Biswajit.HD
Contributor
- Joined
- 5 Aug 2011
- Messages
- 37,813
- Reaction score
- 4,633
Begin with assembly of feedhorn and LNB and checking mounting of the feedhorn. In a center focus (prime focus) system, all legs supporting the feedhorn should be the same length, measure them to be sure, and do any adjustment you can if they are not the same length.
Next, check the distance from three different points on the edge of the dish, to the center of the feedhorn, to be sure feed is set to be in the center of the dish. I like to use a focal finder to determine if the feed is aimed at the center of the dish. It is a plastic cup (dual cups, for standard C and Ku-band feed scalar rings) that fits over the innermost scalar ring with a telescoping pointer which, when extended, will indicate exactly if the feed is directed to the center of the dish. Remember, even if the feed legs have the same length, that does not mean the feed is centered! You might have to 'bend' the feed into the center of the dish, or adjust the feed support legs for the feed to be centered in the dish. In assembly of the feedhorn/LNB, do not place sealant on feed gaskets - they are meant to be installed dry; and the gasket thickness should be such that there is metal-to-metal contact of the flange contacts after bolt tightening. Do not overtighten bolts, flanges will crack. Leave off the elbows, if you want, and let the LNB(s) stick out to the side - you will gain three dB if you do.
If you are using an adjustable noise interference ring (commonly called scalar rings) then set the F/D ratio in accordance with manufacturer's instructions. (If you do not know the F/D ratio, then calculate it using formula in diagram below.) Do not overtighten the screw/bolt that connects the scalar ring to the feed throat as those pieces will also crack. During the feed installation process, be sure do not touch the probe inside the feedhorn nor the probe inside the LNB throat as oil from your fingerprints could interfere with signal reception; and definitely do not bend the probes as they are finely adjusted and any impairment to their shape or position will inhibit their performance.
After feed is mounted, use a measuring tape to ensure focal distance of the feedhorn is properly set; the measurement distance can be found in the instruction manual of the dish. Measure from the absolute center of the dish to the front of the polarotor and adjust the distance to as exactly as possible to what the dish instructions say. A 1/8" either way will greatly affect video reception performance on weaker satellites. Imagine wearing eyeglasses and understand that a dish focuses beam energy into the feed as do glasses focus light into your eyes; your eyes and dish can only see as well as they are focused. Dish to feedhorn misalignments of 0.5" reduces collected C-Band signal power by 50% (-3dB)!!
For instance, the difference in signal gain between a 10 foot dish and 7 foot dish is about 3dB; an improperly aligned feedhorn wastes this gain differential. Without a doubt, an incorrectly placed feedhorn compromises picture quality resulting in detrimental video effects such as picture 'snow' or horizontal lines. If you do not know the focal length, then calculate it. At the end of all feedhorn adjustments and installation, once again check to be sure feed is still centered within the dish. If the face of the scalar is not parallel (equidistant) to the dish face then you will have to carefully bend it into place if the feed legs do not allow for appropriate adjustment .When adjusting the feed, be sure not to touch the probe inside the feedhorn as oil from your fingerprints could interfere with signal reception; in fact, there is no need to remove the plastic cap covering the feed throat probe except to slide on the scalar rings and to measure the focal length, then immediately replace the cap. Now you may connect the coax cable and servo cables to the feed assembly. When connecting any wiring to the dish or receiver, including LNB coax cables, turn off power to the receiver; better yet, unplug the receiver. NOTE: On F/D ratio, the antenna manufacturer's focal length measurement is probably from a center plate (in the middle of the dish) and is not the value used to calculate the curvature of the dish as the center plate will sit atop the center of the dish by some thickness of metal (most center plates are on the order of 1/8" thick); you can use the recommended focal length setting to set the feedhorn location but you can not use it in calculations without adding the thickness of the center plate.
Next, check the distance from three different points on the edge of the dish, to the center of the feedhorn, to be sure feed is set to be in the center of the dish. I like to use a focal finder to determine if the feed is aimed at the center of the dish. It is a plastic cup (dual cups, for standard C and Ku-band feed scalar rings) that fits over the innermost scalar ring with a telescoping pointer which, when extended, will indicate exactly if the feed is directed to the center of the dish. Remember, even if the feed legs have the same length, that does not mean the feed is centered! You might have to 'bend' the feed into the center of the dish, or adjust the feed support legs for the feed to be centered in the dish. In assembly of the feedhorn/LNB, do not place sealant on feed gaskets - they are meant to be installed dry; and the gasket thickness should be such that there is metal-to-metal contact of the flange contacts after bolt tightening. Do not overtighten bolts, flanges will crack. Leave off the elbows, if you want, and let the LNB(s) stick out to the side - you will gain three dB if you do.
If you are using an adjustable noise interference ring (commonly called scalar rings) then set the F/D ratio in accordance with manufacturer's instructions. (If you do not know the F/D ratio, then calculate it using formula in diagram below.) Do not overtighten the screw/bolt that connects the scalar ring to the feed throat as those pieces will also crack. During the feed installation process, be sure do not touch the probe inside the feedhorn nor the probe inside the LNB throat as oil from your fingerprints could interfere with signal reception; and definitely do not bend the probes as they are finely adjusted and any impairment to their shape or position will inhibit their performance.
After feed is mounted, use a measuring tape to ensure focal distance of the feedhorn is properly set; the measurement distance can be found in the instruction manual of the dish. Measure from the absolute center of the dish to the front of the polarotor and adjust the distance to as exactly as possible to what the dish instructions say. A 1/8" either way will greatly affect video reception performance on weaker satellites. Imagine wearing eyeglasses and understand that a dish focuses beam energy into the feed as do glasses focus light into your eyes; your eyes and dish can only see as well as they are focused. Dish to feedhorn misalignments of 0.5" reduces collected C-Band signal power by 50% (-3dB)!!
For instance, the difference in signal gain between a 10 foot dish and 7 foot dish is about 3dB; an improperly aligned feedhorn wastes this gain differential. Without a doubt, an incorrectly placed feedhorn compromises picture quality resulting in detrimental video effects such as picture 'snow' or horizontal lines. If you do not know the focal length, then calculate it. At the end of all feedhorn adjustments and installation, once again check to be sure feed is still centered within the dish. If the face of the scalar is not parallel (equidistant) to the dish face then you will have to carefully bend it into place if the feed legs do not allow for appropriate adjustment .When adjusting the feed, be sure not to touch the probe inside the feedhorn as oil from your fingerprints could interfere with signal reception; in fact, there is no need to remove the plastic cap covering the feed throat probe except to slide on the scalar rings and to measure the focal length, then immediately replace the cap. Now you may connect the coax cable and servo cables to the feed assembly. When connecting any wiring to the dish or receiver, including LNB coax cables, turn off power to the receiver; better yet, unplug the receiver. NOTE: On F/D ratio, the antenna manufacturer's focal length measurement is probably from a center plate (in the middle of the dish) and is not the value used to calculate the curvature of the dish as the center plate will sit atop the center of the dish by some thickness of metal (most center plates are on the order of 1/8" thick); you can use the recommended focal length setting to set the feedhorn location but you can not use it in calculations without adding the thickness of the center plate.
