divibi said:
Hello CH,
Thank you for writing insightful posts and illuminating us.
I've got a few questions. Let's say I've got a perfectly made prime focus dish with f/D = 0.40, matched with a custom-made appropriate feed, rated at exactly f/D= 0.40, so would there be a need for any PEAKING, either for illumination or noise? Is there a rule of thumb, saying, for example, that if you have your dish in the garden you go for maximum illumination and if it's on the roof, then minimum background noise? And how do I know there is too much background noise?
Secondly, how do I position the feed for minimum background noise - does the feed have to be moved towards the face of the dish by half a millimetre for that ???
Peaking of a single piece prime focus dish (even with perfectly matched feedhorn) is required to get the best out of it. All decent manufacturers will have a specification that is optimised using fine tweaking of the reflector and feed .
A visual look of the reflector for dents, and a straight rim can often bring up signs of damage , that even the best feed cannot compensate for.
The most common method is the use of a boresight tool to get the feed exactly at the centre of the reflector, however this needs to be carried out in conjuction with centralising of the feed, which may require lengthening or shortening one or more of the feed arms. Stringing is used to ensure the dish surface is at its minimum distortion.
Next would be the shakedown of any petalised dish, usually by slackening off all bolts whilst the dish is facing skywards, and retightening evenly all round.
Lastly (and this usually only occurs on very large dishes) is a counterbalance behind the mount, securely attached to the backplate that compensates for the changes in elevation and axial rotation, where the weight of the dish can distort the reflector.
On metal dishes, there is also a possibility of the changes in temperature of the reflector between day and night will throw out the beam at one of the extremes, and to reduce this the dish should be assembled and tightened up half way between the hottest and coldest points. Use of cooling or heating panels can also be used to reduce the extremes.
In all assembly the best possible method for optimising would be with a spectrum analyser, taking careful notes between each adjustment.
All the above comes before working on the best illumination for the particular location and position of the dish on the property. Background noise can occur as a result of using the wrong type of dish, ground noise, airborne interference, or even proximity to some other electronic equipment. In all cases the spectrum analyser can be used to find out the best solution, in the worst case I have had to dig a trench to put the dish in and install a microwave filter fence to stop interference from an unknown source of noise on the horizon. On some occasions a change of LNB with a different local oscillator has been required to prevent one particular frequency from swamping the required signals once downconverted to the IF tuner frequencies.
