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The pin in the LNB

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Aug 21st, 2008 at 1:02pm  
I have a Hughes system with the NJR2784H LNB.
When I took the LNB off the head, the pin inside the LNB was on the opposite side of the pin inside the tube coming from the feedhorn. Is this pin supposed to be on the opposite side or should it line up with the pin in the tube?
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Reply #1 - Aug 21st, 2008 at 2:02pm  
What you call the "head" would be the OMT/filter (Ortho-Mode Transducer). The "pin" is actually the radio frequency (RF) antenna; specifically, a dipole. Many not in the industry typically refer to the "dish" as an antenna, but it's really just a passive collector/reflector. That little pin is the actual antenna; the transmitter (BUC) has one, as does the receiver (LNB). Generally, the BUC and/or LNB are positioned so the pin is up and down for vertical polarization, sideways for horizontal. That's if there's a rectangular box in the waveguide flange. In the case of a square box, the polarization may be changed electronically (by the modem).

//greg//
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Reply #2 - Aug 21st, 2008 at 2:11pm  
So, does the antenna in the LNB need to line up on the same side with the antenna in the Filter or does the LNB antenna need to be on the opposite side of the Filter antenna as it was when i pulled it out or does it not really matter?

Thank you for the rundown on key terms USN!
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Reply #3 - Aug 21st, 2008 at 3:04pm  
There is typically no active antenna in OMT/filter, so I'm unsure what this 2nd "pin" is you're talking about. The OMT/filter is a component that's designed in part to prevent the BUC  from interfering with the LNB, so consider it a "pass through" device. Having said that, I'd benefit from seeing yours - so that I could be more specific. Can you post a photo link?

In the case of non-electronically switched polarization, the LNB and BUC "pin" is oriented relative to the polarization scheme specified by your provider. Some linear transponders require horizontal transmit/horizontal receive, some are horizontal/vertical, a few vertical/horizontal, and I suppose vertical/vertical is possible - although I've never seen it used.

Anyway, note that there are an even number of machine screws that hold both the LNB and BUC to the OMT. This permits the LNB and/or the BUC to be rotated in four 90 degree increments; horizontal/vertical/horizontal (again)/vertical (again). If/when the waveguide flange is rectangular, the "pin" will be oriented perpendicular.

By the way - since you've obviously had yours apart - make sure that you coat both sides of the O-ring with a light coat of dielectric grease before reassembling.

//greg//
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« Last Edit: Aug 21st, 2008 at 5:29pm by USN - Retired »  

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Reply #4 - Aug 21st, 2008 at 4:31pm  
These two images below show horizontal polarisation.  It is of no consequence if the half dipole pin is on the left or on the right.   It is still horizontal polarisation if the line of the pin is horizontal.
...            ...

Always join rectangular waveguides so that the apertures match up.  The same applies to the rectangular hole in a gasket, if used.

Here is an external image of a cross-pol OMT with filter/diplexer assembly:
...

I am doubtful about there being a pin inside the filter as transmit reject/diplexer filters are generally complex coupled cavity structures.   If there is something unusual there check it is not some loose metal part dropped in, which would tend to cause degradation around certain resonant frequencies.

Best regards, Eric.
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« Last Edit: Aug 21st, 2008 at 6:21pm by Admin1 »  
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Reply #5 - Aug 21st, 2008 at 5:32pm  
JonnyRotten wrote
Quote:
Try these instead to view pics
Sorry, not enough detail to make out what you think you see in the OMT. But I see Eric has added his skepticism to mine. Don't have any idea of what you're seeing in there.

//greg//
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« Last Edit: Feb 20th, 2015 at 4:43pm by Admin1 »  

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Reply #6 - Aug 22nd, 2008 at 7:24am  
The thing inside the filter is part of the mechanism. It is just a little nub of metal. It is smoothly rounded like the tip of a ball point ink pen. It is roughly 1.5 mm tall. It doesn't look like a part that has been added to the filter. It is a part of the metal that was molded.

Sorry I can't seem to get the lighting adequate to take a highly detailed and focused picture inside the filter.

Thank you both for your assistance. You have been very informative and helpful.
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Reply #7 - Sep 2nd, 2008 at 10:40am  
Such dimples into waveguides are intentional and help the radio waves smoothly transition from one waveguide to the next.  In some cases, such as at the ends of a cross site Andrew elliptiguide, there may be three adjustable tiny screws, on the centre line.  These screws may be adjusted to get a perfect match while running a broadband sweeper and detector connected across ends of the whole waveguide network.   For solid waveguides you can squeeze the guide gently with a G clamp and see what happens to the frequency response and when you have found the sweet spot, remove the clamp and hit the guide at the exact spot with a centre punch marker to make a slight dent. This has the effect of adding a tiny capacitor across the guide.

Best done with great caution and multi $000 test equipment, but worth it to minimise reflected power when working with several 100 watt HPAs.

Best regards, Eric.
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Reply #8 - Sep 3rd, 2008 at 2:47pm  
Here is an interesting picture looking into the end of an old LNB.
...

It is oriented for horizontal polarisation.  The actual receiving antenna is a metal half-dipole pin inside the PTFE cylinder in the middle.  The PTFE helps make the dipole broadband, as it is very difficult to get the transition from waveguide to wire to work over more than a couple of 100 MHz.

The other two objects are a pin right across the guide (looks like an inductor) and a screw which has been adjusted to about half way (looks like a capacitor).  These help with matching and bandwidth.  It is very time consuming to test with these devices in multiple places till you get it right.    I tried different dipole pins and I spent 2 weeks trying a dozens different pin shapes, wire thicknesses and blobs, in 1968, and did not succeed at all.

Note there is a copper gasket with a rectangular aperture, oriented to match the LNB waveguide. You can easily get this wrong so take care with assembly and maybe ask someone to visually check what you are doing when you put it together.

Always have a helper check you if you are connecting up high power TX waveguides.  Also, make sure the HPA is powered off and that you have the key(s) to the HPA mains power switch lock, and any TX waveguide switches,  in your pocket !  As a rule never look into any open ended waveguide.  Treat as quite dangerous.

Best regards, Eric.
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« Last Edit: Sep 4th, 2008 at 1:50pm by Eric Johnston »  
 
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