When we connect our computer to the projector using a long cable, the picture is poor quality. It is fine with a short cable.
The longer the cable run, the higher the risk of signal drop. This is why you need to keep in mind the level of quality for your situation. When we install a Sanyo multimedia projector, sometimes we need to use a cable which is 150 to 200 feet long in order to reach from the computer to the projector. This works well with the quality of cable and projectors we use. With other projectors however this is not always the case.
The first thing I would question is the quality of cable. We use double shielded, high quality VGA cables, from 15 feet to 150 feet long. If your cable is not very good quality, you will lose signal strength over the cable length. You can sometimes determine the quality of cable by the diameter. The outside diameter of your cable should be at least 3/8 of an inch if the cable is 25 feet or longer. If it is any less than this, it is probably not very good.
7456 Series Double Shielded VGA Cable
Assuming your VGA cable is good, you can boost the VGA signal using a VP-200XLN VGA amplifier. To do this, connect the computer to the VP-2XL using a short VGA cable (6 feet) then use your long cable from the VP-2XL output to the projector. The VP-2XL has a gain control to increase the strength of the VGA signal. You can adjust this to obtain the best picture on your projector. The unit also has a second VGA output which enables you to feed a second projector or computer monitor from your single computer output.
VP-200XLN VGA Amplifier
Feeding Video through Cat-5 or Cat-6 Cable
Another approach for feeding Video over long distances, or through areas where it is difficult to use VGA cables, is to use "Baluns". This method uses a transmitter at the computer end to convert VGA to a balanced signal and a receiver at the projector or monitor end to convert back to VGA.