Reply to thread

Message

<blockquote data-quote="a33" data-source="post: 1080912" data-attributes="member: 332642">Only with prime focus dish, this 'center of dish' is right.'Focal distance' is a property of a parabola: distance from vertex to focal point.Only with a prime focus dish, the vertex is at the center of the dish. So for a prime focus dish, your statement is right.With an offset dish, the focal distance is also the distance between focal point and vertex; where vertex means: center of 'mother prime focus dish', where the offset dish 'is cut-out from'. Usually the vertex is not on the offset dish, but below!You are not the first (alas), to use the term confusingly.For offset dishes, there is also an &quot;effective focal length&quot;, used e.g. for multifeedcalculations. But this also has nothing to do with the center of the dish; it is distance from focus to &quot;G-spot&quot; (where bisector line between dishtop to focus to dishbottom, crosses the dish surface).The f/D-ratio also uses the proper focal distance to vertex.For a prime focus dish, there is a simple, straight-forward relation between f/D-ratio and illumination angle of the LNB feedhorn.For an offset dish, the relation is quite a bit more complicated. For better comparison of illumination angles, sometimes the term 'f/D-equivalent' is used, and calculated in a different way than f/D. However, using the illumination angle value is least confusing, I would say.Here endeth the lesson....... <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite53" alt=":confused" title="Confused&nbsp; &nbsp; :confused" loading="lazy" data-shortname=":confused" />&nbsp; Thanks for your attention......greetz,A33</blockquote>

[QUOTE="a33, post: 1080912, member: 332642"] Only with prime focus dish, this 'center of dish' is right. 'Focal distance' is a property of a parabola: [B]distance from vertex to focal point[/B]. Only with a prime focus dish, the vertex is at the center of the dish. So for a prime focus dish, your statement is right. [B]With an offset dish[/B], the focal distance is also the distance between focal point and vertex; where vertex means: center of 'mother prime focus dish', where the offset dish 'is cut-out from'. Usually the vertex is [B]not on[/B] the offset dish, but[B] below[/B]! You are not the first (alas), to use the term confusingly. For offset dishes, there is also an "effective focal length", used e.g. for multifeedcalculations. But this also has nothing to do with the center of the dish; it is distance from focus to "G-spot" (where bisector line between dishtop to focus to dishbottom, crosses the dish surface). The f/D-ratio also uses the proper focal distance to vertex. For a prime focus dish, there is a simple, straight-forward relation between f/D-ratio and illumination angle of the LNB feedhorn. For an offset dish, the relation is quite a bit more complicated. For better comparison of illumination angles, sometimes the term 'f/D-equivalent' is used, and calculated in a different way than f/D. However, using the illumination angle value is least confusing, I would say. Here endeth the lesson....... :confused Thanks for your attention...... greetz, A33 [/QUOTE]

Verification