Hi guys,

What kind of adapter is this one?

https://www.satsig.net/forum-archive/waveguide-adapter.jpg

Thanks,
Regards.

mrninni wrote on Mar 30^th, 2011 at 4:58pm:

Hi guys, What kind of adapter is this one? https://www.satsig.net/forum-archive/waveguide-adapter.jpg Thanks, Regards.

This allows the LNBF (or LNB) to attach to the feed horn.
Feedhorn ---> WR75 adapter >---- LNB.
Most of these are used on receive only antennas.
The feed horn mounts between two clamps and allows you to adjust the skew (or cross-pol) of the satellite you are looking at.  

The "front" side, with the approximately rectangular aperture, goes to the rectangular waveguide hole in the LNB.

The "back" side, with the circular ring, goes to the circular waveguide of the feed horn throat. Was a rubber "O" ring supplied to seal the joint from water ?

It may be described as a "circular to rectangular waveguide adaptor". This one is very short and has a "step" to minimise reflections.  Ideally the two waveguides (circular and rectangular) should be joined using a long gradual taper. That provides the best match over the broadest frequency range.

Dear kingofku and Admin,

Thanks for your reply.
I confirm that the adaptor has a rubber o-ring.

At the moment we are using the feed horn with a OMT, then (i think) the adaptor it is not necessary.

Good bye.

That adaptor is normally used in a receive only, single polarisation, system.

If you have an OMT you must have circular or square waveguide between the OMT and the Feed to allow both polarisations to pass.

A rectangular waveguide will only let one polarisation through.
e.g. vertical polarisation: https://www.satsig.net/images/waveguide-vertical-polarisation.gif

The function of the rubber "O" ring is to stop moisture getting in. This ring may be smeared with a very small amount of silicone grease, but be certain that no silicone gets into the waveguide. Tighten the bolts holding the flanges together equally so that the faces of the waveguide flanges are exactly parallel with each other. If they are not exactly parallel the result is poor cross pol isolation.

Sometimes there is a much narrower and deeper slot ring called a "choke".  Such a ring is quarter wave deep and spaced quarter wave from the waveguide wall.  It produces the equivalent of a short circuit across from the metal wall of one waveguide to the metal wall of the other, when there is actually a small air gap all around.  Such choke joints are always used in circular to circular waveguide joints where rotation is required such as radar or up the co-axial axis of rotating tracking antennas.

Best regards, Eric.