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Re: Which rocket lens ?

Posted: Sat Apr 20, 2019 11:14 am
by 2W0ODS
g4eml wrote:
Sat Apr 20, 2019 9:54 am
3D printing could produce something that looks like a dielectric lens but almost certainly wouldn’t work like one.
The dielectric constant of the material used will be very critical so different plastics or even different batches of the same plastic could vary. Even if you managed to measure and reproduce the exact formulation used in the commercial lenses the layered structure of a 3D printed lens would most likely affect the results.
The commercial lenses are actually made in two interlocking parts. I wouldn’t be surprised if the dielectric constant of the two plastics is different and the transition between them is also part of the design.
That's what I thought normally, but these lenses on the rockets and other LNB's are just caps really and don't seem to be anything special.
Quoting the link above these people http://www.hybridpretender.nl/ state the PLA printed lens performs as well as the milled PA lens.

Re: Which rocket lens ?

Posted: Sat Apr 20, 2019 1:20 pm
by g4eml
The cover on a standard LNB is there mainly for waterproofing, the horn is there to form the correct radiation pattern.
The rocket LNBs do not have a horn and rely on the dielectric lens to form the correct pattern. It is a complex multi ring structure made of two concentric interlocking pieces to provide the needed variation in dielectric constant. Far from ‘just a cap’

The quoted website says the printed lens is as good as the milled one but it has been shown that that neither perform as well as the ones removed from a genuine rocket LNBs. For the narrowband transponder they are probably adequate but for DATV every dB is very valuable.

Re: Which rocket lens ?

Posted: Sat Apr 20, 2019 4:03 pm
by G4WIM
Hi guys,

a useful and informative thread which has prompted me to order one of these LNB's AB LNB 01 as Jen suggested.

I'll do some tests with / without lens and also using a copper 42 / 22mm reducer.

73 Tim

Re: Which rocket lens ?

Posted: Sat Apr 20, 2019 6:11 pm
by g4hiz
Hi all,

I have attached a photo showing the white cap lens feed from the AB LNB 01 in place on my dual band feed. During integration of the antenna feed, I tried reception without and with the lens. Bearing in mind that the prime-focus dish used in this case has an f/d = 0.4, the lens may not be needed - one thought. But in the reception test performed, the 10-12% BER without the lens was reduced to about 4-6% BER with the lens, so the lens was deemed beneficial. The original dish feed had choke rings to reduce feed side-lobes and I believe the the lens does the same job to reduce side-lobe noise pickup.

73
Jen G4HIZ

Re: Which rocket lens ?

Posted: Sat Apr 20, 2019 6:38 pm
by g0mjw
A 0.4 F/d is not really suited to the lens - so glad it is working.

For s-band it will be good. This is what W1GHZ predicts for the S-band patch - its best at 0.5 but OK at 0.4 and 0.6. Don't forget in practice it will be 10-20% less efficient than predicted here as there are no losses included.
patch.jpg
patch.jpg (166.42 KiB) Viewed 782 times
Here is what a typical small horn (my 42mm plumbing fitting) might be like.
42mmHorn.PNG
42mmHorn.PNG (70.01 KiB) Viewed 782 times
Very much designed for a higher F/d. It is about right for a 0.6 f/D which is just as well as that is what it was designed for. I would expect to be at least a dB down on a matched feed. The one to use would be the VE4MA feed like the original. I don't know how much it would detune the patch though. The notched disc would be better.
ve4ma.PNG
ve4ma.PNG (65.33 KiB) Viewed 782 times

If the rocket lens is giving a good result stick with it. Note also that the broadcast transponders and amateur will be different. That's because there are adjacent satellites in Ku band that may limit the signal to interference ratio whereas there are no interferers (yet) at 10.5 GHz so you are looking at pure signal to noise. Under illuminating the dish will reduce sidelobes and reduce the noise, but it will broaden the beam to if there were adjacent signals they would not be rejected as well, but there are none so it doesn't matter. Hoping this makes sense.

Mike

Re: Which rocket lens ?

Posted: Sun Apr 21, 2019 10:53 pm
by 2W0ODS
g4eml wrote:
Sat Apr 20, 2019 1:20 pm
The cover on a standard LNB is there mainly for waterproofing, the horn is there to form the correct radiation pattern.
The rocket LNBs do not have a horn and rely on the dielectric lens to form the correct pattern. It is a complex multi ring structure made of two concentric interlocking pieces to provide the needed variation in dielectric constant. Far from ‘just a cap’

The quoted website says the printed lens is as good as the milled one but it has been shown that that neither perform as well as the ones removed from a genuine rocket LNBs. For the narrowband transponder they are probably adequate but for DATV every dB is very valuable.
That makes perfect sense, I wrongly assumed these were used just to protect from water ingress.

Re: Which rocket lens ?

Posted: Mon Apr 22, 2019 7:19 pm
by g4hiz
Hi all,
as concerns the AB LNB 01 lens shown in my earlier post. You can see typical reception below for my 90cm centre-fed, f/d 0.4, antenna - MER 8dB for the WB beacon. [LNB Octagon with locked 26MHz xtal. Receiver: Sharp Minitiouner running v0.8].

73
Jen

Re: Which rocket lens ?

Posted: Mon Apr 29, 2019 9:56 pm
by G8PEF
Ok, folks - all the pictures in this thread show rocket lenses with a point (presumably where they get their name?)... I have here a similar LNB with a red plastic 'lens' on the end of the W/G - but it has a flat end. is this likely to work with the dual band QO-100 feed in the other thread?

For reference, the LNB I have is the same as this one on eBay: item # 162643222704 The innards look very much the same as the pic of the one with the white plastic case, further up this thread, except that the 'lens' has a flat-ish front face, not conical.

73
John

Re: Which rocket lens ?

Posted: Tue Apr 30, 2019 5:41 am
by g0mjw
That's the one you want, with the flat end and in two pieces that slot together. The pointy thing it for cases where you cannot get a rocket lens, and there is a newer design by HB9PZK of that which is better that will be published soon.

Mike

Re: Which rocket lens ?

Posted: Tue Apr 30, 2019 1:58 pm
by G4WIM
just as a data point I bought and modified one of these AB LNB 01 as recommended by Jen.

I re-used the LNB and lens on my dual feed and found that the lens improved the beacon MER from 5dB to 8dB,

I reduced the diameter of the LNB shaft such that is was a snug fit in the 22mm copper pipe.