Famaval Dishes

[MikeO]

Good morning guys,

I wanted to post some information about Famaval dishes which I hope may be useful to others.
Since we stared to have a few 2D reception issues in Cyprus, I decided to carefully check my dish in order to squeeze every last dB out of it. According to the literature and posts by others on various forums, the focal length of a Famaval is exactly 93cm. So I first decided to confirm this. Measuring the depth and diameter of my setup (including the petals) I obtained the following:

Diameter (D): 307cm
Depth (d): 62cm

Using the equation: F= (D*D)/16d

I obtained a focal length of 95.3cm (giving an F/D of 0.31).

To double check this, I also measured the parameters of the inner portion without the petals as the focal length should be the same. I found the following:

Diameter (D): 231cm
Depth (d): 35cm

Again, using the formula, I got exactly the same focal length of 95.3cm (giving an F/D of 0.41), thereby confirming the first estimation.

I measured the actual focal length which my dish was installed at, and found it to be 94cm.
(For a diagram of these parameters, see attached jpeg).

Thus it appears that we may be taking this magical figure of 93cm as 'red' even if it may not always be totally correct. It may be something we should consider when fine tuning our setups.
This may make sense why I have found that inorder to get a clear picture from 2D my scalar rings have to be fully screwed on as far as they will go, a fact which has been pointed out usually has a negative impact on reception.
Unfortunately it started to rain heavily so I havent had a chance to adjust my setup accordingly, but thought I'd share these findings in case it may help others.

Has anyone else seem something similar?

Cheers,

Mike

 

[Channel Hopper]

Are you using a prime focus Famaval ?

I did have issues (quite a few years back ) on the offset version (and a really odd prototype Gregorian). Never bought more than three of them

 

[MikeO]

Yep, its prime focus. I dunno if its a coincidence but it seems to have made a difference too....no pixellation at all last night on BBC. Even ITV from 10832 was good practically all night except for a few minutes of break up. I need to see if last night was a one off, otherwise for me it seems to have helped.

 

[Huevos]

Those dishes seem to work best with the feedhorn pulled right out so the scalar ring section is almost touching the black plastic holder. Sometimes you even need to put spacers under the LNB holder to get it further from the dish.

 

[T_G]

If possible, could you post some pictures of your dish and feedhorn?