Advice Needed 1.5m GIBERTINI or 1.8m Raven?

[PaulR]

But from satellite's and LNB's point of view, the offset appears as 1.8 x 1.8... doesn't it?

Absolutely - we are comparing prime focus with offset after all.

 

[Channel Hopper]

Absolutely - we are comparing prime focus with offset after all.

The LNB is fed by a matched feedhorn though, and an Offset feed (at least on a 0.6 - 0.8 f/d ratio dish) is going to be looking at far more signal (and somewhat less noise) than a single prime focus reflector system with f/d of 0.3 - 0.5 .

Only one reason why Cassegrains are in high demand.

• Another advantage, important in satellite ground antennas, is that because the feed antenna is directed forward, rather than backward toward the dish as in a front-fed antenna, the spillover sidelobes caused by portions of the beam that miss the secondary reflector are directed upwards toward the cold sky rather than downwards towards the warm earth.[2] In receiving antennas this reduces reception of ground noise, resulting in a lower antenna noise temperature.
• Dual reflector shaping: The presence of a second reflecting surface in the signal path allows additional opportunities for tailoring the radiation pattern for maximum performance. For example, the gain of ordinary parabolic antennas is reduced because the radiation of the feed antenna falls off toward the outer parts of the dish, resulting in lower "illumination" of those parts. In "dual reflector shaping" the shape of the secondary reflector is altered to direct more signal power to outer areas of the dish, resulting in more uniform illumination of the primary, to maximize the gain. However, this results in a secondary that is no longer precisely hyperbolic (though it is still very close), so the constant phase property is lost. This phase error, however, can be compensated for by slightly tweaking the shape of the primary mirror. The result is a higher gain, or gain/spillover ratio, at the cost of surfaces that are trickier to fabricate and test.[3][4] Other dish illumination patterns can also be synthesized, such as patterns with high taper at the dish edge for ultra-low spillover sidelobes, and patterns with a central "hole" to reduce feed shadowing.
• Another reason for using the Cassegrain design is to increase the focal length of the antenna, to improve the field of view[2][5] Parabolic reflectors used in dish antennas have a large curvature and short focal length, to locate the focal point near the mouth of the dish, to reduce the length of the supports required to hold the feed structure or secondary reflector. The focal ratio (f-number, the ratio of the focal length to the dish diameter) of typical parabolic antennas is 0.25 - 0.8, compared to 3 - 8 for parabolic mirrors used in optical systems such as telescopes. A "flatter" parabolic dish with a long focal length would require an impractically elaborate support structure to hold the feed rigid with respect to the dish. However, the drawback of this small focal ratio is that the antenna has a small field of view, the angular width that it can effectively focus. Modern parabolic antennas in radio telescopes and communications satellites often use arrays of feedhorns clustered around the focal point, to create a particular beam pattern. These require good off-axis focusing characteristics. The convex secondary reflector of the Cassegrain increases the focal length, and thus the field of view, so these antennas usually use a Cassegrain design.
• The longer focal length also improves crosspolarization discrimination of off-axis feeds,[2] important in satellite antennas that use the two orthogonal polarization modes to transmit separate channels of information.

http://en.wikipedia.org/wiki/Cassegrain_antenna

 

[archive10]

I recently purchased the raven 1.8m from a certain
online store and it actually has the "channel master" branding on the back of the dish

I have one ASC typ 180 and one Channel Master type 180 dish, they are identical and both say "Channel Master" on the back.

...that's because they've all been made with the same (original) mold.
I talked to a couple of people from Skyware at a tradeshow recently, and they told me that they were still using the same mold (only one ever existed), hence it still says "Channel Master" on the back of a Skyware type 180.

 

[Channel Hopper]

The main reason Skyware use the Channel Master is down to the automatic type approval given to the dish as a standalone device. The cost of approving any new dish design for receive transmit for Ku band use is considerable.

 

[danaddison]

I recently purchased the raven 1.8m from a certain online store and it actually has the "channel master" branding on the back of the dish

can u post pics of your 1.8m dish please front and back thanks