[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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