Problems Installing Chaparral Superfeed Single Polarity Feed Horn
I just purchased a Chaparral Single polarity C-band Feedhorn, (SuperFEED) and am a bit confused how to fine tune polarity with this feedhorn. We only require horizontal polarity on our fixed prime focus dish, however, I was under the impression that the throat would be rotatable much like other Chaparral feed horns I have used. The picture and description on chaparral's website did not indicate it was a single welded piece. This makes it nearly impossible to reposition the LNB one or two degrees in either direction. The only way to adjust is to use the different screw holes to re-position the entire feedhorn - which would change everything by 15 to 20 degrees.
I'm installing a Norsat 3000 series c-band LNB and would appreciate any suggestions on how fine tuning polarity can be accomplished using this feedhorn.
Or.... did I buy the wrong Feedhorn for this dish?
Actually the Chaparral "Superfeed" doesn't have a probe inside the feedhorn, due to the fact that it's a single polarity feedhorn, there's just a large opening at the back end of the unit for placement of the LNB. Nothing inside the cavity.
If you happened to have a buttonhook support, this adjustment could be child's play. If you have support arms, it's sometimes possible to twist the feed a bit back-and-forth, meaning the arms will no longer point at the center of the feed. This will affect the focal point somewhat, but for small adjustments it probably won't matter. If I had to rotate between two polarities with a single polarity feed, I'd probably cobble together some form of slip plate.
One of the other downsides of feeds like this is they have a fixed f/D. I suppose you could cut off the scalar and substitute another. I've picked up extra scalars for around $5-10 from a couple of dealers. The nice thing about feeds like this is they have extremely low losses.
assuming you have a polar mount dish (although without an actuator arm or in other words a fixed dish) I think you better aim the dish to true south and install the Feed horn and LNB as 0 skew (make sure it is properly set for 0 horizontal) then using some sort of pipe or metallic broom simulating an actuator arm simply "move the dish" to the exact position it will have if it had an actuator arm. I know it sounds easy but it is not , but if your polar mount was already setup (it has declination and polar elevation set already) all you have to do is find true south and the "move" the dish manually pushing using the pipe or broom "actuator".