Dish Antenna Design for Es'Hail 2 - some thoughts
The new Es'hail2 satellite with its amateur radio transponder has resulted in a rush of people getting gear to use it. Pretty much an essential item is a dish of some sort. TV dishes are cheap and so they are a popular choice. TV dishes are generally offset-feed types. This means that the LNB and feed support don't obstruct the dish (improving gain) and it also makes them easier to mount. For optimum feed performance offset dishes do require a fairly complex feed design. The photo below (from www.satcure.co.uk) shows two feed types. The one on the left is designed for for a circular dish while the one on the right is for a Sky Mini-dish (offset and wider than it is high). Thus we note that each different design of dish has an optimum feed arrangement. Interestingly different size dishes with the same focal length to diameter ratio can use the same feed design.
It's tempting to think that the feed arrangement is optimised for maximum gain but, in the case of a receiving system, there is a tradeoff between gain and ground noise. Optimum gain comes with a high level of illumination of the dish which gives rise to sidelobes and means that the noise contribution of the ground can be seen by the feed. So the optimum receive performance is often achieved with a small degree of under-illumination. At 10 GHz (for Es'hail 2) this beam-forming for the "perfect" feed is readily achievable with a carefully designed horn antenna. [note to the unwary: the plastic covers on some feeds are part of the beam-forming arrangement]. Es'hail 2 requires linear polarisation (vertical) for the 10 GHz downlink.
For transmission on Es'hail 2 using a dish, the requirements (for amateur radio use at least) are simpler - go for maximum gain. For Es'hail 2, the transmission frequency is 2.4 GHz. At this low frequency, high quality beam-forming is tricky - especially as ideally our feed system will represent a point-source in the focus of the (small WRT wavelength) dish. A further complication is that the uplink requires right-hand circular polarisation. Thus the feed itself needs to be left-hand circular as the sense will be reversed on reflection by the dish surface. Linear polarisation can be used but incurs a 3dB penalty. If a single dish is used, the feed for transmit and receive also need to be co-located giving more challenges.
Simple two-frequency patch feeds have been developed for Es'hail 2. However, they are likely to be suboptimal for transmit and receive. Fortunately this can easily be compensated for with a slightly over-size dish. Unfortunately, if you are looking to develop the smallest possible antenna system, and use the lowest possible power, for Es'hail 2 you will not be able to ignore the design criteria discussed above.