Adding a passive feed to a C band LNB?

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Adding a passive feed to a C band LNB?

Post by radiogareth » Tue Apr 27, 2021 8:41 am

In eager anticipation of being able to send and receive DATV on 3.4GHz I was looking at ways of adding gain/filtering to my receive system and at the same time down-converting so I can use my Minitiouner.
So I have a Titanium C band LNB (C1W PLL) which has two 1/4 wave probes set at 90 degrees to each other and spaced at 1/4 and 1/2 wavelength from the back of the circular waveguide.
For RX and TX I'm using a pair of 4x4 patch aerials salvaged from a couple of Wimax link units, they only needed a SMA adding to provide a feed.
I was wondering how to add a passive feed into the LNB, so I built a cantenna using the handy online calculator that I found here (Thanks Dave for the suggestion...) I found that central heating combi-boiler inner flue pipe was a perfect fit inside the Titanium LNB. A SMA bulkhead plug was modified with a 2mm probe and fitted to the pipe. I made the back face of the cavity adjustable for resonance and matching experiments (on testing it seems to mainly affect resonance and I'd guess that probe length affects the match - if not please suggest what to try). It looks good on my NanoVNAV2 and it certainly has useful gain as tested with a pair of Langstones, one sending and one receiving. It will be passed through a narrow pipe-cap filter centred on 3.404 and showing about 1dB of insertion loss.
Then I thought...."Why not just add a third probe to the LNB waveguide? Spaced at 3/4 wavelength and lined up with one or the other of the existing probes.
Seems a logical thing to try....but before I do, is my thinking flawed? Should I stick with the coupled cantenna concept?
I was fortunate in finding a Stealth 20W amp for the band and I intend to use one panel for TX, the other for RX. Do I need to provide either shielding between the two panels or should I perhaps switch the RX to a dummy load on TX? Although my low power tests show a very clear single lobe off the front of the panel, that's not to say at 20 watts enough will leak across to the RX panel to cause front end harm. Advice and guidance please.....
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Re: Adding a passive feed to a C band LNB?

Post by g4eml » Tue Apr 27, 2021 3:37 pm

Essentially what you have built is a coax to circular waveguide transition. That should be reasonably low loss.


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Re: Adding a passive feed to a C band LNB?

Post by g8gtz » Wed Apr 28, 2021 7:46 am

I agree with Colin and have used similar on 10GHz LNBs

Have a Google around and you find the correct dimensions.


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Re: Adding a passive feed to a C band LNB?

Post by g0mjw » Wed Apr 28, 2021 9:14 am

Yes, a transition. Did you consider making a horn antenna out of it? Same size as the patch array, probably less lossy and rather easy to fabricate. Filtering can be done in the waveguide.


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