Dish Feeds

[g8gtz]

Hi Gareth,

That's right - allow about 15mm above the top plate and whatever is needed behind the back plate.

Common method of mounting LNB seems to mill / file down the o/s diameter so its fits in to a 22m compression back to back coupler - and you use that joint to adjust the LNB skew rather than twist the hole plate.

Just heating up the soldering iron to do mine

73
Noel

[radiogareth]

Progress on two projects today, one of them DATV. Using a new Octagon OSL SO I opened the case (tabbed, two down each side) and then carefully filed off the two die-cast tabs just behind the horn. Then I hacksawed off the concentric horn unit and cleaned up the edge.
This is a very loose fit in a 22mm compression coupler, in fact it passes through the stop-ring in the centre of the coupling. This is helpful as its not a bad fit in there. To make the 22mm olive grip the LNB it needed sectioning and about 4-5 mm removing equally (2.5 each) from the two halves. With some careful assembly the whole assembly can then be tightened up and securely grips the LNB outer body.
Pictures show the outcome.

2019-02-11 17.00.51.jpg (89.96 KiB) Viewed 6538 times

2019-02-11 17.02.28.jpg (66.4 KiB) Viewed 6538 times

Waiting for the 'Rocket' LNB nose-cone to finally size the copper pipe length, in the meantime making up an adaptor ring from HEX central nut to 40mm OD for fitting it all into the LNB holder.
Gas torch at the ready for the soldering job, just a bit beyond my 50 watt Weller
Gareth

[radiogareth]

That was fun

Capture.PNG (19.66 KiB) Viewed 6534 times

30 mins to print.
Gareth

[G7JTT]

Well took the plunge and cut off the horn on my Goobay LNB and fitted it to the dual band feed lost 4MER on the received signal from QO-100 but I don't have the rocket LNB lens/cap yet hopefully it'll improve the signal again.
https://ibb.co/92ByfPL

[g8gtz]

Maybe you've put the feed point too far in to the dish past the feed point - presume the end of the 22mm needs to be at the same point as the end of the LNB horn?

Noel

[gi7ugv]

I had 4/5dB MER with the chopped Venton LNB on my dual feed including the cap then a solid 6dB when swapped to the Octagon.

The positioning of the dual feed was a little too far forward and there was a slight tilt upwards with the weight of the pipe/fitting I was using so there's hopefully a little improvement to be had when I sort out the LNB holder.

[G7JTT]

I tried moving the feed back and fourth but no real difference, I held the cut off horn in front of the 22mm copper and the MER improved to around 6. Will have to experiment some more tomorrow in the light.

John

[radiogareth]

My rocket LNB arrived today, the cone 'feels' like polythene or polypropylene and is quite a complex internal shape which I don't fancy reverse-engineering at this point. The tiny sliver I removed and burned also pointed to one of the two plastics. Neither are 'last forever' outdoors in sunlight unless possessed of UV additives, but for under a tenner delivered its the easiest source of a lens at this point.
Its a 'tight snap-over' fit on the end of the LNB and its a LOT easier to remove if you soak the red bit it in a mug of boiling water then apply a 'twist and lean to one side' motion to get it off.
Its a two piece mounding so if remote mounting some form of retaining method would be a good idea for the outer part. This will make sense when you have one in your hand. The inner is a snug fit inside the 22mm copper.
Gareth

[g8gtz]

I'm another convert to the MJW patch - works really well on transmit, much better on 1.2mt offset dish than an RFhams 1.2 prime with helical feed.

However would welcome comments re receive.

Currently I do have a quad octagon bodged on the back of the 22mm pipe - waiting for a goobay with a longer throat to mount properly.

LNB in standard mount = 8 dB (about right for 1.2mt dish - https://wiki.batc.org.uk/Receiving_Osca ... d_stations)

With open 22mm pipe it is 4 -5 dB down (3dB MER) on the standard LNB.

With a quad octagon feed pushed in to the front of the 22mm pipe it is about 2 dB MER down (6dB MER) but the tx is 4 dB down, clearly blocking / detuning the patch.

With the Rocket red cap lens pushed right in to the end it is about 3dB down (5 dB) and no Tx blocking

Is this normal or should it be better - what do other people do - hopefully it will be better when the LNB is not bodged!

73

Noel

[G7JTT]

Have to agree Noel the patch works very well at 2.4Ghz but the 3cm Rx is somewhat compromised. I also tried other horns and alike and can get the MER up to almost the same as a complete LNB but the match on the 2.4Ghz is not good. Think I'll put a 80cm dish up that I have and just try that.

John G7JTT