How does a BUD do skew?

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luiscolo

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I'm quiet used to Ku band where you set skew by giving the LNB a twist to the corresponding side but what about C band? I guess nothing physicaly moves the LNB but just what goes on?
 
phlatwound

phlatwound

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If you're talking about a fixed C-band dish it's the same deal as the Ku setup you are familiar with., you just manually skew the feed.

If you are talking about a motorized setup (C or Ku) the polar (or H-to-H) mount skews the feed as it travels the arc. :)
 
delta_charlie

delta_charlie

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I'm quiet used to Ku band where you set skew by giving the LNB a twist to the corresponding side but what about C band? I guess nothing physicaly moves the LNB but just what goes on?

Hi, the older feedhorns have a probe inside that moves. It is attached to a servo motor and is controlled by old analog receivers. Or there is at least one modern DVB receiver that can control the servo motor (Pansat 3500 SD)

On these older feeds they have to be mounted on the dish to specification (arrow in manual about 11:00 clock position to the polar axis if I remember correct). Once this is done the probe will select between vertical and horizontal polarity and can fine tune the signal.

Hope this helps, DC
 
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melgarga

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Kind of a trick question. Skew as in the (90°) difference between vertical and horizontal TPs, or "polar offset", which is the relative difference in a given (H or V) polarization from one satellite to the next.
As posted, polarity is selected in legacy C band setups by a polarotor servo (a small bidirectional PWM driven motor with intregal position feedback) that mechanically positions an element inside the LNB feed assy.
Analog (legacy) IRDs, some 1st gen satellite positioner boxes, and one or two Pansat STBs models are the only things commercially built that can control this servo assy. Some newer design (not better IMHO) LNBFs are designed to operate like it's Ku cousins, and use 13 and 18vdc levels to select V or H polarity.
I gather from your post that "quite familiar" and "giving the LNB a twist" your alignment experience is with non-motorized setups. In motorized setups of both flavors, the skew or "polar offset" is compensated for in the mechaniical geometry of the positioning assy.
If you plot the points from E to W you will see that an arc track does not simply pan the sky from one limit to the other, but is "distorted" in that at the E/W ends it is "looking" nearly due E/W (at the horizon) but at the centerpoint (due south) it is looking up into the sky, not at the southern horizon. The amount of "distortion" is relative to the dish's latitude, and more pronounced the closer the dish is to the equator.
Move the dish to it's highest point ( true south) and stand behind it an look where it is looking. Envision crosshairs +, the - is the horizontal signal and the l is the vertical one. Now motor the dish to say 30° or so west. The relationship of LNB to reflector did not change so neither did the crosshairs, but now the crosshairs are rotated on it's centerpoint to the right (clockwise), but is still receiving H and V correctly from the satellite it is pointed to at that orbital location.
True south is a critical point for aligning a C band array. It is the point where H and V signals are correctly parallel or perpendicular to the horizon for the dish's locale, the highest point in the arc for that locale, and the only point where skew, or "polar offset" is zero.
The dish/LNB assy did not rotate in the strictest sense, but due to the declination adjustment, the portion attached to the upper pivot point travels further than it's lower counterpart when the position assy moves from true south to points either side of the arc. It is pretty ingenious.
Offset dish drive assys accomplish the same 'trick' using the 'dogleg bend' in the mounting tube.
This is my favorite reference site for this type of discussion -
http://www.geo-orbit.org/sizepgs/tuningp4.html
It is kind of an "Everything you wanted to know about C band BUDs" type of place. It should be Wiki'd.
The next to the last graphic gives several good two dimentional examples of inclination, declination, and true south adjustment relationships. Unfortunately skew is not accounted for in those examples.
 
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