When it comes to picking up weak satellite channels, especially at the outer edge or even just outside of the required satellite, the user often has little choice but to get a bigger dish. In general, most satellite dishes up to 1 metre are offset dishes and provide similar performance. They also easily fit in most cars and are relatively inexpensive. However, once one goes beyond 1 metre, there are many things to consider to avoid a disaster, especially when a dish is sold for what appears like a great bargain.
Before picking up a large dish, it is worth checking if planning permission is required, especially in a built up area. Generally this is unlikely to be an issue if the dish will be located in the country out of site behind a building, but if it will be clearly visible from the road or especially from a neighbourâ€™s window, this could be a problem. If a nearby neighbour will be able to see it from any of their windows, do check with them first to avoid a potential problem later on. :doh:
The larger the dish, the more it catches the wind. So a strong heavy duty mast is absolutely essential, especially for satellite dishes over 1.2m (4'). For 1.2m or larger dishes, do not use any mast pole less than 750mm (3") in diameter. A good quality mast, especially if professionally installed, may cost as much or even more than the dish itself!
Prime Focus vs. Offset
A prime focus dish has the LNB & feedhorn in the focal point in the centre, where as an offset dish has the LNB at an offset to the centre. As a result, the LNB & feedhorn are in the beam path of the incoming signal on a prime focus dish, thus reducing the effective surface area of the dish. However, as the incoming beam bounces off an offset dish at an angle, the LNB is not in the way of the incoming signal. This issue is worse for smaller prime focus dishes where the LNB & feed cover more of the surface area, which is why there are few (if any) prime focus dishes smaller than 1 metre (~3') in diameter. Unfortunately, offset dishes tend to get very expensive and difficult to make beyond 1.2m (4'), which is why most large dishes are prime focus and large offset dishes are so expensive.
Even though large prime focus dishes are generally cheaper than similar size offset dishes, that they are still rather expensive to buy new, especially solid one-piece dishes. On the other hand, itâ€™s quite common to see the occasional ads where people are selling off their old prime focus dishes for like Â¼ of the price of a brand new dish!
Unfortunately, when the price seems too good to be true, there is usually a catch involved and when it comes to large satellite dishes, it is quite easy to buy a dish that performs worse than a dish half its diameter!
After my experience of dealing with damaged prime focus dishes, here are some useful tips when looking for a bargain, especially a used dish:
String test â€“ Unlike an offset dish, a prime focus dish is not oval and should be perfectly round. To check whether or not it is warped, place a string across its horizontal axis and another string over its vertical axis, placed over the horizontal axis string. With tension on both strings, they should lightly touch at the centre. Repeat again, but with the vertical axis string below the horizontal axis string this time. If the strings do not meet in both tests, the dish is warped.
Surface â€“ Check for any signs of denting on the surface. While a minor dent may not appear that bad, it could be enough to deflect a beam from a nearby strong satellite onto the focal point and interfere with weaker channels.
Feedhorn â€“ All solid prime focus dishes should come with a matching feedhorn. The purpose of the feedhorn is to pick up as much of the reflected signal from the dish as possible, without picking up background noise from outside the edge of the dish. Most Ku band feedhorns have a C120 mount, which works with the widely available C120 LNBs on the market. If the feedhorn is not available, but the bars are present, universal feedhorn kits are available, but they will unlikely be as effective as a matching feedhorn designed for the dish, since they may only see up to a certain diameter of the dishâ€™s surface area or may even see beyond the dish edge, thus picking up background noise.
Solid dish â€“ Most prime focus mesh dishes have been designed for C-band reception only, where they have negligible effect on the signal, but as Ku-band uses a much narrower signal wavelength, these holes may reduce the gain and may even cause refraction from nearby strong satellites to be picked up by the feedhorn. Another drawback with large mesh dishes is that they are easily warped, especially during transport. On the other hand, mesh dishes do have a few advantages over solid dishes: For C-band only reception, such as for free-to-air satellite reception in the US, a new mesh dish is generally a lot cheaper than a new similar size solid dish. A black or dark mesh dish blends in much better with the background than a solid dish.
Mount â€“ A polar mount allows the dish to follow the arc of geosynchronous satellites in the sky when swung left/right. Usually a 36v actuator holds the dish from freely swinging. A 36v actuator can be easily picked up if it is not present. If the mount does not have a motor/actuator or a place to mount one, it is likely a fixed point mount. Note that DiSEqC motors are not suitable for large dishes and should not be used with dishes over 1.2m!
Rust â€“ Most satellite dishes have a steel mount, steel threads and screws. In a wet climate, it is quite common to find 2nd hand dishes with rust built up on these parts. For a fixed point dish, generally a light amount of rust is only an inconvenience when positioning the dish and can be easily treated with periodic cleaning and oiling to prevent it spreading. However, if the mount or threads have significant rust built up, it is worth avoiding the dish unless a replacement mount can be obtained for it. A rusty mount makes it difficult to accurately align the dish, especially with polar mounts and larger dishes that are much more sensitive to accurate alignment. Heavy rust build up also weakens the mount, which makes it likely to crack or even break apart with a strong gust of wind.
Colour â€“ Like their smaller cousins, large dishes come in black, grey, white and even transparent. A dish of the right colour can avoid making it appear unsightly. For example, a white dish will not stand out as strong against a white wall or background. The same for a black dish placed against a dark background. Where the dish is out in the open, a grey dish blends in better with the sky, especially on cloudy days. Finally, where it is important to have a dish that has minimal impact on the area, a transparent dish may be the best choice. Transparent dishes generally cost a lot more than similar size metal dishes. If the dish will be used for C-band only reception or combined C-Band and Ku band reception with stronger KU band satellites, a mesh dish is a much cheaper alternative to a transparent dish.
Size - While larger dishes are required for weaker satellites, it is worth checking out Lyngsat to see if the channels of interest are available on other satellites that can be picked up with a smaller dish. Very large dishes (especially over 1.5m or 5') are often unusable in very windy conditions, especially high up in the open or near the coast. In wintry conditions, prime focus dishes easily collect snow. With a large enough build up, this can cause motor burn out on a motorised dish if an attempt is made to move the dish to another satellite. Excessive build up may even warp the dish or even cause the mount or mast to break or collapse altogether! :eek: