F/D of T-90

[Wescopc]

Does anyone know what the F/D ratio of the Wave Frontier Toroidal T-90 Satellite Dish is. I want to get some new Standard LNBFs that are the best match for this dish. It seems that the angle of the wave guide entrance would make a difference for the db gain of LNBF. I notice that different LNBFs call out different F/D ratios ranging from .5 to .8.
Bob

 

[ftarock]

The focal point for offset dish is pretty much fix. If F/D needs to be adjusted, simply move lnbf forward/backward to get best signal.
The feedhorn type is more importand that needs to be accounted, rather than the F/D.

Does anyone know what the F/D ratio of the Wave Frontier Toroidal T-90 Satellite Dish is. I want to get some new Standard LNBFs that are the best match for this dish. It seems that the angle of the wave guide entrance would make a difference for the db gain of LNBF. I notice that different LNBFs call out different F/D ratios ranging from .5 to .8.
Bob

 

[Mr Tony]

I use a variety of LNB's on mine with no issues

Sadoun .4 single
Sadoun .4 dual
DMSI .5 dual
DMSI .4 single
Xtreme II .3 single
ExpressVu .6 (or higher) dual (when they were KU)

Havent noticed a difference in each one. My .6 seems to be just as strong as the .3

 

[Wescopc]

The focal point for offset dish is pretty much fix. If F/D needs to be adjusted, simply move lnbf forward/backward to get best signal.
The feedhorn type is more importand that needs to be accounted, rather than the F/D.

I am not talking about adjusting the F/D but rather getting a LNBF that matches the F/D of the T-90.

Perhaps some are thinking of the the noise figure that is stated in dB. I am thinking of the Ratio of the Focal distance divided by the Diameter of the dish.

Ice-
It is good to know that you haven't see any difference between you LNBFs, but I wonder about the specs on the XtremeII. I show a F/D Ratio of 0.5 ~ 0.7 in a spec sheet I have for the XtremeII.
Bob

 

[ftarock]

Your are confused. Why you are so concern with the F/D ratio, it's just a factor for calculating Focal Length correctly that is easilly to adjust by move the lnbf forward/backward.
To understand F/D, Focal Length and other things, check out the link below:
http://www.geo-orbit.org/sizepgs/tuningp4.html#anchor799066
That's for C-band, but hopefully that will clear some confusion that on your concerns.

I am not talking about adjusting the F/D but rather getting a LNBF that matches the F/D of the T-90.

Perhaps some are thinking of the the noise figure that is stated in dB. I am thinking of the Ratio of the Focal distance divided by the Diameter of the dish.

Ice-
It is good to know that you haven't see any difference between you LNBFs, but I wonder about the specs on the XtremeII. I show a F/D Ratio of 0.5 ~ 0.7 in a spec sheet I have for the XtremeII.
Bob

 

[Wescopc]

Thanks Ftarock for the link, I think it helps me state my reasoning more clearly.

If F/D ratio determines the feedhorn illumination angle. Then wouldn't it follow, that when the F/D RATIOs of the dish and the feed horn match, the gain would be the highest?

If the F/D ratio doesn't matter - then why do the LNBF manufacturers publish it?

I have looked, and it appears that F/D ratios are not published for LNBs only LNBFs.

Perhaps, this is all to much nit pik, I had just thought that if some one could give me the F/D for the T-90 I could do a better job of matching my LNBFs. I have sent a note to Wave Frontier asking for the F/D. Thanks again for your help.
Bob

 

[mikekohl]

LNBFs contain an LNBF and a feedhorn device, so manufacturers publish specifications such as f/d ratios due to the need to best match the feed device.
Prime Focus antennas are typically between 0.3 and 0.4, while Offset type antennas generally fall between 0.6 and 0.7 f/d ratio.

If you have an LNBF that claims 0.5 to 0.7 f/d, then it would be pretty forgiving about use on just about ANY offset antenna. As was mentioned earlier, the focal point, where all of the signal concentrates, is much more important than the f/d ratio. Formulas are nice to have when figuring out the "big picture" of theoretical focal distance, as derived from the f/d ratio TIMES the diameter in equivalent units such as inches or centimeters...but the reality is that if you have the correct focal distance from the manufacturer, you should need only to double check that with a tape measure, and then use a signal strength or quality meter to see if signals can be improved by moving slightly in or out from that distance (and tweaking polarity for best match while you are at it).

There is no need for an LNB to claim an f/d ratio, because they are physically connected with a feedhorn element, which itself should have f/d specifications.

 

[Wescopc]

Thanks Mike,
We are on the same page. I understand that the focal length is built in and can be adjusted slightly by sliding the LNBF in or out. With out question the horn needs to be placed such that the focus is within it's sweet spot.

Because the T-90 has two reflectors and can accommodate 3° spacing I figured the the illumination angle would be different then a single reflector 90cm dish. If that is the case then it would perhaps be useful to match the LNBF to the F/D of the T-90. Hence my OP question. I waiting to see if the manufacturer will respond to my email.
Bob

 

[ftarock]

Don't mean to offend you but by design the T90 already has a curve bar that hold the lnbf holder. And I am sure the manufacture already do the calculation when they design that, so the lnbfs are place in a position that get the max of signal.
As mentioned earlier, Ku focal length is pretty much fix, if needed to adjust the focal length, simply moving the lnbfs in/out.
Back to the C-band, lnb don't have F/D becuase it's used with the feedhorn/scalar rings. Some feedhorn has the F/D marked but they are pretty much useless because one must go for the focal length when setting up the lnb to find the sweet spot.

Thanks Mike,
We are on the same page. I understand that the focal length is built in and can be adjusted slightly by sliding the LNBF in or out. With out question the horn needs to be placed such that the focus is within it's sweet spot.

Because the T-90 has two reflectors and can accommodate 3° spacing I figured the the illumination angle would be different then a single reflector 90cm dish. If that is the case then it would perhaps be useful to match the LNBF to the F/D of the T-90. Hence my OP question. I waiting to see if the manufacturer will respond to my email.
Bob

 

[Wescopc]

Don't mean to offend you but by design the T90 already has a curve bar that hold the lnbf holder. And I am sure the manufacture already do the calculation when they design that, so the lnbfs are place in a position that get the max of signal.
As mentioned earlier, Ku focal length is pretty much fix, if needed to adjust the focal length, simply moving the lnbfs in/out.

Thanks for you input ftarock, you are correct the T-90 has the curved bar and the focal length already designed in. I don't know the focal length, but I suspect that it is longer to accommodate the waves being reflected twice. So while the LNBFs are certainly in the position to capture the maximum signal, their built-in geometry (scalar rings) might not make the most efficient use of collecting and collating the RF waves.

Like I said before, it is probably not enough difference to matter, but I would like to know the F/D number just to see how much difference there is.