carry the tracking number over from German to English! A new search has to be made in the new language every time I hit the English button. Which means saving the tracking number to paste it into the new box. It's 2014? Don't you believe it!
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Tomorrow was cancelled. At least it was for the UK Spot Beam. Not one single channel showed a signal! I removed the ring thinking this was the cause but this had no effect. The Pan Euro beam was fine but with strange ups and downs in signal Quality. All depending on whether the extension ring was in place, or not. The presumption must be that the extension ring spoils the (supposedly flat) frequency response of the IRTE feedhorn. I have now ordered an Invacom ADF-120 feedhorn to see if it helps to get a better F/D match.I shall have to devise a clamp for the feedhorn neck to adapt it to the central feed boss.
Realigning the dish brought back Astra 28E UKSB for the usual short reception period around lunchtime. The feedhorn extension ring helped.
Now I have to decide whether it is worth the effort to dig a deep hole and fix a pole in concrete. I have the pole but I have the dish hanging temporarily on an angle iron trapezium. Will a firm pole and the original mounting provide any extra signal? It would certainly ensure the dish didn't move. The large dish is extremely sensitive to pointing. The problem with poles is that they are fixed. Access is still required behind the dish for adjustment. Though pushing the dish back as far as possible would be beneficial for other reasons: It takes up slightly less space in the parking area and looks slightly smaller with distance. Height is another important consideration.
Tuesday: My new Invacom feedhorn has now been with DHL Germany for five whole days without moving! [According to their own Track and Trace web page] I paid extra for premium airmail too! Thank you DHL for falling over yourselves to get the item to me! You can look forwards to my negative comments on Trust Pilot. Wednesday: DHL updates to its T&T say the package arrived in Denmark 3 days ago. Though it took them three days to update this information. Still no word whether it is out for delivery today
Wednesday: It seems the package might have been handled by Deutsche Post for delivery via Post Danmark but who knows? DHL's T&T web page can't even carry the tracking number over from German to English! A new search has to be made in the new language every time I hit the English button. Which means saving the tracking number to paste it into the new box. It's 2014? Don't you believe it!
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