I have seen on ebay that there are some 3 degrees monoblock LNB available but I would not want to buy one without first knowing that 3 degrees is what I need exactly to achieve this setup.
Is there a fixed schema to calculate the right degrees factor based on the size of the dish used?
I mean, with an approximate 6 degrees I get two satellites which on the 80cm dish I could only get by considerably separating the 2 LNB's to get 13E and 28.2. Do you think with a 3 degrees separation from the center LNB on 13E would I be likely to get 19.2E also?
For the spacing of LNBs (in inches or cm) you can use the formula:
distance = fG x ( (dA x PI) / 180), with [focal distance = fG, Azimuth difference = dA]
Or easier:
distance = fG x dA x 0,01745
(see
Multifeed installation and exact calculation of LNB position )
For London 51.4N, 0.3W,
dA(13 and 19,2E) = 7,55 degrees (and elevation difference = 1,47 degrees)
For effective focal distance (from LNB to G-spot) you could calculate with the program DishChar (see
Antenna's focal point - "le point G" ); but more easy is the
rule of thumb for a offset dish that you measure the distance between front of LNB-holder and dish-surface at 40% of the dish-height (40% from bottom of dish, so 60% from top of dish), and use that value as the effective focal distance.
Now you can calculate the needed distance between the two LNBs (or the distance between the two necks of a monoblock LNB .
A few mm's off would not be a problem.
Monoblock LNBs are usually characterised as X-degree spacing for Y-diameter dish,
but only for specific regions on earth and only for dishes with standard f/D-ratio!
So the distance calculation gives you much more exact information, and you should ask/measure the distance centre to centre of the "necks" of the wanted monoblock LNB.
In your case you could probably use LNBs with 23mm necks instead of 40 mm necks. But be carefull, some of those LNBs are extremely sensitive to rain! You have to pick them wisely.
BTW: The holder of the two LNBs should be at an angle of exactly 11 degrees to the horizontal in London 51.4N, 0.3W; the LNB for 13E lower than the 19E, as to adjust to the difference in elevation.
BTW2: I don't know if there are monoblocks (or 23mm neck LNBs) that are adjusted for dishes like yours, which are wider than high. If not, then you lose effective dish surface for reception! For a normal monoblock you would need a normal offset dish, which is higher than wide. I agree with
@davemurgtroyd there.
Are you allowed to have a replacement dish with the same surface area, but another form? (And probably with a bigger effective focal distance, which would maybe make it easier to receive 13 and 19E with separate LNBs? ;))
greetz,
A33